% Evaluates the Schrodinger potential corresponding to conductivity
% sigma.m. Uses finite differences to implement the Laplace operator.
%
% Arguments:
% z 		evaluation points, complex.
%
% Calls to:
% sigma.m
%
% Samuli Siltanen February 2012

function ans = poten(z)

h   = 1e-6;
z0  = z;
z1  = z+h;
z2  = z+i*h;
z3  = z-h;
z4  = z-i*h;
sg0 = sqrt(sigma(z0));
sg1 = sqrt(sigma(z1));
sg2 = sqrt(sigma(z2));
sg3 = sqrt(sigma(z3));
sg4 = sqrt(sigma(z4));

ans = 1/(h^2)*(sg1 + sg2 + sg3 + sg4 - 4*sg0)./sg0;


