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., [S. l.], v. 107, p. 15–29, 2020. DOI: 10.60063/gsu.fmi.107.15-29. Disponível em: https://stipendii.uni-sofia.bg/index.php/fmi/article/view/103. Acesso em: 8 may. 2024.