PEETRE, Jaake. Some Remarks on Continuous Orthogonal Expansions, and Eigenfunction Expansions for Positive Self-Adjoint Elliptic Operators with Variable Coefficients. Mathematica Scandinavica, Berkeley, CA, United States, v. 17, p. 56–64, 1965. DOI: 10.7146/math.scand.a-10763. Disponível em: https://journals.msp.org/mscand/article/view/2562. Acesso em: 25 jul. 2026.