is the number of leaves on the tree.
The basic justification is very simple.
is the number of leaves on the tree.
, then the number
of leaves dominates in the recurrence: 
(Rule
1). , then the
first appearance of f(n) in the tree dominates in the recurrence:
(Rule 3).
(There is a ``regularity'' condition on f(n) for this to be true
that holds for nearly all functions that we'll see, so you can ignore it). balance, then f(n) makes a
contribution at every level: 
(Rule 2).
