function drfdt = predator_deriv ( t, rf )

%*****************************************************************************80
%
%% predator_deriv returns the derivative for the predator-prey ODE.
%
%  Licensing:
%
%    This code is distributed under the GNU LGPL license.
%
%  Modified:
%
%    22 February 2020
%
%  Author:
%
%    John Burkardt
%
%  Input:
%
%    real T(:), RF(2), the arguments of the derivative.
%
%  Output:
%
%    real DRFDT(2,:), the value of the derivative.
%
  alpha = 2.0;
  beta = 1.0;
  gamma = 1.0;
  delta = 1.0;

  r = rf(1);
  f = rf(2);

  drdt =   alpha * r - beta  * r * f;
  dfdt = - gamma * f + delta * r * f;

  drfdt = [ drdt; dfdt ];

  return
end

