PEETRE, Jaak. The Euler derivative. An intrinsic approach to the calculus of variations. Mathematica Scandinavica, Berkeley, CA, United States, v. 42, p. 313–333, 1978. DOI: 10.7146/math.scand.a-11756. Disponível em: https://journals.msp.org/mscand/article/view/1893. Acesso em: 25 jul. 2026.