|
%I #16 Jul 08 2021 03:09:37
%S 1,2,8,26,44,167,185,3818,8741,50795
%N Numbers k such that 10^k + 7*R_k - 4 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also numbers k such that (16*10^k - 43)/9 is prime.
%C a(11) > 10^5. - _Robert Price_, Feb 11 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/1/17773.htm#prime">Prime numbers of the form 177...773</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A102027(n) + 1.
%t Do[ If[ PrimeQ[(16*10^n - 43)/9], Print[n]], {n, 0, 10000}]
%Y Cf. A002275, A102027.
%K more,nonn
%O 1,2
%A _Robert G. Wilson v_, Dec 16 2004
%E a(10) from _Robert Price_, Feb 11 2015
| 