PiętkaM. “The Estimation of the Operational Functions of the Type Exp[(Σ_{ν=1}^nβ_νs^{p_ν})λ]”. Annales Mathematicae Silesianae, Vol. 4, Sept. 1990, pp. 25-34, https://www.journals.us.edu.pl/index.php/AMSIL/article/view/14290.