Are there problems that a classical computer can solve and a quantum computer can never solve?

Question: Apologies if this is a silly question. But I've heard quantum computers can solve problems that classical computers can't. What about the converse, are there any problems that a classical computer can solve and a quantum computer can never solve?
Comment: related: https://quantumcomputing.stackexchange.com/q/23/55, https://quantumcomputing.stackexchange.com/q/9951/55 and links therein
Comment: From the article you cite: "For one, theoretical computer scientists already knew that quantum computers can solve any problems that classical computers can." Also check the complexity map they give further down the page.
Answer: The classical computer only manipulates logic 0 and logic 1, composing the essence of the logical circuit. So the question that classical computers can solve is merely by manipulating logic 0 and logic 1, while quantum can easily mimic it by only manipulating $|0\rangle$ and $|1\rangle$. So I think there does not exist any question that classical computers can solve while the quantum computers cannot.
Comment: Nice answer (+1), but I would add that you also need a tool to manipulate qubits in the same way as on a classical computer. Such tool is Toffoli gate - see my answer below.
Answer: Some additional ideas to narip's answer.
Consider a gate-based quantum computer. Such computer allows to implement so-called Toffoli's gate which under specific circumstances (in particular when input to target qubits is $|1\rangle$) behaves like logical gate NAND. Since NAND is a universal gate in classical computing, i.e. it allows to implement any Boolean function and thus any algorithm, also quantum computer is able to perform any classical algorithm.
Another question is speed of the algorithm. When the classical algorithm is implemented with Toffoli's gates (i.e. "quantum NANDs"), its complexity is preserved. In other words, the algorithm runs on a quantum computer at least as fast as on a classical one. Of course, in terms of a O-notation, actual execution time is another question and it depends on speed of quantum gates.
Adding a Hadamard gate to Toffoli's gate we get a universal set of gates for performing any quantum algorithm (note that there are also others sets of universal gates - e.g. set composed of H, T and CNOT gates). Under such setting, we can reach a speed-up for some problems - factoring (Shor's algortihm), linear systems solving (HHL algorihtm), database search (Grover's algoritm) or Monte Caro simulations in finance. While the first two algorithms have exponential speed-up, the latter two have only quadratic. Moreover, there are problems, like evaluation of a binary function parity, where a quantum computer does not bring any advantage.


Did any other early church fathers besides Origen teach the pre-existence of souls?

Question: Origen taught about the pre-existence of our souls, which, among many other of his teachings, was later condemned as heresy. Did any other early church fathers teach this doctrine?
Comment: This is some very useful background on the subject:  http://jewishencyclopedia.com/articles/13933-soul
Answer: It's at least true that others in the early church taught doctrines that resembled Origen's.  However, not all who did would normally be considered "church fathers," and even among those who unambiguously are, there's debate over how similar their teachings actually were to Origen's.
Of course, proponents of the doctrine will naturally find more similarities than others, as will be seen shortly.  However, even a "mainstream" treatment (Philip Schaff, History of the Christian Church) mentions others as holding views similar to Origen:

Even Gregory of Nyssa, although, like Nemesius and Cyril of Alexandria, he supposed the soul to be created before the body, compares Origen's theory to the heathen myths and fables. (Vol. III., page 831)

Schaff also mentions on the same page that Augustine spoke at least relatively favorably of the doctrine in De libero arbitrio, though he later emphatically rejected it.
Louis Berkhof, no friend of the doctrine, says that it was limited to the Alexandrian school and that Origen was its chief representative—but not its only adherent (Systematic Theology, 2.1.2.B).
Turning now to proponents of the doctrine, whose objectivity some might call into question, we find many claims of support from the church fathers.  First, Henry More (1614–1687) wrote a number of works on the subject, and tracks the history of the doctrine in the preface to his Collection of Several Philosophical Writings.  Starting on page xx, he cites Clement of Alexandria (Stromata, Books I and III, and a letter to Julius Cassianus), Basil of Caesarea, Gregory of Nazianzus, Synesius, Arnobius, Prudentius, and Augustine (though he cites only De libero arbitrio, and not Augustine's later works).  Unfortunately, most of More's specific examples are in Latin or Greek, making it difficult for non-scholars to judge the original intent of the quoted authors.  Those who want to examine the quality of his interpretations can review them in the linked work.
A second proponent, I. M. Oderberg, presents several examples in his essay, Reincarnation as Taught by Early Christians (also on WebCite), but does not always clearly source them:

After the original generations of Christians, we find the early Church Fathers, such as Justin Martyr (AD 100–l65), St. Clement of Alexandria (AD 150–220), and Origen (AD 185–254) teaching the pre-existence of souls, taking up reincarnation or one or another aspect of reimbodiment. Examples are scattered through Origen's works, especially Contra Celsum (1, xxxii), where he asks: "Is it not rational that souls should be introduced into bodies, in accordance with their merits and previous deeds . . . ?" And in De Principiis he says that "the soul has neither beginning nor end." St. Jerome (AD 340–420), translator of the Latin version of the Bible known as the Vulgate, in his Letter to Demetrias (a Roman matron), states that some Christian sects in his day taught a form of reincarnation as an esoteric doctrine, imparting it to a few "as a traditional truth which was not to be divulged."
Synesius (AD 370–480), Bishop of Ptolemais, also taught the concept, and in a prayer that has survived, he says: "Father, grant that my soul may merge into the light, and be no more thrust back into the illusion of earth." Others of his Hymns, such as number III, contain lines clearly stating his views, and also pleas that he may be so purified that rebirth on earth will no longer be necessary. In a thesis on dreams, Synesius writes: "It is possible by labor and time, and a transition into other lives, for the imaginative soul to emerge from this dark abode." This passage reminds us of verses in the Revelation of John (3:12), with its symbolic, initiatory language leading into: "Him that overcometh will I make a pillar in the temple of my God, and he shall go no more out."
Comment: Did you find any evidence that Origen himself didn't believe in Origenism, like the [other answer](http://christianity.stackexchange.com/a/32339/4)
Comment: No.  All the secondary sources I examined associate the view with Origen himself; in addition to Schaff and Berkhof above, Gregg R. Allison (*Historical Theology*, p. 325) also attributes it to him. It's not surprising, since the pre-existence of the soul was believed by Platonists, and Origen was influenced by them.
Comment: @Nathaniel I appreciate your point.  The question still feels a little weird.  It's sort of like, "Besides hyenas, what other dogs are scavengers?"  But since not everyone has the same rule (Greek "kanon") for determining what is and isn't heretical, I suppose the situation is unavoidable.
Comment: I've answered [the question on Origen and reincarnation](http://christianity.stackexchange.com/a/41428/21576) as well.  I've found no source for Oderberg's second quote of Origen above, but the first is valid, and additionally Jerome quotes Origen as saying things in support of reincarnation.
Comment: Oderberg seems to equating "Church Father" with "famous early Christian writer."  Origen was anathematized for heresy by an Ecumenical Council and Clement of Alexandria was accused of heretical beliefs about the Incarnation.  Furthermore, the fact that Jerome stated that some Christian sects of the day believed in reincarnation does not imply that he, himself, endorsed the belief.  I would be curious to see which specific work of Justin's he is referring to.  Aside from Origen - whose views were condemned as heretical - he supplies no clear citations.
Comment: @Dialogist Thanks for the feedback.  The question of Origen's status for the purposes of this question isn't really relevant (OP, like others, considers him a church father).  But I've updated my answer to hopefully better a) reflect that some who taught the doctrine are not "church fathers" and b) express even more clearly that More and Oderberg, as proponents of the doctrine, find many more examples than others do.  A question like this demands presenting the views of both sides of the debate, so even though you and I might think Oderberg is wrong, it's still valuable to include his views.
Answer: Origen did teach about the pre-existence of souls as it was widely held during his time among many respected Church fathers.
Origen quoted Romans 9:11-14 as evidence for his position regarding the pre-existence of souls:

11 (For the children being not yet born, neither having done any good
  or evil, that the purpose of God according to election might stand,
  not of works, but of him that calleth;)  12 It was said unto her, The
  elder shall serve the younger. 13 As it is written, Jacob have I
  loved, but Esau have I hated.  14 What shall we say then? Is there
  unrighteousness with God? God forbid.

Origen argued that God could not love Jacob and hate Esau until Jacob had done something worthy of love and Esau had done something worthy of hatred; so then, these verses can only mean that Jacob and Esau had not yet done good or evil In This Life and their conduct Before This Life was the reason why Esau would serve Jacob. Origen rejected the position that God loves or hates a soul based on its inclination toward good or evil, and before the soul actually commits a good or evil act. God, being the perfect creator of all souls and their inclinations, knows perfectly well each soul's inclination toward good or evil.
Jeremiah 1:5

"Before I formed you in the womb I knew you, And before you were born
  I consecrated you; I have appointed you a prophet to the nations."

