PERSSON, A. Bounds for the Discrete Part of the Spectrum of a Semi-Bounded Schrödinger Operator. MATHEMATICA SCANDINAVICA, [S. l.], v. 8, p. 143–153, 1960. DOI: 10.7146/math.scand.a-10602. Disponível em: https://www.mscand.dk/article/view/10602. Acesso em: 2 may. 2024.