On exit:
PERTRB=0, unless a solution to Poisson's equation
λ=0 is required with a combination of periodic or derivative boundary conditions (
LBDCND,
MBDCND and
NBDCND=0 or
3). In this case a solution may not exist.
PERTRB is a constant, calculated and subtracted from the array
F, which ensures that a solution exists. D03FAF then computes this solution, which is a least squares solution to the original approximation. This solution is not unique and is unnormalized. The value of
PERTRB should be small compared to the right-hand side
F, otherwise a solution has been obtained to an essentially different problem. This comparison should always be made to ensure that a meaningful solution has been obtained.