Counting Morse functions on the 2-sphere

Abstract

We count how many 'different' Morse functions exist on the 2-sphere. There are several ways of declaring that two Morse functions f and g are 'indistinguishable' but we concentrate only on two natural equivalence relations: homological (when the regular sublevel sets f and g have identical Betti numbers) and geometric (when f is obtained from g via global, orientation-preserving changes of coordinates on S2 and ). The count of homological classes is reduced to a count of lattice paths confined to the first quadrant. The count of geometric classes is reduced to a count of certain labeled trees, which is encoded by certain elliptic integrals. © 2008 Foundation Compositio Mathematica.