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As laid out in the foundational work of Kolmogorov, a classical probability space (or probability space for short) is a triplet , where is a set, is a -algebra of subsets of , and is a countably additive probability measure on . Given such a space, one can form a number of interesting function spaces, including
- the (real) Hilbert space of square-integrable functions , modulo -almost everywhere equivalence, and with the positive definite inner product ; and
- the unital commutative Banach algebra of essentially bounded functions , modulo -almost everywhere equivalence, with defined as the essential supremum of .
There is also a trace on defined by integration: .
One can form the category of classical probability spaces, by defining a morphism between probability spaces to be a function which is measurable (thus for all ) and measure-preserving (thus for all ).
Let us now abstract the algebraic features of these spaces as follows; for want of a better name, I will refer to this abstraction as an algebraic probability space, and is very similar to the non-commutative probability spaces studied in this previous post, except that these spaces are now commutative (and real).
Definition 1 An algebraic probability space is a pair where
- is a unital commutative real algebra;
- is a homomorphism such that and for all ;
- Every element of is bounded in the sense that . (Technically, this isn't an algebraic property, but I need it for technical reasons.)
A morphism is a homomorphism which is trace-preserving, in the sense that for all .
For want of a better name, I'll denote the category of algebraic probability spaces as . One can view this category as the opposite category to that of (a subcategory of) the category of tracial commutative real algebras. One could emphasise this opposite nature by denoting the algebraic probability space as rather than ; another suggestive (but slightly inaccurate) notation, inspired by the language of schemes, would be rather than . However, we will not adopt these conventions here, and refer to algebraic probability spaces just by the pair .
By the previous discussion, we have a covariant functor that takes a classical probability space to its algebraic counterpart , with a morphism of classical probability spaces mapping to a morphism of the corresponding algebraic probability spaces by the formula
for . One easily verifies that this is a functor.
In this post I would like to describe a functor which partially inverts (up to natural isomorphism), that is to say a recipe for starting with an algebraic probability space and producing a classical probability space . This recipe is not new – it is basically the (commutative) Gelfand-Naimark-Segal construction (discussed in this previous post) combined with the Loomis-Sikorski theorem (discussed in this previous post). However, I wanted to put the construction in a single location for sake of reference. I also wanted to make the point that and are not complete inverses; there is a bit of information in the algebraic probability space (e.g. topological information) which is lost when passing back to the classical probability space. In some future posts, I would like to develop some ergodic theory using the algebraic foundations of probability theory rather than the classical foundations; this turns out to be convenient in the ergodic theory arising from nonstandard analysis (such as that described in this previous post), in which the groups involved are uncountable and the underlying spaces are not standard Borel spaces.
Let us describe how to construct the functor , with details postponed to below the fold.
- Starting with an algebraic probability space , form an inner product on by the formula , and also form the spectral radius .
- The inner product is clearly positive semi-definite. Quotienting out the null vectors and taking completions, we arrive at a real Hilbert space , to which the trace may be extended.
- Somewhat less obviously, the spectral radius is well-defined and gives a norm on . Taking limits of sequences in of bounded spectral radius gives us a subspace of that has the structure of a real commutative Banach algebra.
- The idempotents of the Banach algebra may be indexed by elements of an abstract -algebra .
- The Boolean algebra homomorphisms (or equivalently, the real algebra homomorphisms ) may be indexed by elements of a space .
- Let denote the -algebra on generated by the basic sets for every .
- Let be the -ideal of generated by the sets , where is a sequence with .
- One verifies that is isomorphic to . Using this isomorphism, the trace on can be used to construct a countably additive measure on . The classical probability space is then , and the abstract spaces may now be identified with their concrete counterparts , .
- Every algebraic probability space morphism generates a classical probability morphism via the formula
using a pullback operation on the abstract -algebras that can be defined by density.
Remark 1 The classical probability space constructed by the functor has some additional structure; namely is a -Stone space (a Stone space with the property that the closure of any countable union of clopen sets is clopen), is the Baire -algebra (generated by the clopen sets), and the null sets are the meager sets. However, we will not use this additional structure here.
The partial inversion relationship between the functors and is given by the following assertion:
- There is a natural transformation from to the identity functor .
More informally: if one starts with an algebraic probability space and converts it back into a classical probability space , then there is a trace-preserving algebra homomorphism of to , which respects morphisms of the algebraic probability space. While this relationship is far weaker than an equivalence of categories (which would require that and are both natural isomorphisms), it is still good enough to allow many ergodic theory problems formulated using classical probability spaces to be reformulated instead as an equivalent problem in algebraic probability spaces.
Remark 2 The opposite composition is a little odd: it takes an arbitrary probability space and returns a more complicated probability space , with being the space of homomorphisms . while there is "morally" an embedding of into using the evaluation map, this map does not exist in general because points in may well have zero measure. However, if one takes a "pointless" approach and focuses just on the measure algebras , , then these algebras become naturally isomorphic after quotienting out by null sets.
Remark 3 An algebraic probability space captures a bit more structure than a classical probability space, because may be identified with a proper subset of that describes the "regular" functions (or random variables) of the space. For instance, starting with the unit circle (with the usual Haar measure and the usual trace ), any unital subalgebra of that is dense in will generate the same classical probability space on applying the functor , namely one will get the space of homomorphisms from to (with the measure induced from ). Thus for instance could be the continuous functions , the Wiener algebra or the full space , but the classical space will be unable to distinguish these spaces from each other. In particular, the functor loses information (roughly speaking, this functor takes an algebraic probability space and completes it to a von Neumann algebra, but then forgets exactly what algebra was initially used to create this completion). In ergodic theory, this sort of "extra structure" is traditionally encoded in topological terms, by assuming that the underlying probability space has a nice topological structure (e.g. a standard Borel space); however, with the algebraic perspective one has the freedom to have non-topological notions of extra structure, by choosing to be something other than an algebra of continuous functions on a topological space. I hope to discuss one such example of extra structure (coming from the Gowers-Host-Kra theory of uniformity seminorms) in a later blog post (this generalises the example of the Wiener algebra given previously, which is encoding "Fourier structure").
A small example of how one could use the functors is as follows. Suppose one has a classical probability space with a measure-preserving action of an uncountable group , which is only defined (and an action) up to almost everywhere equivalence; thus for instance for any set and any , and might not be exactly equal, but only equal up to a null set. For similar reasons, an element of the invariant factor might not be exactly invariant with respect to , but instead one only has and equal up to null sets for each . One might like to "clean up" the action of to make it defined everywhere, and a genuine action everywhere, but this is not immediately achievable if is uncountable, since the union of all the null sets where something bad occurs may cease to be a null set. However, by applying the functor , each shift defines a morphism on the associated algebraic probability space (i.e. the Koopman operator), and then applying , we obtain a shift on a new classical probability space which now gives a genuine measure-preserving action of , and which is equivalent to the original action from a measure algebra standpoint. The invariant factor now consists of those sets in which are genuinely -invariant, not just up to null sets. (Basically, the classical probability space contains a Boolean algebra with the property that every measurable set is equivalent up to null sets to precisely one set in , allowing for a canonical "retraction" onto that eliminates all null set issues.)
More indirectly, the functors suggest that one should be able to develop a "pointless" form of ergodic theory, in which the underlying probability spaces are given algebraically rather than classically. I hope to give some more specific examples of this in later posts.
There are a number of ways to construct the real numbers , for instance
- as the metric completion of (thus, is defined as the set of Cauchy sequences of rationals, modulo Cauchy equivalence);
- as the space of Dedekind cuts on the rationals ;
- as the space of quasimorphisms on the integers, quotiented by bounded functions. (I believe this construction first appears in this paper of Street, who credits the idea to Schanuel, though the germ of this construction arguably goes all the way back to Eudoxus.)
There is also a fourth family of constructions that proceeds via nonstandard analysis, as a special case of what is known as the nonstandard hull construction. (Here I will assume some basic familiarity with nonstandard analysis and ultraproducts, as covered for instance in this previous blog post.) Given an unbounded nonstandard natural number , one can define two external additive subgroups of the nonstandard integers :
- The group of all nonstandard integers of magnitude less than or comparable to ; and
- The group of nonstandard integers of magnitude infinitesimally smaller than .
The group is a subgroup of , so we may form the quotient group . This space is isomorphic to the reals , and can in fact be used to construct the reals:
Proposition 1 For any coset of , there is a unique real number with the property that . The map is then an isomorphism between the additive groups and .
Proof: Uniqueness is clear. For existence, observe that the set is a Dedekind cut, and its supremum can be verified to have the required properties for .
In a similar vein, we can view the unit interval in the reals as the quotient
where is the nonstandard (i.e. internal) set ; of course, is not a group, so one should interpret as the image of under the quotient map (or , if one prefers). Or to put it another way, (1) asserts that is the image of with respect to the map .
where is the nonstandard (i.e. internal) set ; of course, is not a group, so one should interpret as the image of under the quotient map (or , if one prefers). Or to put it another way, (1) asserts that is the image of with respect to the map .
In this post I would like to record a nice measure-theoretic version of the equivalence (1), which essentially appears already in standard texts on Loeb measure (see e.g. this text of Cutland). To describe the results, we must first quickly recall the construction of Loeb measure on . Given an internal subset of , we may define the elementary measure of by the formula
This is a finitely additive probability measure on the Boolean algebra of internal subsets of . We can then construct the Loeb outer measure of any subset in complete analogy with Lebesgue outer measure by the formula
where ranges over all sequences of internal subsets of that cover . We say that a subset of is Loeb measurable if, for any (standard) , one can find an internal subset of which differs from by a set of Loeb outer measure at most , and in that case we define the Loeb measure of to be . It is a routine matter to show (e.g. using the Carathéodory extension theorem) that the space of Loeb measurable sets is a -algebra, and that is a countably additive probability measure on this space that extends the elementary measure . Thus now has the structure of a probability space .
