%I
%S 0,1,3,13,45,105,159,297,1443,2977,3699,11709,12357,43165,121995
%N Numbers n such that 2*5^n + 1 is prime.
%C a(n) is odd for n>0. Next term > 17200.  _Julián Aguirre_, Apr 22 2011
%C a(16) > 2*10^5.  _Robert Price_, Mar 14 2015
%e a(3)=13 and 1+2*5^13=2441406251 is prime.
%t Select[Range[0, 16000], PrimeQ[2*5^#+1]&] (* _Julián Aguirre_, Apr 22 2011 *)
%o (Sage)
%o [n for n in range(1500) if is_prime(2*5^n+1)]
%o # _Julián Aguirre_, Apr 22 2011
%o (PARI) is(n)=ispseudoprime(2*5^n+1) \\ _Charles R Greathouse IV_, Jun 12 2017
%Y Cf. A003306 (2*3^n + 1 is prime), A120375 (2*5^n1 is prime).
%K nonn,more
%O 1,3
%A _Labos Elemer_, Jan 12 2001
%E Edited by _Joerg Arndt_, Apr 22 2011
%E Terms 1443,...,12357 from _Julián Aguirre_, Apr 22 2011
%E a(14)a(15) from _Robert Price_, Mar 14 2015
