%I #26 May 04 2024 02:52:49

%S 2,8,12,18,26,32,138,188,222,338,1002,2744,6530,38690,39464,335480,

%T 343734

%N Numbers k such that 5*R_k + 4 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

%C Also numbers k such that (5*10^k + 31)/9 is prime.

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/5/55559.htm#prime">Prime numbers of the form 55...559</a>.

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

%F a(n) = A056687(n) + 1.

%t Do[ If[ PrimeQ[ 5(10^n - 1)/9 + 4], Print[n]], {n, 0, 5000}]

%Y Cf. A002275, A093403, A056687.

%K nonn,more

%O 1,1

%A _Robert G. Wilson v_, Oct 14 2004

%E a(13)-a(15) from Kamada link by _Ray Chandler_, Feb 27 2012

%E a(16)-a(17) from Kamada data by _Tyler Busby_, May 03 2024