Acta Universitatis Matthiae Belii Series Mathematics Number 19 Matej Bel University, Bansk´aBystrica, Slovakia 2011 Acta Universitatis Matthiae Belii, Series Mathematics Editor-in-chief: Miroslav Haviar, Matej Bel University, B. Bystrica, Slovakia Editorial board: A. Breda d'Azevedo, Univesidade de Aveiro, Portugal B. A. Davey, La Trobe University, Melbourne, Australia M. Grend´ar, Matej Bel University, B. Bystrica, Slovakia M. Haviar, Matej Bel University, B. Bystrica, Slovakia P. Hlinˇen´y, Masaryk University, Brno, Czech Republic V. Janiˇs, Matej Bel University, B. Bystrica, Slovakia J. Kratochv´ıl, Charles University, Prague, Czech Republic J. Karab´aˇs, Matej Bel University, B. Bystrica, Slovakia P. Kr´al', Matej Bel University, B. Bystrica, Slovakia V. Jim´enez L´opez, Universidad de Murcia, Spain P. Maliˇck´y, Matej Bel University, B. Bystrica, Slovakia R. Nedela, Matej Bel University, B. Bystrica, Slovakia L'.Snoha, Matej Bel University, B. Bystrica, Slovakia Editorial office: Acta Universitatis Matthiae Belii, ser. Mathematics Institute of Mathematics, Faculty of Natural Sciences Matej Bel University Tajovsk´eho40 974 01 Bansk´aBystrica Slovakia E-mail:
[email protected] Scope of Journal: Acta Universitatis Matthiae Belii, series Mathematics, pub- lishes original research articles and survey papers in all areas of mathematics and theoretical computer science. Occasio- nally valuable papers dealing with applications of mathema- tics in other fields can be published. Manuscripts: Manuscripts submitted for publication must be written in English. Please send the source LATEX-2" file (use please amsart document class) and figures needed to compile the output form as an attachment by e-mail to the address
[email protected]. Subscriptions: Acta Universitatis Matthiae Belii series Mathematics is usu- ally published once a year.