The curves PQ and RS lie in the closed interval (a,b) and. they are given as a set of ordinates.The problem is to solve in the region PQRS. The subroutine contructs a grid in the region PQRS, numbers the grid points alonG the grid lines and sets up the Laplace difference equation at each grid point using 3-point finite difference fcrmulae for the derivatives.The set of these difference equations is banded and is solved by Liebmann's iterative technique. Successive over-relaxation is employed to speed up the convergence of the iteration if invoked the user and if he provides the SOR factor. The subroutine provijes good starting approximations for the potentials at each grid point by assuming linear variation along each vertical grid lineo In most cases this is found to result in fast convergence and consequently considerable saving of computer time
