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Numbers of form 3^i * prime(j), i>=0, j>0.
2

%I #9 Mar 31 2023 02:30:05

%S 2,3,5,6,7,9,11,13,15,17,18,19,21,23,27,29,31,33,37,39,41,43,45,47,51,

%T 53,54,57,59,61,63,67,69,71,73,79,81,83,87,89,93,97,99,101,103,107,

%U 109,111,113,117,123,127,129,131,135,137,139,141,149,151,153,157,159

%N Numbers of form 3^i * prime(j), i>=0, j>0.

%C a(n) is either prime or of form 3*a(m), m<n.

%H Chai Wah Wu, <a href="/A106843/b106843.txt">Table of n, a(n) for n = 1..10000</a>

%o (Python)

%o from itertools import count, islice

%o from sympy import isprime

%o def A106843_gen(startvalue=2): # generator of terms >= startvalue

%o for m in count(max(startvalue,2)):

%o a, b = divmod(m,3)

%o while not b:

%o a, b = divmod(a,3)

%o if (k:=3*a+b)==1 or isprime(k):

%o yield m

%o A106843_list = list(islice(A106843_gen(),20)) # _Chai Wah Wu_, Mar 30 2023

%Y Cf. A093641.

%K nonn

%O 1,1

%A _Ralf Stephan_, May 05 2005