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A320655
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Number of factorizations of n into semiprimes. Number of multiset partitions of the multiset of prime factors of n, into pairs.
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64
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1, 0, 0, 1, 0, 1, 0, 0, 1, 1, 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 1, 1, 0, 1, 1, 1, 0, 0, 0, 0, 0, 0, 1, 1, 1, 2, 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, 1, 1, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 1, 0, 0, 0, 1, 1, 0, 2, 1, 1, 1, 1, 0, 2, 1, 0, 1, 1, 1, 1, 0, 0, 0, 2, 0, 0, 0, 1, 0
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OFFSET
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1,36
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COMMENTS
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LINKS
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EXAMPLE
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The a(900) = 5 factorizations into semiprimes:
900 = (4*9*25)
900 = (4*15*15)
900 = (6*6*25)
900 = (6*10*15)
900 = (9*10*10)
The a(900) = 5 multiset partitions into pairs:
{{1,1},{2,2},{3,3}}
{{1,1},{2,3},{2,3}}
{{1,2},{1,2},{3,3}}
{{1,2},{1,3},{2,3}}
{{2,2},{1,3},{1,3}}
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MATHEMATICA
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semfacs[n_]:=If[n<=1, {{}}, Join@@Table[Map[Prepend[#, d]&, Select[semfacs[n/d], Min@@#>=d&]], {d, Select[Rest[Divisors[n]], PrimeOmega[#]==2&]}]];
Table[Length[semfacs[n]], {n, 100}]
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PROG
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(PARI) A320655(n, m=n) = if(1==n, 1, my(s=0); fordiv(n, d, if((2==bigomega(d)&&(d<=m)), s += A320655(n/d, d))); (s)); \\ Antti Karttunen, Dec 06 2020
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CROSSREFS
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The positions of zeros are A026424.
Cf. A001055, A001222, A001358, A007716, A007717, A056239, A112798, A318871, A318953, A320462, A320656, A320658, A320659.
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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