On the equiconvergence of expansions by Riesz bases formed by eigenfunctions of a linear differential operator of order2n

Abstract

The theory of non-selfadjoint differential operators has great importance in several applications. Developing a fruitful method of V. A. II'in [1], in [21 I. Jod described a new method which makes possible to avoid the use of the basic solution in such investigations. Using this method, a general equiconvergence theorem was proved in [9] for the SchrSdinger operator, having generalized the results of the papers [5], [6], [7] and [8]. This theorem concerns the expansions by Riesz bases formed by eigenfunctions of higher order of the SchrSdinger operator (but seems to be new also for the case of orthonormal bases). The existence of such Riesz bases was proved for a wide class of operators (also of higher order) by V. P. Mikhailov [10] and G. M. Keselman [111. Later on, the method of [2] was generalized for the case of differential operators of higher order in [41, [12] and [13]. These generalizations are based on a new formula which extends the well-known mean-value formula of E. C. Titchmarsh for more general operators. Using the method and the results of these papers, we shall prove a general equiconvergence theorem for linear differential operators of higher order. In this theorem the eigenvalues of the Riesz basis may be arbitrary complex numbers. Our result is new also for orthonormal bases.