Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #26 Jan 17 2019 13:44:06
%S 1,2,3,8,18,20,31,42,103,175,181,531,706,1077,1177,1552,19737,32197,
%T 51508,58275,62233,90033
%N Numbers n such that 5*10^n + 3 is prime.
%C a(23) > 2*10^5. - _Robert Price_, Aug 03 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/5/50003.htm#prime">Prime numbers of the form 500...003</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) = A101568(n) + 1.
%e 1, 2 and 3 are in the list because 53, 503 and 5003 are all prime.
%e 500000003 (5*10^8 + 3) is the next similar prime, so 8 is the next term in the sequence.
%t Do[ If[ PrimeQ[5*10^n + 3], Print[n]], {n, 10000}] (* _Robert G. Wilson v_, Aug 13 2004 *)
%o (PARI) is(n)=isprime(5*10^n+3) \\ _Charles R Greathouse IV_, Sep 27 2016
%Y Cf. A101568.
%K more,nonn
%O 1,2
%A _Carl R. White_, Aug 10 2004
%E a(12)-a(16) from _Robert G. Wilson v_, Aug 13 2004
%E a(17)-a(22) from Kamada data by _Robert Price_, Dec 09 2010