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A239761
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Number of pairs of functions (f, g) on a set of n elements into itself satisfying f(g(x)) = f(x).
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4
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1, 1, 10, 159, 3496, 98345, 3373056, 136535455, 6371523712, 336784920849, 19888195110400, 1297716672601151, 92721494240225280, 7199830049013964921, 603715489091812335616, 54366622743565012989375, 5233114241479255004839936, 536180296483497244155041825
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OFFSET
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0,3
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LINKS
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FORMULA
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a(n) ~ 5^(-1/4) * ((1+sqrt(5))/2)^(3*n-1/2) * n^n / exp(2*n/(1+sqrt(5))). - Vaclav Kotesovec, Aug 07 2014
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MAPLE
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b:= proc(n, i) option remember; `if`(n=0, 1, `if`(i<1, 0,
add(b(n-j, i-1)*binomial(n, j)*j^j, j=0..n)))
end:
a:= n-> b(n$2):
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MATHEMATICA
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f4[n_] := Sum[n^k Sum[Binomial[n - 1, j]*n^(n - 1 - j)*StirlingS1[j + 1, k] *(-1)^(j + k + 1), {j, 0, n - 1}], {k, 1, n}] (* David Einstein, Oct 31 2016 *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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