Petrov, Zhivko, and Tatyana Todorova. “Representation of Natural Numbers by Sum of Four Squares of Almost-Prive Having a Special Form”. Ann. Sofia Univ. Fac. Math. and Inf. 107 (December 12, 2020): 15–29. Accessed December 22, 2024. https://annual.uni-sofia.bg/index.php/fmi/article/view/103.