Department of Mathematics, College of Science, Tianjin Polytechnic University, Tianjin City 300160, China
Copyright © 2010 Feng Qi. This is an open access article distributed under the
Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
By looking back at the long history of bounding the ratio for and , various origins of this topic are clarified, several developed courses are followed, different results are compared, useful methods are summarized, new advances are presented, some related problems are pointed out, and related references are collected.