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%I #24 Sep 08 2022 08:45:16
%S 1,4,6,7,10,16,22,31,1315,2064,6150,8707,12252,18610,21630,41712,
%T 44808,45421
%N Numbers n such that 7*10^n + 4*R_n - 3 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C Also numbers n such that (67*10^n-31)/9 is prime.
%C a(19) > 10^5. - _Robert Price_, Sep 28 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/7/74441.htm#prime">Prime numbers of the form 744...441</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101139(n) + 1.
%t Do[ If[ PrimeQ[(67*10^n - 31)/9], Print[n]], {n, 0, 10000}]
%t Select[Range[10000], PrimeQ[(67 10^# - 31) / 9] &] (* _Vincenzo Librandi_, Sep 29 2015 *)
%o (Magma) [n: n in [0..500] | IsPrime((67*10^n-31) div 9)]; // _Vincenzo Librandi_, Sep 29 2015
%Y Cf. A002275, A101139.
%K more,nonn
%O 1,2
%A _Robert G. Wilson v_, Jan 19 2005
%E a(13)-a(15) from Kamada data by _Robert Price_, Dec 14 2010
%E a(16)-a(18) from Erik Branger May 01 2013 by _Ray Chandler_, Aug 16 2013
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