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%I #28 Apr 14 2024 03:45:04
%S 1,3,5,7,11,12,28,36,384,629,720,1398,2002,2622,6457,19305,38508,
%T 46455,95976,102107,148505,189693
%N Numbers k such that 10^k + 6*R_k + 3 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also numbers k such that (5*10^k + 7)/3 is prime.
%C a(20) > 10^5. - _Robert Price_, Nov 15 2014
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/1/16669.htm#prime">Prime numbers of the form 166...669</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F a(n) = A102025(n) + 1.
%t Do[ If[ PrimeQ[(5*10^n + 7)/3], Print[n]], {n, 0, 10000}]
%Y Cf. A002275, A102025.
%K more,nonn,changed
%O 1,2
%A _Robert G. Wilson v_, Dec 16 2004
%E Addition of a(16) from Kamada data by _Robert Price_, Dec 12 2010
%E a(17)-a(18) from Erik Branger May 01 2013 by _Ray Chandler_, Aug 16 2013
%E a(19) from Kamada data by _Robert Price_, Nov 15 2014
%E a(20)-a(22) from Kamada data by _Tyler Busby_, Apr 14 2024
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