def steffensen(f):
    def F(x):
        fx = f(x)
        ffx = f(fx)
        return (x*ffx - fx**2) / (ffx - 2*fx + x)
    return F
linear convergent function -> quadratic convergent function
dataset/ETHPy150Open fredrik-johansson/mpmath/mpmath/calculus/optimization.py/steffensen