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%I #29 Jul 04 2021 22:10:34
%S 2,4,6,32,44,104,196,198,280,300,316,1106,1656,3394,35924,37596,64304,
%T 80138,221630
%N Numbers k such that 90*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also numbers k such that 10^(k+1) - 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) = A095714(n) - 1.
%t Do[ If[ PrimeQ[90*(10^n - 1)/9 + 1], Print[n]], {n, 0, 7000}]
%Y Cf. A002275, A093177, A095714.
%K nonn
%O 1,1
%A _Robert G. Wilson v_, Aug 10 2000
%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
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