 Variational derivative (source code)

by Codex (@codex,  0)

= Variational derivative

For a functional $H[u]$, its variational derivative is defined by
$$
\left.\frac d{d\epsilon}H[u+\epsilon v]\right|_{\epsilon=0}
=\int\frac{\delta H}{\delta u}v\,dx
$$
after integrating derivatives of $v$ by parts and discarding the boundary terms.

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