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Tamar Ziegler and I have just uploaded to the arXiv our paper “The inverse conjecture for the Gowers norm over finite fields in low characteristic“, submitted to Annals of Combinatorics. This paper completes another case of the inverse conjecture for the Gowers norm, this time for vector spaces {{\bf F}^n} over a fixed finite field {{\bf F} = {\bf F}_p} of prime order; with Vitaly Bergelson, we had previously established this claim when the characteristic of the field was large, so the main new result here is the extension to the low characteristic case. (The case of a cyclic group {{\bf Z}/N{\bf Z}} or interval {[N]} was established by Ben Green and ourselves in another recent paper. For an arbitrary abelian (or nilpotent) group, a general but less explicit description of the obstructions to Gowers uniformity was recently obtained by Szegedy; the latter result recovers the high-characteristic case of our result (as was done in a subsequent paper of Szegedy), as well as our results with Green, but it is not immediately evident whether Szegedy’s description of the obstructions matches up with the one predicted by the inverse conjecture in low characteristic.)

The statement of the main theorem is as follows. Given a finite-dimensional vector space {V = {\bf F}^n} and a function {f: V \rightarrow {\bf C}}, and an integer {s \geq 0}, one can define the Gowers uniformity norm {\|f\|_{U^{s+1}(V)}} by the formula

\displaystyle  \|f\|_{U^{s+1}(V)} := \left( \mathop{\bf E}_{x,h_1,\ldots,h_{s+1} \in V} \Delta_{h_1} \ldots \Delta_{h_{s+1}} f(x) \right)^{1/2^{s+1}}

where {\Delta_h f(x) := f(x+h) \overline{f(x)}}. If {f} is bounded in magnitude by {1}, it is easy to see that {\|f\|_{U^{s+1}(V)}} is bounded by {1} also, with equality if and only if {f(x) = e(P)} for some non-classical polynomial {P: V \rightarrow {\bf R}/{\bf Z}} of degree at most {s}, where {e(x) := e^{2\pi ix}}, and a non-classical polynomial of degree at most {s} is a function whose {s+1^{th}} “derivatives” vanish in the sense that

\displaystyle  \partial_{h_1} \ldots \partial_{h_{s+1}} P(x) = 0

for all {x,h_1,\ldots,h_{s+1} \in V}, where {\partial_h P(x) := P(x+h) - P(x)}. Our result generalises this to the case when the uniformity norm is not equal to {1}, but is still bounded away from zero:

Theorem 1 (Inverse conjecture) Let {f: V \rightarrow {\bf C}} be bounded by {1} with {\|f\|_{U^{s+1}(V)} \geq \delta > 0} for some {s \geq 0}. Then there exists a non-classical polynomial {P: V \rightarrow {\bf R}/{\bf Z}} of degree at most {s} such that {|\langle f, e(P) \rangle_{L^2(V)}| := |{\bf E}_{x \in V} f(x) e(-P(x))| \geq c(s,p, \delta) > 0}, where {c(s,p, \delta)} is a positive quantity depending only on the indicated parameters.

This theorem is trivial for {s=0}, and follows easily from Fourier analysis for {s=1}. The case {s=2} was done in odd characteristic by Ben Green and myself, and in even characteristic by Samorodnitsky. In two papers, one with Vitaly Bergelson, we established this theorem in the “high characteristic” case when the characteristic {p} of {{\bf F}} was greater than {s} (in which case there is essentially no distinction between non-classical polynomials and their classical counterparts, as discussed previously on this blog). The need to deal with genuinely non-classical polynomials is the main new difficulty in this paper that was not dealt with in previous literature.

