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Steklov Mathematical Institute Seminar
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Combinatorial realisation of cycles and small covers A. A. Gaifullin |
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Abstract: To define the homology groups of a space one needs to point out what is supposed to be a cycle and which cycles are supposed to be homologous. Initially, Poincaré defined a cycle in a manifold to be a smooth submanifold without boundary. Later he came to the definition of a cycle as an algebraic sum of singular simplices. This led to the singular homology theory. In the middle of the 20th century it was understood that the definition of a cycle as a smooth manifold leads to another important homology theory, which was called the bordism theory. Naturally, the question about the relationship between the two definitions of a cycle was posed. In particular, Steenrod in the late 1940s asked the following question. Let In 1954 Thom constructed a non-realisable 7-dimensional class. However, he proved that for every dimension In the talk we shall show how, given a singular cycle, to construct a manifold ( We shall describe several properties of the class of all manifolds |