function parabola (n)

  if (nargin != 1)
    usage ("parabola (n)");
  endif

  if (n >= 0.1 && n < 1.0)
    p = n * 10;

    x = y = (-10: 10)';
    [xx, yy] = meshgrid(x, y);

    z = (xx .^ 2 + yy .^ 2) / (4 * p);

    mesh (x, y, z);
  else
    error ("parabola: n must be between 0.1 and 1.0");
  endif

endfunction