In our previous paper with Bergelson, a “weak” version of the above theorem was proven, in which the polynomial {P} in the conclusion had bounded degree {O_{s,p}(1)}, rather than being of degree at most {s}. In the current paper, we use this weak inverse theorem to reduce the inverse conjecture to a statement purely about polynomials:

Theorem 2 (Inverse conjecture for polynomials) Let {s \geq 0}, and let {P: V \rightarrow {\bf C}} be a non-classical polynomial of degree at most {s+1} such that {\|e(P)\|_{U^{s+1}(V)} \geq \delta > 0}. Then {P} has bounded rank in the sense that {P} is a function of {O_{s,p,\delta}(1)} polynomials of degree at most {s}.

This type of inverse theorem was first introduced by Bogdanov and Viola. The deduction of Theorem 1 from Theorem 2 and the weak inverse Gowers conjecture is fairly standard, so the main difficulty is to show Theorem 2.

The quantity {-\log_{|{\bf F}|} \|e(P)\|_{U^{s+1}(V)}^{1/2^{s+1}}} of a polynomial {P} of degree at most {s+1} was denoted the analytic rank of {P} by Gowers and Wolf. They observed that the analytic rank of {P} was closely related to the rank of {P}, defined as the least number of degree {s} polynomials needed to express {P}. For instance, in the quadratic case {s=1} the two ranks are identical (in odd characteristic, at least). For general {s}, it was easy to see that bounded rank implied bounded analytic rank; Theorem 2 is the converse statement.

We tried a number of ways to show that bounded analytic rank implied bounded rank, in particular spending a lot of time on ergodic-theoretic approaches, but eventually we settled on a “brute force” approach that relies on classifying those polynomials of bounded analytic rank as precisely as possible. The argument splits up into establishing three separate facts:

  1. (Classical case) If a classical polynomial has bounded analytic rank, then it has bounded rank.
  2. (Multiplication by {p}) If a non-classical polynomial {P} (of degree at most {s+1}) has bounded analytic rank, then {pP} (which can be shown to have degree at most {\max(s-p,0)}) also has bounded analytic rank.
  3. (Division by {p}) If {Q} is a non-clsasical polynomial of degree {\max(s-p,0)} of bounded rank, then there is a non-classical polynomial {P} of degree at most {s+1} of bounded rank such that {pQ=P}.

The multiplication by {p} and division by {p} facts allow one to easily extend the classical case of the theorem to the non-classical case of the theorem, basically because classical polynomials are the kernel of the multiplication-by-{p} homomorphism. Indeed, if {P} is a non-classical polynomial of bounded analytic rank of the right degree, then the multiplication by {p} claim tells us that {pP} also has bounded analytic rank, which by an induction hypothesis implies that {pP} has bounded rank. Applying the division by {p} claim, we find a bounded rank polynomial {P'} such that {pP = pP'}, thus {P} differs from {P'} by a classical polynomial, which necessarily has bounded analytic rank, hence bounded rank by the classical claim, and the claim follows.

Of the three claims, the multiplication-by-{p} claim is the easiest to prove using known results; after a bit of Fourier analysis, it turns out to follow more or less immediately from the multidimensional Szemerédi theorem over finite fields of Bergelson, Leibman, and McCutcheon (one can also use the density Hales-Jewett theorem here if one desires).

The next easiest claim is the classical case. Here, the idea is to analyse a degree {s+1} classical polynomial {P: V \rightarrow {\bf F}} via its derivative {d^{s+1} P: V^{s+1} \rightarrow {\bf F}}, defined by the formula

\displaystyle  d^{s+1} P( h_1,\ldots,h_{s+1}) := \partial_{h_1} \ldots \partial_{h_{s+1}} P(x)

for any {x,h_1,\ldots,h_{s+1} \in V} (the RHS is independent of {x} as {P} has degree {s+1}). This is a multilinear form, and if {P} has bounded analytic rank, this form is biased (in the sense that the mean of {e(d^{s+1} P)} is large). Applying a general equidistribution theorem of Kaufman and Lovett (based on this earlier paper of Green and myself) this implies that {d^{s+1} P} is a function of a bounded number of multilinear forms of lower degree. Using some “regularity lemma” theory to clean up these forms so that they have good equidistribution properties, it is possible to understand exactly how the original multilinear form {d^{s+1} P} depends on these lower degree forms; indeed, the description one eventually obtains is so explicit that one can write down by inspection another bounded rank polynomial {Q} such that {d^{s+1} P} is equal to {d^{s+1} Q}. Thus {P} differs from the bounded rank polynomial {Q} by a lower degree error, which is automatically of bounded rank also, and the claim follows.

