The hyperbolic dodecahedral space of Weber and Seifert has a natural non-positively curved cubulation obtained by subdividing the dodecahedron into cubes. We show that the hyperbolic dodecahedral space has a 6-sheeted irregular cover with the property that the canonical hypersurfaces made up of the mid-cubes give a very short hierarchy. Moreover, we describe a 60-sheeted cover in which the associated cubulation is special. We also describe the natural cubulation and covers of the spherical dodecahedral space (aka Poincaré homology sphere).In general, it is known through work of Bergeron and Wise [6] that if M is a closed hyperbolic 3-manifold, then π 1 (M) is isomorphic to the fundamental group of an NPC cube complex. However, the dimension of this cube complex may be arbitrarily large and it may not be a manifold. Agol's theorem provides a finite cover that is a special cube complex, and the π 1 -injective surfaces of Kahn and Markovic [13] are quasi-convex and hence have separable fundamental group. Thus, the above outline completes a sketch of the proof that M is virtually Haken. An embedding theorem of Haglund and Wise [11] and Agol's virtual fibring criterion [1] then imply that M is also virtually fibred.Weber and Seifert [16] described two closed 3-manifolds that are obtained by taking a regular dodecahedron in a space of constant curvature and identifying opposite sides by isometries. One is hyperbolic and known as the Weber-Seifert dodecahedral space and the other is spherical and known as the Poincaré homology sphere. Moreover, antipodal identification on the boundary of the dodecahedron yields a third closed 3-manifold which naturally fits into this family: the real projective space.The dodecahedron has a natural decomposition into 20 cubes, which is a NPC cubing in the case of the Weber-Seifert dodecahedral space. The main result of this note can be stated as follows.Theorem 2 The hyperbolic dodecahedral space WS of Weber and Seifert admits a cover of degree 60 in which the lifted natural cubulation of WS is special.In addition, we exhibit a 6-sheeted cover of WS in which the canonical immersed surface consists of six embedded surface components and thus gives a very short hierarchy of WS. The special cover from Theorem 2 is the smallest regular cover of WS that is also a cover of this 6-sheeted cover. Moreover, it is the smallest regular cover of WS that is also a cover of the 5-sheeted cover with positive first Betti number described by Hempel [12].We conclude this introduction by giving an outline of this note. The dodecahedral spaces are described in §3. Covers of the hyperbolic dodecahedral space are described in §4, and all covers of the spherical dodecahedral space and the real projective space in §5.The authors thank Daniel Groves and Alan Reid for their encouragement to write up these results, and the anonymous referee for some insightful questions and comments which triggered us to find a special cover.
Cube complexes, injective surfaces and hierarchiesA cube complex is a space obtained by ...