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Search: id:A159297
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| A159297 |
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Number of 3D matrices with positive integer entries such that sum of all entries equals n |
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+0
1
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| 1, 4, 10, 25, 58, 130, 286, 620, 1329, 2827, 5977, 12559, 26227, 54493, 112849, 233272, 481616, 992955, 2043238, 4194649, 8591014, 17559133, 35833948, 73054885, 148849186, 303171755, 617306563, 1256452642, 2555937826
(list; graph; listen)
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OFFSET
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1,2
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COMMENT
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Equivalently, number of quadruples (i, j, k; P) such that i, j and k are positive integers and P is a composition of n into ijk parts. (A composition of n with m parts is an ordered list of m positive integers that sum to n. The number of compositions of n into m parts is given by the binomial coefficient C(n - 1, m - 1).) [From Joel Brewster Lewis (jblewis(AT)post.harvard.edu), May 07 2009]
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FORMULA
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a(n) = sum(C(n - 1, ijk - 1)) where the sum is over all triples (i, j, k) such that 0 < i, j, k and ijk <= n. [From Joel Brewster Lewis (jblewis(AT)post.harvard.edu), May 07 2009]
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EXAMPLE
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For n=3, the 10 possible matrices are: 3 (1*1*1) (1,2) as three different vectors (1*1*2, 1*2*1, 2*1*1) (2,1) as three different vectors (1*1*2, 1*2*1, 2*1*1) (1,1,1) as three different vectors (1*1*2, 1*2*1, 2*1*1)
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MATHEMATICA
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Table[Sum[Sum[Sum[Binomial[n - 1, i*j*k - 1], {i, 1, n}], {j, 1, n}], {k, 1, n}], {n, 1, 40}] [From Joel Brewster Lewis (jblewis(AT)post.harvard.edu), May 07 2009]
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CROSSREFS
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Cf. A101509
Sequence in context: A145368 A111207 A113412 this_sequence A033539 A020748 A021004
Adjacent sequences: A159294 A159295 A159296 this_sequence A159298 A159299 A159300
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KEYWORD
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nonn
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AUTHOR
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Lior Manor (lior.manor(AT)gmail.com), Apr 09 2009
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EXTENSIONS
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More terms from Joel Brewster Lewis (jblewis(AT)post.harvard.edu), May 07 2009
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