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A239676 Least k such that k*3^n+1 is prime. 2
1, 2, 2, 4, 2, 2, 2, 8, 6, 2, 8, 28, 10, 12, 4, 4, 2, 2, 10, 20, 26, 24, 8, 48, 16, 34, 14, 14, 18, 6, 2, 26, 26, 14, 22, 26, 16, 22, 12, 4, 62, 64, 68, 88, 70, 56, 34, 96, 32, 50, 20, 24, 8, 6, 2, 18, 6, 2, 8, 6, 2, 42, 14, 18, 6, 2, 98, 66, 22, 70, 74, 80, 68, 52 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

All numbers in this sequence, except for a(0), are even.

LINKS

Marius A. Burtea, Table of n, a(n) for n = 0..1000

EXAMPLE

1*3^1+1 = 4 is not prime. 2*3^1+1 = 7 is prime. Thus, a(1) = 2.

1*3^3+1 = 28 is not prime. 2*3^3+1 = 57 is not prime. 3*3^3+1 = 82 is not prime. 4*3^3+1 = 109 is prime. Thus, a(3) = 4.

PROG

(Python)

import sympy

from sympy import isprime

def Pow3(n):

..for k in range(10**4):

....if isprime(k*(3**n)+1):

......return n

n = 1

while n < 100:

..print(Pow3(n))

..n += 1

(PARI)

for(n=0, 100, k=0; while(!isprime(k*3^n+1), k++); print1(k, ", ")) \\ Colin Barker, Mar 24 2014

(MAGMA) sol:=[]; m:=1; for n in [0..73] do k:=0; while not IsPrime(k*3^n+1) do k:=k+1; end while; sol[m]:=k; m:=m+1; end for; sol; // Marius A. Burtea, Jun 05 2019

CROSSREFS

Cf. A003306 (where k=2), A035050 (k*2^n+1 is prime).

Sequence in context: A077636 A215847 A057000 * A295639 A182982 A090047

Adjacent sequences:  A239673 A239674 A239675 * A239677 A239678 A239679

KEYWORD

nonn

AUTHOR

Derek Orr, Mar 23 2014

STATUS

approved

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Last modified July 19 03:54 EDT 2019. Contains 325144 sequences. (Running on oeis4.)