PERSSON, Arne. Bounds for the Discrete Part of the Spectrum of a Semi-Bounded Schrödinger Operator. Mathematica Scandinavica, Berkeley, CA, United States, v. 8, p. 143–153, 1960. DOI: 10.7146/math.scand.a-10602. Disponível em: https://journals.msp.org/mscand/article/view/2747. Acesso em: 25 jul. 2026.