[1] Constantine Philosopher Univ Nitra, Fac Nat Sci, Dept Math, A Hlinku 1, SK-94901 Nitra, Slovakia
[2] Matej Bel Univ, Fac Nat Sci, Dept Math, Tajovskeho 40, SK-97401 Banska Bystrica, Slovakia
[3] Slovak Acad Sci, Math Inst, Stefanikova 49, SK-81473 Bratislava, Slovakia