Retraction to: Mathematical Sciences (2012) 6:55 DOI 10.1186/2251-7456-6-55

This article has been retracted by the Editor in Chief of Mathematical Sciences as it has been brought to his attention that it’s a duplicate of a paper that had first been submitted to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 iss. 8 (2013), no. 1, 39–47, 115.