GARDING, L. On the asymptotic distribution of the eigenvalues and eigenfunctions of elliptic differential operators. MATHEMATICA SCANDINAVICA, [S. l.], v. 1, p. 237–255, 1953. DOI: 10.7146/math.scand.a-10382. Disponível em: https://www.mscand.dk/article/view/10382. Acesso em: 5 may. 2024.