A245348 - OEIS

%I #21 Dec 16 2021 16:48:52

%S 1,1,1,4,3,2,27,15,8,4,256,112,50,22,10,3125,1125,430,166,66,26,46656,

%T 14256,4752,1626,576,206,76,823543,218491,64484,19768,6310,2054,688,

%U 232,16777216,3932160,1040384,288512,83736,24952,7660,2388,764

%N Number T(n,k) of endofunctions f on [n] that are self-inverse on [k]; triangle T(n,k), n>=0, 0<=k<=n, read by rows.

%C T(n,k) counts endofunctions f:{1,...,n}-> {1,...,n} with f(f(i))=i for all i in {1,...,k}.

%H Alois P. Heinz, <a href="/A245348/b245348.txt">Rows n = 0..140, flattened</a>

%F T(n,k) = Sum_{i=0..min(k,n-k)} C(n-k,i)*C(k,i)*i!*A000085(k-i)*n^(n-k-i).

%e T(3,1) = 15: (1,1,1), (2,1,1), (3,1,1), (1,2,1), (3,2,1), (1,3,1), (3,3,1), (1,1,2), (2,1,2), (1,2,2), (1,3,2), (1,1,3), (2,1,3), (1,2,3), (1,3,3).

%e T(3,2) = 8: (2,1,1), (1,2,1), (3,2,1), (2,1,2), (1,2,2), (1,3,2), (2,1,3), (1,2,3).

%e T(3,3) = 4: (3,2,1), (1,3,2), (2,1,3), (1,2,3).

%e Triangle T(n,k) begins:

%e 0 :       1;

%e 1 :       1,      1;

%e 2 :       4,      3,     2;

%e 3 :      27,     15,     8,     4;

%e 4 :     256,    112,    50,    22,   10;

%e 5 :    3125,   1125,   430,   166,   66,   26;

%e 6 :   46656,  14256,  4752,  1626,  576,  206,  76;

%e 7 :  823543, 218491, 64484, 19768, 6310, 2054, 688, 232;

%e      ...

%p g:= proc(n) g(n):= `if`(n<2, 1, g(n-1)+(n-1)*g(n-2)) end:

%p T:= (n, k)-> add(binomial(n-k, i)*binomial(k, i)*i!*

%p              g(k-i)*n^(n-k-i), i=0..min(k, n-k)):

%p seq(seq(T(n,k), k=0..n), n=0..10);

%t g[n_] := g[n] = If[n<2, 1, g[n-1] + (n-1)*g[n-2]]; T[0, 0] = 1; T[n_, k_] := Sum[Binomial[n-k, i]*Binomial[k, i]*i!*g[k-i]*n^(n-k-i), {i, 0, Min[k, n-k]}]; Table[T[n, k], {n, 0, 10}, {k, 0, n}] // Flatten (* _Jean-François Alcover_, Feb 19 2017, translated from Maple *)

%Y Columns k=0-1 give: A000312, A089945(n-1) for n>0.

%Y Main diagonal gives A000085.

%Y T(2n,n) gives A245141.

%Y Cf. A239771, A245692.

%K nonn,tabl

%O 0,4

%A _Alois P. Heinz_, Jul 18 2014