function drawhyperbola(C, a, b, alpha)
%DRAWHYPERBOLA
%
%       drawhyperbola(C, a, b, alpha)
%       draw hyperbola with center C  semiaxis a  and b
%       alpha angle between a and x-axe
%
  s = sin(alpha); c = cos(alpha);
  Q = [c -s; s c];
  fac = (a + b + 2)*log(a + b + 2);
  theta = [-fac:fac / 50:fac];
  u = diag(C)*ones(2,length(theta)) + ...
      Q*[a*cosh(theta); b*sinh(theta)];
  plot(u(1,:),u(2,:));
  u = diag(C)*ones(2,length(theta)) + ...
      Q*[-a*cosh(theta); b*sinh(theta)];
  plot(u(1,:),u(2,:));
  plot(C(1),C(2),'+');

end % drawhyperbola

