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 A206527 #34 Mar 16 2023 12:14:23
%S A206527 11,33,99,2727,8181,243243,729729,21872187,65616561,1968319683,
%T A206527 5904959049,177147177147,531441531441,15943231594323,47829694782969,
%U A206527 1434890714348907,4304672143046721,129140163129140163,387420489387420489
%N A206527 3^n concatenated with itself.
%H A206527 Vincenzo Librandi, <a href="/A206527/b206527.txt">Table of n, a(n) for n = 0..200</a>
%F A206527 a(n) = A020338(3^n). - _Bruno Berselli_, Mar 14 2012
%e A206527 a(1)=33 because 3^1 concatenated with 3^1 is 33.
%t A206527 Table[FromDigits[Join[IntegerDigits[3^n], IntegerDigits[3^n]]], {n, 0, 18}] (* _Bruno Berselli_, Mar 14 2012 *)
%t A206527 Table[3^n 10^IntegerLength[3^n]+3^n,{n,0,20}] (* _Harvey P. Dale_, May 27 2019 *)
%o A206527 (Magma) [Seqint(Intseq(3^n) cat Intseq(3^n)): n in [0..18]]; // _Bruno Berselli_, Mar 14 2012
%Y A206527 Cf. A178664, A000244.
%K A206527 nonn,base,easy
%O A206527 0,1
%A A206527 _Vincenzo Librandi_, Mar 14 2012