# Finding Hamiltonian as Legendre transform on SO(3)

1. Feb 19, 2012

### conquest

1. The problem statement, all variables and given/known data
We need to find the Hamiltonian that corresponds to a given Lagrangian by finding the Legendre transform. The system is a rigid body pinned down in some point. This means the motion is described essentialy by SO(3). So the Lagrangian is given in terms of these matrices and for instance a path that solves it is a path in SO(3).

This means we need to take the derivative with respect to the velocity (where we restrict the Lagrangian to a fiber of the vector bundle TSO(3)) of a curve in SO(3), check that all the second derivatives are positive so that it is convex and then find the canonical momenta for the problem to find the Hamiltonian.

2. Relevant equations
$L(g',g)=-1/2Tr(Ag'gg'g)$ where A is a constant (3x3) matrix g' is the velocity matrix and g is the base point matrix. Tr denotes taking the ordinary trace.

3. The attempt at a solution

I tried to find a suitable basis for $T_gSO(3)$. I think in the Identity element it is equal to the space of 3x3 skew symmetric matrices. So I tried a basis of three elements where one $(e_1)$ had a 1 as 21 entry and a -1 as 12 entry, one $(e_2)$ had a 1 as 31 entry and a -1 as 13 entry and the last $(e_3)$ had a 1 as 32 entry and a -1 as 23 entry. Then differentiation with respect to this basis got me
$-1/2(Tr(Ae_jge_ig)+Tr(Ae_ige_jg)$

for the derivative with respect to the ith and then the jth entry. But This does not seem to be positive definite. Als I don't see quite how to get canonical momenta out of this.