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 A178664 #26 Jun 24 2025 17:01:46
%S A178664 11,22,44,88,1616,3232,6464,128128,256256,512512,10241024,20482048,
%T A178664 40964096,81928192,1638416384,3276832768,6553665536,131072131072,
%U A178664 262144262144,524288524288,10485761048576,20971522097152,41943044194304,83886088388608,1677721616777216
%N A178664 a(n) = 2^n concatenated with itself.
%H A178664 Vincenzo Librandi, <a href="/A178664/b178664.txt">Table of n, a(n) for n = 0..200</a>
%F A178664 a(n) = A020338(2^n). - _Bruno Berselli_, Mar 14 2012
%e A178664 a(1) = 22 because 2^1 concatenated with 2^1 is 22.
%p A178664 a:= n-> parse(cat(2^n$2)):
%p A178664 seq(a(n), n=0..25); # _Alois P. Heinz_, Jun 24 2025
%t A178664 Table[FromDigits[Join[IntegerDigits[2^n],IntegerDigits[2^n]]],{n,0,30}] (* _Vincenzo Librandi_, Mar 14 2012 *)
%t A178664 #*10^IntegerLength[#]+#&/@(2^Range[0,25]) (* _Harvey P. Dale_, Aug 29 2024 *)
%o A178664 (Magma) [Seqint(Intseq(2^n) cat Intseq(2^n)): n in [0..40]]; // _Vincenzo Librandi_, Mar 14 2012
%Y A178664 Cf. A000079.
%K A178664 nonn,easy,base
%O A178664 0,1
%A A178664 _Vincenzo Librandi_, Jul 10 2010