The Hilbert and Bessel properties of systems of eigenfunctions of second-order operators
N. V. Assonova, V. D. Budaev

This publication is cited in the following articles:

1. A. S. Tsareva, “The Bessel and Hilbert properties for root functions of second-order ordinary differential operators with the Carleman condition violated”, Dokl. Math., 77:3 (2008), 375