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 A239768 Number of pairs of functions (f,g) from a set of n elements into itself satisfying f(x) = f(g(f(x))). 3
 1, 1, 10, 195, 6808, 362745, 26848656, 2621263519, 324981308800, 49669569764433, 9146879704748800, 1993011341241988551, 506190915590699695104, 148000190814308473203433, 49289886405448749446514688, 18529196186934893511062427375, 7800708229072237749055062900736, 3652486190893312491910941333813537 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 LINKS Alois P. Heinz, Table of n, a(n) for n = 0..251 FORMULA a(n) = Sum_{k=0..n} (n!/(n-k)!) * C(n,k) * (n*k)^(n-k). - David Einstein, Oct 10 2016 MATHEMATICA A[n_] := If[n == 0, 1, Sum[(n!/(n - k)!) Binomial[n, k] (n k)^(n - k), {k, 1, n}]] Table[A[n], {n, 10}] (* David Einstein, Oct 10 2016 *) PROG (PARI) a(n) = sum(k = 1, n, (n!/(n-k)!)*binomial(n, k)*(n*k)^(n-k)); \\ Michel Marcus, Oct 11 2016 CROSSREFS Cf. A181162. Sequence in context: A126463 A239766 A294047 * A239769 A152561 A224298 Adjacent sequences:  A239765 A239766 A239767 * A239769 A239770 A239771 KEYWORD nonn AUTHOR Chad Brewbaker, Mar 26 2014 EXTENSIONS a(6)-a(7) from Giovanni Resta, Mar 28 2014 a(8)-a(17) from David Einstein, Oct 10 2016 STATUS approved

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Last modified August 2 14:47 EDT 2021. Contains 346428 sequences. (Running on oeis4.)