Euler's Equation for Vanishing Variation in Canonical Variables

From ProofWiki
Jump to navigation Jump to search


This article needs to be linked to other articles.
You can help $\mathsf{Pr} \infty \mathsf{fWiki}$ by adding these links.
To discuss this page in more detail, feel free to use the talk page.
When this work has been completed, you may remove this instance of {{MissingLinks}} from the code.


Theorem


This page has been identified as a candidate for refactoring of advanced complexity.
In particular: There are a number of pages linking here with the presentation of the link set as "momenta". This is going to need a definition of its own, but it is not clear what that is from looking at this page.
Until this has been finished, please leave {{Refactor}} in the code.

New contributors: Refactoring is a task which is expected to be undertaken by experienced editors only.

Because of the underlying complexity of the work needed, it is recommended that you do not embark on a refactoring task until you have become familiar with the structural nature of pages of $\mathsf{Pr} \infty \mathsf{fWiki}$.
To discuss this page in more detail, feel free to use the talk page.
When this work has been completed, you may remove this instance of {{Refactor}} from the code.


Consider the following system of differential equations:

$(1): \quad \begin {cases} F_{y_i} - \dfrac \d {\d x} F_{y_i'} = 0 \\ \dfrac {\d {y_i} } {\d x} = y_i'\end{cases}$

where $i \in \set {1, \ldots, n}$.


Let the coordinates $\tuple {x, \family {y_i}_{1 \mathop \le i \mathop \le n}, \family {y_i'}_{1 \mathop \le i \mathop \le n}, F}$ be transformed to canonical variables:

$\tuple {x, \family {y_i}_{1 \mathop \le i \mathop \le n}, \family {p_i}_{1 \mathop \le i \mathop \le n}, H}$


Then the system $(1)$ is transformed into:

$\begin {cases} \dfrac {\d y_i} {\d x} = \dfrac {\partial H} {\partial p_i} \\ \dfrac {\d p_i} {\d x} = -\dfrac {\partial H} {\partial y_i} \end {cases}$


Proof

Find the full differential of Hamiltonian:

\(\ds \rd H\) \(=\) \(\ds -\rd F + \rd {\sum_{i \mathop = 1}^n y_i' p_i}\) Definition of Hamiltonian
\(\ds \) \(=\) \(\ds -\rd F + \sum_{i \mathop = 1}^n \paren {\rd {y_i'} p_i + y_i' \rd p_i}\) Full differential of a product
\(\ds \) \(=\) \(\ds -\frac {\partial F} {\partial x} \rd x - \sum_{i \mathop = 1}^n \frac {\partial F} {\partial y_i} \rd y_i - \sum_{i \mathop = 1}^n \frac {\partial F} {\partial y_i'} \rd y_i' + {\sum_{i \mathop = 1}^n \rd y_i' p_i} + \sum_{i \mathop = 1}^n y_i'\rd p_i\) Definition of Differential of Real-Valued Function
\(\ds \) \(=\) \(\ds -\frac {\partial F} {\partial x} \rd x - \sum_{i \mathop = 1}^n \frac {\partial F} {\partial y_i} \rd y_i - \sum_{i \mathop = 1}^n p_i \rd y_i' + {\sum_{i \mathop = 1}^n \rd y_i' p_i} + \sum_{i \mathop = 1}^n y_i'\rd p_i\) Definition of Canonical Variable: $p_i$
\(\ds \) \(=\) \(\ds -\frac {\partial F} {\partial x} \rd x - \sum_{i \mathop = 1}^n \frac {\partial F} {\partial y_i} \rd y_i + \sum_{i \mathop = 1}^n y_i' \rd p_i\) Terms with $p_i \rd y_i'$ cancel
\(\ds \) \(=\) \(\ds \frac {\partial H} {\partial x} \rd x + \sum_{i \mathop = 1}^n \frac {\partial H} {\partial y_i} \rd y_i + \sum_{i \mathop = 1}^n \frac {\partial H} {\partial p_i} \rd p_i\) Definition of Differential of Real-Valued Function

By equating coefficients of differentials in last two equations we find that:

$\dfrac {\partial H} {\partial x} = -\dfrac {\partial F} {\partial x}$
$\dfrac {\partial H} {\partial y_i} = -\dfrac {\partial F} {\partial y_i}$
$\dfrac {\partial H} {\partial p_i} = y_i'$

From the third identity it follows that:

$\paren {\dfrac {\d y_i} {\d x} = y_i} \implies \paren {\dfrac {\d y_i} {\d x} = \dfrac {\partial H} {\partial p_i} }$

while the second identity together with the definition of $p_i$ assures that:

$\paren {\dfrac {\partial F} {\partial y_i} - \dfrac \d {\d x} \dfrac {\partial F} {\partial y_i} = 0} \implies \paren {\dfrac {\d p_i} {\d x} = -\dfrac {\partial H} {\partial y_i} }$

$\blacksquare$


Source of Name

This entry was named for Leonhard Paul Euler.


Sources

Retrieved from "https://proofwiki.org/w/index.php?title=Euler%27s_Equation_for_Vanishing_Variation_in_Canonical_Variables&oldid=564633"