Markov, K., and K. Zvyatkov. “ON THE EFFECTIVE BEHAVIOUR OF A NONLINEAR DISPERION OF SPHERES”. Ann. Sofia Univ. Fac. Math. And Inf., vol. 82, no. 2, Dec. 1991, pp. 179-92, https://ftl5.uni-sofia.bg./index.php/fmi/article/view/543.