Sound Cloud custom player - "id property is null" error

Question: I'm having some issues working with the Sound Cloud custom html5 player (http://developers.soundcloud.com/docs/custom-player) specifically on internet explorer (tested on version 7, 8 and 9).
It seems whenever the player is created (right now this is dynamically through JS) it is running into an error with a function in the "sc-player.js" file (provided by sound cloud here).
The error is "Unable to get the value or property 'id': object null or undefined". This is the code that messes up: line 640 character 9
 // selecting tracks in the playlist
  $('.sc-trackslist li').live('click', function(event) {
var $track = $(this),
    $player = $track.closest('.sc-player'),
    trackId = $track.data('sc-track').id,
    play = $player.is(':not(.playing)') || $track.is(':not(.active)');
if (play) {
  onPlay($player, trackId);
}else{
  onPause($player);
}
$track.addClass('active').siblings('li').removeClass('active');
$('.artworks li', $player).each(function(index) {
  $(this).toggleClass('active', index === trackId);
});
return false;
});

I understand this is trying to find the id of a track on the generated list, however I have hidden this list on the page, so really shouldn't that click event should not even fire? I have tried removing the click event all together but that seems to cause a number of other issues with the player.
Just wondering if anyone else has experienced this or knows how to get around it?
EDIT -
I manually attached "test" data the track. And this seems to solve that first error, but of course brings up another. This time the same error is spat out for this line (in the sc-player.js file):
getPlayerData = function(node) {
   return players[$(node).data('sc-player').id];
}

This is the code I use to call the player, the track argument being the link to a music tracks soundcloud page, and #player being the empty div its being created on top of:
function init_player(track,title) {
  $('.open #player').scPlayer({
   links: [{url: track, title: title}],
   beforeRender  :   function(tracksData) {
      $scplayer = $(this);
      $('.sc-trackslist li, .sc-trackslist li a').data({ id: track});
  }
}

When the user wants to change to another section of music, I then have another function which empties the container div, appends #player to it (because it was overwritten by the soundcloud player), and re runs the above function.
My issue is on the second time the init_player function is run, I get the error.
Sorry for the delayed response!
Comment: Alon is right, the error is here: `trackId = $track.data('sc-track').id`. Can you ensure that element has an attribute `data-sc-track`?
Answer: This line is wrong:
trackId = $track.data('sc-track').id,

I can't help you more then that because I need the rest of the code...


Fair or Foul Ruling?

Question: A pop fly is hit along the first base foul line between home plate and first base. It lands in fair territory, but with the spin, it bounces three feet into foul territory. Before the ball is allowed to bounce a second time in foul territory, the pitcher catches it in mid-air. No one touched the ball until it was three feet in foul territory, when the ball was caught in mid-air. Is this a fair or a foul ball, since it took place inside the first base bag?
Answer: The definition for "Fair Ball" in the MLB rules say:

If a fly ball lands in the infield between home and first base, or
  home and third base, and then bounces to foul territory without
  touching a player or umpire and before passing first or third base, it
  is a foul ball; or if the ball settles on foul territory or is touched
  by a player on foul territory, it is a foul ball.

Therefore, this scenario is a foul ball, given the pitcher who catches it in mid-air is also in foul territory.
Answer: It is a foul ball since it went foul before it passed first base. It does not matter that it did not touch the ground in foul territory. It is determined to be foul based on where the ball is when it is touched by the fielder.
Note: it does not matter where the fielder is standing when they touch it, only where the ball is in relation to the foul line. This is the umpire's judgment.

Fair or Foul Ruling?


With Friends or Group DD

Question: Hi so I recently played a campaign with my friends and I have to say I've had a real blast. So much so that I kind of want to make it a weekly thing. But they don't really want to play D&D very much. I live in a huge city and I know there's several clubs around here that play D&D but I don't want to go in and have it be super serious. I was wondering what are the key differences between playing in a formal d&d club versus playing with an informal group of friends? Also what seem to be the most common types of rules or restrictions in these clubs?
Comment: Unfortunately, this question's too broad to usefully answer. RPG clubs aren't homogeneous, and groups formed by groups of friends aren't homogeneous, so there's no general statements we can make about how the two differ.
Comment: Related: [Where can I find other RPG players?](http://rpg.stackexchange.com/questions/1232/where-can-i-find-other-rpg-players)
Comment: Hello Paws, welcome to the site. Check out the [tour] and [help]. RPG.SE is a Q&A site, and consequently isn't good at any sort of discussion, personal opinion and lists. Maybe you want to check out one of the many [forums](http://meta.rpg.stackexchange.com/questions/5449/ive-been-told-my-question-is-better-suited-to-a-forum-but-where-should-i-go) instead? I hope this is not off-putting to you, and if you have a question about the game itself, feel free to ask!
Comment: Hi Paws, and welcome! I made some edits to your question to try to make it fit our format better. If you disagree with the edits or I got something wrong, please feel free to make your own changes!
Answer: Paws! I'm new to D&D too. I was just like you a couple months ago, so let me share my personal experience.

What are D&D clubs/organized groups?

Not sure how to answer this without just repeating back clubs/organized groups that play D&D. So rather than what, I'll tell you where I went to find them. I went to Meetup.com, searched for local D&D groups and found a game store that held Adventure League nights. You just check in, say you'll be there, and then show up. Easy!

What kinds of rules or restrictions are common in these clubs?

Not sure what's common, because I only went to this one game store, but depending on whether or not it's an Adventure League night or something else, the rules and restrictions might be tightly focused on RAW rules (rules as they are written in the source material) or perhaps not. If they aren't, the DM ought to let you know at the beginning so you know what to expect.
I don't know if these qualify as rules per se, but in my experience there is certainly an expectation of your preparedness when you come to the game. Here's a list of things to bring:

a pencil
a PHB
a character sheet
some dice

I put pencil at the top of the list because that was the only thing that everyone at the table had a (jokingly) strong reaction to when I said I only had a pen. I would say the other three might be optional (the game store had dice, extra character sheets, and even a figurine I could borrow), but if you want to make a good first impression, come prepared.

What are the key differences between playing in a formal D&D club and playing in an informal group of friends?

Well, if it's an Adventure League night, you're not going to see anything homebrewed, and all the rules are going to be pretty strict to the book. Groups of friends may be more relaxed about that. Secondly, Adventure League was heavily encounter-driven. You don't really get a chance to roleplay. Or at least we couldn't at my DM's table, which is ultimately why I stopped going to that game shop. But I met a lot of people I gel with, so it's definitely worth going, checking it out, seeing what kind of game you like to play.
Good luck!
Answer: Go find a club, and check out DnD.wizards.com and look for where adventurer league events are hosted. They are usually held weekly. Our group is huge and hosts 4 full tables of DMs and players and we are already looking for a 5th Dm.
You may discover you develop a whole nother group of friends.

With Friends or Group DD


Why do people say they like "art" and "photography", instead of just "art"?

Question: Always puzzle me to hear people list photography (in the context of it being art) and something different than art. Is there a reason for this historically?
Answer: Not all photography is art, not because of some failure, but because it never meant to be that. Some documentary photography may be art, but most isn't. Some of it is just recording a moment. Most photography is decoration, or otherwise functional, not meant to be art, let alone upper-case-a Ahhrt, or Fine Art.
"What is art?" is a constant question —  in fact, it's a question with no fixed answer, but a continuing conversation. See some of this in the context of photography at What makes "fine art" fine art? In fact, a useful definition may be that art is any work which participates in that conversation — it's the dialog itself which makes something art rather than any intrinsic feature of the work. Clearly, that's beside the point of much photography, which simply isn't interested in that.
But you don't have to even be concerned with this question to appreciate the craft of photography, as many people do. Art is important, but the label — or its lack — isn't necessarily a value judgment.
And of course, there's no question that there's plenty of art that isn't photography:

I probably should have made the photography circle much smaller next to all of art, but hey, this is a photography site after all. :)
Comment: @blunders: well, in that case, isn't it just to distinguish the "art photography" from that photography not fitting that category?
Comment: +1 @mattdm: Right, that's why I said "(in the context of it being art)" -- as if even "art photography" was not "art", though I guess what art is and is not is never meant to be understood.
Comment: +1 @mattdm: Maybe, though I've always taken it as meaning no photography is art.
Comment: Some people probably _do_ mean that. Photography has always struggled for that sort of respect, since it is a uniquely accessible. But in general, that debate is long settled and it's clear that some photography at least _does_ stand as part of Art, and it really comes down to arguing about the specifics. Anyone so dismissive of photography in entirety probably just doesn't know what they're talking about.


What are the disadvantages of more suspension travel?

Question: Having some suspension travel is always nice, but what disadvantages are there for the bikes handling as the travel increases? This question applies to both to full and front suspension bikes.
Answer: Although this isn't explicitly part of your question, I'll go ahead and throw it in as it's one of the most important factors to consider in terms of increased suspension if you plan on pedaling your bike- the basic principle behind propelling a bike is to convert a mostly downward force (pedaling) into forward momentum (the drivetrain turning the back wheel which in turn moves you and the bike). The more suspension you add, the more difficult it becomes to transfer the energy of pedal strokes into the drivetrain. The better circles you spin the less you'll suffer from this, but the fact remains that even with clipless pedals the majority of the power input to the drivetrain is coming from your glute, quad, and calf pushing down on the pedal during the powerstroke. Extra suspension soaks this up- it's like running on a trampoline.
Now that that's out of the way, let's talk about how additional suspension affects handling... after we briefly cover the benefits of adding suspension travel. Additional suspension will not only smooth out bigger, abrupt bumps (rocks, roots, your riding buddy that crashed in front of you, etc), but it will also smooth out more gradual terrain changes at higher speeds. Additional suspension basically allows the bike to track better and for the rider to maintain better control of the bike over rougher terrain and at speed. More suspension also helps the bike and the rider absorb bigger drops. Combined with a slacker headtube angle and longer wheelbase (all 3 tend to come as a package) you're also given a larger margin for error; mistakes that may have sent you over the bars on a cross country bike may be recoverable on a bike designed for bigger hits. These benefits come with tradeoffs, however.
Additional suspension will cause "muted" handling- that is that the bike won't feel as sharp or as connected to the trail. It will not turn as quickly or as precisely. This is due to a multitude of factors that come along with longer travel bikes:

