Department of Mathematics, Kyungpook National University, Daegu, Korea
© 2014 Published by Elsevier B.V. on behalf of Korea Centers for Disease Control and Prevention.
This is an Open Access article distributed under the terms of the CC-BY-NC License (http://creativecommons.org/licenses/by-nc/3.0).
^{} This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.