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Numbers k such that 80*R_k + 9 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
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%I #21 Jan 17 2019 13:44:05

%S 1,13,16,34,4174,4471,9811,12259,12340,13759,14575,53410,144682,148327

%N Numbers k such that 80*R_k + 9 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

%C Also numbers k such that (8*10^(k+1) + 1)/9 is prime.

%C a(15) > 2*10^5. - _Robert Price_, Nov 01 2014

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/8/88889.htm#prime">Prime numbers of the form 88...889</a>.

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

%F a(n) = A096508(n) - 1. - _Robert Price_, Nov 01 2014

%t Do[ If[ PrimeQ[ 80*(10^n - 1)/9 + 9 ], Print[n]], {n, 0, 30000}]

%Y Cf. A002275, A096508.

%K hard,nonn

%O 1,2

%A _Robert G. Wilson v_, Aug 09 2000

%E Intermediate missing terms 9811 (from M. Kamada), 12259, 12340, 13759 added by _Serge Batalov_, Dec 14 2008

%E a(12)-a(14) derived from A096508 by _Robert Price_, Nov 01 2014