Kerler-Lyubashenko Functors on 4-Dimensional 2-Handlebodies

Abstract: We construct a braided monoidal functor $J_4$ from Bobtcheva and Piergallini's category $4\mathrm{HB}$ of connected 4-dimensional 2-handlebodies (up to 2-deformations) to an arbitrary unimodular ribbon category $\mathcal{C}$, which is not required to be semisimple. The main example of target category is provided by $H$-mod, the category of left modules over a unimodular ribbon Hopf algebra $H$. The source category $4\mathrm{HB}$ is freely generated, as a braided monoidal category, by a BPH algebra (short for Bobtcheva-Piergallini Hopf algebra), and this is sent by the Kerler-Lyubashenko functor $J_4$ to the end $\int_{X \in \mathcal{C}} X \otimes X^*$ in $\mathcal{C}$, which is given by the adjoint representation in the case of $H$-mod. When $\mathcal{C}$ is factorizable, we show that the construction only depends on the boundary and signature of handlebodies, and thus projects to a functor $J_3^\sigma$ defined on Kerler's category $3\mathrm{Cob}^\sigma$ of connected framed 3-dimensional cobordisms. When $H^*$ is not semisimple and $H$ is not factorizable, our functor $J_4$ has the potential of detecting diffeomorphisms that are not 2-deformations.