Argyros, I. K. “On the Secant Method and the Ptak Error Estimates”. Rev. Anal. Numér. Théor. Approx., vol. 24, no. 1, Aug. 1995, pp. 3-14, https://ictp.acad.ro/jnaat/journal/article/view/1995-vol24-nos1-2-art1.