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A087422
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Number of terms in the expansion of Product(x_i + x_{i+1} + ... + x_j) over 1 <= i < j <= n.
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2
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OFFSET
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0,3
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COMMENTS
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The sum of the coefficients in the expansion of this product are the superfactorials A000178. - Robert G. Wilson v, Aug 02 2005
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LINKS
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EXAMPLE
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a(3) = 8 because the expansion of (x+y)(y+z)(x+y+z) = x^2y + x^2z + 2xy^2 + 3xyz + xz^2 + y^3 + 2y^2z + yz^2 has 8 terms.
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MAPLE
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a:= n-> nops(expand(mul(mul(add(x[k], k=i..j), i=1..j-1), j=2..n))):
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MATHEMATICA
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f[1]=1; f[n_] := Block[{lst = Take[{a, b, c, d, e, f, g, h, i}, n], s = 1}, Do[s = s*Times @@ Plus @@@ Partition[lst, i, 1], {i, 2, n}]; Length@Expand@s]; Do[ Print@ f@n, {n, 9}] (* Robert G. Wilson v, Sep 18 2006 *)
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CROSSREFS
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KEYWORD
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nonn,more
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AUTHOR
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Alex Postnikov (apost(AT)math.mit.edu), Oct 22 2003
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EXTENSIONS
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STATUS
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approved
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