Makarov, V. Yu. 1999. “Relationship Between the Asymptotic Behavior of Exponents of a Multidimensional Exponential Series and the Asymptotic Behavior of Its Coefficients in a Neighborhood of Singular Points”. Ukrains’kyi Matematychnyi Zhurnal 51 (9), 1193–1200. https://umj.imath.kiev.ua/index.php/umj/article/view/4715.