Wheelbase is longer because chainstays are longer. This is partially by design and partially because in order to make more room for more travel in the back, the rear wheel typically has to be moved out so it doesn't buzz the seat tube at the bottom of the suspension travel. Longer wheelbases make for bigger turning radiuses. Think of how much room it takes to turn a truck around vs a sports coup. You'll suffer the same sort of effect in tight turns. For a little more detail on how chainstay length affects bike handling, see my answer to another question here.
A slacker headtube angle almost always goes along with more travel. There are exceptions to this rule, but as you move from cross country to trail to all mountain to downhill and freeride bikes, the front of the bike will get more and more raked out. This stabilizes the bike at higher speeds but makes the bike handle "slower" at lower speeds. Slacker headtubes also make for more margin for error in terms of rider input through the bars, but in trade it takes more input to make the bike actually turn.
Weight increases. Bigger hit bikes use bigger tubes to make up the frame, more rugged components, and bigger, heavier duty forks and rear shocks. This is pretty much inevitable as it improves durability and stiffness. That extra weight equates to extra inertia that you have to move around under you, and in general that slows down bike handling.
More travel equals more time to change directions. One of the biggest reasons for this is that it takes a while to load all of that suspension up. You have to push against the shocks hard enough that they start push back harder than the forces of inertia in order to change direction, and this becomes especially noticeable in tight turns on long travel bikes. Other factors play into this, but suspension is the biggest one.
Brake dive goes up as suspension goes up. If you're not familiar with brake dive, it's when the front compresses down (and for full suspension bikes the rear typically comes up), which changes the geometry of the bike. The more suspension, the more dramatic the change in geometry under braking. Modern high end forks have a multitude of adjustments to combat this (slow speed compression adjustment, threshold adjustment, etc) but there's currently no technology that will fully make up for this, and the longer the travel in the front, the worse this will be.
As travel increases so does bottom bracket height which, as Vincent pointed out, raises the bike's center of gravity. Obviously this isn't a 1:1 ratio in terms of additional suspension and additional bottom bracket height, but it is pretty much unavoidable and does have an effect. As is the case with most of these points, there is typically some form of compensation for this. Some trail and all-mountain bikes come equipped with dropper seatposts (hit a lever on the bar and the seapost telescopes down) and DH/FR bikes have their seat height set much lower than a standard XC bike. Dropping seat height allows the you to drop your center of gravity, but ultimately your feet are on the pedals and that height is set, so the you will start out higher off the ground on longer travel bikes.

I'm sure you've noticed that I've talked a lot about other factors beyond purely the travel of a bike's suspension alone, but it is important to take into consideration the whole package as all of the factors mentioned here work in symphony to create the ride quality of a bike as a whole, and each factor attributes to the ride quality in its own way- both positive and negative. You can't consider suspension travel alone because you can't find a bike where all else is equal beyond travel. You can't take a cross country bike and give it 8 inches of travel and call it a downhill bike anymore than you can take an F1 car and add 3 feet of suspension to it and call it a Baja truck- Each is a purpose built platform in nearly every aspect.
Answer: disadvantages :

more weight (more material, more oil)
more energy required in order to pedal
the bike is less snappier because it "eats" some of the terrain, so dirt jumping tricks (e.g 360s, backflips, frontflips) are harder than on bikes with less suspension
some people believe that more suspension is not appropriate when learning to ride MTB and technical trails because the bike is more forgiving so it helps the rider a lot
Answer: Regarding how increased suspension adversely effects handling.
The first consideration is that to accommodate the increased travel you must make the bike taller so that the pedal/chainrings don't hit the ground as you utilize the increased travel. This results in higher center of gravity and the accompanying adverse handling changes.
Second; assuming there is an optimal suspension travel for the type of riding you are doing and you are asking, what are likly to be the advers handling effects of increasing travel? You also need to make the assumption that you will attempt to tune the suspension so that is used. Effects; Excessive loss of power to the suspension, sluggish dampened handling, difficult to tune and setup, excessive movement over terrain which can decrease stability when cornering.
This is not really an easy question to answer. To some extent you just keep getting more of the beneficial characteristics of suspension until them become detriments.
Answer: Suspension isn't something that can broken down simply, but let's try. First, you're asking about two different scenarios: front suspension and full suspension. Both setups can range from as little as 80mm on a hard-tail dirt jumper to 215mm on a full downhill/freeride bike.
Now, obviously, you're not going to want to grab your downhill bike to become the world's best XC racer. Why? For one, all bikes' geometry is geared around the purpose of the bike and how it's suspension aids that. Bikes with more suspension tend to have lower BB heights and longer wheelbases to aid in keeping the rider on the ground while traveling through the rough stuff. Shorter travel bikes usually have a steeper head angle to allow for greater steering: think of the XC racer weaving through tight trees.
A few of the disadvantages of increasing suspension:

Pedaling efficiency: Ever heard of "pedal bob"? It's the way a bike bounces up and down as you pedal it. Generally this gets worse with more, and more forgiving, suspension.
Jumping: When you try to jump that lip, or rock pile, you'll have to preload a bit with full suspension, not so bad with only front though.
Setup/Maintenance: The more moving parts you have the more complex your bike becomes. This adds more things to your maintenance list and more things that need to be adjusted for you, the rider. Also, this means more parts that can break or malfunction!
Weight: You won't find a XC bike with a big burly shock because it ways as much as a wheelset! (Maybe, depends on the wheels/shock) But the point is the big forks and shocks are just that, big, and they have weight. This can be reduced with things like air-sprung forks and titanium springs, but only so much.
Response: Sometimes you just don't "feel" the trail as much. This is more of a learning thing, but for new riders, they sometimes don't quite appreciate how gnarly that rock garden is when they blast over it with 6-8" of travel.

After all that, there are lots of benefits to more suspension (probably should ask a separate question for that). It really comes down to the rider, the terrain, and what the rider wants to get out of it. Want a super smooth ride and don't mind pedaling? Then 6" of travel could work. Want to boost every lip and root, but still manage to rip some good speed on the flats? Maybe a 4" or 120mm trail bike with full suspension or hard tail is more for you.


How do I encourage my cockatiels to continue feeding their chicks when they stop?

Question: My cockatiels stop feeding their chicks after about 5-6 days. Sometimes they feed them until they are 10 days old and then they'll stop. I have lost so many chicks due to this bad habit. What should I do make them keep feeding their chicks until they're fully grown?
Comment: For reference. A normal time period for the parent cockatiels feeding their chicks should be about 2 weeks, to 2-1/2 weeks before the breeder/owner would take the chicks away to start hand feeding them.
Comment: @MattS. parents stop feeding chicks and sometimes they die if they are not hand fed. what should i do so that parents keeps on feeding the chicks
Answer: There is a delicate balance between overseeing your cockatiels' hatchlings, and causing them stress. If the parents get stressed, or feel like they're in danger, they will stop caring for (even kill) their babies. Usually they will do this when they're still eggs, like if the breeder is looking into their nest too often, but it's possible that something is happening in the couple days that the babies have hatched where the parents don't feel safe caring for them anymore.
Another reason could be food. The link for Cockatiel Cottage explains that part. You need to be feeding your birds double what you normally would, plus regularly offering them soft food. That way they have enough for them, and the babies. If they don't have enough food for both, they will keep the food for themselves. Although there's the delicate balance again where you don't want them overfeeding the babies, as their crops need to be emptied every 24 hours, or they risk impaction.
The only other reason I can think of, is that there is something wrong with the babies, or they think there is something wrong with the babies. Birds can tell when their chicks are born with defects, or are too underdeveloped to survive. So it they don't think the babies will make it, they'll simply ignore them to focus on the ones that will. In this case, since all of the chicks are being ignored, there is something bigger going on.

I would suggest reading up on breeding cockatiels. Breeding any animal is a delicate process and should not be done without proper care. And since I do not specialize in birds, there is a great deal more you can find out from people other than me.
Cockatiel Cottage on Breeding
The American Cockatiel Society on Cockatiel Breeding
Avian Web on Cockatiel Breeding
I'm hoping the couple you're breeding aren't related, as trying to breed parents that are related to each other produces nothing but chicks with birth defects, in which case the parents would ignore them.
I would do some research into crop impaction, as that's my best guess as to what's happening here. If they're being overfed, fed the wrong foods, the temperature isn't right, they have an infection, then their digestive systems will shut down. That would cause the parents to ignore them.
Finally, I think the best thing to do is get someone who breeds cockatiels, or, even better, a vet to take a look at both the adult pair, and the babies. To make sure that there isn't any underlying issues that is causing this. Ultimately I can give you all the advice in the world, but I cannot see the birds, so I'm inevitably missing something. Being able to see them helps a great deal.
Ultimately, you're probably going to have to take the babies yourself and hand-feed them sooner than you would normally. The links I provided should help you with that as well.


Can ice magic freeze blood?

Question: In my story, one of my main characters has ice powers. She can turn water into ice, she can conjure ice (sucking moisture out of air and freezing it). She can also drop someone's body temperature, as she'd killed a whole lot of people with the great tool of hypothermia. But would I be taking it too far if I allowed her to be able to freeze the blood in someone's body, therefore killing them?
Comment: Welcome to Worldbuilding.SE helena, glad you found us.  Please check out our [tour] and [help].
Answer: Its your magic system, you can decide exactly what you think it should be.  Depending on your world, it may be taking it too far, or it may be a natural step.  We cannot decide, only you can.
I would, however, highly recommend reading up on Sanderson's Laws of Magic.  He's got a few laws that are very helpful for making a believable magic system, such as his first law:

Sanderson's First Law of Magics: An author's ability to solve conflict with magic is DIRECTLY PROPORTIONAL to how well the reader understands said magic.

