In order to incorporate a variable step algorithm for the Fehlberg’s method, an equation for error estimation has to be included. The Butcher tableau (ref: Hairer, Nørsett & Wanner, Solving ODEs, Vol. 1, Non-stiff problems) with the ycap coefficients are:

The generalized RK method can then be used with the Kaps-Rentrop algorithm for adaptive step size to solve ODEs.