The commutative ambiguity cambG,X of a context-free grammar G with start symbol X assigns to each Parikh vector v the number of distinct leftmost derivations yielding a word with Parikh vector v. Based on the results on the generalization of Newton’s method to ω-continuous semirings [EKL07b, EKL07a, EKL10], we show how to approximate cambG,X by means of rational formal power series, and give a ...