Pushouts of categories, derived limits and colimits
Abstract
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.
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