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 A100846 #21 Jun 25 2018 10:35:57
%S A100846 1001,1111,1221,1331,1441,1551,1661,1771,1881,1991,110101,111111,
%T A100846 112121,113131,114141,115151,116161,117171,118181,119191,120201,
%U A100846 121211,122221,123231,124241,125251,126261,127271,128281,129291,130301,131311,132321
%N A100846 Concatenate (1,n,n,1).
%H A100846 M. F. Hasler, <a href="/A100846/b100846.txt">Table of n, a(n) for n = 0..10000</a> (Terms a(1..9999) from Robert Israel)
%F A100846 G.f.: 1001 + x*(31-11*x)/(1-x)^2 + Sum_{k>=0} 90*(12*10^(2*k)*(1-x)+10^k*x)*x^(10^k)/(1-x)^2. - _Robert Israel_, Dec 30 2015
%e A100846 For n = 0, concatenate(1,n,n,1) is 1001 = a(0).
%e A100846 For n = 5, concatenate(1,n,n,1) is 1551 = a(5).
%e A100846 For n = 10, concatenate(1,n,n,1) is 110101 = a(10).
%p A100846 seq(seq((10^(2*d+1)+1+(10^(d+1)+10)*n), n=`if`(d>1,10^(d-1),0) .. 10^d-1),d=1..3);
%p A100846 # _Robert Israel_, Dec 30 2015, edited for n=0 by _M. F. Hasler_, Jun 25 2018
%t A100846 For[n = 0, n < 30, n++, l := Floor[Log[10, Min[n,1]] + 1]; gvout := (n*10^l + n)*10 + 1; m := Floor[Log[10, gvout]]; giveout := 10^(m + 1) + out; Print[giveout]] (* _Stefan Steinerberger_, Jan 27 2006, edited for n=0 by _M. F. Hasler_, Jun 25 2018 *)
%o A100846 (PARI) A100846(n)=eval(Str(1,n,n,1)) \\ _M. F. Hasler_, Jun 22 2018
%Y A100846 Cf. A100896 (3nn3), 7nn7 (A100897), 9nn9 (A102484).
%Y A100846 For primes in these sequences: A102496, A102497 (1nn1); A102498, A102499 (3nn3); A102500, A102501 (7nn7); A102502, A102503 (9nn9); A102504 (intersection).
%K A100846 nonn,base
%O A100846 0,1
%A A100846 _Parthasarathy Nambi_, Jan 07 2005
%E A100846 More terms from _Stefan Steinerberger_, Jan 27 2006
%E A100846 Definition reworded and missing 1001 added by _M. F. Hasler_, Jun 22 2018