# Using canonical transform to show area preserving

1. Sep 15, 2013

### bagram

1. The problem statement, all variables and given/known data
Given a certain poincare map, show that it is area preserving for all values of r

2. Relevant equations
$$\binom{x_{n+1}}{y_{n+1}}=\begin{pmatrix}e^{r} & 0 \\ 0 & e^{-r} \end{pmatrix}\begin{pmatrix}cos(\phi+I_{n}) & -sin(\phi+I_{n}) \\ sin(\phi+I_{n}) & cos(\phi+I_{n}) \end{pmatrix}\begin{pmatrix}x_{n}\\ y_{n} \end{pmatrix}$$

where
$$I_{n}=x_{n}^2+y_{n}^2$$

3. The attempt at a solution
I tried to prove that the determinant $$\binom{x_{n+1}}{y_{n+1}}=\begin{pmatrix}e^{r} & 0 \\ 0 & e^{-r} \end{pmatrix}\begin{pmatrix}cos(\phi+I_{n}) & -sin(\phi+I_{n}) \\ sin(\phi+I_{n}) & cos(\phi+I_{n}) \end{pmatrix}$$ is equal to 1, I believe that is wrong since the I in the matrix is X^2+Y^2, so I was wondering should i use the Poisson bracket method?

2. Sep 15, 2013

### TSny

The presence of $I_n$ makes the map nonlinear, so you can't just look at the determinant of the map to see if it is area preserving.

Think of the map as being in the form $$x_{n+1} = f(x_n,y_n)$$ $$y_{n+1} = g(x_n,y_n)$$
Then consider the Jacobian of this transformation.
See for example http://www.math.harvard.edu/archive/118r_spring_05/handouts/henon.pdf

3. Sep 15, 2013

### bagram

Ahhh okay, so then i just do the partial of each entry in the matrix to check if the det=1?

4. Sep 15, 2013

### TSny

Yes. It's a little tedious.