Clearly, God is stating that Jeremiah was selected before he was formed in the womb and before he was born for God's special purpose.
Origen's writings did not support reincarnation. Origen's writings reflected a belief in Christian metempsychosis. The difference is clear if anyone studies it. Reincarnation is the belief that there is an endless automatic recycling of souls into human or animal forms. Origin, on the other hand, taught that human souls only incarnate in human bodies; it was not an automatic event, but determined by the will of God.
Many Christians confuse Origen's uncorrupted and original teachings with the later developments by followers in different geographical locations that became known as "origenism." Origenism is a corruption of Origen's original teachings based upon ignorance and gross misunderstandings as well as slander, gossip, and judgments by those who ignorantly misunderstood what Origen was trying to convey. It was "origenism" that was anathematized and NOT Origen himself. Origen was already dead and in Heaven with God by the time "origenism" was anathematized.
It is common among for people to confuse reincarnation with Origin's unique teachings on Christian metempsychosis which also was distinctly different from Plato's metempsychosis; likewise, it is also common to confuse the followers of Origen and their ignorant corruption of his teachings with Origen himself and his original teachings.
Comment: "widely held during his time among many respected Church fathers."  This answer would be greatly improved by including some citations.
Comment: Welcome to Christianity.SE. For a quick overview, please take the [Site Tour](http://christianity.stackexchange.com/tour). For more on what this site is all about, see: [How we are different than other sites](http://meta.christianity.stackexchange.com/q/1808/20394). In particular, this is a Q&A site, not a discussion site. Answers must address the specific question asked, rather than discussing related issues. In this case, the question is whether any other church fathers *besides* Origen taught the pre-existence of souls--which your answer doesn't address.
Answer: I haven't read everything cited in the long answer above, but I have read Justin Martyr and it isn't even discussed. The prayer cited at the end is clearly not about reincarnation, but about the states of mind and spirit that the individual in this life may go through, coming into light and falling into darkness, discussed by many regarding THIS life only and the possible repurcussions at the last judgement.
Jeremiah's cite above doesn't relate to preexistence - before I formed you doesn't have to mean before incarnation while you existed, but forming after conception, or it could mean the intention for Jeremiah to come into existence and plans for him.
I think a lot of confusion exists relating to the idea that the soul is created separately from but at the same time as the physical conception, and added to it instantly, all of this in a matter of seconds if not less,
Traducianism states that the soul material (for lack of a better term) is passed to the child from the parents like the physical material is, and at the same time. The soul material is what makes the physical material alive. Paul takes this for granted in explaining how Jesus' Melchizedec priesthood (drawing on a statement in Psalms) is superior to that of Aaron and Levi and not derived from them.
Also Adam became (not acquired) a living soul, animals also have the breath of life so are living souls as stated in the part of Genesis describing what died in the Flood, just different kinds of souls from us, and Ecclesiastes refers to the animal spirit speculating on where it goes at death.


How can I create video loopback devices on OS X? Is there a Mac equivalent of v4l2loopback?

Question: In Linux, I can use v4l2loopback to create virtual devices and use them as video inputs/outputs. Is there an equivalent toolset for OS X? How does one create multiple streams of videos on MacBook?
Answer: Audio Loopback
For audio loopback devices I'm familiar with Soundflower. From the looks of it there appears to be an alternative called Loopback which is a paid-for application.

Soundflower GitHub Repo
[Loopback](https://rogueamoeba.com/loopback/(

NOTE: Soundflower is available as a binary build from the Github repo.
Video Loopback
For video loopback devices you have a couple of options. Searching led me to this page on Stackoverflow - gstreamer create virtual device on OSX
.
That Q&A mentions that you can use gstreamer which provides a DMG file to use on OSX.
$ ./gst-launch-1.0 -v videotestsrc pattern=snow ! video/x-raw,width=1280,height=720 ! osxvideosink

There's also another option which mentions the framework CoreMediaIO. As part of that framework is sample code and documentation which covers creating a custom solution to construct a loopback device for video.
References

Rogue Amoeba info page about Soundflower
Record your computer's screen with audio on a Mac
How can I send my computer's audio to multiple outputs?
If we use SoundFlower to record the system's audio output, then we can't hear it at the same time?
Comment: Can you elaborate more on the video part of your answer? Everything you linked to is very vague. (And the command you pasted, in and of itself, does not create a video loopback device).


Can the Senate postpone confirming a new Supreme Court judge virtually indefinitely?

Question: Right now the Republican leadership are refusing to confirm a Supreme Court nominee in order to keep a Democratic President's nomination from being accepted.
Can the Senate postpone confirming a judge indefinitely?
Answer:
What if next year we have a republican president, the senate democrats decide to play the same hand, and refuse to confirm a supreme court nominee for the next four years? If you can delay for a year, who's to say you can't delay for four?

Absolutely nothing in law.  In practice, the Republicans currently have a stronger hand than Democrats would with a Republican President in 2017.  Republicans currently have a majority in the Senate and can thus block a vote from happening.  It's unlikely that Senate Democrats will have a majority if there is a Republican President.  While the votes are distinct, it would be uncommon for the two to go in opposite directions in the same election.
Republicans can also point out examples of Democrats using similar rhetoric in election years.  Democrats wouldn't have the same protection in an appointment immediately following an election.  Democrats already face a tough Senate map in 2018 with twenty-three seats up to the Republicans eight (and two more independents who caucus with the Democrats up).  In a less extreme case in 2014, Republicans picked up nine seats.  And of course if the Republicans have a Senate majority, they could always change the Senate rules to allow a simple majority to confirm an appointment.  Harry Reid already gave up the high ground on that by changing the rules for other confirmations.
The only provision that the constitution has for this situation is recess appointments, where the President appoints someone when the Senate is not in session.  Even if the Senate went out of session, Obama might avoid doing that.  It would weaken his hand at getting Garland confirmed.
Another possible solution would be for Roberts to ask one of the retired Supreme Court justices to sit on the court.  There is precedent for that with lower courts.  Both Souter and O'Connor have sat on Appeals court panels since retiring.  That would provoke a constitutional crisis though, and it's unclear that it would be upheld.
Tactically it also faces the problem that Souter and O'Connor (and Stevens) are more liberal than Roberts, so there is little incentive for him to ask their help in this way.  Doing so would tend to lead to him being in the minority in many decisions that are currently tied.  There is no current retired conservative justice that he could assign.
In the long term, this problem could be resolved by a constitutional amendment changing how Supreme Court justices are confirmed.  It's unclear how exactly the current system would change though.  Note that if a vote were held, the Senate could vote against Garland.  Or he could be filibustered.  Either way, we'd still be effectively in the same spot.
Comment: It's worth noting that while both Republicans and Democrats have delayed an appointment for partisan reasons in the past, the current stonewalling, should it last until the next election like Republicans are threatening, is *by far* the longest time a party has gone on refusing to perform this particular Constitutional duty (with nearly 25% of the President's time in office yet remaining).
Comment: Let us [continue this discussion in chat](https://chat.stackexchange.com/rooms/83948/discussion-between-tylerh-and-elliot-svensson).
Answer: Short answer: Yes.
Longer answer: The president has the power to appoint judges, but only the Senate can confirm them. The senate has in the past sat on nominations for extended periods, although not for the Supreme Court justice. There is no requirement in Article I of the Constitution that the Senate confirm, or even take up a President's nomination. Importantly, there is no requirement that the Supreme Court have 9 people, so technically there isn't an "empty seat" at all, unless the Senate chooses to vote on one. Additionally, as FDRs court stacking showed, there is no reason the President couldn't nominate 2,3, or even 10 people and the Senate just select who they want.
Two other examples of long delays between 'nomination' and confirmation bear mentioning. The oldest signed but unratified treaty by the U.S. according to wikipedia was signed in 1930! The 27th Amendment was passed by Congress at the same time as the Bill of Rights, but because it had no expiration, wasn't ratified by the states until 1992. Another amendment passed at the same time is [still technically pending]! Long durations, therefore, are neither prohibited by law, nor particularly uncommon.
Comment: There's a requirement for the number of seats. Circuit Justice Act of 1869.
Comment: Fair enough, but that isn't in the constitution.
Answer: They can go even further than that.
In 1867 Congress was unwilling to accept an Andrew Johnson appointment, so they passed a law reducing the number of justices from ten to seven.
After Grant had taken office they promptly increased it to nine.
Comment: It's probably online in quite a few places, but one is at https://www.thoughtco.com/how-many-supreme-court-justices-are-there-104778
Comment: Hi Mike, do you have any citations for this? Thanks for answering!
Answer: The correct answer is NO. At the end of any session of congress(2 years), all presidential appointments expire. The president could renominate and have it sit another 2 years, but there is a expiration date on all nominations. For example see Merrick Garland. His nomination expired officially with that congress.


Elitist and Marxist theories on the distribution of power

Question: What are the main differences and similarities between the marxist and elitist theories on the distribution of power in Liberal democracies?
Answer: Elite Theory
Elite theory is a scientific theory from the social sciences. It's most strongly associated with sociology, but has touched political science, economics, and probably other bodies of work as well. Elite theory just says that power is concentrated in the hands of a small number of people in society. So if you want to understand the movements of society, you should focus on them.
So the idea of a "distribution of power" here is that power is concentrated in the hands of some small group of people. Elite theory in general does not posit who exactly those elites are, though individual scholars sometimes do. For example Mills (The Power Elite) identifies key military, economic, and political groups that dominate American society.
Marxism
Marxism is in some sense, far more specific. It's fair to say that while Marxism and Elite Theory are different bodies of work, they do overlap on some key points.
For Marx, all of society is driven by its economy. Capitalist countries all share certain characteristics and follow the same over-all trajectory merely because they are capitalist. In these countries, an elite strata of society called the bourgeoisie control society. They do this by owning the means of production (things like factories, arable land, valuable equipment, etc.). Since they control production, they are able to control all of society.
For a primary source, the Communist Manifesto outlines this in a pretty straight-forward way.
Comment: I would say that Marxism also emphasizes the concept of the group or collective and discourages the concept of the individual.
Comment: @DavidS I would disagree, but that's outside the scope of the question. This isn't a question about individualism or collectivism.
Comment: Your answer covers most of what I would have to to say, though I would argue for inclusion of collectivism as a core component. However, you disagree, so I would have to provide a competing answer. Rather, I'll +1 yours, not enough of a quibble for me.


Why are rockets so big?

