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Can someone help me with the first 4 lines of Page 111 of Local Algebra by Serre?

I would like to know which spectral sequence is being used.

Initially I thought it is the Grothendieck spectral sequence, but I don't think that is the case, and I don't know what a "Triple Tor" is. Any help would be appreciated. Thank you.

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up vote 2 down vote accepted

Basically he's computing the homology of the complex $M \otimes^{\mathbb{L}}N \otimes^{\mathbb{L}} k$ (where tensors are taken over $A$). Associativity of the tensor product tells us that $$ (M\otimes^{\mathbb{L}}N) \otimes^{\mathbb{L}} k = M \otimes^{\mathbb{L}}(N \otimes^{\mathbb{L}} k) $$

And so we get two different spectral sequences... $$ \operatorname{Tor}_p(\operatorname{Tor}_q(M, N), k) \Rightarrow \operatorname{Tor}_{p+q}(M, N, k) $$ $$ \operatorname{Tor}_p(M, \operatorname{Tor}_q(N,k)) \Rightarrow \operatorname{Tor}_{p+q}(M, N, k) $$

which you can think of in many different ways. One comes from the spectral sequence associated to a double complex. That is, in general if we have complexes $A_*$ and $B_*$ and we want to compute $H_*(A_* \otimes^{\mathbb{L}} B_*)$ (this is sometimes called "hypertor" but that makes it sound scarier than it is...) then we can choose Cartan-Eilenberg resolutions $P \rightarrow A$ and $Q \rightarrow B$, and consider the spectral sequence of the double complex $P \otimes Q$. This gives: $$ H_*(H_*(A) \otimes^{\mathbb{L}} H_*(B)) \Rightarrow H_*(A \otimes^{\mathbb{L}} B) $$ which specializes to the aforementioned spectral sequences in your example.

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@Dylan Thanks, yes you are right, I finally got it. – messi Apr 7 '13 at 11:24
Do you know if we have a spectral sequence involving Ext. Something similar to above where all the Tor is replaced by Ext? Thanks – messi May 4 '13 at 5:12
I mean if we have something like "Triple Ext"? – messi May 5 '13 at 6:08
@messi: If you can say what you mean by "triple Hom" then that should tell you what to do with "triple Ext" – Dylan Wilson May 15 '13 at 17:58

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