2014/01/15 by Lukáš Vokřínek, Vokřínek, Lukáš
Mathematics · #Category Theory (math.CT) #FOS: Mathematics #Primary 18G10 #Secondary 18G15 #math.CT #msc:18G10 #msc:18G15
paper · pdf · doi:10.48550/arxiv.1401.3757
6 pages
arxiv created 2014/01/15 · arxiv updated 2014/01/17
In a paper by Ford, it is claimed that to any pushout square of categories with all involved functors injective, there is associated an exact "Mayer--Vietoris" sequence of derived (co)limits. We provide a counter-example to this general statement. Further, we construct the Mayer--Vietoris sequence under some restrictions that cover the well-known case of a pushout square of group monomorphisms.