If your main character has the ability to freeze someone, and she does so to resolve conflict, you will need to make certain the reader has a solid understanding of your magic.
Comment: Which, of course, does NOT necessitate long technical dissertations.
Answer: Blood is able to be frozen, and at a surprisingly high temperature. -.6 celcius and/or 31 fahrenheit.
However, the time it takes to freeze is determined by how fast the body temperature would drop to the numbers mentioned above, if you're going with that. You could say only the blood freezes and the body temperature does not drop, that would result in the blood starting to heat up, but by that time the person would die. (If in non-magic circumstances, that is.)
Comment: Fun addition, if a living creature's cells were to be frozen, when thawed they'll basically explode. As in, rupture with great force, but not explosion kind of explode. (Adding this in case you wanna have fun with the ice magic)
Comment: Depending on the body temperature drop, blood can freeze with it.

Can ice magic freeze blood?


Under the US 2nd Amendment, are there limits to the size or type of firearm?

Question: Are state of the art artillery pieces included, ICBMs, or the private ownership of nuclear weapons? If not, where is the limit in size and firepower drawn?
Comment: Isn't this a question about [law.se] more than [politics.se]?
Comment: the second amendment doesn't go into any specifics. it's perhaps the vaguest amendment
Comment: If you are a member of "a well-regulated militia" you'll need at least SMGs, howitzers, SAMs, flamethrowers etc to defeat [King George](https://en.wikipedia.org/wiki/Prince_George_of_Cambridge)'s military and avoid paying too much for hot beverages.
Comment: It's worth noting that the stance that the 2nd amendment was not about an individual right flies in the face of what the actual founding fathers said about it in things like the Federalist Papers.
Comment: I agree this is probably a better fit for Law.SE. A quick search found [this](https://law.stackexchange.com/questions/6761/what-argument-supports-a-right-to-bear-arms-but-denies-a-right-to-bear-nuclear-w) close duplicate, so I didn't flag for migration. I think this question is probably distinct enough, though.
Comment: This **is** on topic.  It **is** about the policy
Comment: @JimmyJames That is a very interesting and succinctly-written answer and clarifies the position well. I will upvote both that and your own question on the History site.
Answer: The entirety of the second amendment is:

A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed.

That's it. Pretty vague, eh?
It does not define "well regulated Militia". It does not define "security of a free State". It does not define "Arms" or even what "shall not be infringed" encompasses.
As such, it's an amendment that has been argued back and forth for a long time and likely will continue to be argued back and forth for some time to come. What the amendment does and does not pertain to is decided in court cases, for the most part.
There are some regulations out there codified in law. For example, The National Firearms Act does put some restrictions on certain weapons and accessories such as machine guns and silencers.
So, to answer the question: no, there are no real restrictions regarding the size or type of firearm directly under the second amendment. There are restrictions that were created afterwards and codified into various laws.
Comment: The bold-ed text appeared to go beyond OP's context - it somewhat stands on its own.  With the recent edit - it is clear.  Removing my comments.
Comment: @chux ["However, beginning in the 1920s, a series of United States Supreme Court decisions interpreted the Fourteenth Amendment to "incorporate" most portions of the Bill of Rights, making these portions, for the first time, enforceable against the state governments."](https://en.wikipedia.org/wiki/Incorporation_of_the_Bill_of_Rights)
Comment: @chux sure...they are called the 'bill of rights'. As my answer points out, the 2nd amendment makes no specific restrictions on individuals.
Comment: @chux we're talking in the context of restrictions..."to the size or type of firearm". Let me clarify that in my answer
Answer: The 2nd Amendment is subject to government regulations.
This is known as the National Firearms Act
First enacted in 1934 as a reaction to the St Valentine's Day Massacre in Chicago, it required registration with the federal government, and placed a heavy (for the time) transfer tax on what was categorized as NFA firearms: fully automatic, shotguns and rifles with barrels less than 18", firearms that had a bore larger than .5", explosive devices (grenades) and projectiles (mortar and cannon shells), and silencers. Registration was not a right: one can be denied the license for a NFA firearm. The specific target for the NFA of 1934 was the model 1928 Thompson, which had become the weapon of choice for gangsters.
The second part came in 1968, as a reaction to the shootings of Martin Luther King and Robert Kennedy. It banned the sale of firearms through the mail and across state lines, except for licensed dealers, who are required to keep records of all transactions - the Federal Firearms License. Also stopped registration of existing NFA firearms - if you hadn't registered them by then, you couldn't register them.
In 1986, sale of new NFA firearms was banned for the general public - only military and law enforcement. Existing NFA firearms were grandfathered in, and can still be purchased... with the price being substantially higher.
It is quite possible (and highly illegal) to modify existing semi-automatic firearms into fully automatic. The working theory right now is that this is what the Las Vegas shooter did, although there are a number of peculiarities about that incident - the alleged shooter doesn't fit the profile of the usual lone psycho shooter.
Rockets, being flying machines, are subject to FAA regulations. There are hobbyists who operate model rockets capable of reaching over 30,000 feet (carrying almost no payload), but their operation is strictly regulated due to the possibility of hitting an aircraft. One can build their own rocket, but testing it will draw government attention... and ire. Today, getting the materials for rocket fuel in sufficient quantity to build an ICBM will also draw a lot of attention, as those materials can also be used to build large scale explosives.
Nuclear materials in general are subject to other government regulations. It is illegal for a US citizen to possess fissionable material, under NRC regulations, as well as extremely difficult. Fissionable U-235 is very rare, and plutonium can only be made in a functioning reactor.
One ambitious young man tried to build his own reactor, using isotopes salvaged from smoke detectors, camping lanterns, and old clocks. He didn't succeed, but he did create high levels of radiation, enough to have his mother's home declared a superfund site for cleanup.
Comment: "2nd Amendment is subject to government regulations." Is it not the other way around: "government regulations.are subject to the 2nd Amendment" ?  Still good answer.
Comment: "In 1986, sale of new NFA firearms was banned for the general public"

This is incorrect. The Hayes amendment only applies to machine guns. Other NFA firearms (SBR, SBS, AOW, DD) are not so limited.
Comment: "quite possible (and highly illegal) to modify existing semi-automatic firearms..." - yeah, like 10 years in prison illegal. It's up there with manslaughter for sentencing.
Comment: Also, the NFA barrel requirement for rifles is 16 inches. Great answer overall though.
Comment: @chux-ReinstateMonica Presumably there have been challenges to their constitutionality and Supreme Court rulings, haven't there?
Answer: To answer your question in short:
Hard No:

Explosives (Grenades, bombs, det charges)
Chemical weapons (viruses, nerve gas, caustic gas)
Flamethrowers of any significant power
Anything you could not reasonably consider a personal armament (Field
artillery, tank cannons, helicopter gunship missile pods)

Legal, with restrictions (see also NFA Items):
These are items that are regulated by the National Firearms Act, and require paperwork + licenses in order to own, but are generally obtainable by private persons.

Silencers/Suppressors
Short barreled shotguns and rifles
Fully automatic weapons (very rare and expensive)
Exotic, hard to categorize weapons like ballistic knives (can be
registered as an AOW or "Any other Weapon")
"Large bore" firearms with a bore over 0.5 inches in diameter

Federally no problem, but many states regulate:

Semi automatic rifles and shotguns
Belt fed and crank fired weapons
Weapons with detachable magazines (i.e. you can reload them with a new magazine)

Generally OK even in hostile states:

Bolt action rifles
Pump action shotguns
Revolvers

Surprisingly lightly regulated everywhere:

