The inhomogeneous Helmholtz equation is the equation ∇ ∇ 2 A ( x ) + k 2 A ( x ) = − − f ( x ) in R n , where ƒ : R → C is a function with compact support, and n = 1, 2, 3. This equation is very similar to the screened Poisson equation, and would be identical if the plus sign (in front of the k term) were switched to a minus sign.