1,2

Using Knuth's arrow notation, this is n^^^2 (n-penta-2) or n^^n (n-tetra-n). - Andrew Robbins, Apr 16 2009

Comment from Trevor Green (green(AT)dlcwest.com): The fourth term in this sequence has about as many digits - 8.07 * 10^153 - as the *square* of the number of protons in the universe.

There could be a term for n=0, but it is not clear whether this should be 0 or 1.

Cf. A027747, A008868 (which mentions an older estimate for the same number).

Sequence in context: A161405 A147876 A164796 this_sequence A066546 A132653 A115544

Adjacent sequences: A004228 A004229 A004230 this_sequence A004232 A004233 A004234

nonn,bref

wild(AT)edumath.u-strasbg.fr (Daniel Wild)