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 A102931 Numbers n such that 10^n + 2*R_n + 7 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 2

%I

%S 1,3,6,51,57,63,150,393,420,547,42024,43063

%N Numbers n such that 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 (11*10^n+61)/9 is prime.

%C a(13) > 10^5. - _Robert Price_, Jan 17 2015

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/1/12229.htm#prime">Prime numbers of the form 122...229</a>.

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

%F a(n) = A102011(n) + 1.

%t Do[ If[ PrimeQ[(11*10^n + 61)/9], Print[n]], {n, 0, 10000}]

%Y Cf. A002275, A102011.

%K more,nonn

%O 1,2

%A _Robert G. Wilson v_, Dec 16 2004

%E a(11)-a(12) from Erik Branger May 01 2013 by _Ray Chandler_, Aug 16 2013

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Last modified May 24 17:16 EDT 2019. Contains 323533 sequences. (Running on oeis4.)