A103058 - OEIS

%I #18 Jan 17 2019 13:44:07

%S 1,2,19,31,106,26194,39973

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

%C Also numbers n such that (67*10^n-13)/9 is prime.

%C a(8) > 10^5. - _Robert Price_, Sep 25 2015

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/7/74443.htm#prime">Prime numbers of the form 744...443</a>.

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

%F a(n) = A101140(n) + 1.

%t Do[ If[ PrimeQ[(67*10^n - 13)/9], Print[n]], {n, 0, 10000}]

%Y Cf. A002275, A101140.

%K more,nonn

%O 1,2

%A _Robert G. Wilson v_, Jan 19 2005

%E a(6) from Kamada data by _Robert Price_, Dec 14 2010

%E a(7) from _Robert Price_, Sep 25 2015