What are some numerical methods to solve [math]\frac{\delta^2 \phi}{\delta x^2} + \frac{\delta^2 \phi}{\delta y^2} + \frac{A\delta \phi}{\delta x} + \frac{B\delta \phi}{\delta y} = C[/math] for [math]\phi[/math] where A, B and C are constants?
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