On the basis property of systems of eigenfunctions and associated functions of a second-order differential operator
The main result in this announcement is a necessary condition for the system of eigenfunctions and associated functions of \(u''+qu=\lambda u\) on a finite interval to have the usual basis property. This condition was previously known to be necessary for the unconditional basis property. Sufficient conditions are considered.
scientific article; zbMATH DE number 1148143