How to use a discrete f (the value of f are known at nodes) for solving poisson equation?

Answer 1

Try posing your question more mathematically.

Using discrete values does not generally make sense. Fields in the variational form are generally piecewise continuous functions.

It is possible to have Delta functions, but I suspect that this is not what you want.

Comments

I have an 2D matrix as f. I want to solve the poisson equation in unit square by dg method with f (matrix) as RHS.