Higher algebra of $A_\infty$ and $\Omega B As$-algebras in Morse theory I

Abstract

Elaborating on works by Abouzaid and Mescher, we prove that for a Morse function on a smooth compact manifold, its Morse cochain complex can be endowed with an $\Omega B As$-algebra structure by counting moduli spaces of perturbed Morse gradient trees. This rich structure descends to its already known $A_\infty$-algebra structure. We then introduce the notion of $\Omega B As$-morphism between two $\Omega B As$-algebras and prove that given two Morse functions, one can construct an $\Omega B As$-morphism between their associated $\Omega B As$-algebras by counting moduli spaces of two-colored perturbed Morse gradient trees. This morphism induces a standard $A_\infty$-morphism between the induced $A_\infty$-algebras. We work with integer coefficients, and provide to this extent a detailed account on the sign conventions for $A_\infty$ (resp. $\Omega B As$)-algebras and $A_\infty$ (resp. $\Omega B As$)-morphisms, using polytopes (resp. moduli spaces) which explicitly realize the dg-operadic objects encoding them. Our proofs also involve showing at the level of polytopes that an $\Omega B As$-morphism between $\Omega B As$-algebras naturally induces an $A_\infty$-morphism between $A_\infty$-algebras. This paper comes in particular with a short survey on operads, $A_\infty$-algebras and $A_\infty$-morphisms, the associahedra and the multiplihedra. All the details on transversality, gluing maps, signs and orientations for the moduli spaces defining the algebraic structures on the Morse cochains are thorougly carried out. It moreover lays the basis for a second article in which we solve the problem of finding a satisfactory homotopic notion of higher morphisms between $A_\infty$-algebras and between $\Omega B As$-algebras, and show how this higher algebra of $A_\infty$ and $\Omega B As$-algebras naturally arises in the context of Morse theory.