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tried to edit Ext a bit. But this needs to be expanded, eventually.
And a tiny little bit at Tor.
Added a list of notions of cohomology expressible as Ext-groups to Ext.
I have added to Ext a section Contravariant Ext on ordinary objects with the explicit definition by resolutions. But mainly I added after this a bit of explicit discussion of how to see that the standard formula $H^n(Hom(P_\bullet,A) )$ computes indeed the homotopy classes $X \to \mathbf{B}^n A$.
More general abstract discussion along these lines is planned there or at derived functors in homological algebra, but is not done yes.
In the subsubsection ‘1-Extensions over single objects’ it reads:
Should it not be:
?
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