Now, the group acts (Loeb-almost everywhere) on the probability space by the addition map, thus for and (excluding a set of Loeb measure zero where exits ). This action is clearly seen to be measure-preserving. As such, we can form the invariant factor , defined by restricting attention to those Loeb measurable sets with the property that is equal -almost everywhere to for each .
The claim is then that this invariant factor is equivalent (up to almost everywhere equivalence) to the unit interval with Lebesgue measure (and the trivial action of ), by the same factor map used in (1). More precisely:
Theorem 2 Given a set , there exists a Lebesgue measurable set , unique up to -a.e. equivalence, such that is -a.e. equivalent to the set . Conversely, if is Lebesgue measurable, then is in , and .
More informally, we have the measure-theoretic version
of (1).
Proof: We first prove the converse. It is clear that is -invariant, so it suffices to show that is Loeb measurable with Loeb measure . This is easily verified when is an elementary set (a finite union of intervals). By countable subadditivity of outer measure, this implies that Loeb outer measure of is bounded by the Lebesgue outer measure of for any set ; since every Lebesgue measurable set differs from an elementary set by a set of arbitrarily small Lebesgue outer measure, the claim follows.
Now we establish the forward claim. Uniqueness is clear from the converse claim, so it suffices to show existence. Let . Let be an arbitrary standard real number, then we can find an internal set which differs from by a set of Loeb measure at most . As is -invariant, we conclude that for every , and differ by a set of Loeb measure (and hence elementary measure) at most . By the (contrapositive of the) underspill principle, there must exist a standard such that and differ by a set of elementary measure at most for all . If we then define the nonstandard function by the formula
then from the (nonstandard) triangle inequality we have
(say). On the other hand, has the Lipschitz continuity property
and so in particular we see that
for some Lipschitz continuous function . If we then let be the set where , one can check that differs from by a set of Loeb outer measure , and hence does so also. Sending to zero, we see (from the converse claim) that is a Cauchy sequence in and thus converges in for some Lebesgue measurable . The sets then converge in Loeb outer measure to , giving the claim.
Thanks to the Lebesgue differentiation theorem, the conditional expectation of a bounded Loeb-measurable function can be expressed (as a function on , defined -a.e.) as
By the abstract ergodic theorem from the previous post, one can also view this conditional expectation as the element in the closed convex hull of the shifts , of minimal norm. In particular, we obtain a form of the von Neumann ergodic theorem in this context: the averages for converge (as a net, rather than a sequence) in to .
If is (the standard part of) an internal function, that is to say the ultralimit of a sequence of finitary bounded functions, one can view the measurable function as a limit of the that is analogous to the "graphons" that emerge as limits of graphs (see e.g. the recent text of Lovasz on graph limits). Indeed, the measurable function is related to the discrete functions by the formula
for all , where is the nonprincipal ultrafilter used to define the nonstandard universe. In particular, from the Arzela-Ascoli diagonalisation argument there is a subsequence such that
thus is the asymptotic density function of the . For instance, if is the indicator function of a randomly chosen subset of , then the asymptotic density function would equal (almost everywhere, at least).
I'm continuing to look into understanding the ergodic theory of actions, as I believe this may allow one to apply ergodic theory methods to the "single-scale" or "non-asymptotic" setting (in which one averages only over scales comparable to a large parameter , rather than the traditional asymptotic approach of letting the scale go to infinity). I'm planning some further posts in this direction, though this is still a work in progress.
The von Neumann ergodic theorem (the Hilbert space version of the mean ergodic theorem) asserts that if is a unitary operator on a Hilbert space , and is a vector in that Hilbert space, then one has
in the strong topology, where is the -invariant subspace of , and is the orthogonal projection to . (See e.g. these previous lecture notes for a proof.) The same proof extends to more general amenable groups: if is a countable amenable group acting on a Hilbert space by unitary transformations , and is a vector in that Hilbert space, then one has
for any Folner sequence of , where is the -invariant subspace. Thus one can interpret as a certain average of elements of the orbit of .
for any Folner sequence of , where is the -invariant subspace. Thus one can interpret as a certain average of elements of the orbit of .
I recently discovered that there is a simple variant of this ergodic theorem that holds even when the group is not amenable (or not discrete), using a more abstract notion of averaging:
Theorem 1 (Abstract ergodic theorem) Let be an arbitrary group acting unitarily on a Hilbert space , and let be a vector in . Then is the element in the closed convex hull of of minimal norm, and is also the unique element of in this closed convex hull.
Proof: As the closed convex hull of is closed, convex, and non-empty in a Hilbert space, it is a classical fact (see e.g. Proposition 1 of this previous post) that it has a unique element of minimal norm. If for some , then the midpoint of and would be in the closed convex hull and be of smaller norm, a contradiction; thus is -invariant. To finish the first claim, it suffices to show that is orthogonal to every element of . But if this were not the case for some such , we would have for all , and thus on taking convex hulls , a contradiction.
Finally, since is orthogonal to , the same is true for for any in the closed convex hull of , and this gives the second claim.
This result is due to Alaoglu and Birkhoff. It implies the amenable ergodic theorem (1); indeed, given any , Theorem 1 implies that there is a finite convex combination of shifts of which lies within (in the norm) to . By the triangle inequality, all the averages also lie within of , but by the Folner property this implies that the averages are eventually within (say) of , giving the claim.
It turns out to be possible to use Theorem 1 as a substitute for the mean ergodic theorem in a number of contexts, thus removing the need for an amenability hypothesis. Here is a basic application:
Corollary 2 (Relative orthogonality) Let be a group acting unitarily on a Hilbert space , and let be a -invariant subspace of . Then and are relatively orthogonal over their common subspace , that is to say the restrictions of and to the orthogonal complement of are orthogonal to each other.
Proof: By Theorem 1, we have for all , and the claim follows. (Thanks to Gergely Harcos for this short argument.)
Now we give a more advanced application of Theorem 1, to establish some "Mackey theory" over arbitrary groups . Define a -system to be a probability space together with a measure-preserving action of on ; this gives an action of on , which by abuse of notation we also call :
(In this post we follow the usual convention of defining the spaces by quotienting out by almost everywhere equivalence.) We say that a -system is ergodic if consists only of the constants.
(A technical point: the theory becomes slightly cleaner if we interpret our measure spaces abstractly (or "pointlessly"), removing the underlying space and quotienting by the -ideal of null sets, and considering maps such as only on this quotient -algebra (or on the associated von Neumann algebra or Hilbert space ). However, we will stick with the more traditional setting of classical probability spaces here to keep the notation familiar, but with the understanding that many of the statements below should be understood modulo null sets.)
A factor of a -system is another -system together with a factor map which commutes with the -action (thus for all ) and respects the measure in the sense that for all . For instance, the -invariant factor , formed by restricting to the invariant algebra , is a factor of . (This factor is the first factor in an important hierachy, the next element of which is the Kronecker factor , but we will not discuss higher elements of this hierarchy further here.) If is a factor of , we refer to as an extension of .
From Corollary 2 we have
Corollary 3 (Relative independence) Let be a -system for a group , and let be a factor of . Then and are relatively independent over their common factor , in the sense that the spaces and are relatively orthogonal over when all these spaces are embedded into .
This has a simple consequence regarding the product of two -systems and , in the case when the action is trivial:
Lemma 4 If are two -systems, with the action of on trivial, then is isomorphic to in the obvious fashion.
This lemma is immediate for countable , since for a -invariant function , one can ensure that holds simultaneously for all outside of a null set, but is a little trickier for uncountable .
Proof: It is clear that is a factor of . To obtain the reverse inclusion, suppose that it fails, thus there is a non-zero which is orthogonal to . In particular, we have orthogonal to for any . Since lies in , we conclude from Corollary 3 (viewing as a factor of ) that is also orthogonal to . Since is an arbitrary element of , we conclude that is orthogonal to and in particular is orthogonal to itself, a contradiction. (Thanks to Gergely Harcos for this argument.)
Now we discuss the notion of a group extension.
Definition 5 (Group extension) Let be an arbitrary group, let be a -system, and let be a compact metrisable group. A -extension of is an extension whose underlying space is (with the product of and the Borel -algebra on ), the factor map is , and the shift maps are given by
where for each , is a measurable map (known as the cocycle associated to the -extension ).
An important special case of a -extension arises when the measure is the product of with the Haar measure on . In this case, also has a -action that commutes with the -action, making a -system. More generally, could be the product of with the Haar measure of some closed subgroup of , with taking values in ; then is now a system. In this latter case we will call -uniform.
If is a -extension of and is a measurable map, we can define the gauge transform of to be the -extension of whose measure is the pushforward of under the map , and whose cocycles are given by the formula
It is easy to see that is a -extension that is isomorphic to as a -extension of ; we will refer to and as equivalent systems, and as cohomologous to . We then have the following fundamental result of Mackey and of Zimmer:
Theorem 6 (Mackey-Zimmer theorem) Let be an arbitrary group, let be an ergodic -system, and let be a compact metrisable group. Then every ergodic -extension of is equivalent to an -uniform extension of for some closed subgroup of .
This theorem is usually stated for amenable groups , but by using Theorem 1 (or more precisely, Corollary 3) the result is in fact also valid for arbitrary groups; we give the proof below the fold. (In the usual formulations of the theorem, and are also required to be Lebesgue spaces, or at least standard Borel, but again with our abstract approach here, such hypotheses will be unnecessary.) Among other things, this theorem plays an important role in the Furstenberg-Zimmer structural theory of measure-preserving systems (as well as subsequent refinements of this theory by Host and Kra); see this previous blog post for some relevant discussion. One can obtain similar descriptions of non-ergodic extensions via the ergodic decomposition, but the result becomes more complicated to state, and we will not do so here.
