Freely generated $n$-categories, coinserters and presentations of low dimensional categories

Abstract

A presentation records not only a categorical structure but also how it is assembled. This matters for rewriting, coherence, and minimality: freely adjoining a cell with prescribed boundary is different from imposing an equation between cells already constructed. We show that these two operations are governed, respectively, by coinserters and coequifiers, giving a uniform account of computadic presentations from ordinary categories to strict higher categories. For a graph $G$, the free category on $G$ is the coinserter in $\mathsf{Cat}$ of its domain and codomain maps between discrete categories. More generally, for every $n\geq1$, freely adjoining $n$-cells with prescribed parallel boundaries to a strict $(n-1)$-category is a coinserter in the $2$-category of strict $n$-categories, strict $n$-functors, and $n$-icons. This construction is left adjoint to the underlying derivation-scheme functor. It recovers free strict $n$-categories from computads, while coequifiers impose equations between freely generated cells. In dimension two, it also satisfies a bicategorical universal property for normal pseudofunctors and icons. Replacing the walking arrow by the unit interval yields the topological coinserter of a graph. For a groupoidal $2$-computad $C$, attaching one disk for each relation produces a presentation complex $X_C$ whose fundamental groupoid is the groupoid presented by $C$. Consequently, the rank-finite deficiency of a connected groupoid is the classical deficiency of any isotropy group. Homology gives a sharp lower bound on the relations required to present a thin groupoid over a fixed graph, while crossed modules extend the comparison to relations among relations. Finally, for the descent computad, the identity and associativity confluences form a homotopy basis and attain the corresponding homological lower bounds.