Question: I'm curious why rockets are so big in their size. Since both the gravitational potential one need to overcome in order to put thing into orbit, and the chemical energy burned from the fuel, are proportional to the mass, so if we shrink the rocket size, it would seem to be fine to launch satellites. So why not build small rocket say the size of human? I can imagine small rocket would be easier to manufacture in large quantities and easier to transport. And maybe someone can make a business out of small rocket, carrying one's own satellite.
Comment: How much fuel do you need to drive your car at 28,968 kilometers per hour  against headwinds as strong as gravity? 200mph winds!!!! powerful telecoms sattellites weigh 1000 KG. they put the maximum amount of satellites in each payload, including small ones, and if you tried to save energy by going slower, you would have to fight gravitty for longer. the trick is to get it out as fast as possible above 30 miles high. a GPS sat weighs 2080 KG.
Answer: The problem is what Konstantin Tsiolkovsky discovered 100 years ago: as speed increases, the mass required (in fuel) increases exponentially. This relation, specifically, is
$$
\Delta v=v_e\ln\left(\frac{m_i}{m_f}\right)
$$
where $v_e$ is the exhaust velocity, $m_i$ the initial mass and $m_f$ the final mass.
The above can be rearranged to get
$$
m_f=m_ie^{-\Delta v/v_e}\qquad m_i=m_fe^{\Delta v/v_e}
$$
or by taking the difference between the two,
$$
M_f=1-\frac{m_f}{m_i}=1-e^{-\Delta v/v_e}
$$
where $M_f$ is the exhaust mass fraction.
If we assume we are starting from rest to reach 11.2 km/s (i.e., Earth's escape velocity) with a constant $v_e=4$ km/s (typical velocity for NASA rockets), we'd need
$$
M_f=1-e^{-11.2/4}=0.939
$$
which means almost 94% of the mass at launch needs to be fuel! If we have a 2000 kg craft (about the size of a car), we would need nearly 31,000 kg of fuel in a craft that size. The liquid propellant has a density similar to water (so 1000 kg/m$^3$), so you'd need an object with a volume of 31.0 m$^3$ to hold it. Our car sized object's interior would be around 3 m$^3$, a factor of 10 too small!
This means we need a bigger craft which means more fuel! And explains why this mass-speed relation has been dubbed "the tyranny of the rocket problem".
This also explains the fact that modern rockets are multi-staged. In an attempt to alleviate the required fuel, once a stage uses all of its fuel, it is released from the rocket and the next stage is ignited (doing this over land is dangerous for obvious reasons, hence NASA launching rockets over water), and the mass of the craft is lowered by the mass of the (empty) stage. More on this can be found at these two Physics.SE posts:

Why do rockets have multiple stages?
Why do rockets jettison fuel tanks?
Comment: The Tsiolkovsky equation in the form you have stated only applies when the net external force is zero (i.e. no gravity). To accurately calculate the $\Delta v$ required, you need to include an additional term $-g(\frac{m_{propell}}{\dot m})$ on the right hand side of the equation.
Comment: @Asad: this is true, but I think it's (mostly) irrelevant to the point that we still need a boat-load of propellant to get ourselves into space, hence large rockets and not person-size ones.
Comment: @BЈовић: They usually don't burn oil, it's not efficient enough. But fuel actually isn't that expensive. It's often just a few % of launch costs.
Comment: @KyleKanos Yes, the gist of your answer is correct. I was taking issue with the calculation you added, which is flawed. Either you need to consider an **effective** $\Delta v$ which is augmented to approximately account for the retarding effect of gravity as well as the required escape velocity (this is the standard approach) or actually do the calculation taking fuel burn time into account.
Comment: @MSalters - They oftentimes do burn oil. The first stage of the Saturn V rocket used [RP-1](http://en.wikipedia.org/wiki/RP-1), a highly refined kerosene, to launch men to the Moon. RP-1 with liquid oxygen as the oxidizer is very widely used as a propellant.
Comment: @Asad It might have been easier if Kyle Kanos would have use the $\Delta v$ budget needed to get into low Earth orbit, which is about [9.3 - 10 km/s](http://upload.wikimedia.org/wikipedia/commons/7/74/Delta-Vs_for_inner_Solar_System.svg), but this would still return about the same result.
Comment: @fibonatic The delta v budget you are quoting is only a reasonable approximation for rockets with similar burn time to actual rockets. Since this question is specifically about rockets that can be very small, using the gravity inclusive delta v budget for a large rocket will yield poor results.
Comment: holy crap! that means they are burning oil like it is nothing, just to get some junk up there. And they did it lots of times. What kind of maniac is needed to form nasa?!?
Comment: @BЈовић For a better idea of what type of fuel gets used, see the Wikipedia pages for [Solid-fuel Rockets](http://en.wikipedia.org/wiki/Solid-fuel_rocket) and for [Liquid Rocket Propellants](http://en.wikipedia.org/wiki/Liquid_rocket_propellants).
Comment: Might the few downvoters mention what they think is wrong with this answer?
Comment: "as speed increases, the mass required (in fuel) increases exponentially", this is wrong assumption. It depends on the type of fuel that are used for rocket propulsion. Are you talking about liquid fuel, solid fuel or solar fuel. All have different capacities.
Comment: Kyle I found my answer at https://physics.stackexchange.com/questions/487194/masslessness-of-the-electromagnetic-field deleted . I formally object. The reason was that not enough explanation was given, only a link to arxiv.org. You know that arxiv.org is a very stable site. It has been consistent for over 20 years. Also I should be given the opportunity to explain my answer in more detail. Please undelete. Apologies for the cross post. I saw no other means.
Comment: @my2cts https://physics.meta.stackexchange.com/
Answer: TL;DR: This answer arrives at roughly the same conclusion as Kyle Kanos' answer, i.e. in addition to payload considerations, the difficulty lies in stuffing a small rocket with a mass of fuel exceeding the mass of the rocket itself. This answer, however, is more rigorous in how the $\Delta v$ budget is treated.

The rocket equation:
Consider the Tsiolkovsky rocket equation, which describes the motion of vehicles that propel themselves by expelling part of their mass with a certain velocity. A simplified version which only takes (constant) gravity and thrust into account is given below:
$$
\Delta v(t) = v_e \cdot \ln \frac{m_0}{m(t)} - g\left(\frac{m_f}{\dot m}\right)
$$
where $v_e$ is the effective exhaust velocity, $m_f$ is the mass of the fuel aboard, $\dot m$ is the the mass burn rate (constant with respect to time), $m_0$ is the the initial mass of the rocket and $m(t)$ is the current mass of the rocket.
Note that this is essentially a momentum exchange equation: you have a finite amount of momentum available from expulsion of fuel, which you must spend on increasing the velocity of the rocket + remaining fuel system, as well as overcoming gravity (i.e. dragging the planet ever so slightly). A form of the Tsiolkovsky equation that does not take this into account (as in the other answer) will give you non-physical results.

Constrained variables:
Now, what can we play with in this equation? Assuming $t_{escape}$ is the time at which the rocket escapes Earth's gravity:

$\Delta v(t_{escape})$ is simply our desired escape velocity (assuming the rocket starts from rest), which is dictated by where we're trying to send the rocket
$m(t_{escape})$ will optimally be the mass of the rocket without any fuel
The effective exhaust velocity $v_e$ and the rate of mass flow $\dot m$ are a function of the type of engine/propellant available

This means none of these quantities are negotiable; we are constrained by the demands of the mission and the available technology.

Developing a relationship between rocket and fuel mass:
All we are left to play with is the initial masses of the rocket fuel $m_f$ and rocket body $m_r$. Let us substitute in the values of $v$ and $m$ at the instant when the rocket escapes gravity, noting that $m_0 = m_f + m_r$:
$$
\begin{align}
v_{escape} & = v_e \cdot \ln \frac{m_f + m_r}{m_r} - g\left(\frac{m_f}{\dot m}\right)\\
& = v_e \cdot \ln\left(1 + \frac{m_f}{m_r}\right) - g\left(\frac{m_f}{\dot m}\right)
\end{align}
$$
Rearranging, we have:
$$
m_r = m_f \cdot \left(\exp\left(\frac{v_{esc} + g\left(\frac{m_f}{\dot m}\right)}{v_e}\right) -1\right)^{-1}
$$
Note that this is effectively providing $m_r$ as a function of $m_f$, since all the other parameters are fixed by the constraints of the mission and equipment as well as environmental constants. Since the relationship isn't immediately obvious, here is a plot of $m_r$ against $m_f$ for selected values of the constants:

In red, we have a plot of rocket mass versus initial fuel mass, while in blue we have a plot of the ratio of initial fuel mass to total mass. Note that the axis for the blue plot starts at 0.9!! This indicates that regardless of what rocket mass you picked, the net initial mass of your vehicle would have to consist almost entirely of fuel.
So what does this mean?
Filling a vehicle with a mass of fuel exceeding its own is increasingly difficult for small rockets, but not so difficult for much larger rockets (think of how the enclosed volume of a hollow body scales versus mass). This is why making smaller and smaller rockets becomes progressively more difficult.
In addition, a minimum limit on the rocket mass we can choose is imposed by the weight of the payload it must carry, which could be anything from a satellite to a single person.
Upper limit on payload:
A very interesting thing happens near the inflection point of the rocket mass - fuel mass curve. Before the inflection point, adding more fuel allowed us to hoist a larger payload to the desired velocity.
However, somewhere around $4 \cdot 10^6$ kg of fuel mass (for our selected parameter values) we discover that adding more fuel starts to decrease the payload that can be hoisted! What is happening here is that the cost of the additional fuel having to fight against gravity begins to win out against the benefit of having a high fuel to payload mass ratio.
This shows there is a theoretical upper limit to the payload that can be hoisted on Earth using the propellant technology we have available. It is not possible to simply keep increasing the  payload and fuel masses in equal proportion in order to lift arbitrarily large loads, as would be suggested by using the Tsiolkovsky equation with no extra terms for gravity.
Comment: Comments are not for extended discussion; this conversation has been [moved to chat](https://chat.stackexchange.com/rooms/78604/discussion-on-answer-by-asad-saeeduddin-why-are-rockets-so-big).
Answer: Consider the problem in the from of a ratio, what is the ratio of mass used to lift the rocket(fuel), to the mass finally put into orbit(cockpit). That proportion will be much the same regarding smaller objects that must be put into orbit. If you use the same ratio or proportion to calculate the needed fuel mass for a small craft, you will find you can't even carry the device holding your fuel. This is also why rockets use stages.
The type of fuel used also has an impact, but those are details that need a new question.
Comment: this is the correct answer. Also, you need to factor the fact that atmospheric drag grows as the square power of width, while total fuel mass grows with the third power, even assuming constant fuel to dry mass ratio
Answer: Because most payloads are quite heavy. I am not sure what kind of payloads you had in mind, I am no expert on this, but I think that most launches contain satellites, which might be heavier then you think, for instance the satellite in this BBC Documentary weighs 6000 kg. And according to Wikipedia, miniaturized satellites weigh less than 500 kg (so heavier is normal). And some of those miniaturized satellites are using excess capacity on larger launch vehicles.
And I think that smaller rockets will experience the turbulence of our atmosphere much violently. Also think of the relatively higher costs in terms of personnel (such as mission control). And I would also expect that certain aspects do not scale linearly in size, but for be this would just be speculation.
xxxxxx
Answer: Mainly because you need a lot of speed to go into space, and to each that speed, you need to accelerate. If you need a high speed, you will need to accelerate for a long time, thus the need for a large quantity of fuel. You also need to compensate for gravity the whole lift.
There are ways to reduce that fuel requirement, like a horizontal takeoff, you reach a high altitude and then launch, so you keep the engine, but you still need a lot of energy to fight against gravity, and wings can't lift you very high, so that would not be such a good fuel economy, and the plane would still require to be quite big.
Answer: $ E = mc^2 $
The larger the mass, the more energy can be produced. And we still haven't found any fuel which in small quantities gives the needed amount of energy. I know you will be thinking of nuclear energy; we cannot fit a nuclear reactor inside a rocket with current technology, and even if we can fit it I don't think our existing knowledge of nuclear science is sufficient to ensure accident-free reactors at such velocities.
Comment: $E=mc^2$ doesn't really apply here. First, I'm not aware of any practical matter-energy conversion process that comes anywhere near close to that (insofar as I know we still haven't figured out how to build matter/antimatter reactors for power generation purposes, and that'd be about the only way to get anywhere near such amounts of energy). Second, if you look at the rocket equation cited in other answers, you'll see that the critical issue is the exhaust velocity. If you can get insane exhaust velocities, each tiny nugget of fuel packs a lot more punch in terms of total system $\Delta v$.
Comment: We could use the propulsion similar to that of [project Orion](http://en.m.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)), but this probably will not be used at take-off due to the nuclear fallout.
Comment: @fibonatic ...and the fact that you need to worry about nuclear fallout is a pretty good indicator to begin with that you aren't in $E=mc^2$ territory.
Comment: We can put it on an airplane https://en.wikipedia.org/wiki/Nuclear-powered_aircraft

