92 1. Higher order Fourier analysis
The above regularity lemma (or more precisely, a close relative of this
lemma) was also used in [GoWo2010b]:
Theorem 1.5.12 (Gowers-Wolf theorem [GoWo2010b]). Let Ψ =
(ψ1,...,ψt) be a collection of linear forms with integer coeﬃcients, with
no two forms being linearly dependent. Let F have suﬃciently large char-
acteristic, and suppose that f1,...,ft :
→ C are functions bounded in
magnitude by 1 such that
|ΛΨ(f1,...,ft)| ≥ δ
where ΛΨ was the form defined in Section 1.3. Then for each 1 ≤ i ≤ t
there exists a classical polynomial φi of degree at most d such that
where d is the true complexity of the system Ψ as defined in Section 1.3.
This d is best possible.
1.6. The inverse conjecture for the Gowers norm II. The
In Section 1.5, we saw that the Gowers uniformity norms on vector spaces
were controlled by classical polynomial phases e(φ).
Now we study the analogous situation on cyclic groups Z/NZ. Here,
there is an unexpected surprise: the polynomial phases (classical or other-
wise) are no longer suﬃcient to control the Gowers norms U
s exceeds 1. To resolve this problem, one must enlarge the space of poly-
nomials 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
on nilmanifolds G/Γ; 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 → g(n)Γ, and this is the perspective we will take here.
A polynomial phase n → e(φ(n)) on a finite abelian group H is formed
by starting with a polynomial φ: H → R/Z to the unit circle, and then
composing it with the exponential function e: R/Z → C. To create a
nilsequence n → F (g(n)Γ), we generalise this construction by starting with