96 1. Higher order Fourier analysis
In analogy with Exercise 1.6.1, we can now define the general notion of
a polynomial map:
Definition 1.6.7. A map φ: H G between two filtered groups H•,G• is
said to be polynomial if it maps
HKk(H•)
to
HKk(G•)
for each k 0. The
space of all such maps is denoted Poly(H• G•).
Since
HKk(H•), HKk(G•)
are groups, we immediately
obtain20
Theorem 1.6.8 (Lazard-Leibman theorem). Poly(H• G•) forms a group
under pointwise multiplication.
In a similar spirit, we have
Theorem 1.6.9 (Filtered groups and polynomial maps form a category).
If φ: H G and ψ : G K are polynomial maps between filtered groups
H•,G•,K•, then ψ φ: H K is also a polynomial map.
We can also give some basic examples of polynomial maps. Any constant
map from H to G taking values in G≥0 is polynomial, as is any map φ: H
G which is a filtered homomorphism in the sense that it is a homomorphism
from H≥i to G≥i for any i 0.
Now we turn to an alternate definition of a polynomial map. For any h
H and any map φ: H G Define the multiplicative derivative Δhφ: H G
by the formula Δhφ(x) :=
φ(hx)φ(x)−1.
Theorem 1.6.10 (Alternate description of polynomials). Let φ: H G be
a map between two filtered groups H, G. Then φ is polynomial if and only
if, for any i1,...,im 0, x H≥0, and hj H≥ij for j = 1,...,m, one
has Δh1 . . . Δhm φ(x) G≥i1+···+im .
In particular, from Exercise 1.6.1, we see that a non-classical polynomial
of degree d from one additive group H to another G is the same thing as a
polynomial map from (H, 1) to (G, d). More generally, a φ map from
(H, 1) to a filtered group G• is polynomial if and only if
Δh1 . . . Δhi φ(x) G≥i
for all i 0 and x, h1,...,hi H.
Proof. We first prove the “only if” direction. It is clear (by using 0-
dimensional cubes) that a polynomial map must map H≥0 to G≥0. To
obtain the remaining cases, it suffices by induction on m to show that if φ
20From our choice of definitions, this theorem is a triviality, but the theorem is less trivial
when using an alternate but non-trivially equivalent definition of a polynomial, which we will give
shortly. Lazard [La1954] gave a version of this theorem when H was the integers and G was a
nilpotent Lie group; the general problem of multiplying polynomial sequences was considered by
Leibman [Le1998].
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