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 A174212 #16 Nov 11 2020 03:07:12
%S A174212 1,1,4,27,16777216,437893890380859375,
%T A174212 500702078263459319174537025249570888246709955377400223021257741084821677152403456
%N A174212 Ultradoublefactorials: a(n) = (n!!)^(n!!).
%C A174212 The next term (a(8)) has 993 digits. - _Harvey P. Dale_, Aug 17 2017
%F A174212 a(n) = A006882(n)^A006882(n).
%F A174212 Sum_{n>=1} 1/a(n) = A134877. - _Amiram Eldar_, Nov 11 2020
%e A174212 For n=4 the doublefactorial is n!! = 4*2 = 8 and a(n) = n!!^n!! = 8^8 = 16777216.
%p A174212 P:=proc(i) local a,n; for n from 0 by 1 to i do print(doublefactorial(n)^doublefactorial(n)); od; end: P(10);
%t A174212 udf[n_]:=Module[{c=n!!},c^c]; Array[udf,7,0] (* _Harvey P. Dale_, Aug 17 2017 *)
%Y A174212 Cf. A006882, A046882, A134877, A165812.
%K A174212 easy,nonn
%O A174212 0,3
%A A174212 _Paolo P. Lava_ & _Giorgio Balzarotti_, Mar 16 2010