Generating function (physics)

Generating functions which arise in Hamiltonian mechanics are quite different from generating functions in mathematics. In physics, a generating function acts as a bridge between two sets of canonical variables when performing a canonical transformation.

Details

There are four basic generating functions, summarized by the following table:

Generating Function	Its Derivatives
$F=F_{1}(q,Q,t)\,\!$	$p=~~{\frac {\partial F_{1}}{\partial q}}\,\!$ and $P=-{\frac {\partial F_{1}}{\partial Q}}\,\!$
$F=F_{2}(q,P,t)-QP\,\!$	$p=~~{\frac {\partial F_{2}}{\partial q}}\,\!$ and $Q=~~{\frac {\partial F_{2}}{\partial P}}\,\!$
$F=F_{3}(p,Q,t)+qp\,\!$	$q=-{\frac {\partial F_{3}}{\partial p}}\,\!$ and $P=-{\frac {\partial F_{3}}{\partial Q}}\,\!$
$F=F_{4}(p,P,t)+qp-QP\,\!$	$q=-{\frac {\partial F_{4}}{\partial p}}\,\!$ and $Q=~~{\frac {\partial F_{4}}{\partial P}}\,\!$

Example

Sometimes one can turn a given Hamiltonian into one that looks a bit more like the harmonic oscillator Hamiltonian, which is

$H=aP^{2}+bQ^{2}\,$

So, as an example, if one were given the Hamiltonian

$H={\frac {1}{2q^{2}}}+{\frac {p^{2}q^{4}}{2}}\quad \quad \quad \quad (1)\,$

(where p is generalized momentum, and q is the generalized coordinate.)

a good canonical transformation to choose would be

$P=pq^{2}{\text{ and }}Q={\frac {-1}{q}}\quad \quad \quad \quad (2)\,$

This turns the Hamiltonian into

$H={\frac {Q^{2}}{2}}+{\frac {P^{2}}{2}}\,$

which is in the form of the harmonic oscillator Hamiltonian.

The generating function, F, for this transformation is of the 3rd kind,

$F=F_{3}(p,Q).\,$

To find F explicitly, use the equation for its derivative (from the table above),

$P=-{\frac {\partial F_{3}}{\partial Q}}\,$

and substitute the expression for P from equation (2), expressed in terms of p and Q:

${\frac {p}{Q^{2}}}=-{\frac {\partial F_{3}}{\partial Q}}.\,$

Integrating this with respect to Q results in an equation for the generating function of the transformation given by equation (2):

$F_{3}(p,Q)={\frac {p}{Q}}\,$

To confirm that this is the correct generating function, verify that it matches (2):

$q=-{\frac {\partial F_{3}}{\partial p}}={\frac {-1}{Q}}\,$

See also

• Hamilton-Jacobi equation
• Poisson bracket

References

• Goldstein, Herbert (2002). Classical Mechanics. Addison Wesley. ISBN 978-0-201-65702-9.