This generator computes a five-point central finite difference discretization of the two-dimensional variable-coefficient linear elliptic equation -(p u_{x})_{x} -(q u_{y})_{y} + r u_{x} + (r u)_{x} + s u_{y} + (s u)_{y} + t u = f, where p, q, r, s and t are the functions of x and y. The domain is the unit square (0,1)x(0,1), and the boundary condition are Dirichlet.

Although the code handles arbitrary p, q, r, s and t, it is set up initially with p = exp(-xy), Q = exp(xy), R = beta (x+y), S = gamma (x+y), and t = 1/(1+x+y). It is suggested to use values of beta and gamma between 0 and 250. The object is to estimate those eigenvalues with the largest real parts and to determine whether or not there are significant gaps in the spectrum.