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Numbers n such that 6*10^n-7 is prime.
2

%I #18 Jan 17 2019 13:44:07

%S 1,2,5,6,7,11,17,29,42,47,96,108,166,210,331,1022,3593,4426,5704,

%T 13936,16486,85910

%N Numbers n such that 6*10^n-7 is prime.

%C Also numbers n such that 5*10^n + 9*R_n - 6 is prime, where R_n = 11...1 is the repunit (A002275) of length n.

%C a(23) > 2*10^5. - _Robert Price_, Aug 07 2015

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/5/59993.htm#prime">Prime numbers of the form 599...993</a>.

%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.

%F a(n) = A101590(n) + 1.

%t Do[ If[ PrimeQ[6*10^n - 7], Print[n]], {n, 0, 10000}]

%o (PARI) is(n)=ispseudoprime(6*10^n-7) \\ _Charles R Greathouse IV_, Jun 13 2017

%Y Cf. A002275, A101590.

%K more,nonn

%O 1,2

%A _Robert G. Wilson v_, Jan 18 2005

%E Addition of a(20)-a(22) from Kamada data by _Robert Price_, Dec 09 2010