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A097763
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Number of different partitions of the set {1, 2, ..., n} into an even number of blocks such that each block contains at least 2 elements.
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2
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0, 0, 0, 3, 10, 25, 56, 224, 1506, 9951, 57992, 315425, 1761552, 11022180, 78474748, 603715831, 4771273414, 38070877273, 309146434240, 2598546954268, 22887194502518, 211388690471531, 2031261113410564, 20121026325645745
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OFFSET
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1,4
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COMMENTS
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LINKS
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FORMULA
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Exponential generating function: cosh(exp(x)-x-1).
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EXAMPLE
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a(6)=25 since we can partition a set of six elements into two non-singleton blocks, either of sizes four and two (15 ways) or three and three (10 ways); a(6)=15+10=25.
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MAPLE
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seq(coeff(series(cosh(exp(x)-x-1), x=0, 25), x^i)*i!, i=1..24);
# second Maple program:
with(combinat):
b:= proc(n, i, t) option remember; `if`(n=0, t,
`if`(i<2, 0, add(multinomial(n, n-i*j, i$j)/j!*
b(n-i*j, i-1, irem(t+j, 2)), j=0..n/i)))
end:
a:= n-> b(n$2, 1):
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MATHEMATICA
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multinomial[n_, k_List] := n!/Times @@ (k!); b[n_, i_, t_] := b[n, i, t] = If[n == 0, t, If[i < 2, 0, Sum[multinomial[n, Join[{n - i*j}, Array[i &, j]]]/j!*b[n - i*j, i - 1, Mod[t + j, 2]], {j, 0, n/i}]]]; a[n_] := b[n, n, 1]; Table[a[n], {n, 1, 30}] (* Jean-François Alcover, Jan 10 2016, after Alois P. Heinz *)
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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Isabel C. Lugo (izzycat(AT)gmail.com), Aug 23 2004
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STATUS
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approved
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