HALVORSEN, Sigbjorn. On the Quadratic Integrability of Solutions of $d^2x/dt^2 + f(t)x = 0$. MATHEMATICA SCANDINAVICA, [S. l.], v. 14, p. 111–119, 1964. DOI: 10.7146/math.scand.a-10711. Disponível em: https://tidsskrift.dk/math/article/view/10711. Acesso em: 14 aug. 2026.