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A317751
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Number of divisors d of n such that there exists a factorization of n into factors > 1 with GCD d.
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6
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0, 1, 1, 2, 1, 2, 1, 2, 2, 2, 1, 3, 1, 2, 2, 3, 1, 3, 1, 3, 2, 2, 1, 3, 2, 2, 2, 3, 1, 2, 1, 3, 2, 2, 2, 5, 1, 2, 2, 3, 1, 2, 1, 3, 3, 2, 1, 4, 2, 3, 2, 3, 1, 3, 2, 3, 2, 2, 1, 3, 1, 2, 3, 4, 2, 2, 1, 3, 2, 2, 1, 5, 1, 2, 3, 3, 2, 2, 1, 4, 3, 2, 1, 3, 2, 2, 2, 3, 1, 3, 2, 3, 2, 2, 2, 4, 1, 3, 3, 5, 1, 2, 1, 3, 2
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OFFSET
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1,4
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COMMENTS
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Also the number of distinct possible GCDs of factorizations of n into factors > 1.
Also the number of nonzero terms in row n of A317748.
If n is squarefree and composite, a(n) = 2.
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LINKS
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EXAMPLE
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The divisors of 36 that are possible GCDs of factorizations of 36 are {1, 2, 3, 6, 36}, so a(36) = 5.
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MATHEMATICA
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facs[n_]:=If[n<=1, {{}}, Join@@Table[Map[Prepend[#, d]&, Select[facs[n/d], Min@@#>=d&]], {d, Rest[Divisors[n]]}]];
goc[n_, m_]:=Length[Select[facs[n], And[And@@(Divisible[#, m]&/@#), GCD@@(#/m)==1]&]];
Table[Length[Select[Divisors[n], goc[n, #]!=0&]], {n, 100}]
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PROG
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(PARI)
A317751aux(n, m, facs, gcds) = if(1==n, setunion([gcd(Vec(facs))], gcds), my(newfacs); fordiv(n, d, if((d>1)&&(d<=m), newfacs = List(facs); listput(newfacs, d); gcds = setunion(gcds, A317751aux(n/d, d, newfacs, gcds)))); (gcds));
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CROSSREFS
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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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