Mihova, V., and G. Ganchev. “PARTIAL DIFFERENTIAL EQUATIONS OF TIME-LIKE WEINGARTEN SURFACES IN THE THREE-DIMENSIONAL MINKOWSKI SPACE”. Ann. Sofia Univ. Fac. Math. And Inf., vol. 101, Dec. 2013, pp. 143-65, https://annual.uni-sofia.bg/index.php/fmi/article/view/236.