function err2 = delphi(x,y,E)

% xV(X,Y,E) returns (V(x,y)-E)^2 for Delphi potential.  Minimizing
%   this enables one to find values of X for particular Y and E.

V = 1/4*(x.^2+y.^2).^2+0.54433105395182*x.^3.*(y-1)-y.*(x.^2+y.^2) ...
    +1/2*(x.^2+3*y.^2);
err2 = (V-E).^2;