Why are rockets so big?


Force Majeure (2014): significance of scene in the fog

Question: What is the significance of the scene in the fog near the end of Force Majeure (2014)?
Answer: From an interview with Ruben Östlund, the director:

You mentioned the final, foggy ski run as a "group therapy." Did you
  see that scene as a desperate way to restore order in the family?
Yes, I see it as a group therapy ski run. It's Ebba that wants to try
  to restore the family and wants to push to make Tomas the male leader
  of the group and the hero.
So the children can feel at ease, Ebba can feel protected, and Tomas
  can feel like a man and they can continue lying to themselves forever.
I love the moment when he says, "We made it, we made it." In a
  Hollywood movie we would cut at that moment, but instead we have to
  see their everyday life continue.
Comment: Thanks a lot! I find the director's explanation appalling really. The husband goes through the fog to save his wife, so that she "can feel protected". All the while, the children are left alone and it looks like they will lose _both_ their parents. I'm sure they didn't "feel at ease". Amazing film, but awful protagonists.


wiring breaker box

Question: My second question is, there is a main panel with a 100 amp double breaker and two 60 am double breakers. Prior owners run a line off the 60 amp double breaker to the wash room washer and dryer.  Can I run a line with conduit 10 gauge line off of one of the 60 am double breakers? Do I just run a 10 gauge line off the 60 amp breaker to a new breaker box with a 30 amp double breaker?
I appreciate all advice.  Thank You.
Comment: This question is hard to follow, but you can't connect #10 wire to a 60 amp breaker, you need #6 to carry 60 amps.
Comment: What is this 30A circuit servicing? Also, is there a subpanel at the wash room, or did some genius connect the washer and dryer outlets directly to the 60A circuit?  Last but not least -- can I presume that the dryer is electric, or is it gas?
Comment: Also, just *what* kind of breakers do you have already?
Answer: Bottom line:
The breaker MUST be sized to be equal to or less than what the wire permits in ADDITION to what the manufacture of the appliance says to use.*
The breaker can NOT be greater than what the wire permits. Period.
#10 gauge size wire is sized for at most 30 Amps.  Therefore, at most a 30A breaker should be used.
Most washers require no more than a 20A circuit and new code requires them to be GFCI protected.  I've never seen a double pole washing machine, but that doesn't mean they don't exist.
Your existing setup doesn't sound correct at all.  But what I've outlined is pretty standard.
*The breaker/wire can be greater than what the manufacture calls for IF and only IF there is a fuseable disconnected/sub-panel to protect the appliance(s).
Comment: +1 for "your existing setup doesn't sound correct at all."
Answer: You can't hang 30A circuits off a 60A breaker.  The only thing you can do with a 60A breaker - is run wire rated for 60A, to serve a device rated to draw 60A.  One possible device is a sub-panel.
I gather from your other answer, that you're dealing with panels whose breakers are not available at any sane price or reliability, so changing breaker size is not an affordable option for you.  You have a main 120/240V panel (this panel), and also two 120V-only panels each fed by a different pole of "hot".
If you can obtain a 30A breaker for that main panel, yes, you can connect a water heater in the usual way with 10 AWG wire.  You don't need a neutral wire unless the water heater or timer is actually 120/240.  If it's straight 240, 10-2 cable (black/white/ground) will suffice, relabel the white as a hot.
Otherwise: I would install a new sub-panel somewhere, perhaps in the washer-dryer area (cost of wire may be a factor).  Feed the sub-panel off the 60A double breaker now used for the washer and dryer.   Connect the sub-panel to the main panel with wire rated for 60 amps.  If necessary, change wire - because for you, wire is cheaper than breakers.
Then, in the new sub-panel, install the correct breakers for washer (single 15-20A, GFCI) and water heater (typically double, 30A).  If you also want to power other loads off this panel, ask here.
On choice of sub-panel:  "What a fool I was to buy a panel with so many spaces" was said by no one, ever.   Err on the side of "really big" when buying panels.  With panels, it's OK if the panel is rated for more than 60 amps.  The 60A breaker in the main panel will protect the wire to the panel.  Some panels are sold in "combo packs" bundled with breakers, which is far-and-away the cheapest way to buy breakers+panel.  If you have extra breakers, you can sell them on eBay or save them for the happy day when you replace those other panels.  I'd buy this at a real electrical supply house, they will also have the best selection (and often better prices - electricians don't buy there for no reason).
On choice of wire: check your existing wire to see if it is legal in current code. I don't know what your local regulations require, what will easily install, and what's available at your local electrical supply. Normally I'd say consider aluminum wire in AA-8000 alloy; but given the age of your breakers I'd avoid it.
If you DIY, I would figure about $200 depending on the wire distance and the service panel you choose.  If you can spend $30 extra and get more panel, definitely a good idea.

wiring breaker box


What happens when all IP addresses run out?

Question: I recently read an article in which they wrote that that there will be a time when there will be no more IP addresses available. Is that right? Do I need to register a website now just in case?
Comment: I've updated the tags to indicate that this an IPv4 vs IPv6 question
Answer: No, you don't need to worry.
Registering a website (domain name) is totally separate to having an IP address. Registering a domain name just gives you the name, this name will then be entered into DNS servers, which will allow the name to be resolved into an IP address. This will be the IP address of the web server that holds your site. The company running the web server or their ISP will be responsible for ensuring that they have valid IP address, you don't need to worry about it. It can also change, so you don't need to reserve a specific address.
Something called Network address translation (NAT) has relived a lot of the pressure on IPv4 addresses. It allows a single external IP address to be mapped to a large number of IP addresses internally within a smaller network. (Such as a home or company network).
We are moving towards support for IPv6, which will expand the number of available addresses significantly. Most modern operating systems now support it, and a a lot of network hardware is starting to support IPv6 now.
Unless you are a registrar or ISP you do not need to significantly worry about IpV4 addresses running out. Just consider it when buying hardware and try to buy IPv6 compatible hardware where possible.
You can read more about it here.
Comment: All the answers are fine.But i am accepting your answer, which solved my problem.
Answer: Probably not as NAT has freed up a lot of IP addresses, plus once they are scarce the price they command will rise to the point that people will start selling them.
It's the same reason that all commodities in a market are available at some price. Once the price is high enough it tempts people to sell them, find ways to extend them (NAT, Apache Virtual Hosting etc)
I'm assuming that IP addresses can be bought and sold. Not sure if they actually are at the moment.
Also having a web site (you probably meant domain name) doesn't automatically get you an ip address. Your hosting provider will probably use one IP address to host many domains at once, and rely on the header your browser sends to determine the domain name requested.
Comment: NAT = Network Address Translation - http://en.wikipedia.org/wiki/Network_address_translation
Comment: There are utilities floating on the net that can tell you how many web sites are hosted on a single IP address. Some of the IP addresses at GoDaddy are hosting about 600 web sites. But if they have the infrastructure and the pipe to support them, this is just fine.
Answer: Well, people are crying since some years that all IPv4 addresses are used up...as I said, they're saying it now for years. The truth is that 90% of all IPv4 addresses are allocated, but the rest will hold for another 2 to 3 years...and after that there's IPv6. Maybe you wanna read the slashdot story about it.


Does travelling through the US border with encrypted files break the law?

Question: If I have encrypted files and I'm going to US, do I break any encryption import laws? Likewise, if I go out of US with those files, do I break any encryption export laws?
Comment: The encryption-related laws I'm aware of (arms-export control) deal with encryption software, not the result of using that software, but there might be other laws out there.
Comment: Does that include the software that's on the laptop which is used to encrypt and decrypt the files?
Answer: The Electronic Frontier Foundation, a San Francisco-based digital rights non-profit, specifically advises to use encryption when crossing the US borders. Read more at https://www.eff.org/wp/digital-privacy-us-border-2017.
While the EFF doesn't say that using encryption would be illegal, keep in mind that the US borders are a gray area, legally-speaking, and US Government agents have a wide range of powers that would not be available elsewhere - within the US, or outside the US.
The EFF also advises this:

We appreciate and respect technologists' efforts to find ways to help travelers protect their data. However, we recommend against using methods that may be, or even appear to be, calculated to deceive or mislead border agents about what data is present on a device. There is a significant risk that border agents could view deliberately hiding data from them as illegal. Lying to border agents can be a serious crime, and the agents may take a very broad view of what constitutes lying.11 We urge travelers to take that risk very seriously.
Answer: Wikipedia's list of countries with restrictions on import of cryptography does not include the United States of America. Their page on export of crypography from the United States has a list of currently restricted technologies, but files which have been encrypted are by themselves not technologies at all, just data that is the output of some technology.
If you have encryption software on that computer, that may still be subject to restrictions.
The "current status" section on the latter page has a "needs update" badge; but the technologies it currently lists are

Militarized encryption equipment, TEMPEST-approved electronics, custom cryptographic software, and even cryptographic consulting services

So if you have custom decryption software, you are at risk, but bog-standard GPG should not be liable as per this documentation.
(I obviously am in no position to guarantee rational behavior from a country whose current administration seems hellbent on reviving the dark ages. This answer is in whole based on what I could find on Wikipedia, not on personal insights into the matter.)


