Combinatorial realisation of cycles and small covers
Alexander A. GaifullinSteklov Mathematical Institute, Moscow, Russian Federation
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In 1940s Steenrod asked if every homology class of every topological space can be realised by an image of the fundamental class of an oriented closed smooth manifold. Thom found a non-realisable 7-dimensional class and proved that for every , there is a positive integer such that the class is always realisable. The proof was by methods of algebraic topology and gave no information on the topology of the manifold which realises the homology class. We give a purely combinatorial construction of a manifold that realises a multiple of a given homology class. For every , this construction yields a manifold with the following universality property: For any and , a multiple of can be realised by an image of a (non-ramied) finite-sheeted covering of . Manifolds satisfying this property are called URC-manifolds. The manifold is a so-called small cover of the permutahedron, i. e., a manifold glued in a special way out of permutahedra. (The permutahedron is a special convex polytope with ! vertices.) Among small covers over other simple polytopes, we find a broad class of examples of URC-manifolds. In particular, in dimension 4, we find a hyperbolic URC-manifold. Thus we prove a conjecture of Kotschick and Loh claiming that a multiple of every homology class can be realised by an image of a hyperbolic manifold. We also obtain some new results on the relationship of URC-manifolds with theory of simplicial volume.