%I #20 Jan 17 2019 13:44:07

%S 1,2,3,5,8,20,29,86,283,757,1199,2473,2733,24853,39629,53492,71237,

%T 72302,81653,167510

%N Numbers n such that 5*10^n + 9 is prime.

%C a(21) > 2*10^5. - _Robert Price_, Aug 10 2015

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/5/50009.htm#prime">Prime numbers of the form 500...009</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) = A101569(n) + 1.

%t Do[ If[ PrimeQ[5*10^n + 3], Print[n]], {n, 0, 10000}]

%o (PARI) is_ok(n)=isprime(5*10^n + 9);

%o first(m)=my(v=vector(m),t);for(i=1,m,while(!is_ok(t),t++;);v[i]=t;t++);v; /* _Anders Hellström_, Aug 10 2015 */

%Y Cf. A101569.

%K more,nonn

%O 1,2

%A _Robert G. Wilson v_, Jan 18 2005

%E a(14)-a(19) from Kamada data by _Robert Price_, Dec 10 2010

%E a(20) from _Robert Price_, Aug 10 2015