Given two unit vectors in a real inner product space, one can define the correlation between these vectors to be their inner product , or in more geometric terms, the cosine of the angle subtended by and . By the Cauchy-Schwarz inequality, this is a quantity between and , with the extreme positive correlation occurring when are identical, the extreme negative correlation occurring when are diametrically opposite, and the zero correlation occurring when are orthogonal. This notion is closely related to the notion of correlation between two non-constant square-integrable real-valued random variables , which is the same as the correlation between two unit vectors lying in the Hilbert space of square-integrable random variables, with being the normalisation of defined by subtracting off the mean and then dividing by the standard deviation of , and similarly for and .
One can also define correlation for complex (Hermitian) inner product spaces by taking the real part of the complex inner product to recover a real inner product.
While reading the (highly recommended) recent popular maths book "How not to be wrong", by my friend and co-author Jordan Ellenberg, I came across the (important) point that correlation is not necessarily transitive: if correlates with , and correlates with , then this does not imply that correlates with . A simple geometric example is provided by the three unit vectors
in the Euclidean plane : and have a positive correlation of , as does and , but and are not correlated with each other. Or: for a typical undergraduate course, it is generally true that good exam scores are correlated with a deep understanding of the course material, and memorising from flash cards are correlated with good exam scores, but this does not imply that memorising flash cards is correlated with deep understanding of the course material.
However, there are at least two situations in which some partial version of transitivity of correlation can be recovered. The first is in the "99%" regime in which the correlations are very close to : if are unit vectors such that is very highly correlated with , and is very highly correlated with , then this does imply that is very highly correlated with . Indeed, from the identity
(and similarly for and ) and the triangle inequality
we see that
Thus, for instance, if and , then . This is of course closely related to (though slightly weaker than) the triangle inequality for angles:
Thus, for instance, if and , then . This is of course closely related to (though slightly weaker than) the triangle inequality for angles:
Remark 1 (Thanks to Andrew Granville for conversations leading to this observation.) The inequality (1) also holds for sub-unit vectors, i.e. vectors with . This comes by extending in directions orthogonal to all three original vectors and to each other in order to make them unit vectors, enlarging the ambient Hilbert space if necessary. More concretely, one can apply (1) to the unit vectors
in .
But even in the "" regime in which correlations are very weak, there is still a version of transitivity of correlation, known as the van der Corput lemma, which basically asserts that if a unit vector is correlated with many unit vectors , then many of the pairs will then be correlated with each other. Indeed, from the Cauchy-Schwarz inequality
we see that
Thus, for instance, if for at least values of , then must be at least , which implies that for at least pairs . Or as another example: if a random variable exhibits at least positive correlation with other random variables , then if , at least two distinct must have positive correlation with each other (although this argument does not tell you which pair are so correlated). Thus one can view this inequality as a sort of `pigeonhole principle" for correlation.
Thus, for instance, if for at least values of , then must be at least , which implies that for at least pairs . Or as another example: if a random variable exhibits at least positive correlation with other random variables , then if , at least two distinct must have positive correlation with each other (although this argument does not tell you which pair are so correlated). Thus one can view this inequality as a sort of `pigeonhole principle" for correlation.
A similar argument (multiplying each by an appropriate sign ) shows the related van der Corput inequality
and this inequality is also true for complex inner product spaces. (Also, the do not need to be unit vectors for this inequality to hold.)
and this inequality is also true for complex inner product spaces. (Also, the do not need to be unit vectors for this inequality to hold.)
Geometrically, the picture is this: if positively correlates with all of the , then the are all squashed into a somewhat narrow cone centred at . The cone is still wide enough to allow a few pairs to be orthogonal (or even negatively correlated) with each other, but (when is large enough) it is not wide enough to allow all of the to be so widely separated. Remarkably, the bound here does not depend on the dimension of the ambient inner product space; while increasing the number of dimensions should in principle add more "room" to the cone, this effect is counteracted by the fact that in high dimensions, almost all pairs of vectors are close to orthogonal, and the exceptional pairs that are even weakly correlated to each other become exponentially rare. (See this previous blog post for some related discussion; in particular, Lemma 2 from that post is closely related to the van der Corput inequality presented here.)
A particularly common special case of the van der Corput inequality arises when is a unit vector fixed by some unitary operator , and the are shifts of a single unit vector . In this case, the inner products are all equal, and we arrive at the useful van der Corput inequality
(In fact, one can even remove the absolute values from the right-hand side, by using (2) instead of (4).) Thus, to show that has negligible correlation with , it suffices to show that the shifts of have negligible correlation with each other.
(In fact, one can even remove the absolute values from the right-hand side, by using (2) instead of (4).) Thus, to show that has negligible correlation with , it suffices to show that the shifts of have negligible correlation with each other.
Here is a basic application of the van der Corput inequality:
Proposition 1 (Weyl equidistribution estimate) Let be a polynomial with at least one non-constant coefficient irrational. Then one has
where .
Note that this assertion implies the more general assertion
for any non-zero integer (simply by replacing by ), which by the Weyl equidistribution criterion is equivalent to the sequence being asymptotically equidistributed in .
Proof: We induct on the degree of the polynomial , which must be at least one. If is equal to one, the claim is easily established from the geometric series formula, so suppose that and that the claim has already been proven for . If the top coefficient of is rational, say , then by partitioning the natural numbers into residue classes modulo , we see that the claim follows from the induction hypothesis; so we may assume that the top coefficient is irrational.
In order to use the van der Corput inequality as stated above (i.e. in the formalism of inner product spaces) we will need a non-principal ultrafilter (see e.g this previous blog post for basic theory of ultrafilters); we leave it as an exercise to the reader to figure out how to present the argument below without the use of ultrafilters (or similar devices, such as Banach limits). The ultrafilter defines an inner product on bounded complex sequences by setting
Strictly speaking, this inner product is only positive semi-definite rather than positive definite, but one can quotient out by the null vectors to obtain a positive-definite inner product. To establish the claim, it will suffice to show that
for every non-principal ultrafilter .
Note that the space of bounded sequences (modulo null vectors) admits a shift , defined by
This shift becomes unitary once we quotient out by null vectors, and the constant sequence is clearly a unit vector that is invariant with respect to the shift. So by the van der Corput inequality, we have
for any . But we may rewrite . Then observe that if , is a polynomial of degree whose coefficient is irrational, so by induction hypothesis we have for . For we of course have , and so
for any . Letting , we obtain the claim.
Many fluid equations are expected to exhibit turbulence in their solutions, in which a significant portion of their energy ends up in high frequency modes. A typical example arises from the three-dimensional periodic Navier-Stokes equations
where is the velocity field, is a forcing term, is a pressure field, and is the viscosity. To study the dynamics of energy for this system, we first pass to the Fourier transform
so that the system becomes
We may normalise (and ) to have mean zero, so that . Then we introduce the dyadic energies
where ranges over the powers of two, and is shorthand for . Taking the inner product of (1) with , we obtain the energy flow equation
where range over powers of two, is the energy flow rate
where range over powers of two, is the energy flow rate
is the energy dissipation rate
and is the energy injection rate
The Navier-Stokes equations are notoriously difficult to solve in general. Despite this, Kolmogorov in 1941 was able to give a convincing heuristic argument for what the distribution of the dyadic energies should become over long times, assuming that some sort of distributional steady state is reached. It is common to present this argument in the form of dimensional analysis, but one can also give a more "first principles" form Kolmogorov's argument, which I will do here. Heuristically, one can divide the frequency scales into three regimes:
- The injection regime in which the energy injection rate dominates the right-hand side of (2);
- The energy flow regime in which the flow rates dominate the right-hand side of (2); and
- The dissipation regime in which the dissipation dominates the right-hand side of (2).
If we assume a fairly steady and smooth forcing term , then will be supported on the low frequency modes , and so we heuristically expect the injection regime to consist of the low scales . Conversely, if we take the viscosity to be small, we expect the dissipation regime to only occur for very large frequencies , with the energy flow regime occupying the intermediate frequencies.
We can heuristically predict the dividing line between the energy flow regime. Of all the flow rates , it turns out in practice that the terms in which (i.e., interactions between comparable scales, rather than widely separated scales) will dominate the other flow rates, so we will focus just on these terms. It is convenient to return back to physical space, decomposing the velocity field into Littlewood-Paley components
of the velocity field at frequency . By Plancherel's theorem, this field will have an norm of , and as a naive model of turbulence we expect this field to be spread out more or less uniformly on the torus, so we have the heuristic
and a similar heuristic applied to gives
(One can consider modifications of the Kolmogorov model in which is concentrated on a lower-dimensional subset of the three-dimensional torus, leading to some changes in the numerology below, but we will not consider such variants here.) Since
we thus arrive at the heuristic
Of course, there is the possibility that due to significant cancellation, the energy flow is significantly less than , but we will assume that cancellation effects are not that significant, so that we typically have
or (assuming that does not oscillate too much in , and are close to )
or (assuming that does not oscillate too much in , and are close to )
On the other hand, we clearly have
We thus expect to be in the dissipation regime when
and in the energy flow regime when
and in the energy flow regime when
Now we study the energy flow regime further. We assume a "statistically scale-invariant" dynamics in this regime, in particular assuming a power law
for some . From (3), we then expect an average asymptotic of the form
for some structure constants that depend on the exact nature of the turbulence; here we have replaced the factor by the comparable term to make things more symmetric. In order to attain a steady state in the energy flow regime, we thus need a cancellation in the structure constants:
On the other hand, if one is assuming statistical scale invariance, we expect the structure constants to be scale-invariant (in the energy flow regime), in that
for dyadic . Also, since the Euler equations conserve energy, the energy flows symmetrise to zero,
for some . From (3), we then expect an average asymptotic of the form
for some structure constants that depend on the exact nature of the turbulence; here we have replaced the factor by the comparable term to make things more symmetric. In order to attain a steady state in the energy flow regime, we thus need a cancellation in the structure constants:
On the other hand, if one is assuming statistical scale invariance, we expect the structure constants to be scale-invariant (in the energy flow regime), in that
for dyadic . Also, since the Euler equations conserve energy, the energy flows symmetrise to zero,
which from (7) suggests a similar cancellation among the structure constants
Combining this with the scale-invariance (9), we see that for fixed , we may organise the structure constants for dyadic into sextuples which sum to zero (including some degenerate tuples of order less than six). This will automatically guarantee the cancellation (8) required for a steady state energy distribution, provided that
or in other words
for any other value of , there is no particular reason to expect this cancellation (8) to hold. Thus we are led to the heuristic conclusion that the most stable power law distribution for the energies is the law
or in terms of shell energies, we have the famous Kolmogorov 5/3 law
or in terms of shell energies, we have the famous Kolmogorov 5/3 law
Given that frequency interactions tend to cascade from low frequencies to high (if only because there are so many more high frequencies than low ones), the above analysis predicts a stablising effect around this power law: scales at which a law (6) holds for some are likely to lose energy in the near-term, while scales at which a law (6) hold for some are conversely expected to gain energy, thus nudging the exponent of power law towards .
