Povzner 1953

 author = {A. Ya. Povzner},
 year = 1953,
 title = {On the Expansions of Arbitrary Functions in terms

of the eigenfunctions of the operator {$-\Delta u + cu$} (in {Russian})},

 journal = {Mat. Sbornik},
 volume = 32,
 number = 74,
 pages = {109--156}