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 A056657 #32 Jul 07 2024 17:08:39
%S A056657 0,1,5,7,8,10,19,22,40,62,65,118,121,148,251,283,304,591,745,874,1203,
%T A056657 1363,2239,2402,5105,5775,5812,12455,14234,39605,55543,84238,275921
%N A056657 Numbers k such that 60*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A056657 Also numbers k such that (20*10^k+1)/3 is prime.
%H A056657 Makoto Kamada, <a href="https://stdkmd.net/nrr/6/66667.htm#prime">Prime numbers of the form 66...667</a>.
%H A056657 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A056657 a(n) = A096507(n) - 1.
%e A056657 7, 67, 666667, 66666667, 666666667, 66666666667, etc. are primes.
%t A056657 Do[ If[ PrimeQ[ 60*(10^n - 1)/9 + 7 ], Print[n]], {n, 25556}]
%Y A056657 Cf. A002275, A093170, A096507.
%K A056657 nonn
%O A056657 1,3
%A A056657 _Robert G. Wilson v_, Aug 09 2000
%E A056657 More terms from _Robert G. Wilson v_, Oct 22 2003
%E A056657 2239,2402,5105,5775 from _Farideh Firoozbakht_, Dec 23 2003
%E A056657 39605 and 55543 from Serge Batalov, Jun 2009
%E A056657 84238 from Serge Batalov, Jul 06 2009 confirmed as next term by _Ray Chandler_, Feb 23 2012
%E A056657 a(33) derived from A096507 by _Robert Price_, Jul 07 2024