function [x y]=sf2d(n,a)
  u=[0:.001:2*pi];
  raux=abs(1/a(1)*abs(cos(n(1)*u/4))).^n(3)+abs(1/a(2).*abs(sin(n(1)*u/4))).^n(4);
  r=abs(raux).^(-1/n(2));
  x=r.*cos(u);
  y=r.*sin(u);
endfunction;