A239677 Least numbers k such that k*3^n-1 is prime.

1, 2, 2, 8, 4, 6, 2, 2, 16, 8, 6, 2, 16, 10, 4, 14, 8, 18, 6, 2, 18, 6, 2, 20, 18, 6, 2, 38, 30, 10, 16, 20, 18, 6, 2, 60, 20, 10, 10, 40, 58, 48, 16, 12, 4, 32, 90, 30, 10, 8, 130, 62, 26, 10, 6, 2, 30, 10, 32, 18, 6, 2, 74, 28, 46, 18, 6, 2, 30, 10, 46, 80, 94, 52

Offset

1,2

Comments

All the numbers in this sequence, excluding a(1), are even.

Links

Table of n, a(n) for n=1..74.

Example

1*3^2-1 = 8 is not prime. 2*3^2-1 = 17 is prime. Thus, a(2) = 2.
1*3^5-1 = 242 is not prime. 2*3^5-1 = 485 is not prime. 3*3^5-1 = 728 is not prime. 4*3^5-1 = 971 is prime. Thus, a(5) = 4.

Prog

(Python)
import sympy
from sympy import isprime
def Pow_3(n):
..for k in range(10**4):
....if isprime(k*(3**n)-1):
......return n
n = 1
while n < 100:
..print(Pow_3(n))
..n += 1
(PARI)
for(n=1, 100, k=0; while(!isprime(k*3^n-1), k++); print1(k, ", ")) \\ Colin Barker, Mar 24 2014

Crossrefs

Cf. A085427.

Sequence in context: A274041 A049331 A369771 * A331333 A120399 A144816

Adjacent sequences: A239674 A239675 A239676 * A239678 A239679 A239680

Keyword

nonn

Author

Derek Orr, Mar 23 2014

Status

approved