ESSEN, Erik Von. On stability of $C^\infty$ mappings of manifolds with boundary. Mathematica Scandinavica, Berkeley, CA, United States, v. 43, p. 197–203, 1978. DOI: 10.7146/math.scand.a-11775. Disponível em: https://journals.msp.org/mscand/article/view/1870. Acesso em: 25 jul. 2026.