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In mathematics and physics, a Hamiltonian vector field on a symplectic manifold is a vector field, defined for any energy function or Hamiltonian. Named after the physicist and mathematician Sir William Rowan Hamilton, a Hamiltonian vector field is a geometric manifestation of Hamilton's equations in classical mechanics. The integral curves of a Hamiltonian vector field represent solutions to the equations of motion in the Hamiltonian form. The diffeomorphisms of a symplectic manifold arising from the flow of a Hamiltonian vector field are known as canonical transformations in physics and (Hamiltonian) symplectomorphisms in mathematics.

Hamiltonian vector fields can be defined more generally on an arbitrary Poisson manifold. The Lie bracket of two Hamiltonian vector fields corresponding to functions f and g on the manifold is itself a Hamiltonian vector field, with the Hamiltonian given by the Poisson bracket of f and g.

Definition [link]

Suppose that (M,ω) is a symplectic manifold. Since the symplectic form ω is nondegenerate, it sets up a fiberwise-linear isomorphism

Failed to parse (Missing texvc executable; please see math/README to configure.): \omega:TM\to T^*M,

between the tangent bundle TM and the cotangent bundle T*M, with the inverse

Failed to parse (Missing texvc executable; please see math/README to configure.): \Omega:T^*M\to TM, \quad \Omega=\omega^{-1}.

Therefore, one-forms on a symplectic manifold M may be identified with vector fields and every differentiable function H: M → R determines a unique vector field X_H, called the Hamiltonian vector field with the Hamiltonian H, by requiring that for every vector field Y on M, the identity

Failed to parse (Missing texvc executable; please see math/README to configure.): \mathrm{d}H(Y) = \omega(X_H,Y)\,

must hold.

Note: Some authors define the Hamiltonian vector field with the opposite sign. One has to be mindful of varying conventions in physical and mathematical literature.

Examples [link]

Suppose that M is a 2n-dimensional symplectic manifold. Then locally, one may choose canonical coordinates (q¹, ..., qⁿ, p₁, ..., p_n) on M, in which the symplectic form is expressed as

Failed to parse (Missing texvc executable; please see math/README to configure.): \omega=\sum_i \mathrm{d}q^i \wedge \mathrm{d}p_i,

where d denotes the exterior derivative and ∧ denotes the exterior product. Then the Hamiltonian vector field with Hamiltonian H takes the form

Failed to parse (Missing texvc executable; please see math/README to configure.): \Chi_H=\left( \frac{\partial H}{\partial p_i}, - \frac{\partial H}{\partial q^i} \right) = \Omega\,\mathrm{d}H,

where Ω is a 2n by 2n square matrix

Failed to parse (Missing texvc executable; please see math/README to configure.): \Omega = \begin{bmatrix} 0 & I_n \\ -I_n & 0 \\ \end{bmatrix},

and

Failed to parse (Missing texvc executable; please see math/README to configure.): \mathrm{d}H=\begin{bmatrix} \frac{\partial H}{\partial q^i} \\ \frac{\partial H}{\partial p_i} \end{bmatrix}.

Suppose that M = R²ⁿ is the 2n-dimensional symplectic vector space with (global) canonical coordinates.

• If H = p_i then Failed to parse (Missing texvc executable; please see math/README to configure.): X_H=\partial/\partial q^i;

• if H = qⁱ then Failed to parse (Missing texvc executable; please see math/README to configure.): X_H=-\partial/\partial p^i;

• if Failed to parse (Missing texvc executable; please see math/README to configure.): H=1/2\sum (p_i)^2

then  Failed to parse (Missing texvc executable; please see math/README to configure.): X_H=\sum p_i\partial/\partial q^i; 

• if Failed to parse (Missing texvc executable; please see math/README to configure.): H=1/2\sum a_{ij} q^i q^j, a_{ij}=a_{ji}

then  Failed to parse (Missing texvc executable; please see math/README to configure.): X_H=-\sum a_{ij} q_i\partial/\partial p^j. 

Properties [link]

• The assignment f ↦ X_f is linear, so that the sum of two Hamiltonian functions transforms into the sum of the corresponding Hamiltonian vector fields.

• Suppose that (q¹, ..., qⁿ, p₁, ..., p_n) are canonical coordinates on M (see above). Then a curve γ(t)=(q(t),p(t)) is an integral curve of the Hamiltonian vector field X_H if and only if it is a solution of the Hamilton's equations:

Failed to parse (Missing texvc executable; please see math/README to configure.): \dot{q}^i = \frac {\partial H}{\partial p_i}

Failed to parse (Missing texvc executable; please see math/README to configure.): \dot{p}_i = - \frac {\partial H}{\partial q^i}.

• The Hamiltonian H is constant along the integral curves, that is, H(γ(t)) is actually independent of t. This property corresponds to the conservation of energy in Hamiltonian mechanics.

• More generally, if two functions F and H have a zero Poisson bracket (cf. below), then F is constant along the integral curves of H, and similarly, H is constant along the integral curves of F. This fact is the abstract mathematical principle behind Noether's theorem.

• Symplectic form ω is preserved by Hamiltonian flow; or equivalently, Lie derivative Failed to parse (Missing texvc executable; please see math/README to configure.): \mathcal{L}_{X_H} \omega= 0

Poisson bracket [link]

The notion of a Hamiltonian vector field leads to a skew-symmetric, bilinear operation on the differentiable functions on a symplectic manifold M, the Poisson bracket, defined by the formula

Failed to parse (Missing texvc executable; please see math/README to configure.): \{f,g\} = \omega(X_f,X_g)= df(X_g) = \mathcal{L}_{X_g} f

where Failed to parse (Missing texvc executable; please see math/README to configure.): \mathcal{L}_X

denotes the Lie derivative along a vector field X. Moreover, one can check that the following identity holds:

Failed to parse (Missing texvc executable; please see math/README to configure.): X_{\{f,g\}}=-[X_f,X_g],

where the right hand side represents the Lie bracket of the Hamiltonian vector fields with Hamiltonians f and g. As a consequence, the Poisson bracket satisfies the Jacobi identity

Failed to parse (Missing texvc executable; please see math/README to configure.): \{\{f,g\},h\}+\{\{g,h\},f\}+\{\{h,f\},g\}=0,

which means that the vector space of differentiable functions on M, endowed with the Poisson bracket, has the structure of a Lie algebra over R, and the assignment f ↦ X_f is a Lie algebra homomorphism, whose kernel consists of the locally constant functions (constant functions if M is connected).

References [link]

• Abraham, Ralph; Marsden, Jerrold E. (1978). Foundations of Mechanics. London: Benjamin-Cummings. ISBN [[Special:BookSources/0-8053-1012-X|0-8053-1012-X]]. See section 3.2.
• Arnol'd, V.I. (1997). Mathematical Methods of Classical Mechanics. Berlin etc: Springer. ISBN 0-387-96890-3. 
• Frankel, Theodore (1997). The Geometry of Physics. Cambridge: Cambridge University Press. ISBN 0-521-38753-1. 
• McDuff, Dusa; Salamon, D. (1998). Introduction to Symplectic Topology. Oxford Mathematical Monographs. ISBN 0-19-850451-9.