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%I #32 Nov 14 2019 17:54:34
%S 1,2,5,8,24,29,84,110,129,176,593,1137,2675,4992,26904,31572,55077,
%T 81021,122274
%N Numbers n such that 3*10^n+7 is prime.
%C 593 and 1137 both give primes. - Joao da Silva (zxawyh66(AT)yahoo.com), Sep 30 2005
%C a(19) > 10^5. - _Robert Price_, Jan 26 2015
%C a(20) > 2*10^5. - _Robert Price_, Jul 04 2015
%C Some of the results were computed using the PrimeFormGW (PFGW) primality-testing program. - _Hugo Pfoertner_, Nov 14 2019
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/3/30007.htm#prime">Prime numbers of the form 300...007</a>.
%H Sabin Tabirca and Kieran Reynolds, <a href="http://multimedia.ucc.ie/Staff/ST/articles/SNJ03_Tabirca1.ps">Lacunary Prime Numbers</a>.
%F a(n) = A101824(n) + 1.
%e (3*10^5)+7 = (3*100000)+7 = 300000+7 = 300007, which is prime.
%t Do[ If[ PrimeQ[ 3*10^n + 7], Print[ n ]], {n, 0, 20000}]
%o (PARI) is(n)=ispseudoprime(3*10^n+7) \\ _Charles R Greathouse IV_, May 22 2017
%Y Cf. A096774, A101824.
%K nonn,more,hard
%O 1,2
%A Julien Peter Benney (jpbenney(AT)ftml.net), Nov 23 2004
%E a(12) from Mark Hudson (mrmarkhudson(AT)hotmail.com), Nov 26 2004
%E a(13 & a(14) from _Hugo Pfoertner_, Nov 29 2004. The next term is > 20000.
%E a(15) from Kamada data by _Robert Price_, Dec 12 2010
%E a(16)-a(18) from Kamada data by _Robert Price_, Jan 26 2015
%E a(19) from _Robert Price_, Jul 04 2015
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