T Banakh, R Cauty, A Karassev - arXiv preprint arXiv:1002.2802, 2010 - arxiv.org
A closed subset A⊂ X is called a homological (homotopical) Zn-set if for any k< n+ 1 and
any open set U⊂ X the relative homology (homotopy) group Hk (U, U\A)(πk (U, U\A)) …