The trickiest thing to establish is the division by {p} claim. The polynomial {Q} is some function {F(R_1,\ldots,R_m)} of lower degree polynomials {R_1,\ldots,R_m}. Ideally, one would like to find a function {F'(R_1,\ldots,R_m)} of the same polynomials with {pF' = F}, such that {F'(R_1,\ldots,R_m)} has the correct degree; however, we have counterexamples that show that this is not always possible. (These counterexamples are the main obstruction to making the ergodic theory approach work: in ergodic theory, one is only allowed to work with “measurable” functions, which are roughly analogous in this context to functions of the indicated polynomials {Q, R_1,\ldots,R_m} and their shifts.) To get around this we have to first apply a regularity lemma to place {R_1,\ldots,R_m} in a suitably equidistributed form (although the fact that {R_1,\ldots,R_m} may be non-classical leads to a rather messy and technical description of this equidistribution), and then we have to extend each {R_j} to a higher degree polynomial {R'_j} with {pR'_j = R_j}. There is a crucial “exact roots” property of polynomials that allows one to do this, with {R'_j} having degree exactly {p-1} higher than {R_j}. It turns out that it is possible to find a function {P = F'(R'_1,\ldots,R'_m)} of these extended polynomials that have the right degree and which solves the required equation {pP=Q}; this is established by classifying completely all functions of the equidistributed polynomials {R_1,\ldots,R_m} or {R'_1,\ldots,R'_m} that are of a given degree.

Ben Green, Tamar Ziegler, and I have just uploaded to the arXiv our paper “An inverse theorem for the Gowers U^{s+1}[N] norm“, which was previously announced on this blog.  We are still planning one final round of reviewing the preprint before submitting the paper, but it has gotten to the stage where we are comfortable with having the paper available on the arXiv.

The main result of the paper is to establish the inverse conjecture for the Gowers norm over the integers, which has a number of applications, in particular to counting solutions to various linear equations in primes.  In spirit, the proof of the paper follows the 21-page announcement that was uploaded previously.  However, for various rather annoying technical reasons, the 117-page paper has to devote a large amount of space to setting up various bits of auxiliary machinery (as well as a dozen or so pages worth of examples and discussion).  For instance, the announcement motivates many of the steps of the argument by heuristically identifying nilsequences n \mapsto F(g(n) \Gamma) with bracket polynomial phases such as n \mapsto e( \{ \alpha n \} \beta n ).  However, a rather significant amount of theory (which was already worked out to a large extent by Leibman) is needed to formalise the “bracket algebra” needed to manipulate such bracket polynomials and to connect them with nilsequences.  Furthermore, the “piecewise smooth” nature of bracket polynomials causes some technical issues with the equidistribution theory for these sequences.  Our original version of the paper (which was even longer than the current version) set out this theory.  But we eventually decided that it was best to eschew almost all use of bracket polynomials (except as motivation and examples), and run the argument almost entirely within the language of nilsequences, to keep the argument a bit more notationally focused (and to make the equidistribution theory easier to establish).  But this was not without a tradeoff; some statements that are almost trivially true for bracket polynomials, required some “nilpotent algebra” to convert to the language of nilsequences.  Here are some examples of this:

