The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Please make a donation to keep the OEIS running. We are now in our 56th year. In the past year we added 10000 new sequences and reached almost 9000 citations (which often say "discovered thanks to the OEIS"). Other ways to donate

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A058934 Numbers n such that 2*5^n + 1 is prime. 1

%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^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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified November 26 12:37 EST 2020. Contains 338639 sequences. (Running on oeis4.)