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In mathematics, specifically in the calculus of variations, a variation of a function can be concentrated on an arbitrarily small interval, but not a single point. Accordingly, the necessary condition of extremum (functional derivative equal zero) appears in a weak formulation (variational form) integrated with an arbitrary function . The fundamental lemma of the calculus of variations is typically used to transform this weak formulation into the strong formulation (differential equation), free of the integration with arbitrary function. The proof usually exploits the possibility to choose concentrated on an interval on which keeps sign (positive or negative). Several versions of the lemma are in use. Basic versions are easy to formulate and prove. More powerful versions are used when needed. ==Basic version== :If a continuous function ''f'' on an open interval (''a'',''b'') satisfies the equality :: :for all compactly supported smooth functions ''h'' on (''a'',''b''), then ''f'' is identically zero. Here "smooth" may be interpreted as "infinitely differentiable",〔 but often is interpreted as "twice continuously differentiable" or "continuously differentiable" or even just "continuous",〔 since these weaker statements may be strong enough for a given task. "Compactly supported" means "vanishes outside () for some ''c'',''d'' such that ''a''<''c''<''d''<''b''";〔 but often a weaker statement suffices, assuming only that ''h'' (or ''h'' and a number of its derivatives) vanishes at the endpoints ''a'', ''b'';〔 in this case the closed interval () is used. 抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)』 ■ウィキペディアで「Fundamental lemma of calculus of variations」の詳細全文を読む スポンサード リンク
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