# Tangent vector to a curve (Differential geometry/Lie theory).

• B L
Since f is a scalar, df/dc is a real number (which is what I had) but dc/ds is a matrix (which is what you had). In summary, the problem asks to find the tangent vector to a curve in SO(3) at the identity, using the definition of a tangent vector as a differential operator. This involves finding the matrix of the tangent vector by differentiating the curve with respect to its parameter s. The solution is achieved by taking the derivative of the curve and evaluating it at s=0, resulting in a matrix that corresponds to the tangent vector at the identity.

## Homework Statement

Let $c(s) = \left( \begin{array}{ccc} \cos(s) & -\sin(s) & 0 \\ \sin(s) & \cos(s) & 0 \\ 0 & 0 & 1 \end{array} \right)$ be a curve in SO(3). Find the tangent vector to this curve at $I_3$.

## Homework Equations

Presumably, the definition of a tangent vector as a differential operator would be useful here:
If X is the tangent vector to $c(s)$ at $I_3$ (i.e. $s = 0$), then for functions $f: SO(3) \rightarrow \mathbb{R}$,
$X\left[ f \right] = \frac{df\left(c\left(s\right)\right)}{ds}|_{s=0}$

## The Attempt at a Solution

The fact that this problem is found in the section of the textbook (Nakahara, Geometry, Topology and Physics) dealing with Lie groups and Lie algebras, together with the fact that the problem asks for the tangent vector to c(s) at the identity leads me to think that we are supposed to use the Lie algebra in some way. However, I can't see how one would do that, so I've tried proceeding naively from the definition:
$X\left[ f \right] = \frac{df\left(c\left(s\right)\right)}{ds}|_{s=0}$
$= \frac{df}{dc} \frac{dc}{ds}|_{s=0}$
$= \frac{df}{dc} \left( \begin{array}{ccc} 0 & -1 & 0 \\ 1 & 0 & 0 \\ 0 & 0 & 0 \end{array} \right)$

I have no idea how to proceed from here.

Thanks for any and all help!

EDIT: I quickly realized how trivial this is.

Last edited:
You are already done. Sort of. X[f] would indeed be df(c(s))/ds at s=0. But that would be a real number. Writing something like df/dc is a little misguided. That would be the derivative of a real function with respect to a matrix. It would be tough to clearly define that. I think all they really want is the matrix dc/ds at s=0. And that you already have.

Am I correct in thinking that the corresponding tangent vector (i.e. element of the Lie algebra $so(3) \cong T_eSO(3)$) would be $-\frac{\partial}{\partial x^{12}} +\frac{\partial}{\partial x^{21}}$?

Thanks for the help.

B L said:
Am I correct in thinking that the corresponding tangent vector (i.e. element of the Lie algebra $so(3) \cong T_eSO(3)$) would be $-\frac{\partial}{\partial x^{12}} +\frac{\partial}{\partial x^{21}}$?

Thanks for the help.

Yes, you could write it that way. That would tell you how X acts on a function f:SO(3)->R.

I know why I went wrong - what I didn't realize is that by differentiation wrt c, I really meant partial differentiation wrt c^ij (and differentiation of c^ij wrt s) As soon as you do that the problem is trivial.