Nijenhuis geometry of parallel tensors

TL;DR

This paper characterizes the integrability of various tensor fields on manifolds through geometric and analytical methods, linking it to algebraic constancy and Nijenhuis-type tensors, including the classical Nijenhuis tensor.

Contribution

It provides a unified framework for understanding integrability of diverse tensor fields via Nijenhuis-type tensors and algebraic conditions, extending classical results.

Findings

Integrability characterized by algebraic constancy and Nijenhuis-type tensors.

Unified approach for different tensor types including forms, vectors, and complex tensors.

Extension of classical Nijenhuis tensor concepts to broader tensor classes.

Abstract

A tensor -- meaning here a tensor field $\Theta$ of any type $(p,q)$ on a manifold -- may be called integrable if it is parallel relative to some torsion-free connection. We provide analytical and geometric characterizations of integrability for differential $q$-forms, $q=0,1,2,n-1,n$ (in dimension $n$), vectors, bivectors, symmetric $(2,0)$ and $(0,2)$ tensors, as well as complex-diagonalizable and nilpotent tensors of type $(1,1)$. In most cases, integrability is equivalent to algebraic constancy of $\Theta$ coupled with the vanishing of one or more suitably defined Nijenhuis-type tensors, depending on $\Theta$ via a quasilinear first-order differential operator. For $(p,q)=(1,1)$, they include the ordinary Nijenhuis tensor.