For example, a labeled binary tree is a node or a node and two subtrees.
>
|
|
One can use attributes to count the number of leaves.
>
|
|
>
|
|
| (1) |
For example, the series up to order 10 indicates that there are five trees with 7 nodes and 4 leaves.
>
|
|
| (2) |
The internal path length, or sum of distances from nodes to the root, can be defined recursively.
>
|
|
>
|
|
| (3) |
This system can be solved and the average values attained.