It is a manifold $M$ together with an operation $\{-,-\}$ in $C^{\infty}(M)$ satisfying the properties:
The operation is called Poisson bracket. Every symplectic manifold is a Poisson manifold but the converse is not true (@olver86 page 391). The algebra of functions on a Poisson manifold constitutes a Poisson algebra.
Example
An arbitrary $\mathbb R^N\equiv (p_i,q_i,z_j)$, with $i=1,\ldots,m$ and $j=1,\ldots,l$ ($N=2m+l$), can be seen as a Poisson manifold, we can define a Poisson bracket on the space of smooth functions on $\mathbb R^N$ as follows:
For any two smooth functions $f,g:\mathbb R^N\rightarrow \mathbb R$, we define their Poisson bracket ${f,g}$ as:
$$ \{f,g\} = \sum_{i=1}^m \left(\frac{\partial f}{\partial q_i} \frac{\partial g}{\partial p_i} - \frac{\partial f}{\partial p_i} \frac{\partial g}{\partial q_i}\right) $$The Poisson bracket of functions which only depends on $z_j$ with any other function is zero. These are called distinguished functions or Casimir functions, and can be thought as "observables" which do not provide "evolution parameter", since its associated Hamiltonian vector field is null.
Moreover, they remain constant along the evolution parameter of any Hamiltonian vector fields. They are a kind of "generalized constants". This reminds me the centralizer of an algebra, in case that the bracket is given by the commutator of a product...
$\blacksquare$
For every $f$ we have that $\{-,f\}$ is a differential operator, so it is a vector field. We call it the Hamiltonian vector field associated with $f$. It coincides with the notion of Hamiltonian vector field for symplectic manifolds. Also, see Poisson bracket#Relation with Lie bracket.
The Poisson bracket can be seen in local coordinates as a skew-symmetric matrix. If we require that the rank of this matrix is everywhere the same as the dimension of the manifold, we recover the notion of symplectic manifold.
The "morphisms" between Poisson manifolds are called Poisson maps.
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Author of the notes: Antonio J. Pan-Collantes
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