In the mathematical field of differential geometry, a Kenmotsu manifold is an almost-contact manifold endowed with a certain kind of Riemannian metric. They are named after the Japanese mathematician Katsuei Kenmotsu.

Definitions

Let ${\displaystyle (M,\varphi ,\xi ,\eta )}$ be an almost-contact manifold. One says that a Riemannian metric ${\displaystyle g}$ on ${\displaystyle M}$ is adapted to the almost-contact structure ${\displaystyle (\varphi ,\xi ,\eta )}$ if:

$${\displaystyle {\begin{aligned}g_{ij}\xi ^{j}&=\eta _{i}\\g_{pq}\varphi _{i}^{p}\varphi _{j}^{q}&=g_{ij}-\eta _{i}\eta _{j}.\end{aligned))}$$

${\displaystyle g_{p},}$

${\displaystyle \xi _{p))$

${\displaystyle \ker \left(\eta _{p}\right);}$

${\displaystyle g_{p))$

${\displaystyle \ker \left(\eta _{p}\right)}$

${\displaystyle \varphi _{p}{\big \vert }_{\ker \left(\eta _{p}\right)}.}$

${\displaystyle (M,\varphi ,\xi ,\eta ,g)}$

An almost-contact metric manifold ${\displaystyle (M,\varphi ,\xi ,\eta ,g)}$ is said to be a Kenmotsu manifold if^[2]

$${\displaystyle \nabla _{i}\varphi _{j}^{k}=-\eta _{j}\varphi _{i}^{k}-g_{ip}\varphi _{j}^{p}\xi ^{k}.}$$