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%I #35 Jul 08 2021 05:04:55
%S 3,5,7,33,45,105,197,199,281,301,317,1107,1657,3395,35925,37597,64305,
%T 80139,221631
%N Numbers k such that 9*R_k - 8 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also numbers k such that 10^k - 9 is a prime.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/9/99991.htm#prime">Prime numbers of the form 99...991</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F a(n) = A056696(n) + 1.
%e a(2) = 5, since 10^5 - 9 = 99991, which is prime.
%t Do[ If[ PrimeQ[10^n - 9], Print[n]], {n, 0, 7000}]
%Y Cf. A002275, A093177, A056696.
%K nonn
%O 1,1
%A _Alonso del Arte_, Jul 07 2004
%E a(12) - a(14) from _Robert G. Wilson v_, Oct 15 2004
%E a(15) - a(16) from _Jason Earls_, Jan 07 2008
%E a(17) - a(19) from _Alexander Gramolin_, May 13 2011
%E Edited by _Ray Chandler_, Feb 26 2012
%E Title corrected by _Robert Price_, Sep 06 2014
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