Black powder weapons and antique muskets

Now for the long answer:
So, US constitutional law is a complex beast, because there is a lot of effort spent trying to "read the intent" of the authors. This can lead to some interesting results, such as Roe v Wade's implied right to privacy.
There is split opinion over how much to weight the original intent of the founders when writing legislation, and even beyond that, considerable disagreement, even at the level of the Supreme Court, over what the Bill of Rights actually protects.
The "Majority" opinion is that rights in the Bill of Rights are individual, actively protected rights. As in, no state government, nor the federal government, may make laws that are in abrogation of the terms of those amendments. Some justices have historically believed that the Bill of Rights merely ties the hands of the federal government, and not the state government. You can read more on that topic here. As in, if the state of Kentucky wanted to make a law banning free speech, they are free to do so, but Congress may not pass a similar law.
I'm not nearly a studied enough scholar to tell you what the Second Amendment actually means, but I can give you a run-down on how it is commonly interpreted. Other people have given better blow-by-blows leading up to the Hughes Amendment and the NFA, but firearms were not very tightly regulated until rather recently. Generally the distinction was made between "firearms" and things that really weren't "personal arms", like cannons, artillery, rocket launchers and so on. Also "explosives" are tightly regulated under separate laws, so anything that uses explosives is part of a whole different discussion.
But as we approach the present, you had much more effort spent on categorizing and banning weapons by features, usually driven by high-profile criminal incidents. The "high water mark" of regulation was under Bill Clinton, with the Clinton Assault Weapons Ban, that introduced unprecedented restrictions on the sale of weapons.
It set a bunch of frankly arbitrary categories that separated "weapons" from "assault weapons", like bayonet lugs, muzzle brakes, pistol grips on rifles, detachable magazines and so on. The rhetoric being that "assault weapons" were only useful for soldiers in war, and not suitable for a civilian to own. The ATF has a term called "no sporting purpose" that they use to regulate certain weapons. In the US, a weapon with a muzzle bore of greater than 0.5 inches in diameter is considered a "destructive device" (that is, more a cannon than a firearm).
However, you will not that a common 12 gauge shotgun has a bore diameter closer to ~0.60 inches. Shotguns are exempt from this regulation because they are "common sporting weapons", and attempting to ban them would have upset millions more people than regulating semi-automatic rifles. However, many "combat shotguns" like the "Street Sweeper" were determined by the ATF to have "no sporting purpose" and subsequently banned by being re-labeled as "destructive devices".
I bring up "common use", because the biggest piece of law in US Firearms history came in 2008 with DC vs. Heller in a highly controversial 5-4 decision, the Justices found that the Second Amendment was an individual right, and that individuals had the right to own "weapons in common use".
The whole "common use" thing, is more or less their attempt to interpret how the framers of the constitution talked about guns and gun regulation in their various writing and correspondence, as well as to let the "social bellwether" regulate the issue, much like censorship and obscenity laws often have "reasonable person" or "community standard" components to them.
Of course, this brings up a whole other nest of hornets; what is common use? The gun rights folks would tell you the AR-15 is a "common weapon", and in a numerical sense, they are absolutely correct, as they are extremely popular, common, and frequently used by sportsmen and hunters. However, many people are unsatisfied with this, feeling that semi-automatic rifles place too much power in the hands of people.
Now, this is all interesting history, but what does this come down to in the end? Basically, the law is pretty sure Americans have the right to buy guns, but they're not sure how much power the state has to regulate the purchase of them, and the ATF is more or less winging it when it comes to defining "acceptable" vs "unacceptable" weapons.
When you regulate a constitutional right, a common standard is "undue burden", as in, does this law place a substantive barrier between the citizen and the exercise of that right. For example, a 3 cent tax on book shops doesn't stop anyone from distributing literature, but a 100000$ tax on printer ink would place an unacceptable barrier in front of anyone who tried to spread their political views in writing. Similarly, it seems to be fair game to make people get a background check before buying a gun, but some states like New York put into place months of waiting and hundreds of dollars in fees.
All of this is separate from issues about what is moral, and what is right, but New York, California, and other places with stringent gun laws are frequently and repeatedly sued over them, California is currently a hotbed of lawsuits over the raft of gun control bills Jerry Brown signed recently. Interestingly enough, because in the US you have the right to appeal to a higher court if you don't like the decision of a lower court, many times states will find very popular gun control bills struck down by a Federal court who tells them that in the pure legal sense, their gun control laws are unconstitutional.
California, Illinois, and New York have all lost significant lawsuits related to gun regulation, but the payouts are usually not huge enough to cause a stir.
Gun rights as a political lobby is unusual. Most lobbying groups in congress exert power through having a lot of money, like oil and agriculture. Gun groups exert power on politicians by having huge mailing lists of "single-issue voters" who will vote against anyone who wants to pass gun control. The "NRA-ILA", the NRA's lobbying arm, actually doesn't have very much money. But they are the boogeyman of politics, because in any state with a significant gun culture, they can make or break politicians with a single mailing campaign. They give politicians a "letter grade" form A+ to F, . The Supreme Court can be considered somewhat "pro gun" at this moment due to its makeup (Roberts, Thomas, Gorsuch, Alito, Kennedy), something likely to become more pronounced with Trump having the opportunity to appoint two Justices.
EIDT: Whew, that was long, someone feel free to edit me down into coherence, that was a long ramble.
Comment: I was hoping for a link for the "Hard No" section.  Where are those items outright banned?
Comment: This answer would benefit from links
Comment: This reads more like a rant than an answer. And answers should be backed up.
Comment: @DavidStarkey added some links
Comment: @indigochild I'm open to suggestions or edits to get my thoughts into more working order. I did my best to give a brief overview of the complicated interaction of political forces that gives us our current laws on guns.
Answer: Because of the nature of the Second Amendment and its relative ambiguity, the line where something becomes permissible to prohibit is not clear. This was a longstanding question until the case of Heller v. DC (PDF). In the majority opinion written by Justice Scalia, the Court came to a partial conclusion that the right to bear arms is not unlimited, and the Second Amendment applies only to those arms "in common use." Starting on page 52:

We may as well consider at this point (for we will have to consider eventually) what types of weapons Miller permits. Read in isolation, Miller's phrase "part of ordinary military equipment" could mean that only those weapons useful in warfare are protected. That would be a startling reading of the opinion, since it would mean that the National Firearms Act's restrictions on machineguns (not challenged in Miller) might be unconstitutional, machineguns being useful in warfare in 1939. We think that Miller's "ordinary military equipment" language must be read in tandem with what comes after: "[O]rdinarily when called for [militia] service [able-bodied] men were expected to appear bearing arms supplied by themselves and of the kind in common use at the time." The traditional militia was formed from a pool of men bringing arms "in common use at the time" for lawful purposes like self-defense. "In the colonial and revolutionary war era, [small-arms] weapons used by militiamen and weapons used in defense of person and home were one and the same." Indeed, that is precisely the way in which the Second Amendment's operative clause furthers the purpose announced in its preface. We therefore read Miller to say only that the Second Amendment does not protect those weapons not typically possessed by law-abiding citizens for lawful purposes, such as short-barreled shotguns. That accords with the historical understanding of the scope of the right.

The Court went on to further narrow this, and to essentially lock in at current tech levels what kind of arms can be carried. From page 55:

We also recognize another important limitation on the right to keep and carry arms. Miller said, as we have explained, that the sorts of weapons protected were those "in common use at the time." We think that limitation is fairly supported by the historical tradition of prohibiting the carrying of "dangerous and unusual weapons."
It may be objected that if weapons that are most useful in military service—M-16 rifles and the like—may be banned, then the Second Amendment right is completely detached from the prefatory clause. But as we have said, the conception of the militia at the time of the Second Amendment's ratification was the body of all citizens capable of military service, who would bring the sorts of lawful weapons that they possessed at home to militia duty. It may well be true today that a militia, to be as effective as militias in the 18th century, would require sophisticated arms that are highly unusual in society at large. Indeed, it may be true that no amount of small arms could be useful against modern-day bombers and tanks. But the fact that modern developments have limited the degree of fit between the prefatory clause and the protected right cannot change our interpretation of the right.

A legislative body could expand the concept of arms in "common use" by permitting (or at least not prohibiting) civilian ownership, and if it reached a point where the courts found it to be "common," then it would fall under Second Amendment protection.
Comment: This is a pretty good answer but without considering the 14th amendment, it's really impossible to understand how the 2nd amendment applies to individuals.


Need suggestion for WiFi router

Question: I have 100 Mbps broadband plan. I want to limit it to 1 Mbps for guest network so that streaming and other things would work smoothly on my main network. Which router is suitable for me(in India)?
Comment: Can you explain your use case in brief that how are you going to use it?
Answer: You need a router approved for use in your locale with  Quality of Service (QoS), sometimes referred to as Internet Access Policy, features introduced in this article.  Since I am not in India, my shopping results won't tell me what's available to you, but if you search for "WiFi router Internet Access Policy" or "WiFi router Quality of Service", you will see what's available.
Answer: I know in the US that the Netgear Nighthawk series allows for this and works well. I have a Netgear Nighthawk R7800, and that works a charm. We previously had a Netgear Nighthawk R6900P and that also worked a charm. These routers are quality routers but may be pricy in India.
I'd recommend one of these:
Netgear Nighthawk R6700,
Netgear Nighthawk R6900P,
Netgear Nighthawk R7000P, or
Netgear Nighthawk R7800
All of these will last long, well-rated in the US, reliable, user-friendly and have room for expansion. They'll cover a 3-story house well and even outside of the house.
If you want a mesh network (a series of small routers linked together), I would use the Eero mesh system they will cover .7 acres of land extremely well and if there are any slow-spots in the house/property, you can buy more than just one pack of three and hook it up. They are extremely user-friendly and easy to set up. The only issues with them are that you can't set them up with a computer, you must use the Eero app on an Android or iOS device, and you can't limit the speed on the guest network.
Comment: thanks, i found similar one [here](https://www.amazon.in/Netgear-Nighthawk-AC2300-MU-MIMO-Dual-Band/dp/B075JMF84S?th=1)

Need suggestion for WiFi router


Does an HSA have to be used exclusively for medical expenses

Question: I had an HSA last year. This year I have health insurance through ACA. Does the HSA money have to be used exclusively for medical expenses?
Comment: What were you hoping to use it for?
Comment: Side tangent: if you can afford to do so, keep the funds in the account until you have something you can use it on. Surely there are some gaps between ACA and what is HSA eligible...
Comment: If a doctor prescribes a hot tub to help your joints then that would qualify.
Comment: IRS guidance [here](https://www.irs.gov/publications/p969#en_US_2018_publink1000204083). I believe you can use HSA funds to reimburse yourself for the premiums paid for that ACA plan, as well as for deductibles/co-pays/prescriptions & over-the counter medication not covered by your plan, plus things like band-aids and even contraceptives.
Answer: I just want to point out that your question implies a misunderstanding, even though you didn't explicitly mention it:

I had an HSA last year. This year I have health insurance through ACA.

The fact that you have insurance through the ACA may not be relevant. There are many plans in the ACA that are HSA compatible. But even if you don't currently have a compatible HSA plan, that just means you can't contribute any more funds to your HSA account, but you can still spend the funds already in your HSA, and regardless of your insurance plan (or even if you're uninsured) you will almost always have some eligible medical expenses that you could use the funds for.
To answer your specific question, no, you don't have to use the funds for eligible medical expenses, but if you don't you'll have to pay tax + fees on the withdrawals. (Unless you're over 65 in which case you'll pay tax, but no fees.)
Comment: @Upper_Case - good idea, though, technically the answer is "No", not yes. ;) I updated my answer to clarify.
Comment: It might be worth highlighting that the answer to this specific question is *yes*, possibly contrasted against an FSA.
Answer: If you want the tax benefits of the funds, yes, they must be used for qualified medical expenses as defined by the IRS.
You can withdraw funds whenever you want, for any reason, but if they are not spent on qualified medical expenses, then there will be tax implications. If you are under 65 years old, you will pay income tax plus a penalty on any unqualified expenses. But if you are at least 65, you will pay only income tax, without a penalty.
Answer: Yes, it must be for a qualified medical expense.

Qualified medical expenses are those expenses that generally would qualify for the medical and dental expenses deduction. These are
  explained in Pub. 502, Medical and Dental Expenses.
Also, non-prescription medicines (other than insulin) aren't
  considered qualified medical expenses for HSA purposes. A medicine or
  drug will be a qualified medical expense for HSA purposes only if the
  medicine or drug:

Requires a prescription,
Is available without a prescription (an
  over-the-counter medicine or drug) and you get a prescription for it,
  or
Is insulin.

For HSA purposes, expenses incurred before you establish your HSA
  aren't qualified medical expenses. State law determines when an HSA is
  established. An HSA that is funded by amounts rolled over from an
  Archer MSA or another HSA is established on the date the prior account
  was established.
If, under the last-month rule, you are considered to be an eligible
  individual for the entire year for determining the contribution
  amount, only those expenses incurred after you actually establish your
  HSA are qualified medical expenses.
Qualified medical expenses are those incurred by the following
  persons.

