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 A056656 #35 Jul 18 2024 05:29:35
%S A056656 0,1,2,7,10,35,94,100,127,259,350,466,644,1010,1177,1216,2441,3760,
%T A056656 3805,15616,26458,63116,88544,93496
%N A056656 Numbers k such that 20*R_k + 3 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A056656 Also numbers k such that (2*10^(k+1)+7)/9 is prime.
%H A056656 Makoto Kamada, <a href="https://stdkmd.net/nrr/2/22223.htm#prime">Prime numbers of the form 22...223</a>.
%H A056656 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A056656 a(n) = A096506(n) - 1.
%t A056656 Do[ If[ PrimeQ[ 20*(10^n - 1)/9 + 3 ], Print[n]], {n, 7000}]
%Y A056656 Cf. A093162 (corresponding primes), A096506.
%K A056656 nonn
%O A056656 1,3
%A A056656 _Robert G. Wilson v_, Aug 09 2000
%E A056656 2441 from _Rick L. Shepherd_, Mar 27 2004
%E A056656 15616 and 26458 from Erik Branger, Jan 31 2010
%E A056656 63116, 88544 and 93496 from Erik Branger, Mar 14 2011; confirmed as next terms by _Ray Chandler_, Feb 17 2012