Stanilov, G., and V. Videv. “On a Generalization of the Jacobi Operator in the Riemannian Geometry”.
Ann. Sofia Univ. Fac. Math. And Inf.
, vol. 86, no. 1, Dec. 1994, pp. 27-34, https://admin.uni-sofia.bg/index.php/fmi/article/view/432.