Petrov Z, Todorova T. Representation of natural numbers by sum of four squares of almost-prive having a special form. Ann. Sofia Univ. Fac. Math. and Inf. [Internet]. 2020 Dec. 12 [cited 2024 Nov. 24];107:15-29. Available from: https://ftl5.uni-sofia.bg./index.php/fmi/article/view/103