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 A097685 Numbers n such that R_n + 8 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 3

%I

%S 2,5,6,8,17,50,684,720,1452,1679,3146,72821

%N Numbers n such that R_n + 8 is prime, where R_n = 11...1 is the repunit (A002275) of length n.

%C Also numbers n such that (10^n+71)/9 is prime.

%C Values indicate primes of the form "(n-1) ones followed by a nine". Related to the base 10 repunit primes.

%C a(13) > 10^5. - _Robert Price_, Nov 01 2014

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/1/11119.htm#prime">Prime numbers of the form 11...119</a>.

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

%F a(n) = A056659(n) + 1.

%t Do[ If[ PrimeQ[(10^n - 1)/9 + 8], Print[n]], {n, 0, 5000}] (* _Robert G. Wilson v_, Oct 14 2004 *)

%Y Cf. A004023, A056659, A097683, A097684.

%K nonn,more,hard

%O 1,1

%A _Carl R. White_, Aug 19 2004

%E a(12) from _Robert Price_, Nov 01 2014

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Last modified September 27 15:29 EDT 2020. Contains 337383 sequences. (Running on oeis4.)