Frank S. Beale "On the Polynomials Related to the Differential Equation $\frac{1}{y} \frac {dy}{dx} = \frac{a_0 + a_1x}{b_0 + b_1x + b_2x^2} \equiv \frac{N}{D}$," The Annals of Mathematical Statistics, Ann. Math. Statist. 8(4), 206-223, (December, 1937)