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 A074843 #14 Dec 20 2021 13:17:21
%S A074843 1111,2222,3333,4444,5555,6666,7777,8888,9999,10101010,11111111,
%T A074843 12121212,13131313,14141414,15151515,16161616,17171717,18181818,
%U A074843 19191919,20202020,21212121,22222222,23232323,24242424,25252525
%N A074843 Quadruplets: base 10 representation is the juxtaposition of four identical strings.
%C A074843 Doublets in which the index is also a doublet. - _Jamie Robert Creasey_, Jun 23 2021
%H A074843 Harvey P. Dale, <a href="/A074843/b074843.txt">Table of n, a(n) for n = 1..1000</a>
%F A074843 A020338(A020338(n)). - _Jamie Robert Creasey_, Jun 23 2021
%t A074843 Table[FromDigits[Flatten[IntegerDigits/@PadRight[{},4,n]]],{n,30}] (* _Harvey P. Dale_, Dec 20 2021 *)
%o A074843 (UBASIC) 10 cls 30 for I=1 to 100 40 A=str(I) 50 C=A+A+A+A 60 B=val(cutspc(C)) 80 print B 90 next 100 end
%o A074843 (PARI) a(n)=eval(Str(n,n,n,n)) \\ _Charles R Greathouse IV_, Jun 23 2021
%Y A074843 Cf. A020338.
%K A074843 easy,nonn,base
%O A074843 1,1
%A A074843 _Felice Russo_, Sep 10 2002