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A322353
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Number of factorizations of n into distinct semiprimes; a(1) = 1 by convention.
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15
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1, 0, 0, 1, 0, 1, 0, 0, 1, 1, 0, 0, 0, 1, 1, 0, 0, 0, 0, 0, 1, 1, 0, 1, 1, 1, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 0, 1, 1, 1, 0, 0, 0, 0, 0, 1, 0, 0, 1, 0, 1, 0, 0, 1, 1, 1, 1, 1, 0, 2, 0, 1, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 1, 0, 0, 0, 0, 1, 0, 2, 1, 1, 1, 1, 0, 2, 1, 0, 1, 1, 1, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0
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OFFSET
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1,60
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COMMENTS
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A semiprime (A001358) is a product of any two prime numbers. In the even case, these factorizations have A001222(n)/2 factors. - Gus Wiseman, Dec 31 2020
Records 1, 2, 3, 4, 5, 9, 13, 15, 17, ... occur at 1, 60, 210, 840, 1260, 4620, 27720, 30030, 69300, ...
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LINKS
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FORMULA
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EXAMPLE
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a(4) = 1, as there is just one way to factor 4 into distinct semiprimes, namely as {4}.
The a(n) factorizations for n = 60, 210, 840, 1260, 4620, 12600, 18480:
4*15 6*35 4*6*35 4*9*35 4*15*77 4*6*15*35 4*6*10*77
6*10 10*21 4*10*21 4*15*21 4*21*55 4*6*21*25 4*6*14*55
14*15 4*14*15 6*10*21 4*33*35 4*9*10*35 4*6*22*35
6*10*14 6*14*15 6*10*77 4*9*14*25 4*10*14*33
9*10*14 6*14*55 4*10*15*21 4*10*21*22
6*22*35 6*10*14*15 4*14*15*22
10*14*33 6*10*14*22
10*21*22
14*15*22
(End)
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MATHEMATICA
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Table[Count[Subsets[Select[Divisors[n], PrimeOmega[#] == 2 &]], _?(Times @@ # == n &)], {n, 105}] (* Michael De Vlieger, Dec 11 2020 *)
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PROG
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(PARI) A322353(n, m=n) = if(1==n, 1, my(s=0); fordiv(n, d, if((2==bigomega(d)&&(d<=m)), s += A322353(n/d, d-1))); (s)); \\ Antti Karttunen, Dec 10 2020
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CROSSREFS
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Unlabeled multiset partitions of this type are counted by A007717.
The version for partitions is A112020, or A101048 without distinctness.
Positions of zeros include A320892.
Positions of nonzero terms are A320912.
The case of squarefree factors is A339661, or A320656 without distinctness.
Allowing prime factors gives A339839, or A320732 without distinctness.
A037143 lists primes and semiprimes.
A339846 counts even-length factorizations, with ordered version A174725.
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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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