%I #18 Jan 17 2019 13:44:07
%S 0,1,3,4,10,15,25,69,159,166,261,442,1339,1797,2170,3163,3472,4917,
%T 18267
%N Numbers n such that 8*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 (76*10^n-13)/9 is prime.
%C a(20) > 10^5. - _Robert Price_, Oct 20 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/8/84443.htm#prime">Prime numbers of the form 844...443</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101067(n-1) + 1, for n>1.
%t Do[ If[ PrimeQ[(76*10^n - 13)/9], Print[n]], {n, 0, 10000}]
%Y Cf. A002275, A101067.
%K more,nonn
%O 1,3
%A _Robert G. Wilson v_, Jan 19 2005
%E a(18) from Kamada data by _Robert Price_, Dec 14 2010
%E Inserted a(1)=0 by _Robert Price_, Oct 20 2015
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