Laplace operator
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In vector calculus, the Laplace operator or Laplacian is a differential operator equal to the sum of all the second partial derivatives of a dependent variable.

This corresponds to div(grad φ), hence the use of the symbol del to represent it:

It is also written as Δ.

In two-dimensional Cartesian coordinates, the Laplacian is:

In three:

In Spherical coordiantes:

It's linear proprietly can be easly exploited as follows:

It occurs, for example, in Laplace's equation.

The original article can be found at: http://en.wikipedia.org/wiki/Laplace_operator.
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