You and your spouse.
All dependents you claim on your tax return.
Any person you could have claimed as a dependent on your return except
  that:
  a.

The person filed a joint return,
The person had gross income of $4,150 or more, or
You, or your spouse if filing jointly, could be claimed as a dependent on someone else's 2018 return.

https://www.irs.gov/publications/p969#en_US_2018_publink1000204083
Comment: Lots of little gotchas with HSAs.  Make sure to check the guidance (can be found via Google) on *exactly* what's covered and what's not.  You'll find that you can use for some common items that are not prescriptions, then surprise you get a prescription that you reasonably expect to be allowed, but is in fact not (and then you end up repaying it with penalty in your taxes).
Answer: Contrary to what some of the other answers here have said, it is possible to spend your HSA funds on non-medical expenses without penalty, but only if you hold them until after the retirement age of 65:

There is no penalty for non-health care withdraws in retirement though. "It works just like an IRA once you hit (age) 65," Conroy says. At that point, money can be taken from an HSA for any purpose with only income tax due on that amount. That's the same as how withdrawals from traditional IRAs are handled after age 59 1/2.

If your current healthcare plan is comprehensive, you can just sit on your HSA (or even invest it) until you retire. Then, you can spend it on anything you'd like, without tax penalty.


How to stop or even reverse cavities?

Question: This is not a question that requires any knowledge of dentistry. All I ask for is your personal experience.
What do you in case of a cavity and don't want to go dentist because you fear the side effects of drilling?
Comment: Not a real answer, but dentists don't enjoy drilling holes or having their patients being afraid of them. If there were a good alternative way of healing existing cavities, I believe the dentists themselves would be employing it.
Comment: @Doeser, remineralization only works if there's an existing framework for the minerals to attach to.  It can't fix holes, only weak spots.
Comment: @rumtscho, not for mine, he lost patience when I asked about remineralization.
Comment: I'm not sure this is even possible, @doeser, beyond slowing its progression with exceptional oral hygiene.  But I am not a dentist and don't know whether there's evidence for an alternative.
Comment: I'm voting to close this question as off-topic because it's an opinion poll.
Answer: I can speak from personal experience, and from experience working as a physical chemist peripherally associated with a group researching dental care.
My personal experience is that I have a few lesions that my various dentists over the last couple of decades have decided not to fill because they are not serious and not getting any worse. So a dentist will not necessarily reach for their drill at every opportunity.
My research experience is that at least in vitro enamel can remineralise, but you are talking about incremental improvements and nothing approaching total repair of a cavity. Any major cavity will not repair itself.
It is impossible for you to assess the damage to your teeth because you have neither the skills nor the equipment required. Your dentist has both, and you should respect their judgement. Your dentist should probably not have been impatient with your questions, but bear in mind that by questioning him you are implying he does not know what he is talking about and even the most saintly of dentists will get fed up with this eventually.
Comment: Were the findings ever publicized? Are you able to post a link to the abstract?
Comment: @GibralterTop: I'm afraid not. This was industrial research and having paid me and many others lots of money to do the research the company involved was naturally unwilling to give the results away :-)
Comment: So put your tooth into a glass container and treat it, it might remineralise
Answer: There are really two different questions posed here:

How to stop or even reverse cavities?

This is currently only partially possible in the sense of slowing down the further spread and development of caries.
Eliminate acids and sugars from your diet, that includes starches and other cariogenic carbohydrates. Practice impeccable oral hygiene with lots of fluoride toothpaste, interdental cleanig with mouth washes, tooth picks and floss. Drink lots of green tea and milk. All of this – and more, if believe what's floating around the net popularly and mostly falsely – has very limited effects and cannot fully replace a visit to the dentist under any normal circumstances.
Cavities cannot be "reversed", that is rebuilt and filled up to a fully working intact tooth. Not with miracle supplements, exotic foods and not even by a dentist, except that a dentist might have suitable substances to fill the cavity.
That answers hypothetically a question with unrealistic goals.
And the real question:

What do you in case of a cavity and don't want to go dentist because you fear the side effects of drilling?

First thing is of course something of the past in this case: try to prevent the cavity from forming in the first place. this is called prophylaxis and includes the usual: limit carbohydrate contact with teeth, practice oral hygiene and visit a dentist, not to let her drill anything, but to remove calculus and get protective lacquer layers. But as this is apparently too late in a case with developed cavities:
Someone with a cavity who does not want to go to a dentist because of the fear of drilling has to go to the dentist and let her drill and fill. The biggest problem in the scenario presented is not the procedure itself but the anxious anticipation that prevents a necessary and very, very probably best option.
The secret of success is mentioning the anxiety involved and then requesting at least one of the following or even two drugs at once. Most of the time the request will be preempted by an offer:

a large dose of local anaesthetic – for example Lidocaine – that will prevent any pain from being registered in the brain
an adequate dose of a one-time anxiety reliever or even a sedative that will make your brain not only dull to the pain but utterly uninterested in the effects for a while that is long enough until the procedure is over

Dentists can also prescribe medications such as antibiotics, sedatives, and any other drugs used in patient management.
Wikipedia: Dentistry#Dental treatment

Current methods of sedation in dental patients - a systematic review of the literature Med Oral Patol Oral Cir Bucal. 2016 Sep; 21(5): e579–e586.
Published online 2016 Jul 31. doi:10.4317/medoral.20981
WebMD: Sedation Dentistry: Can You Really Relax in the Dentist's Chair?
MouthHealthy, ADA, Anesthesia and Sedation
ADA Guidelines for the Use of Sedation and General Anesthesia by Dentists 2016
SedationDentstry4U
Inform yourself before requesting any of that and do not fixate on a specific combination, but discuss this with your dentist.
Answer: As I understand it, the current consensus among dentists and scientists is that cavities which have reached the dentin layer are too big to heal using any existing technology, but that carious legions (areas of the tooth that have begun to erode through demineralization) can be remineralized with a lot of methods. Your own saliva contains calcium which regularly remineralizes teeth when conditions are right (mostly that pH is high enough). Almost all toothpastes help remineralize by containing some combination of calcium, phosphorous and/or fluoride.
As has been mentioned by other responses, there is some promising research on repairing teeth with substances like AD drug tidesglusib. None of these treatments, to my knowledge, are ready for non-experimental use.
The reason this problem is so tough is that tooth enamel is not a living structure like much of your bone is. It does not contain any living cells. The pulp in your teeth is alive, and can manufacture a small amount of protective dentin (the intermediate layer) to repair damage from the inside, but hasn't been shown to repair the enamel. Once bacteria have reached the dentin by wearing a cavity all the way through your enamel, there is no easy way to kill that bacteria so it is assumed that it will continue to thrive in the cavity until it reaches the pulp and ultimately destroys the tooth. Fillings are created by removing all the weakened and infected enamel and replacing it with an artificial material to protect the rest of the tooth.
Wikipedia provides references for most of this: https://en.wikipedia.org/wiki/Tooth_decay
And here is the information about tidesglusib, which is expected to begin human trials in 2019:
https://epatientfinder.com/human-trials-regrowing-teeth-expected-start-2019/
In answer to your second question, the best thing to do to start out is to get a dental x-ray. This will show you how deep the cavity has penetrated and you may well be able to stop its progress by stepping up your dental care and nutrition, making a filling unnecessary.
Answer: There were two questions here, but I'm only going to respond to the one in the headline "How to stop or even reverse cavities?" with one possibly helpful ingredient of the puzzle.
Besides the regular brushing advice that dentists have been giving for decades, a more recent discovery is the benefits of xylitol on oral health. Studies have shown that oral products with xylitol in them have been "observed to be effective in preventing caries" (cavities). These oral products include xylitol chewing gum, xylitol gummy bear snacks, xylitol mouth rinse, and xylitol toothpaste.
According to a medical article published in 2014, on https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4232036/ it says regarding xylitol:

"The predominant modality for xylitol delivery has been chewing gum. Chewing gum accelerates the processes of rinsing away acid and uptake of beneficial calcium phosphate molecules to remineralize tooth enamel."

It also says:

"A study among Montreal children showed that children who chewed xylitol gum had significantly lower caries progression after 24 months than those who did not use gum. These children exhibited a significantly higher number of reversals of carious lesions than the control group, suggesting that remineralization has occurred."

It also talks about xylitol syrup:

"Twice-daily administration of xylitol oral syrup at a total daily dose of 8 g was observed to be effective in preventing caries."

Regarding xylitol toothpaste, it says:

"Toothpaste with xylitol led to a decrease in S. mutans colonies in saliva, the amount of secreted saliva, and the increase of pH value. It has a positive effect on the quality of the oral environment and it would be useful introducing it into prophylactic programmes."

The study concludes that:

"...more research is needed on the mechanisms of action of xylitol..."

and

"While these issues of xylitol still need to be expanded, the benefits it offers are literally worth salivating over."

Please read the full article for all the details.
An additional medical article from https://www.ncbi.nlm.nih.gov/pubmed/14700079 notes that they did a study where samples were soaked in xylitol solution, and says that:

"These results indicate that xylitol can induce remineralization of deeper layers of demineralized enamel by facilitating Ca2+ movement and accessibility."

Xylitol is a sugar substitute, with 40% less calories and 75% less carbohydrates (than sugar), that naturally occurs in fruits and vegetables, which  doesn't have the negative side effects reported from many artificial sweeteners.
Xylitol chewing gum, mints, mouth rinse can be found at many stores but are especially prevalent in many health food stores.
This answer wasn't meant to replace any recommendations from a dentist, and was only intended to point out some newer things to possibly supplement your usual oral care. You should probably consult a dentist, if you think you have a cavity.
Comment: [Xylitol is a preferred sweetener…, having 40% less calories](https://xylitol.org/xylitol-uses/nutritional-benefits-of-xylitol/), you might want to [edit] that.

How to stop or even reverse cavities?


Constant or variable number of chiasmata during recombination?

