Can all non-concrete categories be constructed by taking a concrete category and identifying some of its morphisms?
Thanks
Can all non-concrete categories be constructed by taking a concrete category and identifying some of its morphisms?
Thanks
The answer to this question is Yes you can. The relevant paper is Ludek Kucera, Every category is a factorization of a concrete one, Journal of Pure and Applied Algebra, Volume 1, Issue 4, December 1971, Pages 373-376. You may read the paper here: http://www.sciencedirect.com/science/journal/00224049/1/4 .
However, this is a formal construction that does not seem to be of much use in concrete problems (such as working in homotopy theory). Also this used the axiom of class choice in Godel-Bernays-Von von neumann set theory.