1.4. Equidistribution in finite fields 73
finite (or limit finite) for d 2. (For d = 1, the analytic rank is infinite if P
is non-constant and zero if P is constant.)
Exercise 1.4.19. Show that if p 2 and P is a (limit) classical polynomial
of degree 2, then rank1(P ) = arank1(P ).
Exercise 1.4.20. Show that if the analytic rank arankd−1(P ) of a limit
classical polynomial P of degree d is unbounded, then P is equidistributed.
Exercise 1.4.21. Suppose we are in the high characteristic case p d.
Using Proposition 1.4.3, show that a limit classical polynomial has bounded
analytic rank if and only if it has bounded rank. (Hint: One direction
follows from the preceding exercise. For the other direction, use the Taylor
formula P (x) =
(x).) This is a special case of the inverse conjecture
for the Gowers norms, which we will discuss in more detail in later sections.
Conclude the following finitary version: if P : V F is a classical poly-
nomial of degree d on a finite-dimensoinal vector space V , and arankd−1(P )
M, then rankd−1(P )
1; conversely, if rankd−1(P ) M, then
arankd−1(P )
Exercise 1.4.22. Show that if P is a (limit) classical polynomial of degree
d, then rankd−1(P ) = rankd−1(cP ) and arankd−1(P ) = arankd−1(cP ) for
all c F\0, and rankd−1(P + Q) = rankd−1(P ) and arankd−1(P + Q) =
arankd−1(P ) for all (limit) classical polynomials Q of degree d 1.
It is clear that the rank obeys the triangle inequality rankd−1(P + Q)
rankd−1(P ) + rankd−1(Q) for all (limit) classical polynomials of degree d.
There is an analogue for analytic rank:
Proposition 1.4.11 (Quasi-triangle inequality for analytic rank). (See
[GoWo2010b].) Let P, Q: V F be (limit) classical polynomials of degree
d. Then arankd−1(P + Q)
) + arankd−1(Q)).
Proof. Let T1(h1,...,hd) be the d-linear form
T1(h1,...,hd) := ∂h1 . . . ∂hd P (x)
(note that the right-hand side is independent of x); similarly define
T2(h1,...,hd) := ∂h1 . . . ∂hd P (x).
By definition, we have
Eh1,...,hd∈V e(T1(h1,...,hd)) =
p− arankd−1(P )
Eh1,...,hd∈V e(T2(h1,...,hd)) =
p− arankd−1(Q)
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