Question: During recombination, is the number of chiasmata consistent for each gamete and are the chiasmata regions consistent within a single organism?
Comment: Welcome to Biology.SE. What do you mean by `[..] are the chiasmata regions consistent within a single organism?`? Do you mean "Are there regions of high recombination rate and other regions of low recombination rate?"?
Comment: Maybe I should rephrase the question, how many recombination events occur in a single germ cell? Is recombination really random or is recombination deterministic? If not, what influences randomness in recombination. Does the cell nucleus secrete some enzyme the randomly glues homologous chromosomes at random sites?
Answer: There is a large amount of variation in the distribution and number of chiasmata or crossing overs (COs). The total number of chiasmata per cell can vary within the same organism.
Where chiasmata occur along the chromosome is also not consistent cell to cell. Hotspots are 1-2kb regions that experience elevated recombination compared to neighboring genomic regions. A way to think of the distribution of COs is that ~80% of recombination happens in hotspots which comprise ~10% of the genome.
Recombination depends on the chromatin context, so chiasmata are rare near the centromere and positive interference results in chiasmata being spaced out along the chromosome. A good prediction of total chiasmata number per cell is at least 1 per chromosome. Some researchers argue that one CO per chromosome arm is more accurate.
Below is a link to a good article that expands on some of the ways recombination varies in humans.
http://www.nature.com/nrg/journal/v8/n1/abs/nrg1947.html
Comment: Thanks!!! Your answer along with the article turned out to be very helpful!!!


Do I need a transit visa for Brazil?

Question: I am an Indian passport holder studying in Guyana, South America. I am going back to India for vacation on August 28, 2018 for 3 months. I am transiting in Panama and Brazil. From Guyana to Panama is one PNR and from Panama to Brazil to Dubai to India is another PNR. I already have transit permission for Panama. Do I need a transit visa for Brazil?
Answer: According to the Brazilian Consulate-General Rotterdam website:

12/04/2017
  The Immigration authorities of Brazil informed that, from October 1st
  2015 onwards, international travelers from countries that do not have
  visa waiver agreements with Brazil will be required to present a
  transit visa (VITRA) when passing through the Brazilian territory with
  reservations from different airline companies to reach the final
  destination.
International travelers bearing a single booking (one airline
  reservation) from the departure country to the final destination will
  not need a transit visa to pass through the Brazilian territory. The
  single booking may include different airline companies considering
  that the companies may have flight sharing agreements.
The transit visa will be required only to the above international
  citizens traveling with more than one booking (more than one airline
  reservation) issued by different airline companies with no flight
  sharing agreements.

or, in my opinion, you can also find an answer in Indian citizen transiting in Brazil on the way to Peru. Is a transit visa required?

Do I need a transit visa for Brazil?


Definition of differential of map (in algebraic geometry)

Question: Given two schemes $X, Y$ with a map $f: X \rightarrow Y$, one should have a map $df: T^*Y \times_Y X \rightarrow T^*X$. How is this map defined (using the language of algebraic geometry, rather than the language of manifolds)?
Comment: One doesn't usually have a cotangent *bundle* so much as a cotangent *sheaf*, in which case it is not appropriate to use fibre product notation for pullback.
Comment: And the cotangent sheaves are relative some basis.
Answer: The map of schemes $f:X\to Y$ comes with a map $f^\sharp:\mathcal O_Y\to\mathcal f_*O_X,$ which induces a local homomorphism $f^\sharp_x:\mathcal O_{Y,f(x)}\to\mathcal O_{X,x}$ of local rings. In particular, $f^\sharp_x$ induces a homomorphism of Zariski cotangent spaces $\varphi:\mathfrak m_{f(x)}/\mathfrak m_{f(x)}^2\to\mathfrak m_x/\mathfrak m_x^2.$ (The dual to this map is usually denoted $d_xf: T_{X,x}\to T_{Y,f(x)}.$ This differential is defined locally.)
In the case that $X,Y$ are nonsingular varieties, we do have cotangent bundles $\Omega_X^1\to X,\Omega^1_Y\to Y,$ and supposing that $x\in X,y\in Y$ are closed points, the fibres are $\Omega^1_{X,x}=\mathfrak m_x/\mathfrak m_x^2$ and $\Omega^1_{Y,y}=\mathfrak m_y/\mathfrak m_y^2.$ We can pull back the cotangent bundle of $Y$ to $X$ along $f,$ which gives a vector bundle $f^*(\Omega_Y^1)=X\times_Y\Omega_Y^1\to X.$ Since we are working over an algebraically closed field, the closed points of $f^*(\Omega_Y^1)$ correspond to pairs of closed points of $X$ and $\Omega_Y^1.$ Thus, we can define a morphism $f^*(\Omega_Y^1)\to\Omega_X^1$ by specifying its values on the closed points $(x,\eta)\in X\times_Y\Omega_Y^1.$ The natural choice is the map $(x,\eta)\mapsto\varphi(\eta),$ and this choice is well-defined because $\eta\in\Omega^1_{Y,f(x)}$ by definition of fibre product. (The details of what I've written are very nicely laid out in Shafarevich BAG 2)
As others mention in the comments, the fact that we have a vector bundle is a special property, relying here on the fact that $X,Y$ are nonsingular varieties. In general we have relative cotangent sheaves, and the techniques used shift away from this directly geometrical perspective.


How to do this Intermediate value theorem proof?

Question:
Use the Intermediate Value Theorem to show that the equation $x^3+x+1=0$ has a solution.

How to do this? :S
Thank you very much!
Comment: Or even, like, $f(0)$ and $f(-1).$
Comment: Define $f(x) = x^3 + x + 1$. How does $f$ behave when $x \rightarrow \infty$? When $x \rightarrow -\infty$? What can you deduce?
Answer: Let $f(x) = x^3 + x + 1$. Then $f$ is a polynomial, so its continuous. Then we notice that
$$f(-1) = -1, \text{ and } f(0) = 1.$$
But then by the intermediate value theorem, this means that for all $y\in[-1,1]$ we have some $x\in [-1,0]$ such that $f(x) = y$. Therefore we have some $z$ such that $f(z) = 0$.
Since $f(z) = 0$ we know that $z^3 + z + 1 = 0$ and so the equation $$x^3 + x + 1 = 0$$ has a solution, namely $z$.
Answer: If you want to use the Intermediate Value Theorem, first you're going to need the statement of the Intermediate Value Theorem.  Once you look at it, it tells you exactly what to do.  From Calculus by Varberg, Purcell, and Rigdon:

Intermediate Value Theorem: Let $f$ be a function defined on $[a, b]$ and let $W$ be a number between $f(a)$ and $f(b)$.  If $f$ is
  continuous on $[a, b]$, then there is at least one number $c$ between
  $a$ and $b$ such that $f(c) = W$.

Now, a very common use for this theorem is to show that a function is 0 somewhere.  How that works is as follows.  Say out of $f(a)$ and $f(b)$, one is positive and one is negative.  Then $0$ is in between $f(a)$ and $f(b)$.  If the function in question is continuous on the closed interval $[a, b]$, then all the conditions of the Intermediate Value Theorem apply.  In that case, we can conclude there is some $c$ between $a$ and $b$ such that $f(c) = 0$.  Even if don't know what $c$ is, and even if we have no idea how to ever figure out what $c$ is, we know there is some $c$ such that $f(c) = 0$.
In your specific example, $f(x) = x^3 + x + 1$ is continuous everywhere, in particular on any closed interval.  So, we have no trouble on that issue.  All we need to know is $f(x)$ is positive somewhere and negative somewhere else.  Just try a few values of $x$ and see if you can find one where $f(x)$ is positive and one where $f(x)$ is negative.  Start by trying the easy values.  In this case, Deven already has shown the easiest ones to try, and it turns out $f(0) = 1$ is positive and $f(-1) = -1$ is negative, so that we know the Intermediate Value Theorem applies and tells us there is some $c$ between $-1$ and $0$ such that $f(c) = 0$.


Can a vowel be a consonant?

Question: So, I know there are certain consonants in the IPA that have vowel-like properties, and can therefor be used as vowels, such as [n], [m], and [l]. Examples include [pnt], or [ʒlf]. So, in the loosest possible sense, can a vowel ever be treated as a consonant? I realize that this is kind of a weird question, and the obvious answer is probably not, but surely, some language might have certain words that sometimes have a vowel in place of a consonant. Thanks!
Comment: Did you forget about semivowels? In the loosest possible sense, those can certainly be described as vowels that are consonants. The following recent question and its answer are also relevant: http://linguistics.stackexchange.com/questions/17747/recording-of-nonsyllabic-voiceless-schwa
Comment: Those consonants [n], [m], [l] can be used as syllable nuclei (ie they have the feature 'syllabic') in some languages but that doesn't make them vowels.
Answer: A vowel has two closely related but inequivalent definitions. One of them looks what's happening in the mouth etc. Vowel is a sound in which the tongue doesn't touch anything and there's no build of pressure anywhere. That's why the sound may last uniformly and that's why it has the potential to be the peak of a syllable. The sound that allows a syllable to exist is the other definition of a vowel.
The consonants you mention – for example, in Czech, most famously, "L" and "R" – may form syllables, so they satisfy the syllabic definition of a vowel (my last name is "Motl" with an "L" and has two syllables; similarly "prcek" with "R"). However, they don't satisfy the tongue-and-pressure-based definition of a vowel so "L" and "R" are still considered consonants (although they are "syllabic nuclei").
On the other hand, vowels (by their sound) may fail to be the bulk of a syllable and they may be treated as consonants. These sounds are called "semivowels" and in English, "y" (in "yes") and "w" (in "west") are examples of such semivowels. By the essence of their sound, "y" and "w" are vowels. After all, "y" is a relative of "i" and "w" is the "double u". But because they're not the most important or long-lasting part of English syllables, they behave as consonants.
Answer: Vowels can't be consonants. Consonants can't be vowels. However, consonants can do things that are more commonly observed with vowels, and vice versa. "Vowel" is specifically a description of the vocal tract constriction of the segment combined with the prosodic property of being a syllable peak ("syllabic"). When a vocalic (vowel-like) segment is not syllabic (thus is more like a consonant, since consonants are typically non-syllabic), it is termed a glide (or semivowel).

Can a vowel be a consonant?