BufferedStream Seek Returns Different Results

Question: Very strange behavior
If I create a bufferedstream on top of a file and then seek to an offset I get a block of bytes back
If I move the debugger back to the seek and re-seek I get an extra two characters
I ve triple checked this
Can there possibly be a bug with this class ?
If I reseek back to position I expect to get the same - The file has not changed - I open it in read only mode and I seek based on Origin
Reproduction:
bufferedStream.Seek(100,0, 100)
bufferedStream.Reade(buffer, 0, 100)

is different to what you get from here
bufferedStream.Seek(100,0, 100)
bufferedStream.Reade(buffer, 0, 100)

Comment: Erm, wait, your code snippets are the same.
Answer: First off, it is hard to know if they are the same without checking the return value fro Read - is it possible they are just choosing different chunks? (perfectly valid; it is your job to ensure you loop over Read until you have enough data, or EOF).
However, I wonder if a BOM is involved here - especially if you are sitting a text-reader on top of this. Simply, a reader expects the BOM at the start; so it may well hide it the first time through. But if you rewind the stream while using the same reader/decoder, it won't be expecting a BOM, so will try to report it as character data (or throw an error, depending on the configuraion).


Are there any tips for helping unhealthy trees?

Question: My new yard has a lot of trees, probably 20-30.  There are apple, pear, peach, pine, plum, and a few others.  Most of them are fairly old (50+ years).  Some of them don't seem to be very healthy (dead branches, withering/yellow leaves, etc.).  Are there any tips or tricks for helping an unhealthy tree?  How do I figure out what is wrong with them?
Comment: What is your hardiness zone?  Have you had any unusual weather is the past few months?  Have the trees been unhealthy for quite some time or just recently?
Comment: Hardiness zone is 6B.  It's been a trend for quite some time.  My grandma has been taking care of them for the last 20 years, and she said that ever since she started they haven't been doing as good as they used to.
Comment: A lot of fruit trees tend to have shorter lives, so they could be "fairly old" as you say. It might be time to think about what you would replace them with when they die/blow over. Successional planning :-)
Answer: Generally speaking, established trees like it just the way it is.  Grade changes, drainage changes, more foot or car traffic causing compaction; these are things that trees don't like.
That sounds like a lot of trees for a backyard.  I've seen cases where previous owners liked trees and planted a lot of them.  "They're so cute when they're small."  After twenty years it can get a bit crowded as the trees fill in.
Things that you can do from easiest to harder:

look at the trees for pests and diseases
decide if there are any you do not want
top dress around each tree with good compost twice a year to a depth of 1/4" to 1/2 inch from the drip line to within 6" from the base of the tree. (Thanks @winwaed)
if the soil is compacted aerate yearly around the drip line of each tree (don't get too close to the trunk)
if it is dry during the summer, water them generously and thoroughly every two weeks
do a maintenance prune yearly or get an arborist in
Comment: Good ideas.  It probably hasn't been aerated ever, and it does get pretty dry during the late summer.  It is a lot of trees, but it's a big yard (almost an acre).  Thanks for the tips!
Comment: Top dressing is popular - perhaps too popular. Do NOT bury the tree's trunk flare. This is a common problem you see in gardens and even parks.


How do I download a particular file from GitHub?

Question: I only need one particular file from a project published on GitHub so it makes little sense to download the whole project. But I just can't find the download button.
Comment: [Download single files from GitHub](https://stackoverflow.com/q/4604663/6521116)
Answer: If you're viewing the contents of the file itself there should be a "Raw" button.

You can either:

Right-click on that and select the "Save content as..." (or similar depending on your browser)
Click through to see the contents dumped onto the screen and save from there
Answer: In the list of files, every filename is a link. A simple right-click and "Save link as..." (or equivalent for your browser) will allow you to download a single file.
This breaks down with HTML files, however. You'll need to use the "Raw" button on the file page as Eight Days of Malaise's answer describes. I'm using Google Chrome. You may have a different experience on a different browser.
Comment: Those links go the the html page for that file, not the file itself. (Unless you're referring to another file list, in which case you should extend your answer to describe how to get to that file list)
Comment: @CodesInChaos: It works just fine. Yes, if you just click the link it will open GitHub's page for that file, but "Save link as..." saves the file in its "native format".
Comment: For me it never works (Using Firefox). It always downloads github's html page for that file.
Comment: Tested it in chrome and it doesn't work either. The downloaded file still contains html. Did you check the *content* of the file?
Comment: This doesn't seem to work with HTML files.
Comment: @Zenadix: So it does. I've updated my answer.
Comment: @Codes: I'm using Chrome, so maybe that has something to do with it. I'll update my answer.
Comment: @Codes: Well I don't know what the difference could be. It works great for me (and apparently other people).
Comment: @CodesInChaos: Do you have any sort of script blocker? I'm pretty sure there's more than just HTML going on here. This could even be turned into a decent question.
Answer: Check out RawGit which is a free service to serve files directly from Github with proper Content-type headers.
Example URL:
https://rawgit.com/user/repo/branch/file
Answer: If you are using the Chromium browser, just hold down the ALT key and left click on the file to download it.
(Doesn't work with Mozilla Firefox, haven't tested other browsers.)
Answer: You can try github-files-fetcher, it is a command line tool which downloads a single folder or file from a GitHub repo.
Think a real scenario: you were visiting the following webpage page and wanna download the async subdirectory alone.

With The github-files-fetcher, you should first copy the url of that page, which is https://github.com/reduxjs/redux/tree/master/examples/async, and then run the command below in command line:
fetcher --url=https://github.com/reduxjs/redux/tree/master/examples/async


Android to Android Bluetooth Tether

Question: My android does a pretty good job doing a bluetooth tether for my PC (in which PC shares my Android's internet), but I was wondering how to get another Android device to connect/tether to it (my first android device)?
Answer: You can do this easily by pairing the devices and enabling data on the first device. Steps are listed below.

Enable bluetooth on both the devices. (Obvious!)

Then, pair them up by clicking "Search for devices" on one of the devices and clicking on the name of other device. You'll be asked to verify a PIN number. It doesn't matter what the PIN number is, as long as it's the same PIN number on both devices.

Go to Settings, choose More then choose Tethering & Portable Hotspot. In there, you have tick Bluetooth tethering. If you don't, the tether will not work!

On the second device, go to Settings, then click on the word Bluetooth (not the toggle – the word). Finally, click on the name of your second device you paired earlier. To get it all working, choose Internet Access.

Now you should be able to use the data connection of your first device.
Comment: I did try that. What version Android is this? On my devices (4.1 & ~2.x) it just shows "File Transfer" (step#4) (on either devices, doesn't matter which phone's connection I share to whom)
Comment: @laggingreflex This should be available the devices running at GB or above. Screenshots aee from a Galaxy Nexus phone.

Android to Android Bluetooth Tether


How is hyperconjugation into antibonding orbitals stabilizing?

Question: So I learned that hyperconjugation is electron delocalization from a $\ce{C-H}$ $\unicode[Times]{x3C3}$ bond into an empty p orbital (on a carbocation for example) or an antibonding orbital. I get how it's stabilizing in the case of an empty p orbital, but how is it stabilizing in terms of delocalization into an antibonding orbital, since by definition electron density in antibonding orbitals is destabilizing?
Answer: This is nothing specific to antibonding or nonbonding orbitals. There is a very general principle behind it:

Mix an occupied and an empty orbital with each other, and the occupied orbital will be stabilised.

