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 A056677 #35 Jan 15 2023 01:45:06
%S A056677 0,2,8,14,27,63,1167,1694,2361,116619
%N A056677 Numbers k such that 20*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A056677 Also numbers k such that (2*10^(k+1)+43)/9 is prime.
%H A056677 Makoto Kamada, <a href="https://stdkmd.net/nrr/2/22227.htm#prime">Prime numbers of the form 22...227</a>.
%H A056677 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A056677 a(n) = A099409(n+1) - 1. - _Robert Price_, Jan 30 2015
%t A056677 Do[ If[ PrimeQ[20*(10^n - 1)/9 + 7], Print[n]], {n, 0, 5000}]
%t A056677 Flatten[Position[Table[FromDigits[PadLeft[{7},n,2]],{n,5000}], _?PrimeQ]]-1 (* _Harvey P. Dale_, May 01 2012 *)
%t A056677 Select[Range[0, 2500], PrimeQ[20*(10^n - 1)/9 + 7]&] (* _Mikk Heidemaa_, Nov 28 2015 *)
%Y A056677 Cf. A002275, A093167, A099409.
%K A056677 hard,nonn,more
%O A056677 1,2
%A A056677 _Robert G. Wilson v_, Aug 10 2000
%E A056677 a(10) from Kamada link entered by _Michael S. Branicky_, Jan 14 2023