%I #22 Jul 08 2021 00:48:16

%S 1,4,13,25,36,357,373,1041,1089,1093,1297,8274,10732,15972,18114,

%T 21823,34519,36096,75498

%N Numbers k such that 4*10^k + 7*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

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

%C a(20) > 10^5. - _Robert Price_, Dec 22 2014

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/4/47777.htm#prime">Prime numbers of the form 477...77</a>.

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

%t Do[ If[ PrimeQ[ 4*10^n + 7*(10^n-1)/9], Print[n]], {n, 0, 6000}]

%Y Cf. A002275, A093940.

%K hard,nonn

%O 1,2

%A _Robert G. Wilson v_, Aug 10 2000

%E a(12) from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

%E a(13)-a(19) from _Robert Price_, Dec 22 2014