Avsnittsöversikt
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Colimits of functors into sets and abelian groups; filtered colimits.
Homology in the top dimension of a manifold and the bootstrap theorem.
tom Dieck, 16.3
Hatcher 3.3 up to p. 239
For colimits of functors, there is a plethora of good sources on- and offline. A classical one is Saunders Mac Lane, "Categories for the working mathematician".