We can solve for in terms of energy dissipation as follows. If we let be the frequency scale demarcating the transition from the energy flow regime (5) to the dissipation regime (4), we have
and hence by (10)
On the other hand, if we let be the energy dissipation at this scale (which we expect to be the dominant scale of energy dissipation), we have
Some simple algebra then lets us solve for and as
Thus, we have the Kolmogorov prediction
with energy dissipation occuring at the high end of this scale, which is counterbalanced by the energy injection at the low end of the scale.
Let be a quasiprojective variety defined over a finite field , thus for instance could be an affine variety
where is -dimensional affine space and are a finite collection of polynomials with coefficients in . Then one can define the set of -rational points, and more generally the set of -rational points for any , since can be viewed as a field extension of . Thus for instance in the affine case (1) we have
where is -dimensional affine space and are a finite collection of polynomials with coefficients in . Then one can define the set of -rational points, and more generally the set of -rational points for any , since can be viewed as a field extension of . Thus for instance in the affine case (1) we have
The Weil conjectures are concerned with understanding the number
of -rational points over a variety . The first of these conjectures was proven by Dwork, and can be phrased as follows.
of -rational points over a variety . The first of these conjectures was proven by Dwork, and can be phrased as follows.
Theorem 1 (Rationality of the zeta function) Let be a quasiprojective variety defined over a finite field , and let be given by (2). Then there exist a finite number of algebraic integers (known as characteristic values of ), such that
for all .
After cancelling, we may of course assume that for any and , and then it is easy to see (as we will see below) that the become uniquely determined up to permutations of the and . These values are known as the characteristic values of . Since is a rational integer (i.e. an element of ) rather than merely an algebraic integer (i.e. an element of the ring of integers of the algebraic closure of ), we conclude from the above-mentioned uniqueness that the set of characteristic values are invariant with respect to the Galois group . To emphasise this Galois invariance, we will not fix a specific embedding of the algebraic numbers into the complex field , but work with all such embeddings simultaneously. (Thus, for instance, contains three cube roots of , but which of these is assigned to the complex numbers , , will depend on the choice of embedding .)
An equivalent way of phrasing Dwork's theorem is that the (-form of the) zeta function
associated to (which is well defined as a formal power series in , at least) is equal to a rational function of (with the and being the poles and zeroes of respectively). Here, we use the formal exponential
Equivalently, the (-form of the) zeta-function is a meromorphic function on the complex numbers which is also periodic with period , and which has only finitely many poles and zeroes up to this periodicity.
Dwork's argument relies primarily on -adic analysis – an analogue of complex analysis, but over an algebraically complete (and metrically complete) extension of the -adic field , rather than over the Archimedean complex numbers . The argument is quite effective, and in particular gives explicit upper bounds for the number of characteristic values in terms of the complexity of the variety ; for instance, in the affine case (1) with of degree , Bombieri used Dwork's methods (in combination with Deligne's theorem below) to obtain the bound , and a subsequent paper of Hooley established the slightly weaker bound purely from Dwork's methods (a similar bound had also been pointed out in unpublished work of Dwork). In particular, one has bounds that are uniform in the field , which is an important fact for many analytic number theory applications.
These -adic arguments stand in contrast with Deligne's resolution of the last (and deepest) of the Weil conjectures:
Theorem 2 (Riemann hypothesis) Let be a quasiprojective variety defined over a finite field , and let be a characteristic value of . Then there exists a natural number such that for every embedding , where denotes the usual absolute value on the complex numbers . (Informally: and all of its Galois conjugates have complex magnitude .)
To put it another way that closely resembles the classical Riemann hypothesis, all the zeroes and poles of the -form lie on the critical lines for . (See this previous blog post for further comparison of various instantiations of the Riemann hypothesis.) Whereas Dwork uses -adic analysis, Deligne uses the essentially orthogonal technique of ell-adic cohomology to establish his theorem. However, ell-adic methods can be used (via the Grothendieck-Lefschetz trace formula) to establish rationality, and conversely, in this paper of Kedlaya p-adic methods are used to establish the Riemann hypothesis. As pointed out by Kedlaya, the ell-adic methods are tied to the intrinsic geometry of (such as the structure of sheaves and covers over ), while the -adic methods are more tied to the extrinsic geometry of (how sits inside its ambient affine or projective space).
In this post, I would like to record my notes on Dwork's proof of Theorem 1, drawing heavily on the expositions of Serre, Hooley, Koblitz, and others.
The basic strategy is to control the rational integers both in an "Archimedean" sense (embedding the rational integers inside the complex numbers with the usual norm ) as well as in the "-adic" sense, with the characteristic of (embedding the integers now in the "complexification" of the -adic numbers , which is equipped with a norm that we will recall later). (This is in contrast to the methods of ell-adic cohomology, in which one primarily works over an -adic field with .) The Archimedean control is trivial:
Proposition 3 (Archimedean control of ) With as above, and any embedding , we have
for all and some independent of .
Proof: Since is a rational integer, is just . By decomposing into affine pieces, we may assume that is of the affine form (1), then we trivially have , and the claim follows.
Another way of thinking about this Archimedean control is that it guarantees that the zeta function can be defined holomorphically on the open disk in of radius centred at the origin.
The -adic control is significantly more difficult, and is the main component of Dwork's argument:
Proposition 4 (-adic control of ) With as above, and using an embedding (defined later) with the characteristic of , we can find for any real a finite number of elements such that
for all .
Another way of thinking about this -adic control is that it guarantees that the zeta function can be defined meromorphically on the entire -adic complex field .
Proposition 4 is ostensibly much weaker than Theorem 1 because of (a) the error term of -adic magnitude at most ; (b) the fact that the number of potential characteristic values here may go to infinity as ; and (c) the potential characteristic values only exist inside the complexified -adics , rather than in the algebraic integers . However, it turns out that by combining -adic control on in Proposition 4 with the trivial control on in Proposition 3, one can obtain Theorem 1 by an elementary argument that does not use any further properties of (other than the obvious fact that the are rational integers), with the in Proposition 4 chosen to exceed the in Proposition 3. We give this argument (essentially due to Borel) below the fold.
The proof of Proposition 4 can be split into two pieces. The first piece, which can be viewed as the number-theoretic component of the proof, uses external descriptions of such as (1) to obtain the following decomposition of :
Proposition 5 (Decomposition of ) With and as above, we can decompose as a finite linear combination (over the integers) of sequences , such that for each such sequence , the zeta functions
are entire in , by which we mean that
as .
This proposition will ultimately be a consequence of the properties of the Teichmuller lifting .
The second piece, which can be viewed as the "-adic complex analytic" component of the proof, relates the -adic entire nature of a zeta function with control on the associated sequence , and can be interpreted (after some manipulation) as a -adic version of the Weierstrass preparation theorem:
Proposition 6 (-adic Weierstrass preparation theorem) Let be a sequence in , such that the zeta function
is entire in . Then for any real , there exist a finite number of elements such that
for all and some .
Clearly, the combination of Proposition 5 and Proposition 6 (and the non-Archimedean nature of the norm) imply Proposition 4.
This is a blog version of a talk I recently gave at the IPAM workshop on "The Kakeya Problem, Restriction Problem, and Sum-product Theory".
Note: the discussion here will be highly non-rigorous in nature, being extremely loose in particular with asymptotic notation and with the notion of dimension. Caveat emptor.
One of the most infamous unsolved problems at the intersection of geometric measure theory, incidence combinatorics, and real-variable harmonic analysis is the Kakeya set conjecture. We will focus on the following three-dimensional case of the conjecture, stated informally as follows:
Conjecture 1 (Kakeya conjecture) Let be a subset of that contains a unit line segment in every direction. Then .
This conjecture is not precisely formulated here, because we have not specified exactly what type of set is (e.g. measurable, Borel, compact, etc.) and what notion of dimension we are using. We will deliberately ignore these technical details in this post. It is slightly more convenient for us here to work with lines instead of unit line segments, so we work with the following slight variant of the conjecture (which is essentially equivalent):
Conjecture 2 (Kakeya conjecture, again) Let be a family of lines in that meet and contain a line in each direction. Let be the union of the restriction to of every line in . Then .
As the space of all directions in is two-dimensional, we thus see that is an (at least) two-dimensional subset of the four-dimensional space of lines in (actually, it lies in a compact subset of this space, since we have constrained the lines to meet ). One could then ask if this is the only property of that is needed to establish the Kakeya conjecture, that is to say if any subset of which contains a two-dimensional family of lines (restricted to , and meeting ) is necessarily three-dimensional. Here we have an easy counterexample, namely a plane in (passing through the origin), which contains a two-dimensional collection of lines. However, we can exclude this case by adding an additional axiom, leading to what one might call a "strong" Kakeya conjecture:
Conjecture 3 (Strong Kakeya conjecture) Let be a two-dimensional family of lines in that meet , and assume the Wolff axiom that no (affine) plane contains more than a one-dimensional family of lines in . Let be the union of the restriction of every line in . Then .
