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A262321
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Number of ways to select a subset s containing n from {1,...,n} and then partition s into blocks of equal size.
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2
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1, 1, 3, 7, 18, 43, 118, 337, 1025, 3479, 13056, 48817, 199477, 898135, 4051128, 18652459, 93872040, 492132207, 2658676056, 14841915049, 84757413959, 517609038551, 3384739112196, 21742333893177, 141230605251082, 1001795869162783, 7387581072984938
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OFFSET
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0,3
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COMMENTS
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a(0) = 1 by convention.
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LINKS
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FORMULA
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E.g.f.: A(x) - Integral_{x} A(x) dx, with A(x) = e.g.f. of A262320.
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EXAMPLE
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a(0) = 1: {}.
a(1) = 1: 1.
a(2) = 3: 2, 12, 1|2.
a(3) = 7: 3, 13, 1|3, 23, 2|3, 123, 1|2|3.
a(4) = 18: 4, 14, 1|4, 24, 2|4, 34, 3|4, 124, 1|2|4, 134, 1|3|4, 234, 2|3|4, 1234, 12|34, 13|24, 14|23, 1|2|3|4.
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MAPLE
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b:= proc(n) option remember; n!*`if`(n=0, 1,
add(1/(d!*(n/d)!^d), d=numtheory[divisors](n)))
end:
a:= n-> add(b(k)*binomial(n-1, k-1), k=0..n):
seq(a(n), n=0..30);
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MATHEMATICA
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b[n_] := b[n] = n!*If[n == 0, 1, DivisorSum[n, 1/(#!*(n/#)!^#)&]];
a[n_] := Sum[b[k]*Binomial[n-1, k-1], {k, 0, n}];
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CROSSREFS
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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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