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 A100896 #14 Sep 08 2022 08:45:15
%S A100896 3003,3113,3223,3333,3443,3553,3663,3773,3883,3993,310103,311113,
%T A100896 312123,313133,314143,315153,316163,317173,318183,319193,320203,
%U A100896 321213,322223,323233,324243,325253,326263,327273,328283,329293,330303,331313,332323
%N A100896 a(n) has decimal expansion 3nn3.
%H A100896 Robert Israel, <a href="/A100896/b100896.txt">Table of n, a(n) for n = 0..10000</a>
%F A100896 a(n) = 3nn3 where n=0, 1, 2, 3, ...
%F A100896 a(n) = 3*(1+10^(2*d+1))+n*(10+10^(d+1)) where d = A055642(n). - _Robert Israel_, Mar 08 2018
%e A100896 If n=5, then 3nn3 is 3553.
%p A100896 f:= proc(n) local d; d:= ilog10(n)+1; 3*(1+10^(2*d+1))+n*(10+10^(d+1)) end proc:
%p A100896 3003, seq(f(n),n=1..100); # _Robert Israel_, Mar 08 2018
%t A100896 FromDigits/@Table[Join[{3},IntegerDigits[i],IntegerDigits[i],{3}],{i,0,40}] (* _Harvey P. Dale_, Sep 22 2014 *)
%o A100896 (Magma) [3003] cat [3*(1+10^(2*#Intseq(n)+1))+n*(10+10^(#Intseq(n)+1)): n in [1..40]]; // _Vincenzo Librandi_, Mar 09 2018
%Y A100896 Cf. A100846.
%K A100896 nonn,base,easy
%O A100896 0,1
%A A100896 _Parthasarathy Nambi_, Jan 10 2005
%E A100896 More terms from Brian Lauer (bel136(AT)psu.edu), Feb 21 2006
%E A100896 Offset changed to 0 by _Robert Israel_, Mar 08 2018