We consider large boolean functions over variables ,
,...,. Any such function can be represented (or computed)
by a complete binary tree with and or or in the internal
nodes and a literal in the external nodes. Many different trees can
represent the same function, so that a fundamental question is related
to the socalled ``complexity'' of a boolean function :
The existence of a
limiting probability distribution on the set of (finite or
infinite) and/or trees was shown by Lefmann and
Savicky [LS97]. We give here an alternative proof of the
existence of this distribution, which leads to effective computation
in simple cases. We also consider the relationship between the
probability and the complexity of a boolean
function . The approach by Lefmann and Savicky leads to the
following inequalities :

(1) 
A
detailed analysis of the functions enumerating some subfamilies of
trees, and of their radius of convergence, allows us to improve the
upper bound, from
to
.
 LS97

Hanno Lefmann and Petr Savický.
Some typical properties of large AND/OR Boolean formulas.
Random Structures Algorithms, 10(3):337351, 1997.
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