User blog comment:Flavio61/Louis Epstein number list/@comment-27173506-20160118190252/@comment-27173506-20160120053006

3 uuuu 4 = 3 uuu 3 u 4~3 uuu 3^^4=3 uu 3 u 3^^4~3 uu 3^^3^^4= 3 u 3 u 3^^3^^4~3^^3^^3^^3^^4

That's about 3^^^5. It is slightly greater actually than 4^^^4, and I admit that I made a mistake. Up arrows surpass ultrexing at hexation (a^^^^b).