Zamkovoy, S., and V. Tzanov. “NON-EXISTENCE OF FLAT PARACONTACT METRIC STRUCTURES IN DIMENSION GREATER THAN OR EQUAL TO FIVE”. Ann. Sofia Univ. Fac. Math. And Inf., vol. 100, Dec. 2011, pp. 27-34, https://ftl5.uni-sofia.bg./index.php/fmi/article/view/244.