1. Badora, R., Przebieracz, B., Volkmann, P.: Stability of the Pexider functional equation. Ann. Math. Silesianae 24(2010), 7–13 (2011)
2. Badora, R., Przebieracz, B., Volkmann, P.: Stability of the functional equation f(xy) = f(yx) on groups, Remark at the Eleventh Katowice-Debrecen Winter Seminar on Functional Equations and Inequalities, Wisła-Malinka 2011. Ann. Math. Silesianae 25(2011), 116 (2012)
3. Ger R., Volkmann P.: On sums of linear and bounded mappings. Abh. Math. Sem. Univ. Hamburg 68, 103–108 (1998)
4. Gilányi, A., Nagatou, K., Volkmann, P.: Stability of a functional equation coming from the characterization of the absolute value of additive functions. Ann. Funct. Anal. 1(2), 1–6 (2011). http://www.emis.de/journals/AFA
5. Golovina L.I.: Linejnaâ algebra i nekotorye ee priloženiâ. Nauka, Moskva (1971)