A335920 Number T(n,k) of binary search trees of height k having n internal nodes; triangle T(n,k), k>=0, k<=n<=2^k-1, read by columns.

1, 1, 2, 1, 4, 6, 6, 4, 1, 8, 20, 40, 68, 94, 114, 116, 94, 60, 28, 8, 1, 16, 56, 152, 376, 844, 1744, 3340, 5976, 10040, 15856, 23460, 32398, 41658, 49700, 54746, 55308, 50788, 41944, 30782, 19788, 10948, 5096, 1932, 568, 120, 16, 1, 32, 144, 480, 1440, 4056

Offset: 0

Author

Alois P. Heinz, Jun 29 2020

Keywords

• nonn
• tabf

Comments

Empty external nodes are counted in determining the height of a search tree.

T(n,k) is defined for n,k >= 0. The triangle contains only the positive terms. Terms not shown are zero.

Examples

			Triangle T(n,k) begins:
  1;
     1;
        2;
        1, 4;
           6,   8;
           6,  20,   16;
           4,  40,   56,   32;
           1,  68,  152,  144,   64;
               94,  376,  480,  352,  128;
              114,  844, 1440, 1376,  832,  256;
              116, 1744, 4056, 4736, 3712, 1920, 512;
  ...
		

Links

• Alois P. Heinz, Columns k = 0..10, flattened
• Wikipedia, Binary search tree
• Index entries for sequences related to rooted trees
• Index entries for sequences related to trees

Crossrefs

Row sums give A000108.

Column sums give A001699.

Main diagonal gives A011782.

T(n+3,n+2) gives A014480.

T(n,max(0,A000523(n)+1)) = A328349(n).

Cf. A073345, A076615, A195581, A244108, A335919 (the same read by rows), A335921, A335922.

Programs

Maple

b:= proc(n, h) option remember; `if`(n=0, 1, `if`(n<2^h,
      add(b(j-1, h-1)*b(n-j, h-1), j=1..n), 0))
    end:
T:= (n, k)-> b(n, k)-`if`(k>0, b(n, k-1), 0):
seq(seq(T(n, k), n=k..2^k-1), k=0..6);

Mathematica

b[n_, h_] := b[n, h] = If[n == 0, 1, If[n < 2^h,
     Sum[b[j - 1, h - 1]*b[n - j, h - 1], {j, 1, n}], 0]];
T[n_, k_] := b[n, k] - If[k > 0, b[n, k - 1], 0];
Table[Table[T[n, k], {n, k, 2^k - 1}], {k, 0, 6}] // Flatten (* Jean-François Alcover, Feb 08 2021, after Alois P. Heinz *)

Formula

Sum_{k=0..n} k * T(n,k) = A335921(n).

Sum_{n=k..2^k-1} n * T(n,k) = A335922(k).