Magnitude, path and coarse homology

Abstract

In this article, we introduce Borel--Moore and uniformly finite versions of magnitude homology and path homology for locally finite graphs by allowing possibly infinite chains subject to local finiteness and boundedness conditions. In both settings, we construct magnitude--path spectral sequences extending the spectral sequence studied by the author. Their E^1-pages are given by the corresponding magnitude homologies, their E^2-pages contain the corresponding path homologies on the p-axis, and they converge to coarse homology and uniformly finite homology, respectively, in the sense of Nowak--Yu. Moreover, the E^{r+1}-page is invariant under r- homotopy, whereas the homology to which the spectral sequence converges is invariant under coarse equivalence. Thus the spectral sequences interpolate between small-scale and large-scale geometry. We also show that the Borel--Moore and uniformly finite magnitude homologies are products and restricted products, respectively, of ordinary magnitude homology groups with a fixed initial point. Hence the E^1-page is assembled from pointwise magnitude-homological data, providing a mechanism for combining local information into global large-scale invariants. As applications, we establish Mayer--Vietoris sequences for magnitude homology, path homology, the associated spectral-sequence pages, and coarse and uniformly finite homology. We also show that diagonality of ordinary magnitude homology implies degeneration at the E^2-page, yielding isomorphisms between path homology and the corresponding coarse homology. Finally, we discuss applications to Cayley graphs of graph products, including right-angled Artin groups and right-angled Coxeter groups.