Equiconvergence property for spectral expansions related to perturbations of the operator - u''(-x) with initial data

Uniform equiconvergence of spectral expansions related to the second-order differential operators with involution: -u''(-x) and -u''(-x) + q(x)u(x) with the initial data u(-1) = 0, u'(-1) = 0 is obtained. Starting with the spectral analysis of the unperturbed operator, the estimates of the Green?s functions are established and then applied via the contour integrating approach to the spectral expansions. As a corollary, it is proved that the root functions of the perturbed operator form the basis in L2(-1,1) for any complex-valued coefficient q(x) ? L2(-1,1).