OFFSET
0,2
LINKS
FORMULA
T(n,0) = 2^n, T(n,k) = 0 if k < 0 or n <= k, otherwise T(n,k) = n^(n-2) if k=n-1, otherwise T(n,k) = Sum_{j=0..k} C(n-1,j)*T(j+1,j)*T(n-1-j,k-j).
EXAMPLE
T(3,1) = 6, because there are 6 forests of trees on 3 or fewer nodes using a subset of labels 1,2,3 and 1 edge:
.1-2. .1... ...2. .1-2. .1.2. .1.2.
..... .|... ../.. ..... .|... ../..
..... .3... .3... .3... .3... .3...
Triangle begins:
1;
2, 0;
4, 1, 0;
8, 6, 3, 0;
16, 24, 27, 16, 0;
32, 80, 150, 190, 125, 0;
MAPLE
T:= proc(n, k) option remember;
if k=0 then 2^n
elif k<0 or n<=k then 0
elif k=n-1 then n^(n-2)
else add(binomial(n-1, j) *T(j+1, j) *T(n-1-j, k-j), j=0..k)
fi
end:
seq(seq(T(n, k), k=0..n), n=0..11);
MATHEMATICA
T[n_, k_] := T[n, k] = Which[k == 0, 2^n, k < 0 || n <= k, 0, k == n-1, n^(n-2), True, Sum[Binomial[n-1, j]*T[j+1, j]*T[n-1-j, k-j], {j, 0, k}]]; Table[T[n, k], {n, 0, 11}, {k, 0, n}] // Flatten (* Jean-François Alcover, Aug 29 2014, translated from Maple *)
CROSSREFS
KEYWORD
nonn,tabl
AUTHOR
Alois P. Heinz, Sep 16 2008
STATUS
approved