Algebraic Presentation of $4$-Dimensional $2$-Handlebodies and $3$-Dimensional Cobordisms

In this paper, we give a new direct proof of a result by Bobtcheva and Piergallini that provides finite algebraic presentations of two categories, denoted $3\mathrm{Cob}$ and $4\mathrm{HB}$, whose morphisms are manifolds of dimension $3$ and $4$, respectively. More precisely, $3\mathrm{Cob}$ is the category of connected oriented $3$-dimensional cobordisms between connected surfaces with connected boundary, while $4\mathrm{HB}$ is the category of connected oriented $4$-dimensional $2$-handlebodies up to $2$-deformations. For this purpose, we explicitly construct the inverse of the functor $\Phi: 4\mathrm{Alg} \to 4\mathrm{HB}$, where $4\mathrm{Alg}$ denotes the free monoidal category generated by a Bobtcheva--Piergallini Hopf algebra. As an application, we deduce an algebraic presentation of $3\mathrm{Cob}$ and show that it is equivalent to the one conjectured by Habiro.