Cherevko, I., and L. Piddubna. “Approximations of Differential Difference Equations and Calculation of Nonasymptotic Roots of Quasipolynomials”. Rev. Anal. Numér. Théor. Approx., vol. 28, no. 1, Feb. 1999, pp. 15-21, https://ictp.acad.ro/jnaat/journal/article/view/1999-vol28-no1-art2.