# Torsion and submanifolds [closed]

EDIT: Let me modify the question then: for what submanifolds $N$ does the torsion $T$ preserve tangent vectors to $N$?

If $\nabla$ is a connection on a manifold $M$, then torsion is defined to be the map $$T(X,Y)=\nabla_XY-\nabla_YX-[X,Y]$$

where $X$ and $Y$ are vector fields on $M$. It can be shown that $T$ is a $2 \choose 1$ tensor on $M$; that is, for all $p\in M$, $$T:T_pM\times T_pM\longrightarrow T_pM$$

where $T_pM$ is the tangent space to $M$ at $p$.

Suppose $N\subset M$ is a submanifold of $M$. Is it the case that $T$ preserves tangent vectors to $N$? That is, does $$T:T_pN\times T_pN\longrightarrow T_pN$$

for $p\in N$?

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## closed as off-topic by Ryan Budney, Daniel Moskovich, Willie Wong, Todd Trimble♦, Peter MichorJul 18 '13 at 13:23

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There's no reason for this to be true, and I'm sure that, for the generic submanifold $N$ of dimension 2 or more (if the dimension of $M$ is at least $3$ and the torsion doesn't satisfy some very special identity) then it won't be true. – Robert Bryant Jul 18 '13 at 0:00
@Robert: Is it the case that $\nabla_XY-\nabla_YX$ lies in the tangent space of $N$? – Oliver Jones Jul 18 '13 at 1:12
Pick a point $p$ in $M$ and any subspace $S\subseteq T_pM$. There is a submanifold $N$ of $M$ such that $p\in N$ and $T_pN=S$. If what you want were true, then the torsion tensor would preserve all subspaces of $T_pM$! – Mariano Suárez-Alvarez Jul 18 '13 at 1:14
@Oliver Jones: Since the Lie bracket respects vector fields which are tangent to $N$, and the torsion does not (in general), so also $\nabla_XY-\nabla_YX$ does not. – Peter Michor Jul 18 '13 at 6:34
Oliver, you shouldn't accept an answer and then change the question. – Ramiro de la Vega Jul 18 '13 at 22:56

A simple example in $M=\mathbb R^3$: Let $N=0\times \mathbb R^2$ and put $$\nabla_XY = dY(X) + \begin{pmatrix}X^T\,A^1\,Y \\ X^T\,A^2\,Y \\ X^T\,A^3\,Y\end{pmatrix}, \quad A=\begin{pmatrix} a^i_{11} & a^i_{12} & a^i_{13}\\ \dots \\a^i_{31} & a^i_{32} & a^i_{33} \end{pmatrix}, \quad a^2_{kl} = a^3_{kl} = 0 \text{ for } 2\le k,l\le 3.$$ Then $Tor^i_{kl} = A^i_{kl}-A^i_{lk}$ maps $T_{(0,x,y)}(0\times \mathbb R^2)\times T_{(0,x,y)}(0\times \mathbb R^2)$ skew linearly into $T_{(0,x,y)}\mathbb R\times 0$.
Perhaps you should point out that this is not an example pertaining to the original question, but the modified one in the comments. After all, the torsion will, by definition, vanish when restricted to any $1$-dimensional subspace. (The modified question doesn't really make much sense anyway, since $\nabla_XY-\nabla_YX$ isn't even a tensorial expression.) – Robert Bryant Jul 18 '13 at 12:15
@Robert: I meant for the expression $\nabla_XY-\nabla_YX$ to be evaluated at a point in the submaniofld. – Oliver Jones Jul 18 '13 at 21:29