function value = hex2_df ( xy )

%*****************************************************************************80
%
%% hex2_df evaluates the hex2 gradient.
%
%  Licensing:
%
%    This code is distributed under the GNU LGPL license.
%
%  Modified:
%
%    03 October 2019
%
%  Input:
%
%    real xy(2): the x and y arguments.
%
%  Output:
%
%    real value(2): the gradient vector.
%
  x = xy(1);
  y = xy(2);
  value(1) = - 400.0 * x * ( y - x.^2 ) - 2.0 * ( 1.0 - x );
  value(2) = 200.0 * ( y - x.^2 );

  return
end

