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 A309809 #15 Jan 30 2024 13:03:33
%S A309809 13,25,37,49,511,613,715,817,919,1021,1123,1225,1327,1429,1531,1633,
%T A309809 1735,1837,1939,2041,2143,2245,2347,2449,2551,2653,2755,2857,2959,
%U A309809 3061,3163,3265,3367,3469,3571,3673,3775,3877,3979,4081,4183,4285,4387,4489,4591,4693,4795,4897
%N A309809 a(n) is the concatenation of n and 2n+1.
%H A309809 Robert Israel, <a href="/A309809/b309809.txt">Table of n, a(n) for n = 1..10000</a>
%F A309809 a(n) = n*10^(ceiling(log_10(2*n+1))) + 2*n + 1.
%e A309809 a(3)=37 is the concatenation of 3 and 7.
%p A309809 seq(n*10^(ceil(log[10](2*n+1)))+2*n+1, n=1..100);
%t A309809 Table[FromDigits[Join[IntegerDigits[n],IntegerDigits[2n+1]]],{n,50}] (* _Harvey P. Dale_, Jan 30 2024 *)
%Y A309809 Cf. A001704, A309808.
%K A309809 nonn,base
%O A309809 1,1
%A A309809 _Robert Israel_, Aug 17 2019