ESSEN, E. V. On stability of $C^\infty$ mappings of manifolds with boundary. MATHEMATICA SCANDINAVICA, [S. l.], v. 43, p. 197–203, 1978. DOI: 10.7146/math.scand.a-11775. Disponível em: https://www.mscand.dk/article/view/11775. Acesso em: 3 may. 2024.