The Simplicial Loop Space of a Simplicial Complex

Abstract

Given a simplicial complex $X$, we construct a simplicial complex $\Omega X$ that may be regarded as a combinatorial version of the based loop space of a topological space. Our construction explicitly describes the simplices of $\Omega X$ directly in terms of the simplices of $X$. Working at a purely combinatorial level, we show two main results that confirm the (combinatorial) algebraic topology of our $\Omega X$ behaves like that of the topological based loop space. Whereas our $\Omega X$ is generally a disconnected simplical complex, each component of $\Omega X$ has the same edge group, up to isomorphism. We show an isomorphism between the edge group of $\Omega X$ and the combinatorial second homotopy group of $X$ as it has been defined in separate work (arxiv:2503.23651). Finally, we enter the topological setting and, relying on prior work of Stone, show a homotopy equivalence between the spatial realization of our $\Omega X$ and the based loop space of the spatial realization of $X$.