Table of contents

In this paper we construct trigonometric null-series with rapidly decreasing coefficients and we also strengthen some of D. E. Men'shov's correction theorems.

Automorphic corrections for the Lorentzian Kac-Moody algebras with the simplest generalized Cartan matrices of rank 3,

       and    

are found. For this correction, which is a generalized Kac-Moody Lie super algebra, is delivered by , the Igusa -modular form of weight 35, while for it is given by some Siegel modular form of weight 30 with respect to a 2-congruence subgroup of . Expansions of and in infinite products are obtained and the multiplicities of all the roots of the corresponding generalized Lorentzian Kac-Moody superalgebras are calculated. These multiplicities are determined by the Fourier coefficients of certain Jacobi forms of weight 0 and index 1. The method adopted for constructing and leads in a natural way to an explicit construction (as infinite products or sums) of Siegel modular forms whose divisors are Humbert surfaces with fixed discriminants. A geometric construction of these forms was proposed by van der Geer in 1982. To show the prospects for further studies, the list of all hyperbolic symmetric generalized Cartan matrices with the following properties is presented: is a matrix of rank 3 and of elliptic or parabolic type, has a lattice Weyl vector, and contains a parabolic submatrix .

In this paper we investigate the stability of surfaces of zero mean curvature in Lorentz manifolds. In the case when the enveloping manifold is a warped Lorentz product and under certain assumptions about the warping function, it is proved that every stable minimal tube or strip is a totally geodesic manifold.

Necessary and sufficient conditions for the existence of limits in the mean of functions polyharmonic in a disc are found.

An operator algebra associated with a smooth embedding is constructed. For elliptic elements of this algebra a finiteness theorem (the Fredholm property) is established, and the index is computed. A connection with Sobolev problems is shown.

A trigonometric polynomial that is positive on a set of small measure is constructed. Several new inequalities in the geometry of numbers are proved using the properties of this polynomial.