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Its not the summation that is wrong, but the lack of indices inside it. Below this expression on their site they define: $$\epsilon(\mathbf{u})=\frac{1}{2}([\nabla\mathbf{u}]+[\nabla\mathbf{u}]^{\mathrm{T}})$$ So $\epsilon(\mathbf{u})$ is a matrix formed from the symmetrized gradient of $\mathbf{u}$. But to get the sum from the earlier expression, we want to ...

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