We prove that if the Walsh bipartite map $ℳ=𝒲\left(\mathscr{H}\right)$ of a regular oriented hypermap $\mathscr{H}$ is also orientably regular then both $ℳ$ and $\mathscr{H}$ have the same chirality group, the covering core of $ℳ$ (the smallest regular map covering $ℳ$) is the Walsh bipartite map of the covering core of $\mathscr{H}$ and the closure cover of $ℳ$ (the greatest regular map covered by $ℳ$) is the Walsh bipartite map of the closure cover of $\mathscr{H}$. We apply these results to the family of toroidal chiral hypermaps ${(3,3,3)}_{b,c}={𝒲}^{-1}{\{6,3\}}_{b,c}$ induced by the family of toroidal bipartite maps...

Let $M$ be a triangulable compact manifold. We prove that, among closed subgroups of ${\mathrm{Homeo}}_0\left(M\right)$ (the identity component of the group of homeomorphisms of $M$), the subgroup consisting of volume preserving elements is maximal.

Let $H_g$ denote the closed 3-manifold obtained as the connected sum of g copies of S² × S¹, with free fundamental group of rank g. We prove that, for a finite group G acting on $H_g$ which induces a faithful action on the fundamental group, there is an upper bound for the order of G which is quadratic in g, but there does not exist a linear bound in g. This implies then a Jordan-type bound for arbitrary finite group actions on $H_g$ which is quadratic in g. For the proofs we develop a calculus for finite group...

We consider finite groups which admit a faithful, smooth action on an acyclic manifold of dimension three, four or five (e.g. Euclidean space). Our first main result states that a finite group acting on an acyclic 3- or 4-manifold is isomorphic to a subgroup of the orthogonal group O(3) or O(4), respectively. The analogous statement remains open in dimension five (where it is not true for arbitrary continuous actions, however). We prove that the only finite nonabelian simple groups admitting a smooth...

We explore the interior geometry of the CAT(0) spaces $X_{\alpha }:0<\alpha \le \pi /2$, constructed by Croke and Kleiner [Topology 39 (2000)]. In particular, we describe a diffraction effect experienced by the family of geodesic rays that emanate from a basepoint and pass through a certain singular point called a triple point, and we describe the shadow this family casts on the boundary. This diffraction effect is codified in the Transformation Rules stated in Section 3 of this paper. The Transformation Rules have various applications....