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%I #33 Sep 08 2022 08:45:16
%S 1,2,3,4,6,16,21,28,48,82,122,130,282,304,4602,12984,13614,42762,
%T 90597,109928,158242
%N Numbers n such that 2*10^n - 7 is prime.
%C Also numbers n such that 10^n + 9*R_n - 6 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C a(20) > 10^5. - _Robert Price_, Nov 16 2014
%C a(22) > 2*10^5. - _Robert Price_, Oct 25 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/1/19993.htm#prime">Prime numbers of the form 199...993</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A102033(n) + 1.
%p select(n -> isprime(2*10^n-7),[$0..10^4]); # _Robert Israel_, Nov 16 2014
%t Do[ If[ PrimeQ[2*10^n - 7], Print[n]], {n, 0, 10000}]
%t Select[Range[1000], PrimeQ[(2 10^# - 7)] &] (* _Vincenzo Librandi_, Nov 17 2014 *)
%o (Magma) [n: n in [1..500] | IsPrime(2*10^n-7)]; // _Vincenzo Librandi_, Nov 17 2014
%o (PARI) is(n)=ispseudoprime(2*10^n-7) \\ _Charles R Greathouse IV_, Jun 06 2017
%Y Cf. A002275, A102033.
%K more,nonn
%O 1,2
%A _Robert G. Wilson v_, Dec 16 2004
%E More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
%E Addition of a(18)-a(19) from Kamada data by _Robert Price_, Dec 10 2010
%E a(20)-a(21) from _Robert Price_, Oct 25 2015