Nijenhuis operators with a unity and F-manifolds
The core object of this paper is a pair $(L, e)$, where $L$ is a Nijenhuis operator and $e$ is a vector field satisfying a specific Lie derivative condition, i.e., $Lie_{e}L=\operatorname{Id}$. Our research unfolds in two parts. In the first part, we establish a Splitting Theorem for Nijenhuis operators with a unity, offering an effective reduction of their study to cases where $L$ has either one real or two complex conjugate eigenvalues at a given point. We further provide the normal forms for $\mathrm{gl}$-regular Nijenhuis operators with a unity around algebraically generic points, along with semi-normal forms for dimensions two and three. In the second part, we establish the relationship between Nijenhuis operators with a unity and $F$-manifolds. Specifically, we prove that the class of regular $F$-manifolds coincides with the class of Nijenhuis manifolds with a cyclic unity. By extending our results from dimension three, we reveal semi-normal forms for corresponding $F$-manifolds around singularities.