Action angles result from a type-2 canonical transformation where the generating function is Hamilton's characteristic function ${\displaystyle W(\mathbf {q} )}$  (not Hamilton's principal function ${\displaystyle S}$ ). Since the original Hamiltonian does not depend on time explicitly, the new Hamiltonian ${\displaystyle K(\mathbf {w} ,\mathbf {J} )}$  is merely the old Hamiltonian ${\displaystyle H(\mathbf {q} ,\mathbf {p} )}$  expressed in terms of the new canonical coordinates, which we denote as ${\displaystyle \mathbf {w} }$  (the action angles, which are the generalized coordinates) and their new generalized momenta ${\displaystyle \mathbf {J} }$ . We will not need to solve here for the generating function ${\displaystyle W}$  itself; instead, we will use it merely as a vehicle for relating the new and old canonical coordinates.

Rather than defining the action angles ${\displaystyle \mathbf {w} }$  directly, we define instead their generalized momenta, which resemble the classical action for each original generalized coordinate

${\displaystyle J_{k}\equiv \oint p_{k}\,\mathrm {d} q_{k}}$ 

where the integration path is implicitly given by the constant energy function ${\displaystyle E=E(q_{k},p_{k})}$ . Since the actual motion is not involved in this integration, these generalized momenta ${\displaystyle J_{k}}$  are constants of the motion, implying that the transformed Hamiltonian ${\displaystyle K}$  does not depend on the conjugate generalized coordinates ${\displaystyle w_{k}}$ 

${\displaystyle {\frac {\mathrm {d} }{\mathrm {d} t}}J_{k}=0={\frac {\partial K}{\partial w_{k}}}}$ 

where the ${\displaystyle w_{k}}$  are given by the typical equation for a type-2 canonical transformation

${\displaystyle w_{k}\equiv {\frac {\partial W}{\partial J_{k}}}}$ 

Hence, the new Hamiltonian ${\displaystyle K=K(\mathbf {J} )}$  depends only on the new generalized momenta ${\displaystyle \mathbf {J} }$ .

The dynamics of the action angles is given by Hamilton's equations

${\displaystyle {\frac {\mathrm {d} }{\mathrm {d} t}}w_{k}={\frac {\partial K}{\partial J_{k}}}\equiv \nu _{k}(\mathbf {J} )}$ 

The right-hand side is a constant of the motion (since all the ${\displaystyle J}$ 's are). Hence, the solution is given by

${\displaystyle w_{k}=\nu _{k}(\mathbf {J} )t+\beta _{k}}$ 

where ${\displaystyle \beta _{k}}$  is a constant of integration. In particular, if the original generalized coordinate undergoes an oscillation or rotation of period ${\displaystyle T}$ , the corresponding action angle ${\displaystyle w_{k}}$  changes by ${\displaystyle \Delta w_{k}=\nu _{k}(\mathbf {J} )T}$ .

These ${\displaystyle \nu _{k}(\mathbf {J} )}$  are the frequencies of oscillation/rotation for the original generalized coordinates ${\displaystyle q_{k}}$ . To show this, we integrate the net change in the action angle ${\displaystyle w_{k}}$  over exactly one complete variation (i.e., oscillation or rotation) of its generalized coordinates ${\displaystyle q_{k}}$ 

${\displaystyle \Delta w_{k}\equiv \oint {\frac {\partial w_{k}}{\partial q_{k}}}\,\mathrm {d} q_{k}=\oint {\frac {\partial ^{2}W}{\partial J_{k}\,\partial q_{k}}}\,\mathrm {d} q_{k}={\frac {\mathrm {d} }{\mathrm {d} J_{k}}}\oint {\frac {\partial W}{\partial q_{k}}}\,\mathrm {d} q_{k}={\frac {\mathrm {d} }{\mathrm {d} J_{k}}}\oint p_{k}\,\mathrm {d} q_{k}={\frac {\mathrm {d} J_{k}}{\mathrm {d} J_{k}}}=1}$ 

Setting the two expressions for ${\displaystyle \Delta w_{k}}$  equal, we obtain the desired equation

${\displaystyle \nu _{k}(\mathbf {J} )={\frac {1}{T}}}$ 

The action angles ${\displaystyle \mathbf {w} }$  are an independent set of generalized coordinates. Thus, in the general case, each original generalized coordinate ${\displaystyle q_{k}}$  can be expressed as a Fourier series in all the action angles

${\displaystyle q_{k}=\sum _{s_{1}=-\infty }^{\infty }\sum _{s_{2}=-\infty }^{\infty }\cdots \sum _{s_{N}=-\infty }^{\infty }A_{s_{1},s_{2},\ldots ,s_{N}}^{k}e^{i2\pi s_{1}w_{1}}e^{i2\pi s_{2}w_{2}}\cdots e^{i2\pi s_{N}w_{N}}}$ 

where ${\displaystyle A_{s_{1},s_{2},\ldots ,s_{N}}^{k}}$  is the Fourier series coefficient. In most practical cases, however, an original generalized coordinate ${\displaystyle q_{k}}$  will be expressible as a Fourier series in only its own action angles ${\displaystyle w_{k}}$ 

${\displaystyle q_{k}=\sum _{s_{k}=-\infty }^{\infty }A_{s_{k}}^{k}e^{i2\pi s_{k}w_{k}}}$ 

The general procedure has three steps:

1. Calculate the new generalized momenta ${\displaystyle J_{k}}$ 
2. Express the original Hamiltonian entirely in terms of these variables.
3. Take the derivatives of the Hamiltonian with respect to these momenta to obtain the frequencies ${\displaystyle \nu _{k}}$ 

In some cases, the frequencies of two different generalized coordinates are identical, i.e., ${\displaystyle \nu _{k}=\nu _{l}}$  for ${\displaystyle k\neq l}$ . In such cases, the motion is called degenerate.

Degenerate motion signals that there are additional general conserved quantities; for example, the frequencies of the Kepler problem are degenerate, corresponding to the conservation of the Laplace–Runge–Lenz vector.

Degenerate motion also signals that the Hamilton–Jacobi equations are completely separable in more than one coordinate system; for example, the Kepler problem is completely separable in both spherical coordinates and parabolic coordinates.

• Integrable system
• Tautological one-form
• Superintegrable Hamiltonian system
• Einstein–Brillouin–Keller method

• Landau, L. D.; Lifshitz, E. M. (1976), Mechanics (3rd ed.), Pergamon Press, ISBN 0-08-021022-8 (hardcover) and ISBN 0-08-029141-4 (softcover)
• Goldstein, H. (1980), Classical Mechanics (2nd ed.), Addison-Wesley, ISBN 0-201-02918-9
• Sardanashvily, G. (2015), Handbook of Integrable Hamiltonian Systems, URSS, ISBN 978-5-396-00687-4
• Previato, Emma (2003), Dictionary of Applied Math for Engineers and Scientists, CRC Press, Bibcode:2003dame.book.....P, ISBN 978-1-58488-053-0