On $(N(k),\xi)$-semi-Riemannian manifolds: Semisymmetries
  $(N(k),\xi)$-semi-Riemannian manifolds are defined. Examples and properties of $(N(k),\xi)$-semi-Riemannian manifolds are given. Some relations involving ${\cal T}_{a}$-curvature tensor in $(N(k),\xi)$-semi-Riemannian manifolds are proved. $\xi $-${\cal T}_{a}$-flat $(N(k),\xi)$-semi-Riemannian manifolds are defined. It is proved that if $M$ is an $n$-dimensional $\xi $-${\cal T}_{a}$-flat $(N(k),\xi)$-semi-Riemannian manifold, then it is $\eta $-Einstein under an algebraic condition. We prove that a semi-Riemannian manifold, which is $T$-recurrent or $T$-symmetric, is always $T$-semisymmetric, where $T$ is any tensor of type $(1,3)$. $({\cal T}_{a}, {\cal T}_{b}) $-semisymmetric semi-Riemannian manifold is defined and studied. The results for ${\cal T}_{a}$-semisymmetric, ${\cal T}_{a}$-symmetric, ${\cal T}_{a}$-recurrent $(N(k),\xi)$-semi-Riemannian manifolds are obtained. The definition of $({\cal T}_{a},S_{{\cal T}_{b}})$-semisymmetric semi-Riemannian manifold is given. $({\cal T}_{a},S_{{\cal T}_{b}})$-semisymmetric $(N(k),\xi)$-semi-Riemannian manifolds are classified. Some results for ${\cal T}_{a}$-Ricci-semisymmetric $(N(k),\xi)$-semi-Riemannian manifolds are obtained. 
Mukut Mani Tripathi and Punam Gupta
math.DG