@article{MZ_1976__146_172309,
     author = {Walter Eberhard},
     title = {Zur {Vollst\"andigkeit} des {Biorthogonalsystems} von {Eigenfunktionen} irregull;rer {Eigenwertprobleme.}},
     journal = {Mathematische Zeitschrift},
     pages = {213--222},
     publisher = {mathdoc},
     volume = {146},
     year = {1976},
     zbl = {0302.34026},
     url = {http://geodesic.mathdoc.fr/item/MZ_1976__146_172309/}
}