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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

u-invariant

The universal invariant or u-invariant of a field describes the structure of quadratic forms over the field.

The universal invariant u(F) of a field F is the largest dimension of an anisotropic quadratic space over F, or ∞ if this does not exist. Since formally real fields have anisotropic quadratic forms (sums of squares) in every dimension, the invariant is only of interest for other fields. An equivalent formulation is that u is the smallest number such that every form of dimension greater than u is isotropic, or that every form of dimension at least u is universal.

Examples

• For the complex numbers, u(C) = 1.
• If F is quadratically closed then u(F) = 1.
• The function field of an algebraic curve over an algebraically closed field has u ≤ 2; this follows from Tsen's theorem that such a field is quasi-algebraically closed.
• If F is a nonreal global or local field, or more generally a linked field, then u(F) = 1,2,4 or 8.

Properties

• If F is not formally real then u(F) is at most \(q(F) = \left|{F^\star / F^{\star2}}\right|\), the index of the squares in the multiplicative group of F.
• Every even integer occurs as the value of u(F) for some F.
• u(F) cannot take the values 3, 5, or 7. A field exists with u = 9.

The general u-invariant

Since the u-invariant is of little interest in the case of formally real fields, we define a general u-invariant to be the maximum dimension of an anisotropic form in the torsion subgroup of the Witt ring of F, or ∞ if this does exist. For non-formally real fields, the Witt ring is torsion, so this agrees with the previous definition. For a formally real field, the general u-invariant is either even or ∞.

Properties

• u(F) ≤ 1 if and only if F is a Pythagorean field.

References

• Lam, Tsit-Yuen; Introduction to Quadratic Forms over Fields, ser. Graduate Studies in Mathematics 67 (2005), American Mathematical Society, Zbl 1068.11023 ISBN: 0-8218-1095-2
• Rajwade, A. R.; Squares, ser. London Mathematical Society Lecture Note Series 171 (1993), Cambridge University Press, Zbl 0785.11022 ISBN: 0-521-42668-5

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