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%I #19 Jul 04 2021 22:08:19
%S 0,1,2,16,139,989,1886,3529,5995,13819,21872,26044,87719,232598
%N Numbers k such that 90*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also numbers k such that 10^(k+1) - 3 is a prime number.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/9/99997.htm#prime">Prime numbers of the form 99...997</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F a(n) = A089675(n) - 1. - _Robert Price_, Nov 01 2014
%t Do[ If[ PrimeQ[ 90*(10^n - 1)/9 + 7 ], Print[n]], {n, 0, 1250}]
%Y Cf. A002275, A089675.
%K hard,nonn
%O 1,3
%A _Robert G. Wilson v_, Aug 09 2000
%E More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
%E a(13)-a(14) derived from A089675 by _Robert Price_, Nov 01 2014
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