A239677 - OEIS

%I #8 Mar 25 2014 10:23:24

%S 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,

%T 10,16,20,18,6,2,60,20,10,10,40,58,48,16,12,4,32,90,30,10,8,130,62,26,

%U 10,6,2,30,10,32,18,6,2,74,28,46,18,6,2,30,10,46,80,94,52

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

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

%e 1*3^2-1 = 8 is not prime. 2*3^2-1 = 17 is prime. Thus, a(2) = 2.

%e 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.

%o (Python)

%o import sympy

%o from sympy import isprime

%o def Pow_3(n):

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

%o ....if isprime(k*(3**n)-1):

%o ......return n

%o n = 1

%o while n < 100:

%o ..print(Pow_3(n))

%o ..n += 1

%o (PARI)

%o for(n=1, 100, k=0; while(!isprime(k*3^n-1), k++); print1(k, ", ")) \\ _Colin Barker_, Mar 24 2014

%Y Cf. A085427.

%K nonn

%O 1,2

%A _Derek Orr_, Mar 23 2014