function value = rk4vec_step_test_f ( t, n, u )

%*****************************************************************************80
%
%% rk4vec_step_test_f evaluates the right hand side of a particular ODE.
%
%  Licensing:
%
%    This code is distributed under the GNU LGPL license. 
%
%  Modified:
%
%    09 October 2013
%
%  Author:
%
%    John Burkardt
%
%  Parameters:
%
%    Input, real T, the current time.
%
%    Input, real U(N), the current solution value.
%
%    Output, real VALUE, the value of the derivative, dU/dT.
%  
  value(1) =   u(2);
  value(2) = - u(1);
  
  return
end