Actually, to make things work out we need a more quantitative version of the Wolff axiom in which we constrain the metric entropy (and not just dimension) of lines that lie close to a plane, rather than exactly on the plane. However, for the informal discussion here we will ignore these technical details. Families of lines that lie in different directions will obey the Wolff axiom, but the converse is not true in general.
In 1995, Wolff established the important lower bound (for various notions of dimension, e.g. Hausdorff dimension) for sets in Conjecture 3 (and hence also for the other forms of the Kakeya problem). However, there is a key obstruction to going beyond the barrier, coming from the possible existence of half-dimensional (approximate) subfields of the reals . To explain this problem, it easiest to first discuss the complex version of the strong Kakeya conjecture, in which all relevant (real) dimensions are doubled:
Conjecture 4 (Strong Kakeya conjecture over ) Let be a four (real) dimensional family of complex lines in that meet the unit ball in , and assume the Wolff axiom that no four (real) dimensional (affine) subspace contains more than a two (real) dimensional family of complex lines in . Let be the union of the restriction of every complex line in . Then has real dimension .
The argument of Wolff can be adapted to the complex case to show that all sets occuring in Conjecture 4 have real dimension at least . Unfortunately, this is sharp, due to the following fundamental counterexample:
Proposition 5 (Heisenberg group counterexample) Let be the Heisenberg group
and let be the family of complex lines
with and . Then is a five (real) dimensional subset of that contains every line in the four (real) dimensional set ; however each four real dimensional (affine) subspace contains at most a two (real) dimensional set of lines in . In particular, the strong Kakeya conjecture over the complex numbers is false.
This proposition is proven by a routine computation, which we omit here. The group structure on is given by the group law
giving the structure of a -step simply-connected nilpotent Lie group, isomorphic to the usual Heisenberg group over . Note that while the Heisenberg group is a counterexample to the complex strong Kakeya conjecture, it is not a counterexample to the complex form of the original Kakeya conjecture, because the complex lines in the Heisenberg counterexample do not point in distinct directions, but instead only point in a three (real) dimensional subset of the four (real) dimensional space of available directions for complex lines. For instance, one has the one real-dimensional family of parallel lines
with ; multiplying this family of lines on the right by a group element in gives other families of parallel lines, which in fact sweep out all of .
The Heisenberg counterexample ultimately arises from the "half-dimensional" (and hence degree two) subfield of , which induces an involution which can then be used to define the Heisenberg group through the formula
Analogous Heisenberg counterexamples can also be constructed if one works over finite fields that contain a "half-dimensional" subfield ; we leave the details to the interested reader. Morally speaking, if in turn contained a subfield of dimension (or even a subring or "approximate subring"), then one ought to be able to use this field to generate a counterexample to the strong Kakeya conjecture over the reals. Fortunately, such subfields do not exist; this was a conjecture of Erdos and Volkmann that was proven by Edgar and Miller, and more quantitatively by Bourgain (answering a question of Nets Katz and myself). However, this fact is not entirely trivial to prove, being a key example of the sum-product phenomenon.
We thus see that to go beyond the dimension bound of Wolff for the 3D Kakeya problem over the reals, one must do at least one of two things:
- (a) Exploit the distinct directions of the lines in in a way that goes beyond the Wolff axiom; or
- (b) Exploit the fact that does not contain half-dimensional subfields (or more generally, intermediate-dimensional approximate subrings).
(The situation is more complicated in higher dimensions, as there are more obstructions than the Heisenberg group; for instance, in four dimensions quadric surfaces are an important obstruction, as discussed in this paper of mine.)
Various partial or complete results on the Kakeya problem over various fields have been obtained through route (a) or route (b). For instance, in 2000, Nets Katz, Izabella Laba and myself used route (a) to improve Wolff's lower bound of for Kakeya sets very slightly to (for a weak notion of dimension, namely upper Minkowski dimension). In 2004, Bourgain, Katz, and myself established a sum-product estimate which (among other things) ruled out approximate intermediate-dimensional subrings of , and then pursued route (b) to obtain a corresponding improvement to the Kakeya conjecture over finite fields of prime order. The analogous (discretised) sum-product estimate over the reals was established by Bourgain in 2003, which in principle would allow one to extend the result of Katz, Laba and myself to the strong Kakeya setting, but this has not been carried out in the literature. Finally, in 2009, Dvir used route (a) and introduced the polynomial method (as discussed previously here) to completely settle the Kakeya conjecture in finite fields.
Below the fold, I present a heuristic argument of Nets Katz and myself, which in principle would use route (b) to establish the full (strong) Kakeya conjecture. In broad terms, the strategy is as follows:
- Assume that the (strong) Kakeya conjecture fails, so that there are sets of the form in Conjecture 3 of dimension for some . Assume that is "optimal", in the sense that is as large as possible.
- Use the optimality of (and suitable non-isotropic rescalings) to establish strong forms of standard structural properties expected of such sets , namely "stickiness", "planiness", "local graininess" and "global graininess" (we will roughly describe these properties below). Heuristically, these properties are constraining to "behave like" a putative Heisenberg group counterexample.
- By playing all these structural properties off of each other, show that can be parameterised locally by a one-dimensional set which generates a counterexample to Bourgain's sum-product theorem. This contradiction establishes the Kakeya conjecture.
Nets and I have had an informal version of argument for many years, but were never able to make a satisfactory theorem (or even a partial Kakeya result) out of it, because we could not rigorously establish anywhere near enough of the necessary structural properties (stickiness, planiness, etc.) on the optimal set for a large number of reasons (one of which being that we did not have a good notion of dimension that did everything that we wished to demand of it). However, there is beginning to be movement in these directions (e.g. in this recent result of Guth using the polynomial method obtaining a weak version of local graininess on certain Kakeya sets). In view of this (and given that neither Nets or I have been actively working in this direction for some time now, due to many other projects), we've decided to distribute these ideas more widely than before, and in particular on this blog.
Let be a finite field of order , and let be an absolutely irreducible smooth projective curve defined over (and hence over the algebraic closure of that field). For instance, could be the projective elliptic curve
in the projective plane , where are coefficients whose discriminant is non-vanishing, which is the projective version of the affine elliptic curve
To each such curve one can associate a genus , which we will define later; for instance, elliptic curves have genus . We can also count the cardinality of the set of -points of . The Hasse-Weil bound relates the two:
The usual proofs of this bound proceed by first establishing a trace formula of the form
for some complex numbers independent of ; this is in fact a special case of the Lefschetz-Grothendieck trace formula, and can be interpreted as an assertion that the zeta function associated to the curve is rational. The task is then to establish a bound for all ; this (or more precisely, the slightly stronger assertion ) is the Riemann hypothesis for such curves. This can be done either by passing to the Jacobian variety of and using a certain duality available on the cohomology of such varieties, known as Rosati involution; alternatively, one can pass to the product surface and apply the Riemann-Roch theorem for that surface.
In 1969, Stepanov introduced an elementary method (a version of what is now known as the polynomial method) to count (or at least to upper bound) the quantity . The method was initially restricted to hyperelliptic curves, but was soon extended to general curves. In particular, Bombieri used this method to give a short proof of the following weaker version of the Hasse-Weil bound:
Theorem 2 (Weak Hasse-Weil bound) If is a perfect square, and , then .
In fact, the bound on can be sharpened a little bit further, as we will soon see.
Theorem 2 is only an upper bound on , but there is a Galois-theoretic trick to convert (a slight generalisation of) this upper bound to a matching lower bound, and if one then uses the trace formula (1) (and the "tensor power trick" of sending to infinity to control the weights ) one can then recover the full Hasse-Weil bound. We discuss these steps below the fold.
I've discussed Bombieri's proof of Theorem 2 in this previous post (in the special case of hyperelliptic curves), but now wish to present the full proof, with some minor simplifications from Bombieri's original presentation; it is mostly elementary, with the deepest fact from algebraic geometry needed being Riemann's inequality (a weak form of the Riemann-Roch theorem).
The first step is to reinterpret as the number of points of intersection between two curves in the surface . Indeed, if we define the Frobenius endomorphism on any projective space by
then this map preserves the curve , and the fixed points of this map are precisely the points of :
Thus one can interpret as the number of points of intersection between the diagonal curve
and the Frobenius graph
which are copies of inside . But we can use the additional hypothesis that is a perfect square to write this more symmetrically, by taking advantage of the fact that the Frobenius map has a square root
with also preserving . One can then also interpret as the number of points of intersection between the curve
Let be the field of rational functions on (with coefficients in ), and define , , and analogously )(although is likely to be disconnected, so will just be a ring rather than a field. We then (morally) have the commuting square
if we ignore the issue that a rational function on, say, , might blow up on all of and thus not have a well-defined restriction to . We use and to denote the restriction maps. Furthermore, we have obvious isomorphisms , coming from composing with the graphing maps and .
The idea now is to find a rational function on the surface of controlled degree which vanishes when restricted to , but is non-vanishing (and not blowing up) when restricted to . On , we thus get a non-zero rational function of controlled degree which vanishes on – which then lets us bound the cardinality of in terms of the degree of . (In Bombieri's original argument, one required vanishing to high order on the side, but in our presentation, we have factored out a term which removes this high order vanishing condition.)
To find this , we will use linear algebra. Namely, we will locate a finite-dimensional subspace of (consisting of certain "controlled degree" rational functions) which projects injectively to , but whose projection to has strictly smaller dimension than itself. The rank-nullity theorem then forces the existence of a non-zero element of whose projection to vanishes, but whose projection to is non-zero.
Now we build . Pick a point of , which we will think of as being a point at infinity. (For the purposes of proving Theorem 2, we may clearly assume that is non-empty.) Thus is fixed by . To simplify the exposition, we will also assume that is fixed by the square root of ; in the opposite case when has order two when acting on , the argument is essentially the same, but all references to in the second factor of need to be replaced by (we leave the details to the interested reader).
