function [x,y,z] = hyper(a,b,n,step)
% hyperbolic paraboloid
%  [x,y,z] = hyper(a,b,n,step)

a2 = a*a;
b2 = b*b;
index = 1;
for x1 = -n:step:n
  x(index) = x1;
  y(index) = x1;
  index = index + 1;
end
for i1 = 1:index-1
  for j1 = 1:index-1
    z(i1,j1) = (x(i1)*x(i1)/a2)-(y(j1)*y(j1)/b2);
  end
end
