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Not sure if this is the best place to post this question but..

Why can we expand [tex](\partial_\mu \phi)^2[/tex] in this way:

[tex](\partial_\mu \phi)^2=(\partial_\mu \phi)(\partial^\mu \phi)[/tex]

I mean [anything]^2 should equal [anything]*[anything] - why have be raised one of the indices above?

thanks.

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# Raising and Lowering Indices and expansion

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