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• CommentRowNumber1.
• CommentAuthorUrs
• CommentTimeJul 27th 2010

tried to edit Ext a bit. But this needs to be expanded, eventually.

• CommentRowNumber2.
• CommentAuthorUrs
• CommentTimeJul 27th 2010

And a tiny little bit at Tor.

• CommentRowNumber3.
• CommentAuthorUrs
• CommentTimeAug 29th 2012

Added a list of notions of cohomology expressible as Ext-groups to Ext.

• CommentRowNumber4.
• CommentAuthorUrs
• CommentTimeAug 30th 2012

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.