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Search: id:A070925
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| A070925 |
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Number of subsets of A={1,2,...,n} that have the same center of gravity as A, i.e. (n+1)/2. |
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+0
2
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| 1, 1, 3, 3, 7, 7, 19, 17, 51, 47, 151, 137, 471, 427, 1519, 1391, 5043, 4651, 17111, 15883, 59007, 55123, 206259, 193723, 729095, 688007, 2601639, 2465133, 9358943, 8899699, 33904323, 32342235, 123580883
(list; graph; listen)
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OFFSET
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1,3
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EXAMPLE
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Of the 32 (2^5) sets which can be constructed from the set A = (1,2,3,4,5} only the sets {3}, {2, 3, 4}, {2, 4}, {1, 2, 4, 5}, {1, 2, 3, 4, 5}, {1, 3, 5}, {1, 5} give an average of 3.
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MATHEMATICA
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Needs["DiscreteMath`Combinatorica`"]; f[n_] := Block[{s = Subsets[n], c = 0, k = 2}, While[k < 2^n + 1, If[ (Plus @@ s[[k]]) / Length[s[[k]]] == (n + 1)/2, c++ ]; k++ ]; c]; Table[ f[n], {n, 1, 20}]
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CROSSREFS
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Sequence in context: A147144 A152113 A146149 this_sequence A146687 A146655 A146812
Adjacent sequences: A070922 A070923 A070924 this_sequence A070926 A070927 A070928
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KEYWORD
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nonn
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AUTHOR
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Sharon Sela (sharonsela(AT)hotmail.com), May 20 2002
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EXTENSIONS
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Edited by Robert G. Wilson v (rgwv(AT)rgwv.com) and John W. Layman (layman(AT)math.vt.edu), May 25 2002
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