1. "Babes-Bolyai University, Faculty of Mathematics and Computer Science Kogalniceanu Str. no. 1, 400084 Cluj-Napoca, Romania and Academy of Romanian Scientists Bucharest, Romania e-mail: petrusel@math.ubbcluj.ro"
2. "Babes-Bolyai University, Faculty of Mathematics and Computer Science Kogalniceanu Str. no. 1, 400084 Cluj-Napoca, Romania e-mail: iarus@math.ubbcluj.ro"
3. "Babes-Bolyai University, Faculty of Mathematics and Computer Science Kogalniceanu Str. no. 1, 400084 Cluj-Napoca, Romania e-mail: mserban@math.ubbcluj.ro"