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A219557
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Sum of numbers of bipartite partitions of (n,k) into distinct pairs for 0<=k<=n.
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2
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1, 3, 9, 33, 96, 273, 749, 1953, 4916, 12023, 28642, 66575, 151544, 338294, 741880, 1601048, 3403936, 7137386, 14774713, 30219025, 61115184, 122300146, 242312186, 475589389, 925158391, 1784529840, 3414565313, 6483608230, 12221370425, 22876263089, 42534593868
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OFFSET
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0,2
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LINKS
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FORMULA
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a(n) = Sum_{k=0..n} [x^n*y^k] 1/2 * Product_{i,j>=0} (1+x^i*y^j).
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EXAMPLE
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a(2) = 9: [(2,0)], [(2,1)], [(2,0),(0,1)], [(1,1),(1,0)], [(2,2)], [(2,1),(0,1)], [(2,0),(0,2)], [(1,2),(1,0)], [(1,1),(1,0),(0,1)].
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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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