def fitfunc_e2(epsilon, a, b):
    """
       Fit function for epsilon dependence of NSPT results.

       By construction the Runge-Kutta used in the calculations the
       part linear in epsilon is cancelled.

       Parameters
       ----------
       epsilon : float
           NSPT time step epsilon
       a : float
           Fit parameter for quadratic term
       b : float
           Fit parameter for constant term

       Returns
       -------
       a*epsilon**2+b : float

    """
    return a*epsilon**2 + b