A305824 Number of endofunctions on [n] whose cycle lengths are triangular numbers.

1, 1, 3, 18, 157, 1776, 24807, 413344, 8004537, 176630400, 4374300331, 120136735104, 3623854678677, 119102912981248, 4236492477409935, 162152320065532416, 6645233337842716273, 290321208589666369536, 13469914225467040015827, 661442143465113960448000

Offset

0,3

Links

Alois P. Heinz, Table of n, a(n) for n = 0..386

Maple

b:= proc(n) option remember; local r, f, g;
      if n=0 then 1 else r, f, g:=$0..2;
      while f<=n do r, f, g:= r+(f-1)!*
         b(n-f)*binomial(n-1, f-1), f+g, g+1
      od; r fi
    end:
a:= n-> add(b(j)*n^(n-j)*binomial(n-1, j-1), j=0..n):
seq(a(n), n=0..20);

Mathematica

b[n_] := b[n] = Module[{r, f, g}, If[n == 0, 1, {r, f, g} = {0, 1, 2}; While[f <= n, {r, f, g} = {r + (f - 1)!*b[n - f]*Binomial[n - 1, f - 1], f + g, g + 1}]; r]];
a[0] = 1; a[n_] := Sum[b[j]*n^(n - j)*Binomial[n - 1, j - 1], {j, 0, n}];
Table[a[n], {n, 0, 20}] (* Jean-François Alcover, Jun 15 2018, after Alois P. Heinz *)

Crossrefs

Cf. A000217, A060435, A116956, A193374, A205799, A273994, A273996, A273998.

Sequence in context: A060913 A246523 A246529 * A116956 A166887 A075678

Adjacent sequences: A305821 A305822 A305823 * A305825 A305826 A305827

Keyword

nonn

Author

Alois P. Heinz, Jun 10 2018

Status

approved