On the Average State Complexity of Partial derivative Automata: an analytic Combinatorics Approach

The partial derivative automaton (Apd) is usually smaller than other nondeterministic finite automata constructed from a regular expression, and it can be seen as a quotient of the Glushkov automaton (Apos). By estimating the number of regular expressions that have ε as a partial derivative, we compute a lower bound of the average number of mergings of states in Apos and describe its asymptotic behaviour. This depends on the alphabet size, k, and for growing k’s its limit approaches half the number of states in Apos. The lower bound corresponds to consider the Apd automaton for the marked version of the regular expression, i.e. where all its letters are made different. Experimental results suggest that the average number of states of this automaton, and of the Apd automaton for the unmarked regular expression, are very close to each other.