Reading attribute values using PyQGIS

Question: What is the way to  get the information contained in the attribute table, say in column 2, row 2 using PyQGIS?
Comment: This is helpful, but this only seems to give you the value of the attributes. It seems attributes need to be a name/value pair. How do you get the name for each attribute? Is that what the "fields" is all about?
Comment: Please, do not forget about ["What should I do when someone answers my question?"](https://stackoverflow.com/help/someone-answers)
Answer: To loop over every feature in a layer, use the getFeatures() generator. This returns an iterator of features (QgsFeature) in the layer.
for feature in layer.getFeatures():
    pass # do something with feature

If you're only interested in a particular feature, instead of all of the features in the layer, you can access it using the QgsFeatureRequest and its "FID":
fid = 1 # the second feature (zero-based indexing!)
iterator = layer.getFeatures(QgsFeatureRequest().setFilterFid(fid))
feature = next(iterator)

Once you have a QgsFeature object you can use the attributes() method to retrieve the attributes (a.k.a. columns or fields) associated with that feature (a.k.a. row), e.g.:
# get the feature's attributes
attrs = feature.attributes()
# print the second attribute (note zero-based indexing of Python lists)
print(attrs[1])

If you want to index the field by its name, rather than a number, you need to use the field mappings:
idx = layer.fieldNameIndex('name')
print(feature.attributes()[idx])

The field index shouldn't change during the loop, so you only need to call it once.
There is more information and examples in the PyQGIS cookbook:
http://www.qgis.org/pyqgis-cookbook/vector.html#iterating-over-vector-layer
You can access feature attributes much more easily by using the QgsFeature like a dict, e.g.
for feature in layer.getFeatures():
    name = feature["name"]

I'm not sure which version came in or if it's always been there.
Answer: In order to get the information in a table you would first have to access the layer's features. In my case, I put the features in a variable.
Then you loop through the features, and for each feature you call its attributes, which can then be printed using their column index.
For example, if I would like to get all the values in the second column (which corresponds to the first index), then I would do this:
layer = iface.activeLayer()

features = layer.getFeatures()

for feat in features:
    attrs = feat.attributes()
    print(attrs[1])

It uses the getFeatures() method of the QgsVectorLayer class and the attributes() method of the QgsFeature class.
Answer: The answers above only show how to do this for the active layer. I would think, in many cases, you might want to find said attribute for a layer that is not the active layer or a layer checked in the Layers window.
The code below will get a list of all layers that you have added to your Layers window (regardless if they are checked or active) and find the attribute at row 2, column 2.
from qgis.core import QgsProject

layer = QgsProject.instance().mapLayersByName("myLayerName")[0]
value = layer.getFeature(2).attribute(2)

Note mapLayersByName() returns a list of layers with the given name. I am assuming one exists for this example.

Reading attribute values using PyQGIS


Is it necessary to turn off the engine while refuelling?

Question: At petrol stations in Australia, there are always several signs that say "Turn off engine while refuelling" (or something of the sorts). Is this necessary to do, and why, or is just something that is required in older cars and no longer required in new cars? My question is regarding petrol engines, but I assume diesel is the same.
Comment: @cory, if you would like to answer the question, you should do that in an answer, not a comment.
Comment: Heh... okay man. Good work
Answer: Here is a link from the API (American Petroleum Institute) website.
http://www.api.org/oil-and-natural-gas/consumer-information/consumer-resources/staying-safe-pump
The article is called Staying safe at the pump. It talks a lot about static discharge among other things as sources of ignition. In most jurisdictions it is illegal to have an engine running while refueling. It is also against the law to smoke a cigarette within 25 feet (7.62 meters) of a fuel dispenser while in operation. As always it is better to be safe than dead.
When an engine is running there are multiple possible sources of ignition. Alternator or electric motor contact arcing. In older vehicles distributors could trigger flash fires or explosions. Fuel vapor is worse than liquid gasoline in terms of volatility.  Considering what is directly underneath ... multiple large containers of fuel at about 20000 liters a pop.
Is it worth the risk to yourself and others?
Comment: Any petrol engine is spark-ignition so even minus a distributor there's sparks being made under your bonnet. Modern diesel injectors can use high voltages to actuate quickly too. Alternators, cooling fans, relays all have moving electrical contacts which DO create sparks.
Comment: ...it's POSSIBLE to fill a vehicle with the engine running but not a great idea.
Answer: Yes, it is necessary. When fueling, fuel vapor escapes, potentially creating a combustible mix. Plus there's always the possibility of an accidental spill.
A running engine adds an ignition source to this environment. Switching the engine off makes a fire less likely.
Comment: Agreed, also the use of mobile phones is not allowed (in some countries) for the same reason.
Answer: Mostly a 'just to be safe'.  Modern cars will throw a check engine light is you pull the gas cap while the engine is running.
Comment: It is very uncommon for the evaporative system monitor to Run tests at idle. It takes two failure events to get a CEL on.


model for intersection of two circles in the complex projective plane

Question: There is a well-known model of 2-dimensional projective  geometry points and lines including points at infinity and line at infinity in terms of three dimensional Euclidean geometry, namely, ordinary projective points represented by Euclidean straight lines through the origin and intersecting the Euclidean plane z = 1; ordinary projective lines by Euclidean planes through the origin intersecting the Euclidean plane z= 1; and the projective points at infinity by Euclidean straight lines through the origin but lying on the Euclidean plane z = 0; and the line at infinity by the Euclidean plane z = 0, itself. This model also shows that parallel projective lines on the Euclidean z = 1, do meet in a point at infinity, namely, the Euclidean line of intersection of the two Euclidean planes representing the two parallel projective lines. Is there a similarly explicit representational model that shows that any two circles in the complex projective plane intersecting at two fixed imaginary points?
Answer: To get a perfect complex geometric analogy with the mental picture you've drawn would involve setting the whole mess in complex three-space, so to our real eyes, six-dimensional. You can do it, especially if you're very geometrical.
But the usual way is to think of a point in the projective plane as given by a vector $v=(x,y,z)\in\mathbb{R}^3$, or equally well by any nonzero scalar multiple of $v$. (This gives you your line through the origin.) The traditional way is to write $(x:y:z)$. The ordinary point $(x,y)$ in the finite plane has the "homogeneous coordinates" $(x:y:1)$. A line now is described by a nonzero linear homogeneous form in the three variables $X$, $Y$, $Z$, I mean $AX+BY+CZ$, not all coefficients to be zero. Then the line at infinity is represented as the zero-set of the linear form $Z$.
A circle or any other conic section will be represented by a nonzero quadratic linear form, for instance the unit circle is given by $X^2+Y^2-Z^2$. and the circle whose ordinary equation we write $(x-a)^2+(y-b)^2=r^2$ will be represented by the quadratic form $(X-aZ)^2+(Y-bZ)^2-r^2Z^2$. You see that the unit point on the $x$-axis has homogeneous coordinates $(1:0:1)$, for example. You see also that both of these circles contain the points $(i:1:0)$ and $(-i:1:0)$ on the line at infinity once we allow complex coordinates.
I know that this isn't what you asked for, but it is the best way to look at things unless you have redoubtable geometric intution.


Internal and external pressure during a gas expansion

Question: There is a solid substance in a rigid cylinder with a piston placed on top such that the distance between the piston and the bottom of the cylinder is essentially zero, and then the substance decomposes and forms gas that expands and pushes the piston up against a constant external pressure to a maximum point. It then falls slowly as heat from the reaction is lost to the surroundings. Is it true that at the maximum point, the internal pressure is equal to the external pressure?
Comment: Welcome to Chemistry Stack Exchange! Please add what you have attempted towards solving the problem into the body of your question.  For more information, see the site's [homework policy](http://meta.chemistry.stackexchange.com/questions/141/how-do-i-ask-homework-questions-on-chemistry-stack-exchange) for how to ask homework questions. Thanks!
Answer: If the frictionless massless piston is not moving, then internal and external pressures must be equal.  If the piston has mass, the internal pressure must be greater when it is stationary, to support the weight.  If there is static friction, all bets are off.
Suppose the piston were a right cone rather than a right cylinder.  Would the excess surface area change the outcome, all other things being equal?


Should I err on the side of overdressing for my first day of work?

Question: I have accepted a job offer as a software developer after two phone interviews, and they told me that they have a relaxed dress code. Specifically, they mentioned wearing jeans. However, I'm not sure whether that's jeans with a polo, or a button-down shirt, or a T-shirt; and I want to communicate on my first day that I take the job seriously. Is it acceptable for me to show up on my first day in (nice) jeans (particularly if I wear a nice shirt)? Is it a bigger risk for them to wonder why I'm in jeans on my first day, or for them to feel like I'm overdressed?
Comment: ... you could just ask your contact with the company (or more helpful, your soon to be manager). I probably have done this every job I have taken. In this case, I'd probably wear a nice pair of jeans - or at least not really tattered - and a polo.
Answer: Since they said jeans, I would wear (nice) jeans.
As for the shirt, you could ask (or look for photos of their workplace on their web site).  Failing that, your best bet is to pick something "in the middle" -- not a t-shirt and not a high-end dress shirt, but a nice polo shirt, short-sleeved buttoned shirt, or (climate permitting) sweater would probably fit in.  A sweater that you wear over a t-shirt gives you the option to shed the sweater.
Don't wear something way out of line with what they told you (like a suit), though -- you'll stand out, but not in the way you want to.  I've seen people do that and they end up feeling awkward.
Comment: Save the 8 year old Slayer t-shirt for the second week :)
Comment: There is no harm in overdressing the first day just so you can see what everyone else wears. It is much easier to tone down than to dress too casually and get a bad impression. If you dress up too much on the first day people will understand as you adjust on the subsequent days.
Comment: and make sure the clothes are clean and relatively new. No torn clothes, frayed cuffs and seams, however fancy those might be in some circles.
Answer: Context is King
What was the context of the statement, "they have a relaxed dress code. Specifically, they mentioned wearing jeans."
If it was during the "our company is awesome, work for us!" stage, it could have been a bit exaggerated (using the most casually dressed member as an example of "average" for the company, when it isn't).
Since you seem to be unclear on what the actual dress code is, I'd urge caution on that front.
Check the Website
Assuming this company has a website, they likely have photos of the office somewhere on there (assuming they aren't stock photographs, who knows). At any rate this can give helpful pointers on:

