arXiv subjects

Robert M. Guralnick

Publications and source records attributed to Robert M. Guralnick.

76 records · Page 5Linked to original sources

arXiv2005

Decompositions of Small Tensor Powers and Larsen's Conjecture

We classify all pairs (G,V) with G a closed subgroup in a classical group with natural module V over the complex numbers such that G has the same composition factors on the kth tensor power of V, for a fixed (small) k. In particular, we prove Larsen's conjecture stating that for dim(V) > 6 and k = 4, there are no such G aside from those containing the derived subgroup of the classical group. We also find all the examples where this fails for dim(V) < 7. As a consequence of our results, we obtain a short proof of a related conjecture of Katz. These conjectures are used in Katz's recent works on monodromy groups attached to Lefschetz pencils and to character sums over finite fields. Modular versions of these conjectures are also studied, with a particular application to random generation in finite groups of Lie type.

math.GR↗

arXiv2004

A Question about Pic(X) as a G-module

Let G be a finite group acting faithfully on an irreducible non-singular projective curve defined over an algebraically closed field F. Does every G-invariant divisor class contain a G-invariant divisor? The answer depends only on G and not on the curve. We answer the same question for degree 0 divisor (classes). We investigate the question for cycles on varieties.

math.AG↗

arXiv2003

Inequalities for finite group permutation modules

If f is a nonzero complex-valued function defined on a finite abelian group A and \hat f is its Fourier transform, then |Supp (f)||Supp {\hat f)| \ge |A|, where Supp (f) and Supp (\hat f) are the supports of f and \hat f. In this paper we generalize this known result in several directions. In particular, we prove an analogous inequality where the abelian group A is replaced by a transitive right G-set, where G is an arbitrary finite group. We obtain stronger inequalities when the G-set is primitive and we determine the primitive groups for which equality holds. We also explore connections between inequalities of this type and a result of Chebotarëv on complex roots of unity, and we thereby obtain a new proof of Chebotarëv's theorem.

math.GR↗

arXiv2002

The rational function analogue of a question of Schur and exceptionality of permutation representations

In 1923 Schur considered the following problem. Let f(X) be a polynomial with integer coefficients that induces a bijection on the residue fields Z/pZ for infinitely many primes p. His conjecture, that such polynomials are compositions of linear and Dickson polynomials, was proved by M. Fried in 1970. Here we investigate the analogous question for rational functions, also we allow the base field to be any number field. As a result, there are many more rational functions for which the analogous property holds. Some infinite series come from rational isogenies of elliptic curves and deformations. There are several sporadic examples which do not fit in any of the series we obtain. First we translate the arithmetic property to a question about finite permutation groups, and classify those groups which fulfill the necessary conditions. The proofs depend on the classification of the finite simple groups. Then we use arithmetic arguments to either rule out many cases, or to prove that the remaining cases indeed give rise to examples. This part is based on Mazur's classical results about rational points on the modular curves X_0(p) and X_1(p), results about Galois images in GL_2(p) coming from action of the absolute Galois group of Q on p-torsion points of elliptic curves, the theory of complex multiplication, and the techniques used in the inverse regular Galois problem.

math.GR↗