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Calc of variation - the variation of a derivative?

  1. May 15, 2013 #1
    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]?
  2. jcsd
  3. May 15, 2013 #2
    Maybe this equality holds whenever we are not varying the surface of the region of integration? Can someone confirm?
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