%I #21 Jan 17 2019 13:44:07
%S 1,2,5,7,23,32,38,47,116,140,229,247,734,931,1346,2162,2546,6980,
%T 10283,10543,12128,13741,105320
%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-41)/9 is prime.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/7/75551.htm#prime">Prime numbers of the form 755...551</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101143(n) + 1. - _Robert Price_, Oct 13 2013
%t Do[ If[ PrimeQ[(68*10^n - 41)/9], Print[n]], {n, 0, 10000}]
%Y Cf. A000533, A002275, A101143.
%K more,nonn
%O 1,2
%A _Robert G. Wilson v_, Jan 19 2005
%E a(19)-a(22) from Kamada data by _Robert Price_, Dec 14 2010
%E a(23) from _Robert Price_, Oct 13 2013