Belkhelfa, M., and F. Mahi. 2019. “Second Order Parallel Tensors on $S$ -Manifolds and Semi-Parallel Hypersurfaces of $S$ -Space Forms”. Ukrains’kyi Matematychnyi Zhurnal 71 (10), 1422-29. https://umj.imath.kiev.ua/index.php/umj/article/view/1524.