Definition (to be confirmed)
Given a functional $J$ defined on a space of functions $\Omega$ (Banach space?), an extrema is a function $f\in\Omega$ such that there exist a neighbourhood $U$ of $f$ such that
$$ J(f)\geq J(g) $$for every $g\in U$.
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Author of the notes: Antonio J. Pan-Collantes
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