You're not actually 'transferring electron density into an antibonding orbital'. You are simply linearcombining two orbitals: one occupied one with a rather low energy and one unoccupied one with a rather high energy. With this linear combination, you receive two new orbitals, where the unoccupied one is further destabilised while the occupied one is further stabilised.
Your electron density is still in the (now more stable) occupied orbital and the antibonding/nonbonding/otherwise empty orbital (now less stable) is still empty. Only their shapes have changed a bit, because we mixed them.
Comment: @Michael No it doesn't. It *is* the concept of hyperconjugation.
Comment: What you just said contradicts the concept of hyperconjugation, because the occupied orbital is not associated with the atom that has the antibonding orbital and is being stabilized. You're basically saying you are gonna destabilize the antibonding orbital even more
Comment: if that were true, then the more alkyl groups that are substituted in an alkene,  the less stable the alkene would be because what you are saying that the double bond is more unstable. The opposite is true though the alkyl groups make the enthalpy of the reaction of breaking a double bond in an alkene more positive, not negative
Comment: @Michael No, you just got the argument the wrong way around. The (stable and occupied) $\pi$-orbital of the alkene mixes with the (instable and unoccupied) $\sigma^*$-orbital of the $\ce{C-H}$ bond. This interaction stabilises the $\pi$ bond and destabilises the $\sigma^*$
Answer: The electron density increasing in the antibonding orbital only means that the original bond is reduced in strength. Consider propene, the hyperconjugative structure  adds electron density to the $\pi^*$ orbital of carbon. Let us say that the double bond was between C1 and C2.Now the electron density from the alpha C-H bond at C3 moves into the antibonding orbital, thus weakening the C2 -C1 pi bond, and begins to create a double bond between C3 and C2. Thus one bond is weakened and another is strengthened. The stability is only on account of the delocalization, similar to resonance stability.
Answer: In organic chemistry, hyperconjugation is the interaction of the electrons in a sigma bond (usually C–H or C–C) with an adjacent empty (or partially filled) non-bonding p-orbital, antibonding σ or π orbital, or filled π orbital, to give an extended molecular orbital that increases the stability of the statement.
No of canonical structure= no of hyperconjugable $H$ atom +1
Higher is the no of canonical structures formed Higher is the stability.
It effects are by following ways:
Bond length: Hyperconjugation is suggested as a key factor in shortening of sigma bonds (σ bonds).
The heat of formation of molecules with hyperconjugation are greater than sum of their bond energies and the heats of hydrogenation per double bond are less than the heat of hydrogenation of ethylene
Stability of carbocations:
$\ce{(CH3)3C+ > (CH3)2CH+ > (CH3)CH2+ > CH3+}$
The C–C σ bond adjacent to the cation is free to rotate, and, as it does so, the three C–H σ bonds of the methyl group in turn undergoes the stabilization interaction. The more adjacent C-H bonds there are the larger hyperconjugation stabilization is.
In chemical bonding theory, an antibonding orbital is a type of molecular orbital that, if occupied by electrons, weakens the bond between two atoms and helps to raise the energy of the molecule relative to the separated atoms. Such an orbital has one or more nodes in the bonding region between the nuclei.
Antibonding molecular orbitals (MOs) are normally higher in energy than bonding molecular orbitals. Bonding and antibonding orbitals form when atoms combine into molecules.
as the spacing between the two atoms becomes smaller, the electron wave functions begin to overlap. Each energy level of the isolated atoms slits into two molecular orbitals belonging to the pair, one lower in energy than the original atomic level and one higher. The orbital which is lower than the orbitals of the separate atoms is the bonding orbital, which is more stable and promotes the bending.
A molecular orbital becomes antibonding when there is less electron density between the two nuclei than there would be if there were no bonding interaction at all. When a molecular orbital changes sign (from positive to negative) at a nodal plane between two atoms, it is said to be antibonding with respect to those atoms.
For example, the He2 molecule, in which both the 1sσ and 1sσ* orbitals are filled. Since the antibonding orbital is more antibonding than the bonding orbital is bonding, the molecule has a higher energy than two separated helium atoms, and it is therefore unstable.
negative hyperconjugation is the donation of electron density from a filled π- or p-orbital to a neighboring σ*-orbital.This phenomenon, a type of resonance, can stabilize the molecule or transition state.  It also causes an elongation of the σ-bond by adding electron density to its antibonding orbital.And hence helping in  delocalization of orbitals.
Delocalizationelectrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or a covalent bond . its also the Distribution of electron density beyond a fixed place such as a single atom, lone pair, or covalent bond via resonance or inductive effects.
For example ,
1.The oxygen lone pairs of ethoxide ion are not delocalized.
2.The oxygen lone pairs of trifluoroethoxide ion are delocalized via the trifluoromethyl group's electron-withdrawing inductive effect.
Negative hyperconjugationis most commonly observed when the σ*-orbital is located on certain C–F or C–O bonds,and does not occur to an appreciable extent with normal C–H bonds.
In negative hyperconjugation, the electron density flows in the opposite direction (from π- or p-orbital to empty σ*-orbital) than it does in the more common hyperconjugation (from σ-orbital to empty p-orbital).
decolalization of electron is though conjujation of $\ce{CH σ-bond}$ with π - orbital or atomic orbital( π - orbital which does not participate in resonance) in hyperconjugation
Increase in delocalization, increases the stability, by decreasing the total (potential) energy.
I hope this will help.
Comment: Why does adding electron density to the antibonding orbitals increase stability though? *That* is what the question is about. Most of this answer is only vaguely relevant and it could also do with using better grammar.


On a 777 what would happen if the aft pressurization switch was accidentally left closed?

Question: The Boeing 777 has fore and aft pressurisation switches (see example here). If the aft switch was forgotten on takeoff, could this cause a loss of cabin pressurisation when the aircraft reaches altitude?
Comment: There would most certainly be a visible alert on the screen that it had been left in the close position, since this is not the normal configuration, and the pilots would have seen that. Otherwise, they would also have gotten plenty of warnings about cabin pressure had it been too low.
Comment: My question was edited and the point of it is now lost. My basic idea is: Is there any MISTAKE the pilots could have made that would have done in all the rest of the pasengers without the pilots noticing it until it was too late? The rest of the flight characteristics would then make sense....
Comment: So it can't be pressure. What about the air conditioning? Somewhere in the "Source" document, I read that passengers are airconditioned from both sides but the flight deck just from one (I think that it was the left).
Comment: if it was unintentional, any deviation from the normal operation of the pressurization the pilots would have been alerted about problems on multiple levels. This was most probably a deliberate act by the pilots, where they might or might not have switched off the pressurization entirely.
Answer: From my previous source in that incapacitation question:

Two outflow valves are installed: one forward and one aft. Normally,
  most of the outflow is through the aft outflow valve. This improves
  ventilation and smoke removal. Cabin altitude and full ventilation
  rates can be maintained by either valve. Source
Answer: There is only one pressurization system for the whole aircraft. The two outflow valves regulate the airflow moving through the cockpit and cabin respectively, the intake air comes from engine bleed air, which is quite hot, and is mixed with outside ambient air to regulate the temperature through two or more heat exchangers, depending on the make and size of the aircraft. If there were two pressurization systems, one for the cockpit and one for the cabin, there would have to be a pressure bulkhead between the cockpit and the cabin, and there isn't.


Is there a relation that is irreflexive, anti-symmetric and not transitive?

Question: from the set $\{a, b, c, d\}$?
Of the one's I have tried, it at best is two of the three, but never all.
Comment: @KevinCarlson No, $\{(a,b)\}$ would be transitive.
Comment: Doesn't $\{(a,b)\}$ work?
Comment: Ah, yes, that's right. One needs a case in which $R(a,b)$ and $R(b,c)$.
Answer: Consider the ordered pair, such that $\{(x,y)\,|\,x,y\in\{a,b,c,d\}\}$.  The following relation satisfies those conditions:
$$\{(a,b) ,(b,c), (c,d), (d,a)\}$$
Clearly, this relation is not reflexive since there is no ordered pair with same members i.e. $(x,x)$.  This relation is anti-symmetric since for instance, there is no ordered pair $(b,a)$.  This relation is also not transitive (which is left for you to work out).
Comment: I think even $\{(a,b),(b,c)\}$ is enough.
Comment: I believe you took "anti-symmetric" to mean asymmetric, but this is also anti-symmetric trivially.
Answer: If non-transitive means "never transitive for any triple", that is impossible for a complete tournament (irreflexive, never-symmetric relation) with $4$ or more players.  The question sounds like an exercise of rediscovering that fact.
If  partial tournaments are allowed then it can be done with any number of players, just partition them into 3 categories A,B,C, and have every A player always beat the B players who always beat C who always beat A, and no matches within any category.


food safety regarding re freezing twice cooked chicken

Question: I am defrosting a cooked chicken to use in a curry it will be thoroughly heated at a high temperature. If I have too much curry will i be able to freeze it, as the cooked chicken was frozen before?
Answer: As long as you treat the food safely in between, and the communicative time in the danger zone of 40-140 F / 4 - 60 C does not exceed about 2 hours, you can thaw and refreeze foods an unlimited number of times.  Whether they are raw or cooked doesn't matte for safety.
On the other hand, multiple thawing and refreezing may degrade the quality of the food, especially its texture, and is generally avoided for that reason.
Thawing cooked chicken once, cooking in a curry, then freezing the cooked product is probably well within the tolerances of most people's palette, and you probably will not notice any quality loss, especially in an application like curry.  Just don't over cook your chicken--that will definitely result in dry, tough chicken and won't be pleasant.


Should a site that 'cannot handle the truth' be labelled Christianity?

Question: I notice that the most relevant questions to Christianity are put on hold or closed as being unclear when even a fifth grader could tell what is really being asked. So truth is smoothered, as rightly confessed by a 'higher-up' on the website.
Christians come here wanting to know what is in christianity, to them its the ONLY shopping place for truth, but the website admits as 'unable to handle the truth'.
So should a site that cannot handle the truth be labelled 'Christianity'?
Comment: I agree.Christianity is all about truth.And personal revelation is a big part of How apostels and prophets bring God's word to us,very biblical. Still I do understand the problem with to much opinion based stuff,there is alot of  questions that are very hard to answer and understand where they are coming from.But I agree The label is problematic.
Comment: But I like to add.There are thousands of sites out there,if you want to talk about opinion based Christianity without doctrine,go to another site.Just not this one.
Comment: @Aigle,  Thx for the advice. Only care to explain what you mean by 'Christianity without doctrine' in light of voting expressly 'scriptural 'questions as opinion based.
Comment: Are you aware the "we can't handle the truth" is a bit of a joke in reference to a movie called  [A Few Good Men?](https://www.youtube.com/watch?v=9FnO3igOkOk)  About 45 seconds into the video ...
Comment: @KorvinStarmast No I wasn't aware. If in the midst of the 9/11 chaos any instruction for evacuation could hardly be given as joke, should things that deal with damnation of souls for eternal be so lightly treated?
Comment: @Witness I believe that laughter and a sense of humor was and remains a divine gift.   I realize that some denominations (who I'll not name) have a history of being more grim faced than joyful in the exercise of our faith.  Our church tries to emphasize joy, with good theological foundation.  It is not by accident that we sing "Joy to the World" at Christmas time.  The angels brought us tidings of great joy.
Comment: @KorvinStarmast **laughter and a sense of humor was and remains a divine gift**, true, it's also written that at His right hand there are pleasures for evermore. And I confess to you, a stranger, that though advanced in my years, 'some' cartoons for children I still find very humorous, notwithstanding these things have their place.
When it comes to prospects of an eternity in heaven or hell, no amount of gravity is undue to a careful handling of what's in the Bible, no matter what level or platform. So I find quite upsetting any levity in handling things of unspeakable importance to men.
Comment: In a nutshell: Christianity.SE, by its structure, can serve a very useful role in inter-denominational understanding and dialogue.  The chosen method is to humbly limit the scope of each topic.  "On this matter of the faith, what do (denomination X believe)?"  Rather than trying to be a source of "universal truth" (Jesus has that role sewed up already) the aim of improved understanding is a noble one.  If that is achieved, an opening of hearts across denominational lines can grow, which brings us, as Christians, a small step closer to unifying The Body of Christ (his church)
Answer: All SE sites prohibit questions which are highly opinion based, and there's nothing people are more opinionated about than religion!
You say people come here wanting to know "what is in Christianity", but what is "Christianity"? There are over 40 thousands denominations and a dozen major branches of Christianity, and almost every topic will have some level of disagreement among those branches. Our solution is simple: require questions to specify which perspective they want answers from. No truth is being smothered, just organised into topics.
Comment: Thanks for your response. Christianity refers to all that esteem what Christ has on offer, different views of what that means, granted. But let those that ask the question be the judge of what is mere opinion or not, because the HolySpirit is involved in their search for the truth, albeit in the background.
Answer: SE sites do not try to be all things to all people.
Plenty of other sites on the internet do.  What SE sites try to do is make a favorable signal to noise ratio by posting questions of a limited scope and getting answers that are confined to the question.  A favorable signal to noise ratio is something generally lacking on the internet.  That means that there are limitations in scope here as a design imperative.  This is consistent with SE/SO guidelines.
How Christianity.SE got to where it is today
If you go through the history of discussion on this meta, which shows how this site could conform to SE general principles, what this site will or won't do well to achieve a favorable signal-to-noise ratio was discussed in detail.  It took a few years to better refine how to get it right.  You'll see, if you work through the history, how the caretakers of the site arrived at how to:

