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Gowers norm

For the function field norm, see uniform norm; for uniformity in topology, see uniform space.

In mathematics, in the field of additive combinatorics, a Gowers norm or uniformity norm is a class of norm on functions on a finite group or group-like object which are used in the study of arithmetic progressions in the group. They are named after Timothy Gowers who introduced them in his work on Szemerédi's theorem.

Let f be a complex-valued function on a group G and let J denote complex conjugation. The Gowers d-norm is

\[ \Vert f \Vert_{U^d(g)} = \mathbf{E}_{x,h_1,\ldots,h_d \in G} \prod_{\omega_1,\ldots,\omega_d \in \{0,1\}} J^{\omega_1+\cdots+\omega_d} f\left({x + h_1\omega_1 + \cdots + h_d\omega_d}\right) \ . \]

The inverse conjecture for these norms is the statement that if f has L-infinity norm (uniform norm in the usual sense) equal to 1 then the Gowers s-norm is bounded above by 1, with equality if and only if f is of the form exp(2πi g) with g a polynomial of degree at most s. This can be interpreted as saying that the Gowers norm is controlled by polynomial phases.

The inverse conjecture holds for vector spaces over a finite field. However, for cyclic groups Z/N this is not so, and the class of polynomial phases has to be extended to control the norm.

References

• Tao, Terence; Higher order Fourier analysis, ser. Graduate Studies in Mathematics 142 (2012), American Mathematical Society, Zbl pre06110460URL: http://terrytao.wordpress.com/books/higher-order-fourier-analysis/] ISBN: 978-0-8218-8986-2

Zimmert set

In mathematics, a Zimmert set is a set of positive integers associated with the structure of quotients of hyperbolic three-space by a Bianchi group.

Definition

Fix an integer d and let D be the discriminant of the imaginary quadratic field Q(√-d). The Zimmert set Z(d) is the set of positive integers n such that 4n² < -D-3 and n ≠ 2; D is a quadratic non-residue of all odd primes in d; n is odd if D is not congruent to 5 modulo 8. The cardinality of Z(d) may be denoted by z(d).

Property

For all but a finite number of d we have z(d) > 1: indeed this is true for all d > 10⁴⁷⁶.

Application

Let Γ_d denote the Bianchi group PSL(2,O_d), where O_d is the ring of integers of. As a subgroup of PSL(2,C), there is an action of Γ_d on hyperbolic 3-space H₃, with a fundamental domain. It is a theorem that there are only finitely many values of d for which Γ_d can contain an arithmetic subgroup G for which the quotient H₃/G is a link complement. Zimmert sets are used to obtain results in this direction: z(d) is a lower bound for the rank of the largest free quotient of Γ_d and so the result above implies that almost all Bianchi groups have non-cyclic free quotients.

References

• Maclachlan, Colin; Reid, Alan W.; The Arithmetic of Hyperbolic 3-Manifolds, ser. Graduate Texts in Mathematics 219 (2003), Springer-Verlag, Zbl 1025.57001 ISBN: 0-387-98386-4