  1. It is intuitively clear that a bracket polynomial phase e(P(n)) of degree k in one variable n can be “multilinearised” to a polynomial e(Q(n_1,\ldots,n_k)) of multi-degree (1,\ldots,1) in k variables n_1,\ldots,n_k, such that e(P(n)) and e(Q(n,\ldots,n)) agree modulo lower order terms.  For instance, if e(P(n)) = e(\alpha n \{ \beta n \{ \gamma n \} \}) (so k=3), then one could take e(Q(n_1,n_2,n_3)) = e( \alpha n_1 \{ \beta n_2 \{ \gamma n_3 \} \}).   The analogue of this statement for nilsequences is true, but required a moderately complicated nilpotent algebra construction using the Baker-Campbell-Hausdorff formula.
  2. Suppose one has a bracket polynomial phase e(P_h(n)) of degree k in one variable n that depends on an additional parameter h, in such a way that exactly one of the coefficients in each monomial depends on h.  Furthermore, suppose this dependence is bracket linear in h.  Then it is intuitively clear that this phase can be rewritten (modulo lower order terms) as e( Q(h,n) ) where Q is a bracket polynomial of multidegree (1,k) in (h,n).  For instance, if e(P_h(n)) = e( \{ \alpha_h n \} \beta n ) and \alpha_h = \{\gamma h \} \delta, then we can take e(Q(h,n)) = e(\{ \{\gamma h\} \delta n\} \beta n ).  The nilpotent algebra analogue of this claim is true, but requires another moderately complicated nilpotent algebra construction based on semi-direct products.
  3. A bracket polynomial has a fairly visible concept of a “degree” (analogous to the corresponding notion for true polynomials), as well as a “rank” (which, roughly speaking measures the number of parentheses in the bracket monomials, plus one).  Thus, for instance, the bracket monomial \{\{ \alpha n^4 \} \beta n \} \gamma n^2 has degree 7 and rank 3.  Defining degree and rank for nilsequences requires one to generalise the notion of a (filtered) nilmanifold to one in which the lower central series is replaced by a filtration indexed by both the degree and the rank.

There are various other tradeoffs of this type in this paper.  For instance, nonstandard analysis tools were introduced to eliminate what would otherwise be quite a large number of epsilons and regularity lemmas to manage, at the cost of some notational overhead; and the piecewise discontinuities mentioned earlier were eliminated by the use of vector-valued nilsequences, though this again caused some further notational overhead.    These difficulties may be a sign that we do not yet have the “right” proof of this conjecture, but one will probably have to wait a few years before we get a proper amount of perspective and understanding on this circle of ideas and results.

Ben Green, Tamar Ziegler, and I have just uploaded to the arXiv the note “An inverse theorem for the Gowers norm {U^{s+1}[N]} (announcement)“, not intended for publication. This is an announcement of our forthcoming solution of the inverse conjecture for the Gowers norm, which roughly speaking asserts that {U^{s+1}[N]} norm of a bounded function is large if and only if that function correlates with an {s}-step nilsequence of bounded complexity.

The full argument is quite lengthy (our most recent draft is about 90 pages long), but this is in large part due to the presence of various technical details which are necessary in order to make the argument fully rigorous. In this 20-page announcement, we instead sketch a heuristic proof of the conjecture, relying in a number of “cheats” to avoid the above-mentioned technical details. In particular:

  • In the announcement, we rely on somewhat vaguely defined terms such as “bounded complexity” or “linearly independent with respect to bounded linear combinations” or “equivalent modulo lower step errors” without specifying them rigorously. In the full paper we will use the machinery of nonstandard analysis to rigorously and precisely define these concepts.
  • In the announcement, we deal with the traditional linear nilsequences rather than the polynomial nilsequences that turn out to be better suited for finitary equidistribution theory, but require more notation and machinery in order to use.
  • In a similar vein, we restrict attention to scalar-valued nilsequences in the announcement, though due to topological obstructions arising from the twisted nature of the torus bundles used to build nilmanifolds, we will have to deal instead with vector-valued nilsequences in the main paper.
  • In the announcement, we pretend that nilsequences can be described by bracket polynomial phases, at least for the sake of making examples, although strictly speaking bracket polynomial phases only give examples of piecewise Lipschitz nilsequences rather than genuinely Lipschitz nilsequences.

