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 A056654 #38 Jul 14 2025 06:21:31
%S A056654 0,1,2,4,8,10,23,83,220,1313,2951,20015,51053
%N A056654 Numbers k such that 10*R_k + 3 is prime, where R_k is the repunit (A002275) of length k.
%C A056654 Also numbers k such that (10^(k+1)+17)/9 is prime.
%C A056654 a(14) > 10^5. - _Robert Price_, Nov 01 2014
%H A056654 Makoto Kamada, <a href="https://stdkmd.net/nrr/1/11113.htm#prime">Prime numbers of the form 11...113</a>.
%H A056654 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A056654 a(n) = A097683(n+1) - 1. - _Robert Price_, Nov 01 2014
%e A056654 8 is a term because 111111113 is a prime.
%t A056654 Do[ If[ PrimeQ[ 10*(10^n - 1)/9 + 3 ], Print[ n ] ], {n, 0, 1350} ]
%o A056654 (PARI) is(n)=ispseudoprime(10^n\9*10+3) \\ _Charles R Greathouse IV_, Nov 10 2021
%Y A056654 Cf. A093011 (corresponding primes), A097683.
%K A056654 hard,nonn,more
%O A056654 1,3
%A A056654 _Robert G. Wilson v_, Aug 09 2000
%E A056654 a(11) (only a probable prime) from _Rick L. Shepherd_, Mar 14 2004
%E A056654 a(12)-a(13) derived from A097683 by _Robert Price_, Nov 01 2014