Kahlig, Peter, and Jens Schwaiger. “Transforming the Functional Equation of Gołąb-Schinzel into One of Cauchy”. Annales Mathematicae Silesianae, vol. 8, Sept. 1994, pp. 33-38, https://trrest.vot.pl/ojsus/index.php/AMSIL/article/view/14215.