# Laplacian in Polar Coordinates

July 9 2016

## Verify Laplace's Equation in Polar Coordinates

We verify the following equation is correct by expanding u_(r r), u_r, and u_(theta theta) in the right side of the equation.

u_(x x) + u_(y y) = u_(r r) + (1 / r) u_r + (1 / r^2) u_(theta theta)

## Derive Laplace's Equation in Polar Coordinates

Laplacian for general coordinates is defined with covariant derivative in Riemann geometry.

Delta = g^(i j) grad_i grad_j

This formula is transformed as follow.

Delta = (1 / sqrt(g)) del_i sqrt(g) g^(i j) del_j

In the following program, we calculate Laplacian using this formula.