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A226466
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The number of endomorphisms of the symmetric inverse monoid on the set {1..n}.
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1
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1, 3, 14, 54, 306, 918, 4524, 25560, 168828, 1315764, 11998584, 125358192, 1466062200, 18835528104, 262376016912, 3927709053792, 62795044663056, 1067217284481840, 19208039592749472, 364940846656681536, 7298740111856220576, 153273039685710932448, 3372003538678423189440
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OFFSET
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0,2
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LINKS
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PROG
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(PARI) a(n)=if(n==0, return (1), if(n<5, return([3, 14, 54, 306][n]))); n!*(3+sum(m=0, n, sum(k=0, m\2, 2^(m-3*k)/((n-m)!*(m-2*k)!*k!))))
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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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