HALVORSEN, Sigbjorn. On the Quadratic Integrability of Solutions of $d^2x/dt^2 + f(t)x = 0$. Mathematica Scandinavica, Berkeley, CA, United States, v. 14, p. 111–119, 1964. DOI: 10.7146/math.scand.a-10711. Disponível em: https://journals.msp.org/mscand/article/view/2589. Acesso em: 25 jul. 2026.