Tangent bundle is always orientable

• Feb 4th 2010, 06:37 PM
AtticusRyan
Tangent bundle is always orientable
Hello all, trying to prove that the tangent bundle TM of a smooth n-manifold M is always orientable. To make it easier to write down Im doing n=2.

Let the differentiable structure on M be given by $\{U_\alpha,\phi_\alpha\}$. This then induces a differentiable structure on TM given by $\{U_\alpha\times\mathbb{R}^2,\psi_\alpha\}$ where we define $\psi_\alpha:U_\alpha\times \mathbb{R}^2\to TM,\;\; (u_\alpha,v_\alpha,x,y)\mapsto \left(\phi(u_\alpha,v_\alpha),x\frac{\partial}{\pa rtial u_\alpha}+y\frac{\partial}{\partial v_\alpha}\right)$ i.e. we take as the coordinates of a point (p,w) in TM the coordinates of the point p in U, plus the coordinates of w in the basis $\{\partial/\partial u_\alpha,\partial/\partial v_\alpha\}.$

Once you've proved that TM is a smooth 4-manifold (which isn't hard) you then have to show that for every change of coordinate function the determinant of its jacobian matrix is positive, i.e.
$\det(d(\psi_\beta^{-1}\circ\psi_\alpha))>0$ for all $\alpha,\beta$.

To write out the 4x4 matrix $d(\psi^{-1}_\beta\circ\psi_\alpha)$ is where I've come unstuck. The first two columns will be the partial derivatives of the 4 component functions of $\psi^{-1}_\beta\circ\psi_\alpha$ with respect to $u_\alpha,v_\alpha$, i.e. the top-left 2x2 block will just be $d(\phi_\beta^{-1}\circ\phi_\alpha)$ and the bottom-left will be zero because $d^2=0$.

I don't know how to work out the two 2x2 blocks on the right hand side of the matrix. Basically I don't know what to take the partial derivative with respect to, and just how to write it down. Hopefully the top-right will be zero and the bottom-right will be $d(\phi_\beta^{-1}\circ\phi_\alpha)$ as well, so then we have as a determinant the square of a real number which will always be positive.

Any help anyone could give me would be great!

Cheers,

Atticus
• Feb 8th 2010, 05:46 PM
AtticusRyan
Giving this a bump in case anyone has any ideas. Getting some help on it today though.

Atticus