Rationally limit the scope of a given question
Avoid opinion wars

Ever since the disagreement between Peter and Paul as documented in Galatians, Christianity has had its share of opinion wars  -- to put it mildly.  That has not changed recently.  It is not Christianity.SE's aim to simply add more noise than is already echoing around the internet.
Comment: thanks, it makes alot more sense seen from that point of view, any other way to explain why so many questions that seem to qualify as being within it's scope yet are closed or on hold, in my opinion, is unconvincing.
Answer: This site is for the academic study of religion (and that is narrowed to specifically Christianity). Believers can certainly find that useful, but the topic is to be approached unemotionally, much like the sciences study their topics without attachment. This is a study with your mind, whereas a study for truth is a study with your heart.
Please see

Religious studies, alternately known as the study of religion, is the multi-disciplinary academic field devoted to research into religious beliefs, behaviors, and institutions. It describes, compares, interprets, and explains religion, emphasizing systematic, historically based, and cross-cultural perspectives.
While theology attempts to understand the nature of transcendent or supernatural forces (such as deities), religious studies tries to study religious behavior and belief from outside any particular religious viewpoint. Religious studies draws upon multiple disciplines and their methodologies including anthropology, sociology, psychology, philosophy, and history of religion.
Wikipedia - Religious Studies

The academic study of religion is fundamentally an
  anthropological enterprise. That is, it is primarily
  concerned with studying people ..., their
  beliefs, behaviors, and institutions, rather than
  assessing "the truth" or "truths" of their various beliefs
  or behaviors. An anthropological approach to the study
  of religion (which is not to say that the study of religion
  is simply a sub-field of anthropology) is distinguished
  from a confessional, religious, or theological approach
  ...
  which is generally concerned with determining the
  nature, will, or wishes of a god or the gods.
What is the Academic Study of Religion? by Russell T. McCutcheon, Department of Religious Studies, University of Alabama.

So should the name change? Well, that's been talked about here since about the site's beginning in 2011. If we answer yes, the question is what should the name be? The truth of the matter [bad pun] is that Christianity is exactly the right name. The religion is called Christianity, and this site studies it. I can't think of any other name that would help clear up this common misunderstanding for new users.


How do I record piano and voice music to the best of quality?

Question: My phone records piano music quality very poorly. I am  using the built-in microphone.
I am humbly asking whether you could suggest an app for me that could record high quality music for my playing technique and my voice too.
Answer: It's not the app, it's the phone's hardware that is limiting you. Your phone has a mic and an analog-to-digital converter inside of it that picks up the sound and converts it to digital for whatever app to record. Because your phone is small and engineered to be of use to ordinary people and not audio engineers, those hardware components are not very good relative to professional recording standards.
No matter what app you use, it will be getting the same low quality input from the same hardware. So the solution is to get some kind of external hardware that is of better quality. There are audio interfaces for phones and computers and there are standalone recorders that would all work.
Answer: You definitely will want a good external microphone.
There are several types available, from the usual sources.
I recommend you begin your search by looking for a good lapel mic to start with.  They plug in to your regular 1/8" jack, just like the earbuds with microphones in them, but are of much greater quality.
They work well, and you would want to clip it on to the shirt of the singer and let it pick up the piano in the background.
Here is the one I use:  Quality Lavalier Lapel Clip On Microphone
For about the same price you can go el-cheapo and try a small add-on microphone for your phone like this one:  Olympus Noise Cancelling Microphone - I haven't tried this because I like to have a cord between my tablet and the microphone so I don't get rustling noises as I control the tablet.
Another option is to get one that can plug into the USB port.  I have good success with one of those.  I am using a tablet with a regular USB port so can't really address how well it would work on a phone.  At the very least you would need an adapter that allows a full-size USB plug to go into your phone.  Not all USB mics can work with phones, but here is a decent one that might work:  USB Microphone with stand
In your question you mention that you are looking for the best quality.  Obviously, the best answer would be to set up an actual mixer so you can have separate microphones for the piano and the singer.  They are not all that expensive, and if you are serious about recording you may well decide it is worth it.  To use a mixer you would want an adapter cable for the output that can plug in to the 1/8" microphone port on the phone, plus microphones and their associated cables.  (To professionally mic a piano, you want the microphone to be inside.  For a grand piano, I open the top then use a microphone stand to position the microphone about a foot above the sounding board near the center)
Here is one inexpensive example:  Audio Mixer  - It takes some setting up but this is how the studios do it.
You asked about apps.  Instead of suggesting one in particular, I'd suggest you go to the play store and look around. There are a lot available, and they can help with effects and other parameters.  Try some and see if you think they help.  You might find one that you like.
I have tried some but still use the stock recording app that came with my tablet.
There are also some good playback apps that can do things like boost your bass or give full equalization control.  Those are good for listening to any kind of music.  But in the end, I don't use them.
Instead, I picked up an excellent Bluetooth speaker: OontZ Angle 3 that provides very good sound all by itself.  It is well worth the price.  I use it all the time.
In summary, I would suggest you start with the lapel microphone first, then go from there.  1) you know for sure it will work with your phone.  2) they are easy to use, 3) the price is right, 4) they produce surprisingly good recordings.
In the final analysis, nothing beats getting a good recording in the first place, so step one is to find a decent microphone that suits you.  But playback is also important, so step two is to get a good speaker system that enhances the sound.


Effective ways to dry out books?

Question: I just spilled water on my history textbook. It only hit the first 30 pages and the cover, but I still feel bad that it got wet. Any way to restore it completely so that the wet pages aren't noticeably different from the dry ones?
Comment: This question is also on-topic on [literature.se].
Answer: The passive solution

Put paper towels, tissue paper, toilet paper or anything that absorbs water well between every single page that got damp. Make sure that part of the paper towels stick out of the book to transport moisture from inside the book to outside.
Close the book and turn it so the damp side faces down.
If possible, but a sturdy board that is bigger than the book on top. This could be a (clean) kitchen cutting board, a shelf board or some crafting wood you have laying around.
Put something very heavy on top. A stack of 3 books is not enough, it should be 10 thick books, or a big, heavy flower pot (take care it doesn't tip over), or a stack of packs of sugar, flour or whatever is heavy and not perishable. The weight must be enough to force all pages of the book to lay completely flat. Make sure the weight is evenly distributed over the whole area of the book.
Wait 3 - 5 days (depending on the relative humidity where you live)

The active solution
Warning! This could damage the printing of the book. Please test this solution on an area of unimportant text first (like a page count or some note on the inside cover).

Heat an iron to the lowest setting and make sure the steam function is deactivated.
Open the book to a dampened page and lay it on a flat and sturdy surface
Lay a paper towel on top of the page
Press the iron down onto the paper towel without moving it around. Apply pressure to the iron to straighten out the paper. The heat will dry the page at the same time.

If the book was printed with toner (laser printer technology) then heat might melt the toner and make it stick to the paper towel. Toner melts at approximately 110°C / 230°F. If you notice small black spots on the paper towel, then the heat of your iron is too much.
Comment: Getting the pages to lay flat is key! If they're wrinkled or wavy at all, they'll end up that way permanently.
Comment: @bta unless you fancy wetting it again for a do-over of course :)
Answer: Freeze your book.
I did kind of remembered it, a google search did confirm it, this is the first site I found. This PDF file from a university an other.
If the getting wet was from clean water you should put your book in the freezer as soon as you can, and leave it there for a while. No need to package it, if you leave it in the freezer till it is dry you do not need to do anything about it.
Most people will take it out while frozen and dry it in an other way.
If the water was not clean, like coffee, the book will not become clean from freezing. Taking off as much colored 'water' before you freeze will be better.
Drying a book in the freezer can take long. Think in months for a book gone really wet. Both sites I link to have alternative methods as well, which are basically also what is covered in the other answer.
Comment: Indeed, we have the term "freeze dried" and if you ever noticed food left in the freezer for months too long is all dry and horrible when it's thawed..
Comment: Well I have put 3 perfectly dry books originating from the library into the freezer for 3 days, out of hygienic considerations (whatever). After removing them from the freezer, as they came back to room temperature — to my shock — they got incredibly damp and the pages had thrown some serious waves. Needless to say, for most of the lending period they were sitting under heavy weights as per [Elmy's passive method](https://lifehacks.stackexchange.com/a/24630/34631). So there is something additional that needs to be considered with the freezer.
Answer: While I support the notions in Elmy's answer I wanted to point out that there's a risk with the passive solution that the pages will end up bound together depending on contaminants in the liquid/whether the pages are coated or impregnated with anything to make them glossy (etc), that is water soluble. If you think about papier-mâché modelling; wet paper with a gluey water soluble substance as a binder might well be what you've effectively ended up with
As such, if you believe that your pages will stick to whatever you interleave them with I would suggest perhaps aluminum foil or sarin (cling) wrap as an interleaving material and relying on some forced airflow on the book edges after it had been considerably weighted down - moisture will gradually diffuse to the dry page edge but it will take a looong time
Also prepare your expectations; your desire of "so I can't tell it got wet" may never be achievable in the same way the MDF wood expands when it gets wet and your Ikea cabinet door ends up ruined, so do paper fibers and you may forever be able to tell where the wet spot was from appearance or feel. You should be able to control some of the waviness that will otherwise occur but an exact reversion will likely be unattainable

Effective ways to dry out books?


