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4
votes
Accepted
Finite universal delta-functors
I don't think $F^{d-\bullet}$ will often be universal, even when $\mathcal{A}$ does have enough projectives.
Let $A$ be any ring with finite global dimension that is not hereditary, so that there is …
16
votes
Accepted
When the restriction of a derived functor to a subcategory is the derived functor of the res...
In the example where $\mathcal{D}$ is the category of abelian groups and $\mathcal{C}$ is the category of finite abelian groups, take $F(X)=X\otimes_\mathbb{Z}\mathbb{Q}/\mathbb{Z}$. Then the restrict …
8
votes
Accepted
Left exact functor $F$ preserves quasi-isomorphism between $F$-acyclics
It's not true, without boundedness conditions, that a left exact functor always preserves quasi-isomorphisms between complexes of $F$-acyclic objects.
As alluded to in the question, a chain map is a q …
2
votes
Accepted
Vanishing of $\operatorname{Ext}_R^{1}(M,R)$ when $R$ is a Gorenstein local ring of dimensio...
Take $R=\mathbb{Z}_{(p)}$ for some prime $p$, with $x=p$, and
$M=\mathbb{Q}\oplus R$.
To show that this is a counterexample, the only nonobvious thing to show is that
$\operatorname{Ext}^{1}_{R}(\math …
2
votes
Accepted
A perfect complex over a local Cohen--Macaulay ring whose canonical dual is concentrated in ...
Not even for a Gorenstein ring.
Let $R$ be $\mathbb{Z}_p$, the ring of $p$-adic integers. It is Gorenstein, and therefore its own dualizing module.
Let $M$ be the direct sum of the two complexes
$$\cd …