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A272515
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Number of set partitions of [n] into three blocks with distinct sizes.
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2
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60, 105, 448, 2016, 4980, 15675, 61644, 155155, 482573, 1733550, 4549808, 13890360, 48104628, 128949675, 392009140, 1322692581, 3607864403, 10929721440, 36245555284, 100109572875, 302709337515, 990788537700, 2763564406113, 8344789976616, 27039048750600
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OFFSET
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6,1
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LINKS
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FORMULA
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a(n) = n! * [x^n*y^3] Product_{n>=1} (1+y*x^n/n!).
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MAPLE
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b:= proc(n, i, t) option remember; `if`(t>i or t*(t+1)/2>n
or t*(2*i+1-t)/2<n, 0, `if`(n=0, 1, b(n, i-1, t)+
`if`(i>n, 0, b(n-i, i-1, t-1)*binomial(n, i))))
end:
a:= n-> b(n$2, 3):
seq(a(n), n=6..40);
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MATHEMATICA
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b[n_, i_, t_] := b[n, i, t] = If[t > i || t(t+1)/2 > n || t(2i+1-t)/2 < n, 0, If[n == 0, 1, b[n, i - 1, t] + If[i > n, 0, b[n - i, i - 1, t - 1]* Binomial[n, i]]]];
a[n_] := b[n, n, 3];
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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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