Quasi mean value theorem

Question:
Let $f$ be differentiable on $(a,b)$ and $\lim\limits_{x\to{a^+}}f(x)=\lim\limits_{x\to{b^-}}f(x)$. Prove $f\,'$ has at least one zero on $(a, b)$.
Comment: See [Rolle's theorem](http://en.wikipedia.org/wiki/Rolle%27s_theorem)
Answer: If the one-sided limits are finite, then define $f$ at $a$ and $b$ to be the common  value of the limit (this extended function will be continuous on $[a,b]$). Then use Rolle's Theorem.
If the one sided limits are infinite, there's a bit more work to do. Here, you could mimic the proof of Rolle's Theorem.
For example, in the case where
$\lim\limits_{x\rightarrow a^+}f(x)=\lim\limits_{x\rightarrow b^-}f(x)=\infty$:
Let $d\in(a,b)$. Choose $a^+>a$ close to $a$ and $b^-<b$ close to $b$ such that
$d\in(a^+,b^-)$ and both $f(a^+)$ and $f(b^-)$ exceed $f(d)$. $f$ is continuous on $[a^+,b^-]$, and so has a minimum value over $[a^+,b^-]$ at some point $c\in(a^+,b^-)$ (the minimum cannot occur at the endpoints). Since $f$ is differentiable over $(a^+,b^-)$, we have $f'(c)=0$.
A similar argument can be made when $\lim\limits_{x\rightarrow a^+}f(x)=\lim\limits_{x\rightarrow b^-}f(x)=-\infty$.
Answer: Set $f(a)=f(b)=m$, where m is the limit. Then use mean value theorem on the close interval $[a,b]$. I think it might be some homework.

Quasi mean value theorem


Will the block reward continue halving beyond 1 satoshi if bitcoin precision goes beyond 8 decimal places?

Question: If bitcoin precision is increased beyond 8 decimals, will the block reward continue halving forever instead of eventually going from 1 satoshi to 0 satoshi?
Comment: Where'd you get the idea that having 8 decimals was temporary?
Answer: The "8 decimals limit" is not that, but rather bitcoins are a human readable representation for 100,000,000 satoshi: The software natively computes in satoshi.
What happens under the hood is that the limit of satoshi that a miner may claim by creating a block halves. Obviously, only whole satoshi can be claimed, (as currently they are indivisible), so after block 6,930,000 when the reward limit drops from 1.164… satoshi to 0.582… satoshi per block, one cannot claim a whole satoshi anymore, as it would be greater than the limit.
If a new smallest unit of Bitcoin would have been introduced by then, we could continue to claim parts of a satoshi until the reward limit drops below that new unit.
In fact, we will be losing out on part of a satoshi in every block starting with block 2,310,001 when the reward drops from 9,765,625 satoshi to 4,882,812.5 satoshi in approximately 2049.
Answer: There are several places where the 8 decimals property is listed as something that can change over time. This is true, but not in any meaningful way.
Of course Bitcoin can change. It's a piece of software that people run. If all those people agree to suddenly start using changed software, anything is possible.
Changing the number of decimals is not technically difficult. It would just require an extreme degree of consensus and coordination to make it happen. So unless not changing this is seen as a clear and inevitable danger to the Bitcoin ecosystem, it is very unlikely to ever happen (nor do I think it is necessary, but who knows what the future brings).
Once we decide to change the number of decimals, anything can be changed. Including, how the mining subsidy changes in the long term.
So, the answer to your question is: whatever the future Bitcoin ecosystem agrees upon.
Comment: That's all true, but it doesn't change the fact that whatever "everyone" agrees upon, can change. Obviously not everything is equally likely to be agreed upon :)
Comment: I'm not sure that changing the number of decimals is on par with "anything could be changed": Increasing divisibility would of course require a lot of changes to the protocol and programming, but conceptually is it a very small change, and not one that significantly changes how Bitcoin works: The total amount of money in the system wouldn't change (well, by a few satoshi that could additionally be mined), and most importantly, it could be done without redistributing any holdings. I get why it would still be hard, but isn't it magnitudes easier than e.g. changing the reward schedule?


Is it possible to run an application as superuser within boot without input?

Question: I have an Ubuntu 12.04 Desktop machine running as a server with no input devices accessible after initial setup. I need to place any application that I want to run within the boot process. I have one application that I want to run that requires superuser access to run. Is there a way that I can automate that boot so I don't have to enter my password?
Answer: Since Ubuntu uses Upstart, you need to write a custom job for Upstart: http://upstart.ubuntu.com/getting-started.html
Processes started during the boot sequence have superuser privileges, so you don't need to bother with sudo, passwords and stuff.


use my vmware ubuntu as a non-virtual OS

Question: I like ubuntu. For lots of my development work its far less of a hassle than compiling software on osx. Is there any shot of moving my existing ubuntu vm to my hard drive? That would shave quite a bit of time getting comfortable again... I've seen some docs describing the process, but its windows only...
Answer: You can save your whole home directory, make sure all hidden files/folders are transferred. But you'll have to install the programs and packages again as they may have been scattered around the whole os and would be extremely long to get it how you want.
Answer: It is really easy to just copy your home directory over via ssh file transfer, or anther file copy method.  In fact, there is a blog with 3 simple steps to move to another Ubuntu computer.  Great script there for getting a list of all your PPA's and other packages installed.
This is also one of the first ideas suggested on the Ubuntu Brainstorm site.  There are some good comments in there as well.
Comment: terrific, that's some precise and succinct advice, thanks!


security and hacking of mysql-servers

Question: i'm very very new in questions such hacking, security, servers.
Where i can read some documentation about security, and hacking of php(mysql) servers.
Maybe you will give some links, where  can learn about such things?
Thanks
Answer: I follow this  :
MySQL : SecureInstallation - Tips&Tricks
Answer: SQL databases are very often left vulnerable by the applications that use them. For example, your PHP app that's connecting to the database should be very careful with user input that reaches the database. Spend some time learning about "SQL injection" - it's one of the most common and dangerous attacks, and unless you are aware of it, you might end up building a web app that's vulnerable.
Connect to your database using least privilege. Avoid connecting as the database owner, for example. This is an example of "defense in depth". Say you have a SQL injection vulnerability, but your database only allows your public facing web app to run SELECT statements. An attacker who exploits your SQL injection vulnerability will be able to read data, but not update or delete data. Obviously it'd be best not to have the vulnerability in the first place, but in case one slips through, you have that second line of defense - you've connected to the database using a limited amount of privilege.
"Never connect to the database as a superuser or as the database owner. Use always customized users with very limited privileges"
This is an excerpt from an article I think you should read:
http://phpbuilder.com/manual/en/security.database.sql-injection.php
HTH

security and hacking of mysql-servers


Should I close my credit cards before leaving United States?

Question: I have been living in the US for ~5 years and have around 6 credit cards and 2 checking accounts. I was on a student visa (and then OPT) here which will also expire in June, 2020.  I will be moving to Australia permanently in a month and don't think I will move back to the US to live in the near future. Should I close my credit card accounts?
I wish to keep at least on checking account just to make sure I don't need to send or receive any payments (like taxes next year).
Comment: I don't live in the US but have a US bank account and debit card (and would have a credit card if they'd let me) - they come in handy when I travel to the US, or even just when I want to buy things online as many places require a US card for the purchase to proceed
Answer: Some credit cards have annual fees that needs to be paid and if you no longer have an income in the country it might be difficult to know when and how to settle those and in some cases they can then be forwarded to Debt collection agencies.
This will then cause you an immense amount of trouble.
The above is the only drawback I can think of, if you think you can stay on top of these things there is no reason to cancel them, in fact it can be quite useful to have credit cards from other countries sometimes.
For example I currently live in china and many US ECOM sites do not accept credit cards from main land China, in this case it is very useful to have a US credit card to use.
For me personally I have credit cards from 3 different countries because I travel a lot but I also have property in each of those countries so I can receive mail/bills/notifications and I have people looking at my mail weekly to ensure it does not simply pile up.
In general it is always best to leave a country with a clean sheet and leave nothing behind that can cause you problems later on, but please bare the above in mind based on my experience.
Answer: The following would be some good reasons to keep your credit cards:

Credit score/history - Even if you don't use the credit cards, you still build up your credit history, in case you want to take out a loan, buy a house/car in the future.
Rewards - Does your card offer specific rewards? If you have a store-specific card that you actually use and you get good deals, why not keep it? If you travel often and you have a travel credit card with miles, you can still use that and benefit from it.
Backup credit card - Sometimes a country may not accept certain cards from other countries. It's good to have a backup, and it might help if you need to pay for something in dollars or from the US (online order, streaming service, etc.)

If you have a card that you do not plan on using but that has an annual fee, you can consider closing that card, but if there is no annual fee, just keep it. The credit score is a good enough reason to just do nothing.


Huawei E682 modem not working in 12.04

Question: I'm a user of Ubuntu in Kenya, I've a Huawei E682 modem. When I use it in Ubuntu 11.04 it connects without any problems to the Internet. But in Ubuntu 12.04 I can hardly connect to the Internet using it. Does anyone have an idea what the problem is? Any help will be appreciated.
Answer: In Kiswahili language, for my Kenyan friend "Niaje, ata me niko Kenya, kumbe hii shida si yangu pekee."
Ok, about the querry,
I'm experiencing the same problem, I've been searching for the solution for a while now. I've gathered information from the net and I think I might have a potential solution (though I have not tried it yet)
First, download the Huawei Linux Driver Tool from here. The guide to installing the Driver tool can be downloaded from here (it is the last one on the list, in PDF form). When installation is successful, your Huawei 3G modem should work just fine.
If it still does not work, it is probably because the Huawei Driver Tool modified the USB-modeswitch Data Files. To re-install these data files, run the following commands in the terminal (Ctril+Alt+T):
<code>sudo apt-get remove usb-modeswitch usb-modeswitch-data
sudo apt-get install usb-modeswitch
</code>

Huawei E682 modem not working in 12.04