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%I #17 Jan 17 2019 13:44:06
%S 0,1,3,9,39,2323
%N Numbers n such that 4*10^n+7 is prime.
%C No further terms < 50000.
%C a(7) > 2*10^5. - _Robert Price_ May 16 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/4/40007.htm#prime">Prime numbers of the form 400...007</a>.
%H Sabin Tabirca and Kieran Reynolds, <a href="http://multimedia.ucc.ie/Staff/ST/articles/SNJ03_Tabirca1.ps">Lacunary Prime Numbers</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101714(n-1) + 1.
%e n = 1, 3, 9 are members since 47, 4007 and 4000000007 are primes.
%t Do[ If[ PrimeQ[4*10^n + 7], Print[n]], {n, 0, 10000}]
%o (PARI) is(n)=ispseudoprime(4*10^n+7) \\ _Charles R Greathouse IV_, Jun 12 2017
%Y Cf. A056806, A100501, A088274, A101714.
%K hard,nonn
%O 1,3
%A Julien Peter Benney (jpbenney(AT)ftml.net), Jan 15 2005