# General Variation of Integral Functional/Dependent on N Functions/Canonical Variables

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## Theorem

Let $\delta J$ be a general variation of integral functional dependent on n functions.

Suppose a following coordinate transformation is done:

- $\set {x, \ldots y_i, \ldots, \ldots, y_i', \ldots, F} \to \set {x, \ldots, y_i, \ldots, \ldots p', \ldots, H}, i = \left({1, \ldots, n}\right)$

Then, in canonical variables:

- $\displaystyle \delta J = \int_{x_0}^{x_1} \sum_{i \mathop = 1}^n \paren {F_{y_i} - \dfrac {\d {p_i} } {\d x} } \map {h_i} x \rd x + \intlimits {\sum_{i \mathop = 1}^n p_i \delta y_i - H \delta x} {x \mathop = x_0} {x \mathop = x_1}$

where:

- $\delta x \rvert_{x \mathop = x_j} = \delta x_j$
- $\delta y_i \rvert_{x \mathop = x_j} = \delta_i^j$
- $j = \tuple {0, 1}$

## Proof

## Sources

- 1963: I.M. Gelfand and S.V. Fomin:
*Calculus of Variations*... (previous) ... (next): $\S 3.13$: Derivation of the Basic Formula