WU, Shengjian. On the Location of Zeros of Solutions of $f’’ + A(z)f = 0$ where $A(z)$ is Entire. Mathematica Scandinavica, Berkeley, CA, United States, v. 74, p. 293–312, 1994. DOI: 10.7146/math.scand.a-12498. Disponível em: https://journals.msp.org/mscand/article/view/1133. Acesso em: 25 jul. 2026.