A058934 - OEIS

%I #30 Dec 12 2021 20:07:54

%S 0,1,3,13,45,105,159,297,1443,2977,3699,11709,12357,43165,121995

%N Numbers k such that 2*5^k + 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^n-1 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