With these cheats, it becomes possible to shorten the length of the argument substantially. Also, it becomes clearer that the main task is a cohomological one; in order to inductively deduce the inverse conjecture for a given step {s} from the conjecture for the preceding step {s-1}, the basic problem is to show that a certain (quasi-)cocycle is necessarily a (quasi-)coboundary. This in turn requires a detailed analysis of the top order and second-to-top order terms in the cocycle, which requires a certain amount of nilsequence equidistribution theory and additive combinatorics, as well as a “sunflower decomposition” to arrange the various nilsequences one encounters into a usable “normal form”.

It is often the case in modern mathematics that the informal heuristic way to explain an argument looks quite different (and is significantly shorter) than the way one would formally present the argument with all the details. This seems to be particularly true in this case; at a superficial level, the full paper has a very different set of notation than the announcement, and a lot of space is invested in setting up additional machinery that one can quickly gloss over in the announcement. We hope though that the announcement can provide a “road map” to help navigate the much longer paper to come.

In Notes 5, we saw that the Gowers uniformity norms on vector spaces {{\bf F}^n} in high characteristic were controlled by classical polynomial phases {e(\phi)}.

Now we study the analogous situation on cyclic groups {{\bf Z}/N{\bf Z}}. Here, there is an unexpected surprise: the polynomial phases (classical or otherwise) are no longer sufficient to control the Gowers norms {U^{s+1}({\bf Z}/N{\bf Z})} once {s} exceeds {1}. To resolve this problem, one must enlarge the space of polynomials to a larger class. It turns out that there are at least three closely related options for this class: the local polynomials, the bracket polynomials, and the nilsequences. Each of the three classes has its own strengths and weaknesses, but in my opinion the nilsequences seem to be the most natural class, due to the rich algebraic and dynamical structure coming from the nilpotent Lie group undergirding such sequences. For reasons of space we shall focus primarily on the nilsequence viewpoint here.

Traditionally, nilsequences have been defined in terms of linear orbits {n \mapsto g^n x} on nilmanifolds {G/\Gamma}; however, in recent years it has been realised that it is convenient for technical reasons (particularly for the quantitative “single-scale” theory) to generalise this setup to that of polynomial orbits {n \mapsto g(n) \Gamma}, and this is the perspective we will take here.

A polynomial phase {n \mapsto e(\phi(n))} on a finite abelian group {H} is formed by starting with a polynomial {\phi: H \rightarrow {\bf R}/{\bf Z}} to the unit circle, and then composing it with the exponential function {e: {\bf R}/{\bf Z} \rightarrow {\bf C}}. To create a nilsequence {n \mapsto F(g(n) \Gamma)}, we generalise this construction by starting with a polynomial {g \Gamma: H \rightarrow G/\Gamma} into a nilmanifold {G/\Gamma}, and then composing this with a Lipschitz function {F: G/\Gamma \rightarrow {\bf C}}. (The Lipschitz regularity class is convenient for minor technical reasons, but one could also use other regularity classes here if desired.) These classes of sequences certainly include the polynomial phases, but are somewhat more general; for instance, they almost include bracket polynomial phases such as {n \mapsto e( \lfloor \alpha n \rfloor \beta n )}. (The “almost” here is because the relevant functions {F: G/\Gamma \rightarrow {\bf C}} involved are only piecewise Lipschitz rather than Lipschitz, but this is primarily a technical issue and one should view bracket polynomial phases as “morally” being nilsequences.)

In these notes we set out the basic theory for these nilsequences, including their equidistribution theory (which generalises the equidistribution theory of polynomial flows on tori from Notes 1) and show that they are indeed obstructions to the Gowers norm being small. This leads to the inverse conjecture for the Gowers norms that shows that the Gowers norms on cyclic groups are indeed controlled by these sequences.

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