A099420 - OEIS

%I #19 Jul 08 2021 03:12:28

%S 1,2,3,5,9,12,15,21,264,383,2720,4494,21167,45825,55851,64568,70727

%N Numbers k such that 7*R_k - 4 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

%C Also numbers k such that (7*10^k - 43)/9 is prime.

%C a(18) > 10^5. - _Robert Price_, Oct 19 2014

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/7/77773.htm#prime">Prime numbers of the form 77...773</a>.

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

%F a(n) = A056689(n) + 1.

%t Do[ If[ PrimeQ[ 7(10^n - 1)/9 - 4], Print[n]], {n, 0, 5000}]

%Y Cf. A002275, A056689, A093165.

%K nonn,more

%O 1,2

%A _Robert G. Wilson v_, Oct 14 2004

%E a(13)-a(17) from _Robert Price_, Oct 19 2014