1. Faculty of Mathematics and Computer Science, Babeş-Bolyai University, Strada Mihail Kogălniceanu 1, 400084 Cluj-Napoca, Romania
2. Department of Mathematics, Swansea University Bay Campus, Fabian Way, Swansea SA1 8EN, UK
3. Faculty of Mathematics, University of Białystok, Konstantego Ciołkowskiego 1M, 15-245 Białystok, Poland