|
%I #20 Jul 08 2021 03:12:05
%S 0,1,2,4,10,20,26,722,1310,3170,28934,66284,67796
%N Numbers k such that 4*R_k + 3 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also numbers k such that (4*10^k + 23)/9 is prime.
%C a(14) > 10^5. - _Robert Price_, Nov 26 2014
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/4/44447.htm#prime">Prime numbers of the form 44...447</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F a(n) = A056682(n-1) + 1. - _Robert Price_, Nov 26 2014
%t Do[ If[ PrimeQ[ 4(10^n - 1)/9 + 3], Print[n]], {n, 5000}]
%Y Cf. A002275, A056682.
%K nonn,hard,more
%O 1,3
%A _Robert G. Wilson v_, Oct 14 2004
%E a(11)-a(13) from _Robert Price_, Nov 26 2014
|
