The article [ibid. 6, Paper No. 55, 5 p. (2012; Zbl 1271.39024)] has been retracted by the Editor in Chief of Mathematical Sciences as it has been brought to his attention that it’s a duplicate of a paper that had first been submitted to Iranian Journal of Mathematical Sciences and Informatics and published there under the same title in [Iran. J. Math. Sci. Inform. 8, No. 1, 39–47 (2013; Zbl 1359.39017)].