1. [2] Baron K. and Volkmann P. , ‘Characterization of the absolute value of complex linear functionals by functional equations’, Seminar LV, No. 28 (2006), 10 pp., http://www.math.us.edu.pl/smdk.
2. [5] Jarczyk W. and Volkmann P. , ‘On functional equations in connection with the absolute value of additive functions’, Ser. Math. Catovic. Debrecen., 33 (2010), http://www.math.us.edu.pl/smdk.
3. Functional Equations in Several Variables
4. ON SOME PEXIDER-TYPE FUNCTIONAL EQUATIONS CONNECTED WITH THE ABSOLUTE VALUE OF ADDITIVE FUNCTIONS. PART II