PEETRE, Jaak. The Euler derivative. An intrinsic approach to the calculus of variations. MATHEMATICA SCANDINAVICA, [S. l.], v. 42, p. 313–333, 1978. DOI: 10.7146/math.scand.a-11756. Disponível em: https://tidsskrift.dk/math/article/view/11756. Acesso em: 14 aug. 2026.