A056686 - OEIS

%I #27 Jul 04 2021 22:09:33

%S 0,2,3,5,9,14,21,87,206,527,959,2099,18521,36782,38516

%N Numbers k such that 50*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

%C Also numbers k such that (5*10^(k+1)+13)/9 is prime.

%C a(16) > 10^5. - _Robert Price_, Nov 01 2014

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/5/55557.htm#prime">Prime numbers of the form 55...557</a>.

%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>

%F a(n) = A099417(n+1) - 1. - _Robert Price_, Nov 01 2014

%t Do[ If[ PrimeQ[50*(10^n - 1)/9 + 7], Print[n]], {n, 0, 5000}]

%Y Cf. A002275, A099417.

%K hard,nonn

%O 1,2

%A _Robert G. Wilson v_, Aug 10 2000

%E a(13)-a(15) derived from A099417 by _Robert Price_, Nov 01 2014