First variation
= First variation
{wiki}
The first variation of a <functional> $L$ at $y$ in the direction $\eta$ is
$$
\delta L[y;\eta]
=\left.\frac d{d\varepsilon}L[y+\varepsilon\eta]\right|_{\varepsilon=0}.
$$
= First variation
{wiki}
The first variation of a <functional> $L$ at $y$ in the direction $\eta$ is
$$
\delta L[y;\eta]
=\left.\frac d{d\varepsilon}L[y+\varepsilon\eta]\right|_{\varepsilon=0}.
$$