Belkhelfa, M., and F. Mahi. “Second Order Parallel Tensors on $S$ -Manifolds and Semi-Parallel Hypersurfaces of $S$ -Space Forms”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 71, no. 10, Oct. 2019, pp. 1422-9, http://umj.imath.kiev.ua/index.php/umj/article/view/1524.