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A068605
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Number of functions from [1,2,...,n] to [1,2,...,n] such that the image contains exactly two elements.
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4
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2, 18, 84, 300, 930, 2646, 7112, 18360, 45990, 112530, 270204, 638820, 1490762, 3440430, 7864080, 17825520, 40107726, 89652906, 199229060, 440401500, 968883762, 2122317318, 4630511064, 10066329000, 21810380150, 47110421826, 101468601612, 217969589460, 467077692570
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OFFSET
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2,1
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COMMENTS
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LINKS
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FORMULA
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a(n) = C(n, 2) * (2^n - 2).
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MATHEMATICA
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Table[ Binomial[n, 2]*(2^n - 2), {n, 2, 30}]
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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Sharon Sela (sharonsela(AT)hotmail.com), Mar 29 2002
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EXTENSIONS
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STATUS
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approved
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