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## Homework Statement

This is from lecture, not a homework problem per se. But I need assistance.

The problem was to write this form of a flow equation in Einstein's notation:

## Homework Equations

[itex]\frac{\partial }{ \partial x_1}(K_1 \frac{\partial h}{\partial x_1}) + \frac{\partial }{ \partial x_2}(K_2 \frac{\partial h}{\partial x_2})+ \frac{\partial }{ \partial x_3}(K_3 \frac{\partial h}{\partial x_3}) = 0 [/itex]

Where [itex]K_1, K_2, K_3[/itex] are from a diagonalized form of the K tensor.

## The Attempt at a Solution

The given solution is [itex]\frac {\partial} {\partial x_i} (K_i \frac {\partial h} { \partial x_i}) = 0 [/itex], where i = 1,..,3

Which is supposed to be related to

[itex]\frac {\partial} {\partial x_i} (K_{ij} \frac {\partial h } { \partial x_j }) = 0[/itex] when [itex] K_{ij} = 0[/itex] if i ≠ j

I'm confused because there's an index repeated twice (appears three times), which I learned is bad in a different class. Is there a better way to write this equation?