A103062 - OEIS

%I #20 Jan 17 2019 13:44:07

%S 0,1,3,237,243,351,967,1843,7405,9633,59821

%N Numbers n such that 7*10^n + 5*R_n + 4 is prime, where R_n = 11...1 is the repunit (A002275) of length n.

%C Also numbers n such that (68*10^n + 31)/9 is prime.

%C a(12) > 10^5. - _Robert Price_, Oct 03 2015

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/7/75559.htm#prime">Prime numbers of the form 755...559</a>.

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

%F a(n) = A101146(n-1) + 1 for n>1.

%e For n=0, (68*10^n + 31)/9 = 11 which is a prime, so 0 is a term.

%t Do[ If[ PrimeQ[(68*10^n + 31)/9], Print[n]], {n, 0, 10000}]

%Y Cf. A002275, A101146.

%K more,nonn

%O 1,3

%A _Robert G. Wilson v_, Jan 19 2005

%E a(1)=0 inserted and a(11) added by _Robert Price_, Oct 03 2015