Markov, K., and K. Ilieva. “A Note on the $C^{2}$-Term of the Effective Conductivity for Random Dispersions”. Ann. Sofia Univ. Fac. Math. And Inf., vol. 84, Dec. 1993, pp. 123-38, https://ftl5.uni-sofia.bg./index.php/fmi/article/view/475.