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 A102484 #9 May 10 2017 03:29:07
%S A102484 9009,9119,9229,9339,9449,9559,9669,9779,9889,9999,910109,911119,
%T A102484 912129,913139,914149,915159,916169,917179,918189,919199,920209,
%U A102484 921219,922229,923239,924249,925259,926269,927279,928289,929299,930309
%N A102484 Numbers of the concatenated form 9nn9.
%H A102484 Robert Israel, <a href="/A102484/b102484.txt">Table of n, a(n) for n = 1..10000</a>
%F A102484 a(n) = 9*(1 + 10^(2*A055642(n-1)+1))+(n-1)*(10+10^(1+A055642(n-1))). - _Robert Israel_, May 09 2017
%e A102484 For n=0 we concatenate 3, 0, 0 and 3 to get 3003.
%e A102484 For n=10 we concatenate 3, 10, 10 and 3 to get 310103.
%p A102484 f:= n -> 9 + 9*10^(2*ilog10(n-1)+3)+(n-1)*(10+10^(2+ilog10(n-1))):
%p A102484 f(1):= 9009;
%p A102484 map(f, [$1..200]); # _Robert Israel_, May 09 2017
%t A102484 With[{k = {9}}, Table[FromDigits@ Join[k, #, #, k] &@ IntegerDigits[n], {n, 0, 30}]] (* _Michael De Vlieger_, May 09 2017 *)
%Y A102484 Cf. A100846 for 1nn1; A100896 for 3nn3; A100897 for 7nn7.
%Y A102484 Cf. A055642.
%K A102484 easy,nonn,base
%O A102484 1,1
%A A102484 Mark Hudson (mrmarkhudson(AT)hotmail.com), Jan 12 2005