Correction to: Acta Scientiarum Mathematicarum 81:1–2 https://doi.org/10.14232/actasm-013-526-x

The publication of this article unfortunately contained a mistake. The affiliation of the author was not correct in the html version. Correct it must be:

Department of Mathematics, University of Thessaloniki, 54124 Thessaloniki, Greece; e-mail: ath.arvanitidis@yahoo.gr

The original article has been corrected.