%I #31 Jan 11 2020 15:57:47
%S 1,1,1,1,2,5,19729
%N Number of digits of 2^^n.
%C Next term is too large to fit in the data section.
%C 2^^n is _Don Knuth_'s up arrow notation for the power tower 2^2^...^2 (n times).
%H Wikipedia, <a href="http://en.wikipedia.org/wiki/Knuth's_up-arrow_notation">Knuth's up-arrow notation</a>
%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Tetration">Tetration</a>
%e 2^^-1=0, so a(-1)=1;
%e 2^^0=1, so a(0)=1;
%e 2^^1=2, so a(1)=1;
%e 2^^2=4, so a(2)=1;
%e 2^^3=16, so a(3)=2;
%e 2^^4=65536, so a(4)=5.
%Y Equals number of digits of A014221.
%K nonn,base
%O -1,5
%A _Natan Arie Consigli_, Dec 15 2018