I. Dimitrov, S., and T. L. Todorova. “DIOPHANTINE APPROXIMATION BY PRIME NUMBERS OF A SPECIAL FORM”.
Ann. Sofia Univ. Fac. Math. And Inf.
, vol. 102, Dec. 2015, pp. 71-90, https://ftl5.uni-sofia.bg/index.php/fmi/article/view/64.