Before revision: It is a long standing open problem whether Yao-Yao graphs YY_{k} are all spanners ~\mbox{%DIFAUXCMD li2002sparse . Bauer and Damian ~\mbox{%DIFAUXCMD bauer2013infinite showed that all YY_{6k} for k \geq 6 are spanners.
After revision: It is a long standing open problem whether Yao-Yao graphs YY_{k} are all spanners ~\mbox{%DIFAUXCMD li2002sparse . Bauer and Damian bauer2013infinite showed that all YY_{6k} for k \geq 6 are spanners.