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%I #19 Jan 17 2019 13:44:07
%S 0,1,2,4,7,14,20,39,102,156,457,1190,2308,5870,8314,9703,12828,14959,
%T 17995,19730,26480,31440
%N Numbers n such that 2*10^n + 6*R_n + 3 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C Also numbers n such that (8*10^n+7)/3 is prime.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/2/26669.htm#prime">Prime numbers of the form 266...669</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101965(n-1) + 1.
%t Do[ If[ PrimeQ[(8*10^n + 7)/3], Print[n]], {n, 0, 10000}]
%Y Cf. A002275, A101965.
%K more,nonn
%O 1,3
%A _Robert G. Wilson v_, Dec 17 2004
%E Addition of a(17)-a(21) from Kamada data by _Robert Price_, Dec 10 2010
%E 0 added by _Arkadiusz Wesolowski_, Mar 10 2011
%E a(22) from Erik Branger May 01 2013 by _Ray Chandler_, Aug 16 2013
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