Gordon's Conjectures: Pontryagin-van Kampen duality and the Fourier transform in hyperfinite setting

Abstract

Using the ideas of E.I. Gordon we present and farther advance an approach, based on nonstandard analysis, to simultaneous approximations of locally compact abelian groups and their duals by (hyper)finite abelian groups, as well as to approximations of various types of Fourier transforms on them by the discrete Fourier transform. Combining some methods of nonstandard analysis and additive combinatorics we prove the three Gordon's Conjectures which were open since 1991 and are crucial both in the formulations and proofs of the LCA groups and Fourier transform approximation theorems.