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%I #27 Jul 13 2023 12:27:12
%S 1,2,3,6,8,18,19,21,32,48,65,72,99,120,201,308,717,968,1344,2132,3567,
%T 3968,8327,10598,12465,15875,18895,28611,29418,83693,114962,145040,
%U 256654,262324,295581
%N Numbers k such that 7*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 (23*10^k+7)/3 is prime.
%C a(36) > 3*10^5. - _Robert Price_, Jul 13 2023
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/7/76669.htm#prime">Prime numbers of the form 766...669</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101150(n) + 1.
%t Do[ If[ PrimeQ[(23*10^n + 7)/3], Print[n]], {n, 0, 10000}]
%o (PARI) for(n=1, 10^5, if(isprime((23*10^n+7)/3), print1(n", "))) \\ _Altug Alkan_, Oct 08 2015
%Y Cf. A002275, A101150.
%K more,nonn
%O 1,2
%A _Robert G. Wilson v_, Jan 19 2005
%E More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
%E a(28)-a(29) from Kamada data by _Robert Price_, Dec 14 2010
%E a(30) from _Robert Price_, Oct 08 2015
%E a(31)-a(35) from _Robert Price_, Jul 13 2023
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