This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A014222 #44 Dec 27 2021 22:16:04 %S A014222 0,1,3,27,7625597484987 %N A014222 a(0) = 0; thereafter a(n+1) = 3^a(n). %C A014222 Alternative definition: %C A014222 a(n+1) = H_4(3,n) the tetration (repeated exponentiation) of 3 times n. %C A014222 For definition of and references concerning H_n(x,y) see A054871. %C A014222 Next term is 12580...39387 and has 3638334640025 digits. - Shawn Ligocki (sligocki(AT)gmail.com), Oct 08 2009 %F A014222 a(n+1) = H_4(3,n) = 3^^n; %F A014222 a(0) = 0, a(1) = 1, a(n+1) = 3^3^...^3 (n times). %e A014222 a(0) = H_4(3,-1)= 0; %e A014222 a(1) = H_4(3,0) = 1; %e A014222 a(2) = H_4(3,1) = 3; %e A014222 a(3) = H_4(3,2) = 3^3 = 27; %e A014222 a(4) = H_4(3,3) = 3^3^3 = 7625597484987; %e A014222 a(5) = H_4(3,4) = 3^3^3^3 = 3^7625597484987 > 10^3638334640025. %t A014222 NestList[3^#&,0,4] (* _Harvey P. Dale_, Apr 04 2013 *) %Y A014222 Cf. A014221 (a(n+1) = 2^a(n), or H_4(2,n)), A081651, A114561, A266199 (H_5(3,n)). %K A014222 nonn,easy %O A014222 0,3 %A A014222 _N. J. A. Sloane_ %E A014222 Revised using hyperoperation notation by _Natan Arie Consigli_, Jan 16 2016