It belongs to the famous equations.
In the 3D case is
$$ \frac{\partial^2 f}{\partial x^2}+\frac{\partial^2 f}{\partial y^2}+\frac{\partial^2 f}{\partial z^2}=0. $$The operator
$$ \frac{\partial^2 }{\partial x^2}+\frac{\partial^2 }{\partial y^2}+\frac{\partial^2 }{\partial z^2} $$is called the Laplacian operator.
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Author of the notes: Antonio J. Pan-Collantes
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