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I know that for a scalar [tex]\nabla^2\phi=\nabla_a\nabla^a\phi=\nabla^a\nabla_a\phi[/tex]. However what is [tex]\nabla^2[/tex] for a tensor? For example, is [tex]\nabla^2T_a=\nabla_b\nabla^bT_a[/tex] or is it [tex]\nabla^2T_a=\nabla^b\nabla_bT_a[/tex]? Because I don't think they're the same thing.
Thanks.
Thanks.