Concircularly recurrent nearly Kähler manifolds

In this paper, two concircular invariants of a nearly Kähler manifold are considered. It is proved that a nearly Kähler manifold is concircularly flat if and only if the first concircular invariant is zero. A formula for calculating the second concircular invariant is obtained, and a subclass of nearly Kähler manifolds is distinguished, called the class of concircular-paraKähler manifolds. A concircular-paraKähler manifold of zero scalar curvature is isometric to the complex Euclidean space $\mathbb{C}^n$ equipped with the standard Hermitian metric. The class of concircular-paraKähler manifolds of nonzero constant type coincides with the class of six-dimensional proper nearly Kähler manifolds. It is proved that a concircular-paraKähler nearly Kähler manifold is a Riemannian manifold of constant nonnegative scalar curvature. In this case, its scalar curvature is zero if and only if it is a Kähler manifold. A complete local characterization of concircular-paraKähler nearly Kähler manifolds and concircular-recurrent nearly Kähler manifolds is obtained.