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 A099409 #33 May 03 2024 15:03:17
%S A099409 0,1,3,9,15,28,64,1168,1695,2362,116620,336405
%N A099409 Numbers k such that 2*R_k + 5 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A099409 Also numbers k such that (2*10^k + 43)/9 is prime.
%C A099409 a(11) > 50000. - _Robert Price_, Oct 27 2014
%H A099409 Makoto Kamada, <a href="https://stdkmd.net/nrr/2/22227.htm#prime">Prime numbers of the form 22...227</a>.
%H A099409 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A099409 a(n) = A056677(n-1) + 1.
%t A099409 Do[ If[ PrimeQ[ 2(10^n - 1)/9 + 5], Print[n]], {n, 0, 5000}]
%o A099409 (Magma) [n: n in [0..1000] | IsPrime( 2*(10^n - 1) div 9 + 5)]; // _Vincenzo Librandi_, Oct 28 2014
%Y A099409 Cf. A002275, A056677, A093167.
%K A099409 nonn
%O A099409 1,3
%A A099409 _Robert G. Wilson v_, Oct 14 2004
%E A099409 Added a(1)=0, adapted Mathematica program, _Vincenzo Librandi_, Oct 28 2014
%E A099409 a(11)-a(12) from Kamada data by _Tyler Busby_, May 03 2024