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In his classical textbook on algebraic topology Edwin Spanier developed the theory of covering spaces within a more general framework of lifting spaces (i.e., Hurewicz fibrations with unique path-lifting property). Among other, Spanier proved that for every space $X$ there exists a universal lifting space, which however need not be simply connected, unless the base space $X$ is semi-locally simply connected. The question on what exactly is the fundamental group of the universal space was left unanswered. The main source of lifting spaces are inverse limits of covering spaces over $X$, or more generally, over some inverse system of spaces converging to $X$. Every metric space $X$ can be obtained as a limit of an inverse system of polyhedra, and so inverse limits of covering spaces over the system yield lifting spaces over $X$. They are related to the geometry (in particular the fundamental group) of $X$ in a similar way as the covering spaces over polyhedra are related to the fundamental group of their base. Thus lifting spaces appear as a natural replacement for the concept of covering spaces over base spaces with bad local properties. In this paper we develop a general theory of lifting spaces and prove that they are preserved by products, inverse limits and other important constructions. We show that maps from $X$ to polyhedra give rise to coverings over $X$ and use that to prove that for a connected, locally path connected and paracompact $X$, the fundamental group of the above-mentioned Spanier's universal space is precisely the intersection of all Spanier groups associated to open covers of $X$, and that the later coincides with the shape kernel of $X$. Furthermore, we examine in more detail lifting spaces over $X$ that arise as inverse limits of coverings over some approximations of $X$.

We say A is a quasi-normal subgroup of the group G if the commensurator of A in G is all of G. We develop geometric versions of commensurators in finitely generated groups. In particular, g is an element of the commensurator of A in G iff the Hausdorff distance between A and gA is finite. We show that a quasi-normal subgroup of a group is the kernel of a certain map, and a subgroup of a finitely generated group is quasi-normal iff the natural coset graph is locally finite. This last equivalence is particularly useful for deriving asymptotic results for finitely generated groups. Our primary goal in this paper is to develop the basic theory of quasi-normal subgroups, comparing analogous results for normal subgroups and isolating differences between quasi-normal and normal subgroups., Comment: 23 pages, 0 figures

This paper deals with a parabolic partial differential equation that incorporates diffusion and convection terms and that previously has been shown to have solutions that become unbounded at a single point in finite time. The results presented here describe the limiting behavior of the solution in a neighborhood of the blowup point, as well as the asymptotic growth rate as the blowup time is approached. Rigorous estimates are proved, and some supplementary numerical calculations are presented.