A058934 Numbers k such that 2*5^k + 1 is prime.

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

Offset

1,3

Comments

a(n) is odd for n > 0. Next term > 17200. - Julián Aguirre, Apr 22 2011

a(16) > 2*10^5. - Robert Price, Mar 14 2015

Links

Table of n, a(n) for n=1..15.

Example

a(3)=13 and 1 + 2*5^13 = 2441406251 is prime.

Mathematica

Select[Range[0, 16000], PrimeQ[2*5^#+1]&] (* Julián Aguirre, Apr 22 2011 *)

Prog

(Sage)
[n for n in range(1500) if is_prime(2*5^n+1)]
# Julián Aguirre, Apr 22 2011
(PARI) is(n)=ispseudoprime(2*5^n+1) \\ Charles R Greathouse IV, Jun 12 2017

Crossrefs

Cf. A003306 (2*3^n + 1 is prime), A120375 (2*5^n-1 is prime).

Sequence in context: A095863 A136520 A212416 * A141088 A187915 A115128

Adjacent sequences: A058931 A058932 A058933 * A058935 A058936 A058937

Keyword

nonn,more

Author

Labos Elemer, Jan 12 2001

Extensions

Edited by Joerg Arndt, Apr 22 2011

Terms 1443,...,12357 from Julián Aguirre, Apr 22 2011

a(14)-a(15) from Robert Price, Mar 14 2015

Status

approved