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A103774
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Number of ways to write n! as product of squarefree numbers.
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4
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1, 1, 2, 2, 6, 10, 42, 42, 82, 204, 1196, 1556, 10324, 34668, 104948, 104964, 873540, 1309396, 11855027, 25238220, 91193575, 453628255, 5002616219, 5902762219, 21142729523, 122981607092, 189706055368, 547296181656, 7291700021313, 14330422534833, 202498591157970
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OFFSET
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1,3
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COMMENTS
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Also the number of set multipartitions (multisets of sets) of the multiset of prime factors of n!. For example, The a(2) = 1 through a(6) = 10 set multipartitions are:
{1} {12} {1}{1}{12} {1}{1}{123} {1}{1}{12}{123}
{1}{2} {1}{1}{1}{2} {1}{12}{13} {1}{12}{12}{13}
{1}{1}{1}{23} {1}{1}{1}{12}{23}
{1}{1}{2}{13} {1}{1}{1}{2}{123}
{1}{1}{3}{12} {1}{1}{2}{12}{13}
{1}{1}{1}{2}{3} {1}{1}{3}{12}{12}
{1}{1}{1}{1}{2}{23}
{1}{1}{1}{2}{2}{13}
{1}{1}{1}{2}{3}{12}
{1}{1}{1}{1}{2}{2}{3}
(End)
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LINKS
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EXAMPLE
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n=5, 5! = 1*2*3*4*5 = 120 = 2 * 2 * 2 * 3 * 5: a(5)=#{2*2*2*3*5,2*2*2*15,2*2*6*5,2*2*30,2*2*3*10,2*6*10}=6.
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MATHEMATICA
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sub[w_, e_] := Block[{v=w}, v[[e]]--; v]; ric[w_, k_] := ric[w, k] = If[Max[w] == 0, 1, Block[{e, s, p = Flatten@ Position[Sign@w, 1]}, s = Select[ Prepend[#, First@p] & /@ Subsets[Rest@p], Total[1/2^#] <= k &]; Sum[ric[sub[w, e], Total[1/2^e]], {e, s}]]]; a[n_] := ric[ Sort[ Last /@ FactorInteger[n!]], 1]; Array[a, 22] (* Giovanni Resta, Sep 30 2019 *)
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CROSSREFS
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A157612 is the case of superprimorials.
A045778 counts strict factorizations.
A048656 counts squarefree divisors of factorials.
A050320 counts factorizations into squarefree numbers.
A050326 counts strict factorizations into squarefree numbers.
A076716 counts factorizations of factorials.
A089259 counts set multipartitions of integer partitions.
A116540 counts normal set multipartitions.
A157612 counts strict factorizations of factorials.
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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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