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 A056655 #30 Jul 14 2025 10:03:43
%S A056655 0,1,3,4,7,22,28,39,130,135,214,610,766,2152,2575,22972,42688,85711,
%T A056655 85863,112066,538507,631714
%N A056655 Numbers k such that 10*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A056655 Also numbers k such that (10^(k+1)+53)/9 is prime.
%C A056655 2575 also produces a probable prime.
%C A056655 a(20) > 10^5. - _Robert Price_, Jan 13 2015
%C A056655 a(23) > 670000 (per the Kamada link). - _Bill McEachen_, Mar 02 2024
%H A056655 Makoto Kamada, <a href="https://stdkmd.net/nrr/1/11117.htm#prime">Prime numbers of the form 11...117</a>.
%H A056655 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A056655 a(n) = A097684(n) - 1 for all n >= 0. - _Rick L. Shepherd_, Aug 23 2004
%t A056655 Do[ If[ PrimeQ[ 10*(10^n - 1)/9 + 7 ], Print[ n ] ], {n, 0, 1250} ]
%Y A056655 Cf. A093139 (corresponding primes), A097684.
%K A056655 hard,nonn,more
%O A056655 1,3
%A A056655 _Robert G. Wilson v_, Aug 09 2000
%E A056655 a(14) (giving a probable prime) from _Rick L. Shepherd_, Mar 23 2004
%E A056655 a(15) from _Rick L. Shepherd_, Aug 23 2004
%E A056655 a(16)-a(19) derived from A097684 by _Robert Price_, Jan 13 2015
%E A056655 a(20)-a(22) from the Kamada link by _Bill McEachen_, Mar 02 2024