Editing Exponential formula (section)

==Combinatorial interpretation==

In combinatorial applications, the numbers <math>a_n</math> count the number of some sort of "connected" structure on an <math>n</math>-point set, and the numbers <math>b_n</math> count the number of (possibly disconnected) structures. The numbers <math>b_n/n!</math> count the number of [[isomorphism class]]es of structures on <math>n</math> points, with each structure being weighted by the reciprocal of its [[automorphism group]], and the numbers <math>a_n/n!</math> count isomorphism classes of connected structures in the same way.