- #1
pellman
- 684
- 5
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]?