Calc of variation - the variation of a derivative?

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    Derivative Variation
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The discussion focuses on the conditions under which the equality \(\delta \partial_j \phi = \partial_j (\delta \phi)\) holds in the context of functional variation. Participants agree that this equality is valid when the surface of the region of integration remains unchanged. The implications of this equality are significant for variational calculus and the analysis of functionals.

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pellman
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If we take the variation of a functional of some function [itex]\phi(x_1,...,x_n)[/itex] with [itex]\partial_{j}\phi[/itex] being the partial deriviative of phi with respect x_j, when is it ok to set [itex]\delta \partial_j \phi[/itex] equal to [itex]\partial_j (\delta\phi)[/itex]?
 
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Maybe this equality holds whenever we are not varying the surface of the region of integration? Can someone confirm?
 

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