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%I #16 Jan 17 2019 13:44:07
%S 0,3,15,339
%N Numbers n such that 6*10^n + 2*R_n + 7 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C Also numbers n such that (56*10^n+61)/9 is prime.
%C a(5) > 10^5. - _Robert Price_, Sep 07 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/6/62229.htm#prime">Prime numbers of the form 622...229</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101524(n-1) + 1 for n>1.
%e For n=0, (56*10^n+61)/9=13 which is prime, so 0 is in the sequence.
%t Do[ If[ PrimeQ[(56*10^n + 61)/9], Print[n]], {n, 0, 10000}]
%Y Cf. A002275, A101524.
%K bref,more,nonn
%O 1,2
%A _Robert G. Wilson v_, Jan 18 2005
%E Inserted a(1)=0 by _Robert Price_, Sep 07 2015
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