For any natural number , define to be the set of rational functions which are allowed to have a pole of order up to at , but have no other poles on ; note that as we are assuming to be smooth, it is unambiguous what a pole is (and what order it will have). (In the fancier language of divisors and Cech cohomology, we have .) The space is clearly a vector space over ; one can view intuitively as the space of "polynomials" on of "degree" at most . When , consists just of the constant functions. Indeed, if , then the image of avoids and so lies in the affine line ; but as is projective, the image needs to be compact (hence closed) in , and must therefore be a point, giving the claim.
For higher , we have the easy relations
The former inequality just comes from the trivial inclusion . For the latter, observe that if two functions lie in , so that they each have a pole of order at most at , then some linear combination of these functions must have a pole of order at most at ; thus has codimension at most one in , giving the claim.
From (3) and induction we see that each of the are finite dimensional, with the trivial upper bound
Riemann's inequality complements this with the lower bound
thus one has for all but at most exceptions (in fact, exactly exceptions as it turns out). This is a consequence of the Riemann-Roch theorem; it can be proven from abstract nonsense (the snake lemma) if one defines the genus in a non-standard fashion (as the dimension of the first Cech cohomology of the structure sheaf of ), but to obtain this inequality with a standard definition of (e.g. as the dimension of the zeroth Cech cohomolgy of the line bundle of differentials) requires the more non-trivial tool of Serre duality.
At any rate, now that we have these vector spaces , we will define to be a tensor product space
for some natural numbers which we will optimise in later. That is to say, is spanned by functions of the form with and . This is clearly a linear subspace of of dimension , and hence by Rieman's inequality we have
Observe that maps a tensor product to a function . If and , then we see that the function has a pole of order at most at . We conclude that
and in particular by (4)
We will choose to be a bit bigger than , to make the image of smaller than that of . From (6), (10) we see that if we have the inequality
(together with (7)) then cannot be injective.
On the other hand, we have the following basic fact:
Proof: From (3), we can find a linear basis of such that each of the has a distinct order of pole at (somewhere between and inclusive). Similarly, we may find a linear basis of such that each of the has a distinct order of pole at (somewhere between and inclusive). The functions then span , and the order of pole at is . But since , these orders are all distinct, and so these functions must be linearly independent. The claim follows.
This gives us the following bound:
Proposition 4 Let be natural numbers such that (7), (11), (12) hold. Then .
Proof: As is not injective, we can find with vanishing. By the above lemma, the function is then non-zero, but it must also vanish on , which has cardinality . On the other hand, by (8), has a pole of order at most at and no other poles. Since the number of poles and zeroes of a rational function on a projective curve must add up to zero, the claim follows.
If , we may make the explicit choice
and a brief calculation then gives Theorem 2. In some cases one can optimise things a bit further. For instance, in the genus zero case (e.g. if is just the projective line ) one may take and conclude the absolutely sharp bound in this case; in the case of the projective line , the function is in fact the very concrete function .
Remark 1 When is not a perfect square, one can try to run the above argument using the factorisation instead of . This gives a weaker version of the above bound, of the shape . In the hyperelliptic case at least, one can erase this loss by working with a variant of the argument in which one requires to vanish to high order at , rather than just to first order; see this survey article of mine for details.
Roth's theorem on arithmetic progressions asserts that every subset of the integers of positive upper density contains infinitely many arithmetic progressions of length three. There are many versions and variants of this theorem. Here is one of them:
Theorem 1 (Roth's theorem) Let be a compact abelian group, with Haar probability measure , which is -divisible (i.e. the map is surjective) and let be a measurable subset of with for some . Then we have
where denotes the bound for some depending only on .
This theorem is usually formulated in the case that is a finite abelian group of odd order (in which case the result is essentially due to Meshulam) or more specifically a cyclic group of odd order (in which case it is essentially due to Varnavides), but is also valid for the more general setting of -divisible compact abelian groups, as we shall shortly see. One can be more precise about the dependence of the implied constant on , but to keep the exposition simple we will work at the qualitative level here, without trying at all to get good quantitative bounds. The theorem is also true without the -divisibility hypothesis, but the proof we will discuss runs into some technical issues due to the degeneracy of the shift in that case.
We can deduce Theorem 1 from the following more general Khintchine-type statement. Let denote the Pontryagin dual of a compact abelian group , that is to say the set of all continuous homomorphisms from to the (additive) unit circle . Thus is a discrete abelian group, and functions have a Fourier transform defined by
If is -divisible, then is -torsion-free in the sense that the map is injective. For any finite set and any radius , define the Bohr set
where denotes the distance of to the nearest integer. We refer to the cardinality of as the rank of the Bohr set. We record a simple volume bound on Bohr sets:
Lemma 2 (Volume packing bound) Let be a compact abelian group with Haar probability measure . For any Bohr set , we have
Proof: We can cover the torus by translates of the cube . Then the sets form an cover of . But all of these sets lie in a translate of , and the claim then follows from the translation invariance of .
Given any Bohr set , we define a normalised "Lipschitz" cutoff function by the formula
where is the constant such that
where is the constant such that
The function should be viewed as an -normalised "tent function" cutoff to . Note from Lemma 2 that
We then have the following sharper version of Theorem 1:
Theorem 3 (Roth-Khintchine theorem) Let be a -divisible compact abelian group, with Haar probability measure , and let . Then for any measurable function , there exists a Bohr set with and such that
where denotes the convolution operation
A variant of this result (expressed in the language of ergodic theory) appears in this paper of Bergelson, Host, and Kra; a combinatorial version of the Bergelson-Host-Kra result that is closer to Theorem 3 subsequently appeared in this paper of Ben Green and myself, but this theorem arguably appears implicitly in a much older paper of Bourgain. To see why Theorem 3 implies Theorem 1, we apply the theorem with and equal to a small multiple of to conclude that there is a Bohr set with and such that
But from (2) we have the pointwise bound , and Theorem 1 follows.
Below the fold, we give a short proof of Theorem 3, using an "energy pigeonholing" argument that essentially dates back to the 1986 paper of Bourgain mentioned previously (not to be confused with a later 1999 paper of Bourgain on Roth's theorem that was highly influential, for instance in emphasising the importance of Bohr sets). The idea is to use the pigeonhole principle to choose the Bohr set to capture all the "large Fourier coefficients" of , but such that a certain "dilate" of does not capture much more Fourier energy of than itself. The bound (3) may then be obtained through elementary Fourier analysis, without much need to explicitly compute things like the Fourier transform of an indicator function of a Bohr set. (However, the bound obtained by this argument is going to be quite poor – of tower-exponential type.) To do this we perform a structural decomposition of into "structured", "small", and "highly pseudorandom" components, as is common in the subject (e.g. in this previous blog post), but even though we crucially need to retain non-negativity of one of the components in this decomposition, we can avoid recourse to conditional expectation with respect to a partition (or "factor") of the space, using instead convolution with one of the considered above to achieve a similar effect.
Let be an irreducible polynomial in three variables. As is not algebraically closed, the zero set can split into various components of dimension between and . For instance, if , the zero set is a line; more interestingly, if , then is the union of a line and a surface (or the product of an acnodal cubic curve with a line). We will assume that the -dimensional component is non-empty, thus defining a real surface in . In particular, this hypothesis implies that is not just irreducible over , but is in fact absolutely irreducible (i.e. irreducible over ), since otherwise one could use the complex factorisation of to contain inside the intersection of the complex zero locus of complex polynomial and its complex conjugate, with having no common factor, forcing to be at most one-dimensional. (For instance, in the case , one can take .) Among other things, this makes a Zariski-dense subset of , thus any polynomial identity which holds true at every point of , also holds true on all of . This allows us to easily use tools from algebraic geometry in this real setting, even though the reals are not quite algebraically closed.
The surface is said to be ruled if, for a Zariski open dense set of points , there exists a line through for some non-zero which is completely contained in , thus
for all . Also, a point is said to be a flecnode if there exists a line through for some non-zero which is tangent to to third order, in the sense that
as , or equivalently that
for . Clearly, if is a ruled surface, then a Zariski open dense set of points on are a flecnode. We then have the remarkable theorem of Cayley and Salmon asserting the converse:
for . Clearly, if is a ruled surface, then a Zariski open dense set of points on are a flecnode. We then have the remarkable theorem of Cayley and Salmon asserting the converse:
Theorem 1 (Cayley-Salmon theorem) Let be an irreducible polynomial with non-empty. Suppose that a Zariski dense set of points in are flecnodes. Then is a ruled surface.
Among other things, this theorem was used in the celebrated result of Guth and Katz that almost solved the Erdos distance problem in two dimensions, as discussed in this previous blog post. Vanishing to third order is necessary: observe that in a surface of negative curvature, such as the saddle , every point on the surface is tangent to second order to a line (the line in the direction for which the second fundamental form vanishes).
The original proof of the Cayley-Salmon theorem, dating back to at least 1915, is not easily accessible and not written in modern language. A modern proof of this theorem (together with substantial generalisations, for instance to higher dimensions) is given by Landsberg; the proof uses the machinery of modern algebraic geometry. The purpose of this post is to record an alternate proof of the Cayley-Salmon theorem based on classical differential geometry (in particular, the notion of torsion of a curve) and basic ODE methods (in particular, Gronwall's inequality and the Picard existence theorem). The idea is to "integrate" the lines indicated by the flecnode to produce smooth curves on the surface ; one then uses the vanishing (1) and some basic calculus to conclude that these curves have zero torsion and are thus planar curves. Some further manipulation using (1) (now just to second order instead of third) then shows that these curves are in fact straight lines, giving the ruling on the surface.
Update: Janos Kollar has informed me that the above theorem was essentially known to Monge in 1809; see his recent arXiv note for more details.
I thank Larry Guth and Micha Sharir for conversations leading to this post.


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Batman's Shameful Secret is our forum all about comic books and "graphic novels" (which is just a fancy way of saying "comic books for people who think they're smart"). What up with it, muscles like this?
