function u1 = rk4 ( t0, u0, dt, f )

%*****************************************************************************80
%
%% RK4 takes one Runge-Kutta step.
%
%  Discussion:
%
%    It is assumed that an initial value problem, of the form
%
%      du/dt = f ( t, u )
%      u(t0) = u0
%
%    is being solved.
%
%    If the user can supply current values of t, u, a stepsize dt, and a
%    function to evaluate the derivative, this function can compute the
%    fourth-order Runge Kutta estimate to the solution at time t+dt.
%
%  Licensing:
%
%    This code is distributed under the GNU LGPL license. 
%
%  Modified:
%
%    31 January 2012
%
%  Author:
%
%    John Burkardt
%
%  Parameters:
%
%    Input, real T0, the current time.
%
%    Input, real U0, the solution estimate at the current time.
%
%    Input, real DT, the time step.
%
%    Input, function value = F ( T, U ), a function which evaluates
%    the derivative, or right hand side of the problem.
%
%    Output, real U1, the fourth-order Runge-Kutta solution estimate
%    at time T0+DT.
%

%
%  Get four sample values of the derivative.
%
  f1 = f ( t0,          u0 );
  f2 = f ( t0 + dt / 2, u0 + dt * f1 / 2 );
  f3 = f ( t0 + dt / 2, u0 + dt * f2 / 2 );
  f4 = f ( t0 + dt,     u0 + dt * f3 );
%
%  Combine them to estimate the solution U1 at time T1 = T0 + DT.
%
  u1 = u0 + dt * ( f1 + 2.0 * f2 + 2.0 * f3 + f4 ) / 6.0;

  return
end