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%I #40 Oct 01 2021 01:46:04
%S 1,5,71,112,115,173,263,310,358,10499,22925,37957,254195,463649
%N Numbers k such that 3*10^k + 7*R_k - 6 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also numbers k such that (34*10^k-61)/9 is prime.
%C a(15) > 8*10^5. - _Jason H Parker_, Sep 30 2021
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/3/37771.htm#prime">Prime numbers of the form 377...771</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101841(n) + 1.
%t Do[ If[ PrimeQ[(34*10^k-61)/9], Print[k]], {k, 1, 10000}]
%Y Cf. A002275, A101841.
%K nonn,more
%O 1,2
%A _Robert G. Wilson v_, Dec 17 2004
%E Addition of a(10)-a(11) from Kamada data by _Robert Price_, Dec 13 2010
%E a(12) from Erik Branger May 01 2013 by _Ray Chandler_, Aug 16 2013
%E a(13) from _Jason H Parker_, Jun 14 2019
%E 0 removed by _Georg Fischer_, Jan 03 2021
%E a(14) from _Jason H Parker_, Sep 30 2021