1. Institute of Mathematics , Jan Kochanowski University , Świętokrzyska 15, 25-406 Kielce and Institute of Mathematics , University of Silesia in Katowice , Bankowa 14, 40-007 Katowice , Poland
2. Institute of Mathematics , University of Silesia in Katowice , Bankowa 14, 40-007 Katowice , Poland