The link here shows a proof from Tim Chow that the moser-number is much smaller than grahams number.
I do not understand the inequality
3^^...^^3 (3^^^^^3×2-1 arrows) << G 2
What does G 2 mean here ?
If the sequence
$$G_1 = 3\uparrow^4 3$$
$$G_{n+1} = 3 \uparrow ^{G_n} 3$$
for all $n \ge 1$
is meant, I understand that the left number is much smaller then $G_3$, but I do not understand that it must be smaller than $G_2$.
What am I missing here ?