Cherevko, Igor, and Larisa Piddubna. “ Approximations of Differential Difference Equations and Calculation of Nonasymptotic Roots of Quasipolynomials”. Rev. Anal. Numér. Théor. Approx. 28, no. 1 (February 1, 1999): 15–21. Accessed April 14, 2025. https://ictp.acad.ro/jnaat/journal/article/view/1999-vol28-no1-art2.