%I #17 Jul 08 2021 00:45:48

%S 0,1,11,17,773,18155

%N Numbers k such that 3*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 (34*10^k - 7)/9 is prime.

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/3/37777.htm#prime">Prime numbers of the form 377...77</a>.

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

%t Do[ If[ PrimeQ[ 3*10^n + 7*(10^n-1)/9], Print[n]], {n, 0, 30070}]

%Y Cf. A002275, A093939.

%K hard,nonn

%O 1,3

%A _Robert G. Wilson v_, Aug 10 2000