Torsion of a linear connection

Definition

Given data

• A differential manifold $M$
• A linear connection $\nabla$ on $M$ (viz a connection on the tangent bundle $TM$).

Definition part

The torsion of $\nabla$, denoted as $\tau (\nabla )$, is defined as a map that takes as input 2 vector fields and outputs a third vector field, as follows:

$\tau (\nabla )(X,Y)=\nabla _{X}Y-\nabla _{Y}X-[X,Y]$

A linear connection whose torsion is zero is termed a torsion-free linear connection.

Tensoriality

The torsion map is a $(1,2)$ tensor. It is tensorial in both $X$ and $Y$. This can actually be easily checked by hand.