What they want the outside world to think they wear
How that image matches up with what you were told they wear

Just note the range of dress in the photo (are they all wearing jeans?) and take a mental note.
Comfort is King
At the end of the day, wearing something that makes you feel comfortable will be best. If you try to over/under do it and aren't comfortable as a result, you may be spending more time worrying about your dress than making the right impression on your coworkers.
So whatever you end up picking, make sure that you feel comfortable in it.
I always come in to a first day worried that I will be introduced to the CEO, and want to make sure that I am at least dressed as well as my boss when I go to meet them.
If all indicators point to very relaxed dress, I'd probably still wear khaki pants and a button-down shirt. You can roll up the sleeves and unbutton the collar to look more "casual", but you can also pack a tie in your bag just in case. If you wear a t-shirt underneath you can also unbutton the front of the shirt to look even more casual.
It's like wearing layers during a New England winter. You can adapt yourself to the atmosphere more easily.
That's my style, just get what information you can and decide on yours. Then don't worry about it. A nice smile and confident greeting is far more important than what you wear.
Answer: Yes always err on the side of overdressing on your first day. After you see how others you work with dress, then you can tone it down. I like how you said err on the side of over dressing, don't totally overshoot it. If they said it's relaxed obviously don't wear a suit and tie. If you have one wear a short sleeved shirt with buttons all the way down the front, and make sure it has a collar. Also a solid collar is considered more formal so you may want to choose one with a few stripes.
Comment: I'd personally consider stripes more formal than any solid color except maybe white.
Answer: Since you were specifically told that they dress code is relaxed and jeans were mentioned, then that's what you should wear.
In regards to the shirt - by all means, dress smart-casual - in this regards, there will be no harm in dressing up and you will be able to see what others are wearing and adjust accordingly.
If you are still unsure - by all means go into work in a suit on the first day. This will certainly give the right message - I have done this in the past and had the COO come over to tell me that's the last day I come in to work in a suit.
Answer: Personally, I'd go for smart jeans, a casual shirt (button-down or otherwise) and shoes.
The thing to remember is that there will likely be people at both ends of the spectrum. Some will look like they're ready to haul a few tyres in the local garage whilst others will almost look like they have an interview somewhere. As long as you're not at the extreme end of casual, I think you'll be fine.
As well as fitting in with the accepted standard of dress, it is also important for you to feel comfortable with what you're wearing.
Comment: Yes, certainly you should wear shoes ;). Unless the job is in Mississippi (I can say that, as I was born there LOL).
Comment: Lol @AmyBlankenship. Sorry, I meant shoes as opposed to trainers/sneakers... :)
Answer: Every job you take, you should do two of three things.
1) ALWAYS obtain the contact information of your soon-to-be manager if they weren't the ones to interview you.  Send them a quick email a few days before starting, introducing yourself to them and such.
The next two you need to do one of.
2) during your interview, try not to leave without at least being walked through where you might be sitting/working.  Meet the team if only briefly, and PAY ATTENTION!!  This not only is a great way to get your face and name known to the company (improves chances of being hired) but also gives you the opportunity to learn some stuff about the environment (what are people wearing, how cluttered are peoples' desks, how much "personalization" is there to individual areas).
or
3) Ask about the dress code.  It is not a problem to ask a quick clarifying question, especially if you reface it by explaining your reasoning ("a previous company had some very strange meanings when they said jeans were fine, i just wanted to clarify it real quick here").  this is why step 1 was so important.  You're not going to ask HR, because unless you're applying for an HR job, they won't know.  They'll know the 'official' policy, but sometimes (especially in technical departments such as IT or AppDev) dress code is more relaxed to facilitate better work.
In the event you're unable to do these things, then I would err on the side of caution.  A button-up shirt is a good call, and remember, if it turns out you're a bit overdressed, you can always untuck your shirt to "casual-down" your attire on the fly.
Comment: Once tucked, a shirt should *never* be untucked until it's been ironed again. Many dress shirts designed for tucking are also too long to be untucked.


Increase speed in nmap UDP scan?

Question: How can I get nmap quicker in scanning UDP ports?
This kind of scan is incredibly more slow than TCP ports (at least 2-3 times)
Answer: UDP scanning is likely to be slower than TCP due to the differences in how the protocol works (i.e. with TCP it's easier to establish that a port is open due to the three-way handshake).
That said it's possible to increase the speed of UDP scans in a couple of ways.  one thing to watch for here is that if you apply too many timing options you risk reducing the accuracy of your scan (i.e. nmap will miss things).  Depending on your goal this may or may not be a problem.
One of the best places to get information about these is in the timing & performance section of the nmap book
From that, ones I tend to use
--max-rtt-timeout

This controls the time that nmap will wait for responses and is pretty key for scanning speed.  What you can do here is measure the typical round trip time of a request using ping or something similar and use that as a base for this.
--max-retries

As mentioned in another answer this is useful.  I wouldn't set 0 or you could miss things but setting 1 or 2 if you're really keen to see a speed-up will help
--max-scan-delay

If nmap things that a host is slow responding it'll start adding this in, and it can really slow the scan down.  Set this low around 10ms and it'll speed things up, but if you have flaky/slow hosts it'll likely miss things
--min-hostgroup

If you're not too concerned about getting results as you go and you have a lot of hosts to do ramping this up will improve things.  Something like 32 or 64 will speed up the overall scan.
Another key point to remember is that for UDP you'll likely only get good results if you use version scanning.  this is because a lot of UDP services only respond if the packet they receive is in the right format.  However version scanning will slow things down so if you're in a hurry set
--version-intensity 0

when using version scanning.
Comment: [Nmap 7.40](https://nmap.org/changelog.html#7.40) includes a new option that provides really fast UDP scanning, at the cost of not catching lots of services. If you want a fast check for some common UDP services, use `--defeat-icmp-ratelimit`, but don't consider it a thorough scan!
Answer: Try using the min-rate command line option:

When the --min-rate option is given Nmap will do its best to send packets as fast as or faster than the given rate. The argument is a positive real number representing a packet rate in packets per second. For example, specifying --min-rate 300 means that Nmap will try to keep the sending rate at or above 300 packets per second.

Example: nmap -sU --min-rate 5000 <targets>
Answer: There's a fundamental reason why UDP scans are slower than TCP scans: UDP is connectionless.  This means that Nmap cannot tell the difference between an open port with a program that's silently eating the scan packets, a port blocked by a firewall that's silently dropping the packets, or packets being lost in transit due to network congestion.  In the closed-port case, Nmap cannot tell the difference between a port that's firewalled, a lost packet, or rate-limiting of ICMP Unreachable messages.  Consequently, unless a port returns either data or an ICMP Unreachable message, Nmap needs to send multiple packets to eliminate the possibility of rate-limiting or lost packets.
Answer: In addition to what @Tate Hansen suggested consider using --max-retries 0.  You'll also need to consider the target.  [Some] Linux, for example, limits ICMP Port Unreachable responses so no matter what you do with your scanner the target is still a speed factor.  You could also -F and re-scan the full port range if deemed necessary based on the results of the -F scan.
Hopefully that helps.
Answer: To begin with, don't use something like 'nmap -sUVC' especially when scanning a large port range. For if you do this, the result is most likely that many ports are marked as open|filtered, and nmap will pound on each of them in the actual UDP-scan and in the service version detection - each up to max-retries many times and with nmap's progressively slower scanning.
Instead, you need to portscan UDP in two or more stages, where stage 1 looks like
nmap -n -sU -p[...] --max-retries 1 targets

and possibly further tuning parameters as mentioned by Rory McCune above.
If a host shows that all ports are open|filtered (or some closed and almost all open|filtered), then skip this host.
If a host shows many closed ports and only a few that are open or open|filtered, then investigate on those ports with nmap -sUV -pLIST target where LIST is the list of ports that are (possibly) open.
Depending on the context of the scan a quick sweep on very few ports like 53, 123, 161, and possibly others is often feasible.

Increase speed in nmap UDP scan?


Motion track markers go red and stop tracking

Question: I'm trying to do motion tracking for a clip but it can't track past a certain point. The trackers all go red and I can't figure out why. Any help would be greatly appreciated, thanks!
Comment: Related: [How can I get better results when doing camera motion tracking?](https://blender.stackexchange.com/questions/42329/how-can-i-get-better-results-when-doing-camera-motion-tracking)
Answer: The tracking process stops when blender cannot accurately track an element. In other words: tracking fails when the tracked element cannot be found accurately within the search box area.
This can be because the element has moved too much and is outside the search area, or because is blurred, deformed, occluded or too close to the edge of the frame. The tracking process also stops when it has reached the last frame defined for the scene (or the first one if tracking backwards).
What are the blue and red lines?
When you track there is a path created showing the motion of the tracked element across the frame.
Red indicates motion on the previous frames, blue is for the path on future frames.
If you track forwards you will see a red track being created. If you track backwards you will create a blue path.
When you play back you can see both.

What can you do when the tracking stops?
Go back one frame (or where the tracking stopped) Change some settings and re-start the tracking process.
If the tracked object is growing in size or has moved beyond the search area make the search box larger.
(To view the search box enable the Search Marker Display or preass Alt+S)

Change the motion model. Usually changing from Loc to LocRotScale or perspective works well.

For tracked features that change in shape because of the perspective or because they change size, you might want to change the match setting from keyframe (where the tracker uses the original shape as reference to find the matching features) to Previous Frame (where it will try to match the features to those in the previous frame only)
Answer: I found the answer. I needed to go to user preferences. System tab and them at the bottom increase the memory cache limit to maximum
Comment: You don't necessarily need to increase the cache limit. Blender will still track but it will be slower. When you cache it only means that blender will load the video frames into RAM, accessing the information faster, when you reach the cache limit,  the images will be accesed from the hard drive, making the process a bit slower. I doubt this is the reason the tracking stopped in this case. Tracking stops when blender cannot determine where the tracked element is within the search box area, or when the tracked element is blurred or too close to the edge of the frame,