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 A096506 #25 Sep 14 2024 02:22:45
%S A096506 1,2,3,8,11,36,95,101,128,260,351,467,645,1011,1178,1217,2442,3761,
%T A096506 3806,15617,26459,63117,88545,93497
%N A096506 Numbers n for which 2*R_n + 1 is a prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C A096506 Also numbers n such that (2*10^n + 7)/9 is prime.
%C A096506 Per Kamada link, 181457, 202059, 262874 are also terms, found by Rytis Slatkevicius. - _Michael S. Branicky_, Sep 13 2024
%H A096506 Makoto Kamada, <a href="https://stdkmd.net/nrr/2/22223.htm#prime">Prime numbers of the form 22...223</a>.
%H A096506 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A096506 a(n) = A056656(n) + 1.
%e A096506 n=36: 222222222222222222222222222222222223 is a prime number.
%t A096506 Do[ If[ PrimeQ[ 2(10^n - 1)/9 + 1], Print[n]], {n, 7000}] (* _Robert G. Wilson v_, Oct 14 2004 *)
%Y A096506 Cf. A056656, A093162, A096503, A096504, A096505, A096507, A096508.
%K A096506 nonn
%O A096506 1,2
%A A096506 _Labos Elemer_, Jul 12 2004
%E A096506 a(20)-a(24) from Kamada link by _Ray Chandler_, Feb 27 2012