Piętka, Marek. “A Note on the Increase of the Operational Function exp(λΣ_{k=1}^{n}A_{k}s^{p_k})”. Annales Mathematicae Silesianae, vol. 3, Jan. 1990, pp. 107-14, https://trrest.vot.pl/ojsus/index.php/AMSIL/article/view/14309.