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 A102946 Numbers n such that 2*10^n - 7 is prime. 2

%I

%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

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Last modified April 3 17:03 EDT 2020. Contains 333197 sequences. (Running on oeis4.)