Invariant affine connections on odd-dimensional spheres

TL;DR

This paper classifies invariant affine connections on odd-dimensional spheres viewed as homogeneous spaces, describing their algebraic structure, geometric properties, and special Einstein-like solutions.

Contribution

It provides a complete algebraic description of invariant affine connections on odd-dimensional spheres, including those with Einstein-type properties, using invariant tensors from special geometric structures.

Findings

Classified all invariant affine connections on odd-dimensional spheres.

Identified connections sharing geodesics with Levi-Civita connection.

Characterized spheres admitting Einstein-type invariant connections.

Abstract

A Riemann-Cartan manifold is a Riemannian manifold endowed with an affine connection which is compatible with the metric tensor. This affine connection is not necessarily torsion free. Under the assumption that the manifold is a homogeneous space, the notion of homogeneous Riemann-Cartan space is introduced in a natural way. For the case of the odd dimensional spheres $\mathbb{S}^{2n+1}$ viewed as homogeneous spaces of the special unitary groups, the classical Nomizu's Theorem on invariant connections has permitted to obtain an algebraical description of all the connections which turn the spheres $\mathbb{S}^{2n+1}$ into homogeneous Riemann-Cartan spaces. The expressions of such connections as covariant derivatives are given by means of several invariant tensors: the ones of the usual Sasakian structure of the sphere; an invariant 3-differential form coming from a $3$-Sasakian structure on $\mathbb{S}^7$; and the involved ones in the almost contact metric structure of $\mathbb{S}^5$ provided by its natural embedding into the nearly K\"ahler manifold $\mathbb{S}^6$. Furthermore, the invariant connections sharing geodesics with the Levi-Civita one have also been completely described. Finally, $\mathbb{S}^3$ and $\mathbb{S}^7$ are characterized as the unique odd-dimensional spheres which admit nontrivial invariant connections satisfying an Einstein-type equation.