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Numbers n such that 3*10^n-7 is prime.
2

%I #27 Jan 17 2019 13:44:07

%S 1,2,5,10,29,39,114,484,865,4180,5385,13122,13832,22676,25020,30057,

%T 35910,37935,42295,50194,110076,184124,191152

%N Numbers n such that 3*10^n-7 is prime.

%C Also numbers n such that 2*10^n + 9*R_n - 6 is prime, where R_n = 11...1 is the repunit (A002275) of length n.

%C a(21) > 10^5. - _Robert Price_, Jan 25 2015

%C a(24) > 2*10^5. - _Robert Price_, Aug 09 2015

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/2/29993.htm#prime">Prime numbers of the form 299...993</a>.

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

%F a(n) = A101973(n) + 1.

%e For n=1,2,5 the corresponding primes are 23,293,29993.

%t Do[ If[ PrimeQ[3*10^n-7], Print[n]], {n, 0, 10000}]

%o (PARI) is_ok(n)=isprime(3*10^n-7); /* _Anders Hellström_, Aug 09 2015 */

%Y Cf. A002275, A101973.

%K more,nonn

%O 1,2

%A _Robert G. Wilson v_, Dec 17 2004

%E Addition of a(12)-a(15) from Kamada data by _Robert Price_, Dec 12 2010

%E a(16)-a(20) from Kamada data by _Robert Price_, Jan 25 2015

%E a(21)-a(23) from _Robert Price_, Aug 09 2015