Welcome to BSS of Future Pasts, Hermanos. ITT Everybody's BANNED! - There's a new sherrif in BSS and his name is hermanos! BSS gets a new mod. Note: hermanos means brother in espanol.
51...50...49... The Countdown (and therefore entire current DCU) Thread - The final countdown! dun dun dun dun dun dun! DC has a new weekly series starting next week and all signs are pointing to mediocre. Check out the first post in the thread for the first 10 pages of issue 51. See, because the series is called "Countdown" the first issue is 51 and the numbering goes backwards from it.
Tell me what I need to know to read Infinite Crisis - Answer? Don't.
I don't get Superman - Eh, whats to get? Super strong, flies around, is jerk to poor Jimmy Olsen. Oh wait, that was Silver Age Superman! faker has a problem understanding the moral motivations for Earth's mightiest hero, BSS comes to the rescue!
Science, Philosophy & Education is where the smarty-farty-bo-barty cool kids like to post and think that they're so smart. Well, I've got news for them: They're not! Booya! QuantumCat's been kickin' it in there and holding it up for us.
When do you become a 'mathematician'? - SPE's math goons debate on what it takes to earn the title of 'Mathematician'.
Best math textbooks? - Find out which math textbook is the best for you!
Let's talk Wittgenstein - It's time for some philosophy, with Euphoria 5L's interesting introduction to the works and ideas of Austrian philosopher Ludwig Wittgenstein.
General Bullshit (as presented by my main man SpecialOlympian) is our delightfully general forum with as many interesting facets as wadded-up chunks of Chinese newspapers.
Oh, Jesus Christ!!! - SadWhaleFamily received an enchanted prayer rug imbued with the power of Jesus and the Gospel of Wealth. Yes, simply fill out form on the back and God will take your order.
The Ghanian government will not stop calling me. - Ferretball is having regrets about passing up on his opportunity to be the Last King of Scotland and contracting HIV. But before he passes away, he wants to share the wisdom he acquired after sleeping with 38 Muslim girls. 1 in the U.S., 2 in Africa, and the rest are hookups. This is the scientific criteria we will be using. Now let us discuss vaginal funk, gentlemen.
It's a cat with really big ears, we swear. Disney who? - On behalf of the People's Republic of China, Three Olives would like to introduce you to the legally distinct and completely original cast of Dosneyland. Page 3 Bonus: Coal Black and de Sebben Dwarves
MSPaint awkward bathroom situations - speiseh's thread title is not titular but a direct command. MSPaint your awkward bathroom situations now. If you have none, go outside and LIVE.
Rock legend David Bowie has changed his identity with almost every album. Can you remember all these classic Bowie characters?
Tucker Carlson's idiot brother just called New York mayor Bill de Blasio's spokeswoman a "LabiaFace."
Video games are serious business, but Goons save room for fun stuff, like making Mega Man fan art!
A psychic terrorist travels to the 1990s to win his true love by murdering her entire wedding party!
The SA Goons deliver another knockout round of soccer GIFs!
The Something Awful Forums: the last bastion of sanity on the Internet. "Forum Fridays" glances at some of the most interesting and popular threads from nearly each forum, highlighting a handful of threads each week.
Copyright ©2015 Rich "Lowtax" Kyanka & Something Awful LLC.

subset: commoncrawl


text:
Town Square
Magician's tale creates magic onstage
Original post made on Dec 15, 2012
Read the full story here Web Link posted Friday, December 14, 2012, 12:00 AM
Comments (1)
a resident of another community
on Dec 15, 2012 at 1:17 pm
Thank you for the fascinating REPORT that makes me want to know more. To give a critical review of the production would be meaningful. Thank you.
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text:
Adding sand to paver patio
I recently put in a paver patio following the most common instructions. Dug the hole, added 6 inches of jagged gravel, tamped the gravel level, added 1 inch of sand, and laid the pavers. I then poured sand on top of the patio, and used a coarse broom to push the sand into the cracks while spraying it with water. I also hammered in a plastic border.
Since I have been done, it has rained almost every day. Whenever it rains, the sand in the cracks is washed out. Each time I have added and swept new sand into the cracks. The pavers have shifted slightly, but nothing too much(I used 16x16 inch pavers). Is this common with a new patio and will the sand/ground eventually settle so I can stop adding new sand? If so, how long should I expect to keep adding sand? Or is this a sign of an eventual problem that I need to fix?
You might want to try polymeric sand by Techniseal. It hardens up, but looks just like sand. It lasts about two years before a re-apply. My DH and I did a re-apply to our flagtone walkway in about an hour yesterday. It looks like new except where a few birds walked through it! We have wider joints since we have flagstone.
Last summer, I put the polyermic sand on an 18x20 flagstone patio in just a few hours by myself.
You need 2 days of dry weather on either side of the application. Don't walk on it for 24 hours. You lightly mist the sand between layers. It comes in grey or taupe. A bag costs around $19.00 here at the local John Deere Landscaping store.


text:
- Age / Gender:
- 25, Male
- Location:
- Hungary
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- All Stats >
User Statshot
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Level 22 Animator
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Ranked as Staff Sergeant
Contact Info / Websites
Favorite Movies
Pandora's Book Rated 4.3029 Stars Pandora and Kaluh enjoy a lazy afternoon at Shepherd's Crook. . . Spam Bitey of Brackenwood Rated 4.3298 Stars Bitey is a nasty piece of work... but he'll get what's coming! Other The Reward Rated 4.5176 Stars The Reward is a film about bromance, fantasy, killing stuff... and HIGH FIVES!! Comedy - Original LUCKY DAY FOREVER Rated 4.4779 Stars Win your happiness! Drama
Favorite Games
GemCraft Labyrinth Rated 4.4733 Stars After decades of preparation, the Labyrinth has finally appeared before you... Strategy - Tower Defense Madness: Project Nexus Rated 4.4562 Stars Blaze through the Madness Universe in this sidescrolling Shoot 'Em Up! Action - Shooter - Run 'n Gun Mastermind: WC Rated 4.4378 Stars Manage your minions, defend your base, and conquer this puny planet ONCE AND FOR ALL! Strategy - Real-time (RTS) William and Sly 2 Rated 4.2574 Stars Gnomes have stolen William's journal and scattered pages all over the mountainside! Action - Platformer - Other
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Recent Game Medals
The Governor 10 Points You are even worse than the undead... Medal Stats.
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The Merciful 10 Points Make sure Bill passes away calmly. Medal Stats.
The Savior 25 Points Survive and save two of your friends. Medal Stats.
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Wile E. Ways 10 Points Knock an opponent out of the ring three times in one match. Medal Stats.
The Legend 25 Points Go through the whole intro without skipping. Medal Stats.
To infinity and beyond! 50 Points Travel a minimum of 15,000 meters in "Survival" mode! Medal Stats.
Bad boy, bad! 10 Points Beat the crap out of the second opponent! Medal Stats.
Total Medals Earned: 887 (From 118 different games.)


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Jesse Johnson will discuss some of the fundamental flaws that can occur in attempting to make a one-size-fits-all method of evangelism in a seminar this March at the 2012 Shepherds' Conference.
Visit shepherdsconference.org for more information
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text:
By Jonathan Weber
(Reuters) - Montana Senator Jon Tester appeared poised for victory over Republican Representative Denny Rehberg early Wednesday in what would be a major win for the national Democratic Party as it solidifies its majority in the U.S. Senate.
With more than three-quarters of the vote counted, Tester was leading by 49 percent to 44 percent.
A Tester win would end a ferocious battle in which an estimated $40 million was spent in a right-leaning state with fewer than 1 million people. The majority of those funds came from groups outside Montana.
Tester, a farmer and former state legislator, won the seat in a 3,000-vote upset in 2006. This year, he stressed his rural Montana roots and his independence from President Barack Obama, who lost the state by a wide margin. Tester touted his advocacy for veterans and women's health while attacking Rehberg as irresponsible and untrustworthy.
Rehberg, a veteran politician and real estate developer who has held the state's single House seat for a decade, aimed to paint Tester as a true-blue liberal supporter of the president who was out of touch with Montana issues. Rehberg stressed his budget-cutting acumen and aimed to ride Mitt Romney's coattails in a state with a strong fiscal conservative streak.
Dave Parker, a Montana State University political scientist, pointed to a big turnout in the college towns of Missoula and Bozeman and unexpectedly soft support for Rehberg among older, rural voters as key factors.
Dan Cox, a Libertarian candidate, may also have hurt the Republican, Parker said.
"The spoiler may have been Dan Cox," Parker said, noting that Cox was drawing about 6% of the vote and was especially strong in rural areas that are normally heavily Republican.
The contest between two well-known incumbents was a toss-up throughout, polls showed. Both parties, along with the anonymous independent groups that have flourished since the Supreme Court threw out limits on corporate campaign spending, had long anticipated a tight contest that could decide the balance of power in the Senate - and they spent accordingly.
(Reporting By Jonathan Weber; Editing by Ciro Scotti)
Due to the overwhelming enthusiasm of our readers it has become necessary to transfer our commenting system to a more scalable system in order handle the content.
Despite Amending The Bill, Montana Democrats Still Oppose Pro-Second Amendment Referendum | Matt Vespa
Ex-Clinton Aide Had Secret Intelligence Network, Raises Questions If Hillary Used This 'Undisclosed Back Channel' | Matt Vespa
Fox Poll: 83 percent call Iran untrustworthy, huge majority says Obama must get Congressional approval for deal | Guy Benson


text: No recent wiki edits to this page. As Nemesis prepares his plan to gain freedom from the Global Peace Agency, he's dealt one last fatal setback. He hasn't been able to trust anybody in Electric City, and now he's not even sure if he can trust himself. Is he doomed to a life of torture and paranoia?


