1. On a characteristic property of the kernel of a linear operator;Gohberg, I. C.;Dokl. Akad. Nauk SSSR (N.S.),1955
2. On a characteristic property of the kernel of a linear operator;Markus, A. S.;Dokl. Akad. Nauk SSSR (N.S.),1955
3. Über lineare Funktionalgleichungen;Riesz, Friedrich;Acta Math.,1916