Multiplicativity of Connes' calculus

Abstract

We consider the quadruples $\,(\mathcal{A},\mathbb{V},D,\gamma)$ where $\mathcal{A}$ is a unital, associative $\mathbb{K}\,$-algebra represented on the $\mathbb{K}\,$-vector space $\mathbb{V}$, $D\in \mathcal{E}nd(\mathbb{V})$, $\gamma\in\mathcal{E}nd(\mathbb{V})$ is a $\mathbb{Z}_2$-grading operator which commutes with $\mathcal{A}$ and anticommutes with $D$. We prove that the collection of such quadruples, denoted by $\,\widetilde{\mathcal{S}pec}\,$, is a monoidal category. We consider the monoidal subcategory $\,\widetilde{\mathcal{S}pec_{sub}}\,$ of objects of $\,\widetilde{\mathcal{S}pec}\,$ for which $\gamma\in\pi(\mathcal{A})$. We show that there is a covariant functor $\,\mathcal{G}:\widetilde{\mathcal{S}pec}\longrightarrow\widetilde{\mathcal{S}pec_{sub}}\,$. Let $\,\Omega_D^\bullet\,$ be the differential graded algebra defined by Connes ([Con2]) and $DGA$ denotes the category of differential graded algebras over the field $\mathbb{K}\,$. We show that $\mathcal{F}:\widetilde{\mathcal{S}pec_{sub}}\longrightarrow DGA\,$, given by $(\mathcal{A},\mathbb{V},D,\gamma)\longmapsto\Omega_D^\bullet(\mathcal{A})$, is a monoidal functor. To show that $\,\mathcal{F}\circ\mathcal{G}\,$ is not trivial we explicitly compute it for the cases of compact manifold and the noncommutative torus along with the associated cohomologies.