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A239677
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Least numbers k such that k*3^n-1 is prime.
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0
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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
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OFFSET
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1,2
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COMMENTS
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All the numbers in this sequence, excluding a(1), are even.
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LINKS
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EXAMPLE
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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.
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PROG
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(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
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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