Hristov, A., and G. Kostadinov. “MANIFOLDS ADMITTING A STRUCTURE OF FOUR DIMENTIONAL ALGEBRA OF AFFINORS”.
Ann. Sofia Univ. Fac. Math. And Inf.
, vol. 103, Dec. 2016, pp. 89-95, https://ftl5.uni-sofia.bg/index.php/fmi/article/view/57.