Before revision:  Li and Zhan ~\mbox{%DIFAUXCMD li2016almost generalized their result and proved that all even Yao-Yao graphs YY_{2k} are spanners (for k\geq 42).
After revision:  Li and Zhan li2016almost generalized their result and proved that all even Yao-Yao graphs YY_{2k} are spanners (for k\geq 42).