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:<math>\nabla \cdot (\varphi \varepsilon \, \nabla \varphi )=\varepsilon \, (\nabla \varphi )^2 + \varphi \, \nabla \cdot (\varepsilon \, \nabla \varphi )</math>
And noticing that the second term is zero, one can rewrite this as
:<math>\mathbf{\nabla}\cdot(\varphi\varepsilon \, \mathbf{\nabla}\varphi)= \varepsilon (\mathbf{\nabla}\varphi)^2</math>
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