# Non-symplectic Hamiltonian systems

I'm wondering when the phase space of a Hamiltonian system looses its symplectic structure. I think it happens when the Hamiltonian $$H$$ depends on a set of other variables $$S_1,...,S_k$$ as well as on the set of canonical variables $$q_i, p_i$$.
So, besides having $$\begin{gather} \dot{q}_i = \bigl\{ q_i, H \bigr\} = \frac{\partial H}{\partial p_i}\\ \dot{p}_i= \bigl\{ p_i, H \bigr\} = - \frac{\partial H}{\partial q_i} \end{gather}$$ we have also some $$\dot{S}_i = \bigl\{ S_i, H \bigr\}$$, which is not a Hamilton equation.
Is it right? Could anyone make this clearer?

1. Well, more generally, a Hamiltonian system $$\dot{z}^I=\{z^I,H\}$$ with a Hamiltonian function $$H:M\times \mathbb{R}\to \mathbb{R}$$ is defined on a (not necessarily invertible) Poisson manifold rather than a symplectic manifold.

2. A (not necessarily invertible) Poisson manifold might not have local canonical/Darboux coordinates, cf. OP's example.

3. An important example of a non-invertible Poisson bracket is the Dirac bracket for constrained systems.