This article has been retracted: please see Elsevier Policy
on Article Withdrawal (http://www.elsevier.com/locate/
withdrawalpolicy). The authors have plagiarized part of a paper that had already appeared in Afrika Mathematika, 2012, 1–9. http:// dx.doi.org/10.1007/s13370-012-0083-9. One of the conditions of submission of a paper for publication is that authors declare
explicitly that their work is original and has not appeared in a
publication elsewhere. Re-use of any data should be appropriately
cited. As such this article represents a severe abuse of the scientific publishing system. The scientific community takes a very strong view on this matter and apologies are offered to
readers of the journal that this was not detected during the submission process.