Colimits and Tor
Of the two functors defined in Chapter 3, Ext is the more “universal”; Section 6.6 describes how Tor can, in principle at least, be defined using Ext. On the other hand, Tor has more properties. Up to now, this has only been reflected in the fact that Tor mixes rightR -modules with left R -modules. But there’s more. Tor behaves well with respect to certain colimits, and that is the general subject of this chapter.
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