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[itex]\partial_\mu j^\mu = \partial_0 j^0 + \vec{\nabla} \cdot \vec{j}[/itex]

surely when you take a dot product of four vectors you get a subtraction as in

[itex]a^\mu b_\mu = a^0 b_0 - \vec{a} \cdot \vec{b}[/itex]

Maybe I'm forgetting something