PEETRE, J. Some Remarks on Continuous Orthogonal Expansions, and Eigenfunction Expansions for Positive Self-Adjoint Elliptic Operators with Variable Coefficients. MATHEMATICA SCANDINAVICA, [S. l.], v. 17, p. 56–64, 1965. DOI: 10.7146/math.scand.a-10763. Disponível em: https://www.mscand.dk/article/view/10763. Acesso em: 5 may. 2024.