login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified April 8 14:52 EDT 2020. Contains 333314 sequences. (Running on oeis4.)