NILSSON, Nils. Some Estimates for Eigenfunction Expansions and Spectral Functions Corresponding to Elliptic Differential Operators. Mathematica Scandinavica, Berkeley, CA, United States, v. 9, p. 107–121, 1961. DOI: 10.7146/math.scand.a-10629. Disponível em: https://journals.msp.org/mscand/article/view/2719. Acesso em: 25 jul. 2026.