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Freeness of chain groups cannot simply be dropped from the classical Kunneth statement
Statement
The classical free-complex Kunneth argument cannot simply omit freeness of chain groups: it uses freeness to make cycle and boundary modules flat and to control the Tor edge.
Refutation
Given: the complexes concentrated in degree zero, viewed as complexes of nonfree abelian groups.
Their ordinary tensor complex is concentrated in degree zero, so . On the other hand, and .
If the classical free-complex Kunneth short exact sequence were asserted unchanged after simply deleting freeness, then in total degree one it would surject from the zero group onto the nonzero Tor group from step 1.1. That is impossible. Hence freeness cannot simply be dropped without replacing ordinary tensor by a derived construction or adding suitable flatness hypotheses.
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Used by
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Sources
- Weibel, An Introduction to Homological Algebra (standard reference, not scraped)