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 A206528 #7 Sep 08 2022 08:46:01
%S A206528 11,55,2525,125125,625625,31253125,1562515625,7812578125,390625390625,
%T A206528 19531251953125,97656259765625,4882812548828125,244140625244140625,
%U A206528 12207031251220703125,61035156256103515625,3051757812530517578125,152587890625152587890625
%N A206528 5^n concatenated with itself.
%H A206528 Vincenzo Librandi, <a href="/A206528/b206528.txt">Table of n, a(n) for n = 0..200</a>
%F A206528 a(n) = A020338(5^n).
%e A206528 a(1)=55 because 5^1 concatenated with 5^1 is 55.
%t A206528 Table[FromDigits[Join[IntegerDigits[5^n],IntegerDigits[5^n]]],{n,0,20}]
%o A206528 (Magma) [Seqint(Intseq(5^n) cat Intseq(5^n)): n in [0..20]];
%Y A206528 Cf. A000351, A178664, A206527.
%K A206528 nonn,base,easy
%O A206528 0,1
%A A206528 _Vincenzo Librandi_, Mar 17 2012