Let M 2n+1 be an almost Kenmotsu manifold with the characteristic vector field belonging to the (k, µ) ′ -nullity distribution. We prove that the curvature tensor of M 2n+1 is harmonic if and only if M 2n+1 is locally isometric to either a product space H n+1 (−4) × R n , or an Einstein warped product C × f N 2n of an open interval and a Ricci-flat almost Kähler manifold.
IntroductionA tensor field of type (1, 3) is called an algebraic curvature tensor field if it has symmetric properties of the curvature tensor field of a Riemannian manifold (M, g). It is well known [17] that an algebraic curvature tensor field R on a Riemannian manifold (M, g) is said to be harmonic if (divR)(X, Y, Z) = 0 for any vector fields X, Y, Z on M, where div denotes the divergence operator with respect to the metric g. Following [16], an algebraic curvature tensor field satisfying the second Bianchi identity is harmonic if and only if the associated Ricci operator is of Codazzi type, that is,