NILSSON, N. Some Estimates for Eigenfunction Expansions and Spectral Functions Corresponding to Elliptic Differential Operators. MATHEMATICA SCANDINAVICA, [S. l.], v. 9, p. 107–121, 1961. DOI: 10.7146/math.scand.a-10629. Disponível em: https://www.mscand.dk/article/view/10629. Acesso em: 28 apr. 2024.