LAPTEV, G. Strong elliptic equations of second order in Hilbert space. Lithuanian Mathematical Journal, [S. l.], v. 8, n. 1, p. 87–99, 1968. DOI: 10.15388/LMJ.1968.20183. Disponível em: https://journals.vu.lt./LMJ/article/view/20183. Acesso em: 27 apr. 2025.