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 A056661 #26 Jul 04 2021 19:04:45
%S A056661 0,1,2,5,8,11,29,31,182,296,491,41315
%N A056661 Numbers k such that 40*R_k + 3 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A056661 Also numbers k such that (4*10^(k+1)-13)/9 is prime.
%C A056661 a(13) > 10^5. - _Robert Price_, Nov 01 2014
%H A056661 Makoto Kamada, <a href="https://stdkmd.net/nrr/4/44443.htm#prime">Prime numbers of the form 44...443</a>.
%H A056661 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A056661 a(n) = A096845(n) - 1. - _Robert Price_, Nov 01 2014
%t A056661 Do[ If[ PrimeQ[ 40*(10^n - 1)/9 + 3], Print[n]], {n, 0, 5000}]
%Y A056661 Cf. A002275, A096845.
%K A056661 hard,more,nonn
%O A056661 1,3
%A A056661 _Robert G. Wilson v_, Aug 09 2000
%E A056661 a(12) derived from A096845 by _Robert Price_, Nov 01 2014