# 45. Torsion-free Connections

The *torsion tensor*** **of a connection $ \nabla$ on $M$ is the alternating tensor of type $(1,2)$ defined by $$ T^{\nabla}(X,Y) := \nabla_XY - \nabla_YX - [X,Y].$$

A connection is said to be *torsion-free*** **if $ T^{\nabla} = 0$. We prove that a connection is uniquely determined by its geodesics and its torsion tensor. We then prove that if $ \mathbb{S}$ is any spray on $M$ and $T$ is any alternating tensor of type $(1,2)$ then there exists a unique connection $ \nabla$ on $M$ with geodesic spray $ \mathbb{S}$ and torsion tensor equal to $T$.

Finally, we briefly survey the question as to which Lie groups can occur as holonomy groups of torsion-free connections, and introduce the notion of a *Berger group*.

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