Student Seminar: Classical and Quantum Integrable Systems
Student Seminar: Classical and Quantum Integrable Systems
Student Seminar: Classical and Quantum Integrable Systems
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The last two equations can be rewritten in terms of the single equation. Introduce<br />
two 2n-dimensional vectors<br />
( ) p<br />
x = , ∇H =<br />
q<br />
(<br />
∂H<br />
)<br />
∂p j<br />
∂H<br />
∂q j<br />
<strong>and</strong> 2n × 2n matrix J:<br />
J =<br />
( ) 0 −I<br />
I 0<br />
Then the Hamiltonian equations can be written in the form<br />
ẋ = J · ∇H , or J · ẋ = −∇H .<br />
In this form the Hamiltonian equations were written for the first time by Lagrange<br />
in 1808.<br />
Vector x = (x 1 , . . . , x 2n ) defines a state of a system in classical mechanics. The<br />
set of all these vectors form a phase space M = {x} of the system which in the present<br />
case is just the 2n-dimensional Euclidean space with the metric (x, y) = ∑ 2n<br />
i=1 xi y i .<br />
The matrix J serves to define the so-called Poisson brackets on the space F(M)<br />
of differentiable functions on M:<br />
{F, G}(x) = (∇F, J∇G) = J ij ∂ i F ∂ j G =<br />
n∑<br />
j=1<br />
( ∂F<br />
∂p j<br />
∂G<br />
∂q j − ∂F<br />
∂q j ∂G<br />
∂p j<br />
)<br />
.<br />
Problem. Check that the Poisson bracket satisfies the following conditions<br />
{F, G} = −{G, F } ,<br />
{F, {G, H}} + {G, {H, F }} + {H, {F, G}} = 0<br />
for arbitrary functions F, G, H.<br />
Thus, the Poisson bracket introduces on F(M) the structure of an infinitedimensional<br />
Lie algebra. The bracket also satisfies the Leibnitz rule<br />
{F, GH} = {F, G}H + G{F, H}<br />
<strong>and</strong>, therefore, it is completely determined by its values on the basis elements x i :<br />
{x j , x k } = J jk<br />
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