Peetre J. Some Remarks on Continuous Orthogonal Expansions, and Eigenfunction Expansions for Positive Self-Adjoint Elliptic Operators with Variable Coefficients. Math. Scand. [Internet]. 1965 Dec. 1 [cited 2024 Jul. 3];17:56-64. Available from: https://www.mscand.dk/article/view/10763