text:
Thats true Dale. A couple of dumps just before bottling/kegging works well. Just not sure how clear each glass would be for the first third or so of the brewhemoth if you fermented and served out of it....by the glass.
Thats nothing negative about the brewhemoth specifically, i get the same thing from kegs when i dont use a clearing agent. First third of the keg is a bit cloudy, but clears up over time and pours.
Without co2 back pressure you'll end up with dribble pours and flat beer.
Download a psi vs temp for volumes of co2 you want in your beer and adjust your beer line length and size accordingly.http://www.homebrew.com/pdfs/CO2chart.pdf
So example, if you serve your beer at 36F and want your beer carbonated to 2.5 volumes of co2, you would want to maintain a keg psi of 10-11psi. This is static and shouldnt change for the whole keg dispensing.
You also want a good consistent pour from your tap, like filling a pint in 6-8 seconds. To do this you need to adjust the total length of your beer line...shorter the beer line, less restriction and a faster pour. Longer means more restriction and a slower pour.
Most beer line runs of under 8ish feet would typically use 3/16" beer line. Start with 8 feet and check your pour. If its too slow, cut a foot off and try again until you get the pour you want while maintaining the 10-11 psi in your keg.
Make sense?
If you make a wide range of beer and want every keg to pour nice and have the proper volumes of co2 in it, you'll need several beer lines. A shorter one for stout for instance, and a long one for wheat beers..
Double post
I ordered mine from morebeer.
They also have an inch and a half one as well.
Doc, isn't it easier to oxygenate when running through your chiller with an inline stone on your chiller output? Just thinking if you add something like this to your fermenter you have to have one on each fermenter.
One way or another, this week will end with a homebrew.......https://www.facebook.com/pourhousebrews


text:
Please call 01733 224 841 to speak to one of our advisors who will help you with your holiday needs.
Please call 01733 224 841 to speak to one of our advisors who will help you with your holiday needs.
Sorry, there are currently no holidays available that meet your search criteria. Please change your departure date and/or duration options, or change your star rating and/or board basis options.
Alternatively, for flexible departure airports and flight durations, please visit our flight + hotel page.
If you still experience problems, please call 01733 224 800 (Mon – Sun: 8am – 10pm) to speak to a travel advisor. [Error Code: 1009]
With a gorgeous cliff-top location, fantastic sea views and a lively variety of daytime fun and evening entertainment, the Calas de Mallorca Complex is a popular holiday destination made up of three hotels: Mastines, Chihuahuas and Balmoral.
Please note: where possible, guests will be allocated rooms in the Mastines or Chihuahuas buildings. The hotel does not accept bookings of minors aged 17yrs or younger, unless they are travelling with someone 18yrs of age or older.
300m to the beach; 600m to the restaurants, bars and shops in the resort centre
Rooms 3 adults or 2 adults and 2 children have ceiling fans, fridge*, safety deposit box*, bathroom with shower over bath and balcony or terrace. *Local charge payable. Sea view rooms, single rooms and twin rooms for sole occupancy are available at a supplement.


text:
How Important is it to You that a PB be a member of the BBB?
We just met with a PB who isn't a member of the BBB. He said the reason for this is that they don't want to pay the money for their reference. Would you run from this PB?
BBB can be a guide if the PB has belonged for a number of years. It's hard to get a bad rating from the BBB because they want to keep memberships resigning up every year.
I would ask for references and not use BBB as an up or down for using a builder.
The BBB is a pay for play organization. If you pay there dues you get a good rating even if your bad. Do a google search. There was a big scandal about it in L.A. With the BBB.
Get references and do a google search on the company.
I equate the BBB along the same lines as the mob.
I never had a problem with them but I don't like being extorted.
We turned down a pool builder because they weren't a member of the BBB. We ultimately went with one that received a stellar rating from the BBB, and we're so glad we did. The quality work from the BP justified their BBB rating. Now⦠on the other handâ¦...
We turned down a pool builder because they weren't a member of the BBB. We ultimately went with one that received a stellar rating from the BBB, and we're so glad we did. The quality work from the BP justified their BBB rating. Now⦠on the other handâ¦...
we initially went with a PB that was accredited and had an A+ rating. to make the story short, he has thousands of my money and did almost nothing. So, this is to say that most of the time, but not always, being accredited is a good thing for us consumers but not necessarily a deal breaker. I agree that fees add to overhead but if a PB's operating budget is that tight, how do I know he won't be cutting corners with my pool? BBB should only be one of your vetting tools. I urge you to check references, making sure they're not brothers or cousins, LOL. As you get down to 1-2 finalists, do a little deeper research to ensure there is no pending litigation or judgments against the PB. Ask about who does the work, his employees or subcontractors. Get the facts straight concerning warranties for work done by PB employees and/ or subcontractors. Finally, please go see at least one pool or more if you can. The pool might not be to your liking but it will speak volumes on workmanship and help you decide if this PB and you are a match.
I could write all day about my bad and good experiences with PBs, but attorneys are cautioning about how much I say for now.
Once you are down to 1-2 finalists, it wouldn't hurt you to visit or at least drive by PB place of biz. You could assess amount of activity and "life" of biz. You won't want to select a guy that talks a good talk and turns out is operating out of a closet, with no equipment, no employees. Somebody like that may rely entirely on subcontractors and unless you know he has a fantastic relationship with them, then his priority in terms of how quickly he can get things done is no better than if you thumbed thru the phone book looking for help. In short, try to evaluate that his talk about good working relations is more than lip service. Naturally, customer references will give you a lot of insight as to those aspects.
I don't mean to scare the pants off you or any other reader but you need to know that, at least in TX, the industry is not regulated. There are good builders out there, but with little regulation, the shysters abound. Not only that, but I feel comfortable saying that at least 75% of them get away with their shenanigans with zero consequences. If you are spending anywhere from $5-100K in remodeling/building the pool of your dreams, you don't want to be stung by a scoundrel, do you? DO YOUR HOMEWORK, ASK, ASK, ASK WITHOUT FEAR. Anybody that acts elusive or insulted is no one you want to deal with.


text:
JD Sports
|Public limited company
|Traded as
|Bury, Greater Manchester
Key people
|Peter Cowgill, (Chairman)
Barry Bown, (CEO)
Sportswear Accessories
|£1,258.9 million (2013)[1]
|£56.0 million (2013)[1]
|£41.2 million (2013)[1]
|Pentland Group (57%)
Sports Direct (13%)
Aberforth Partners (10%)
Fidelity Management (5%)
Other Minor Shareholders (15%)
JD Sports Fashion plc, more commonly known as just JD (stylized as jD[2]), is a sports-fashion retail company based in Bury, Greater Manchester, England with shops throughout the United Kingdom and now has 25 stores in Ireland, after taking over Irish sports retailer Champion Sports. It is listed on the London Stock Exchange and is a constituent of the FTSE 250 Index. JD Sports Fashion Plc is a subsidiary of the Pentland Group.
The letters JD in JD Sports stand for the initials of the founders of the company, John and David. The company was founded by John Wardle and David Makin in 1981 trading from a single shop in Bury.[3]
In 1983, the company opened a store in the Arndale Centre in Manchester.[4] It opened its first store in Oxford Street in London in 1989 and was first listed on the London Stock Exchange in 1996.[4]
In 2002, it acquired nearly 200 further stores with the acquisition of First Sport from Blacks Leisure Group and in 2005 it bought 70 stores from the administrators of Allsports.[4]
In May 2005, Pentland Group bought Wardle's and Makin's shares for £44.6m; the pair later resigned from the board.[5]
In December 2007, the company bought out Bank Stores, which sells fashion clothing such as Firetrap, Alu, Henleys and Adidas Originals and the exclusive TFB collection by Tulisa among others, for around £19 million.[6]
In June and July 2008, JD Sports announced sponsorship deals with Bournemouth, Charlton Athletic, Dundee United, Blackpool, Luton Town and Oldham Athletic; these sponsorship deals allow certain clubs to use the Carbrini brand.[7]
2009 saw further expansion of the JD Group through the acquisition of Chausport which operates from 75 small stores in France. In addition, JD acquired the world famous rugby heritage brands 'Canterbury' and 'Canterbury of New Zealand' as well as 'The Duffer of St. George' and 'Kooga Rugby' brands.[8]
In January 2011, it was announced that JD Sports Fashion would buy Irish sports goods chain Champion Sports for €19.6 million, pending approval by the Irish Competition Authority.[9]
In January 2012, JD Sports purchased the troubled Blacks Leisure Group from administration for a total of £20 million.[10]
In February 2012, JD Sports acquired streetwear clothing brand FLY53 for an undisclosed sum.[11]
In February 2013, it purchased Cloggs, a shoe retailer, out of administration.[12]
- ^ a b c Annual Report 2013
- ^ "New distribution warehouse, Rochdale". LSLav.
- ^ Stevenson, Rachel (7 July 2004). "JD Sports founder sells 11per cent stake to firm behind Speedo". London: Daily Telegraph.
- ^ a b c "History". JD Sports.
- ^ "JD Sports founders sell out for £44.6m". London: Times Online. 5 November 2007.
- ^ "John David Group makes a Bank statement". Yorkshire Post. 11 December 2007.
- ^ "Oldham Athletic announce JD Sports Carbrini sponsor deal". Football Shirt Culture. 6 August 2008.
- ^ JD Sports Fashion History
- ^ Champion Sports to be bought by UK retailer for €19.6m The Irish Times, 26 January 2011
- ^ Blacks Leisure sold for £20m while La Senza finds buyer BBC News, 9 January 2012
- ^ JD Sports acquires FLY53 Insider Media Limited, 15 February 2012
- ^ JD Sports Fashion acquires Cloggs Manchester Evening News, 15 February 2013
External links[edit]


text:
Senior Corps Week celebrates area foster grandparents
Every day around the country, seniors volunteer in schools, assist with disaster relief, help elders with daily tasks and more. These volunteers, members of Senior Corps, are "Making a Difference for Generations."
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