%I #25 Jan 17 2019 13:44:07
%S 0,1,4,9,10,31,70,160,171,277,630,724,3717,5542,5634,10656,12724,31954
%N Numbers n such that 4*10^n + 2*R_n + 1 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C Also numbers n such that (38*10^n+7)/9 is prime.
%C a(19) > 10^5. - _Robert Price_, May 09 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/4/42223.htm#prime">Prime numbers of the form 422...223</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101720(n-1) + 1.
%t Do[ If[ PrimeQ[(38*10^n + 7)/9], Print[n]], {n, 0, 10000}]
%Y Cf. A002275, A101720.
%K more,nonn
%O 1,3
%A _Robert G. Wilson v_, Jan 17 2005
%E Addition of a(16)-a(17) from Kamada data by _Robert Price_, Dec 12 2010
%E a(18) from Erik Branger, May 01 2013, submitted by _Ray Chandler_, Aug 16 2013
|