If $\partial_j u^i = \partial u^i / \partial x^j$ is a rank 2 tensor and represents nine different numbers, and $\partial_i u^i = \nabla \cdot \vec{u}$ contracts the upper and lower indices via a divergence and yields a scalar, what exactly does $\partial_j \partial^j u^i$ represent? (Please note that I am adopting a notation used in quantum field theory.)
2026-03-24 20:30:25.1774384225
What does $\partial_j \partial^j u^i$ represent?
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