Retrieval of algebra structure from spectral sequence - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-22T23:41:35Z http://mathoverflow.net/feeds/question/101467 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/101467/retrieval-of-algebra-structure-from-spectral-sequence Retrieval of algebra structure from spectral sequence George 2012-07-06T08:52:04Z 2012-07-08T14:57:56Z <p>Suppose we have a spectral sequence of algebras and know that it degenerates at some $E_r$, take for example the cohomology Leray Serre spectral sequence associated to some fibration $F\hookrightarrow E\rightarrow B$. Suppose we are working over a field so there is no extension problem and so $H^n(E)\cong\oplus_{r+s=n} E_{\infty}^{r,s}$. Under what conditions can we read off algebra structure of $H^*(E)$ from the $E_{\infty}$ page? (e.g. one very special case is when the $E_{\infty}$ page is free as an algebra.)</p> <p>I think it is not possible in general, since we can have two different algebras with some filtrations such that the associated graded objects are isomorphic, but I would like to know if there are certain conditions under which one can compute the algebra structure. </p> http://mathoverflow.net/questions/101467/retrieval-of-algebra-structure-from-spectral-sequence/101675#101675 Answer by Ralph for Retrieval of algebra structure from spectral sequence Ralph 2012-07-08T14:57:56Z 2012-07-08T14:57:56Z <p>I want to highlight a case that is similar to the free case but is used in several cohomology computations in literature: Let $C=(C^i)_{i\ge 0}$ be a cochain complex with filtration $$C^i=F^0C^i \supseteq \cdots \supseteq F^iC^i \supseteq F^{i+1}C^i=0.$$</p> <p>Assume that $E_\infty^{\ast,\ast}=E_\infty^{\ast,0}\otimes_k E_\infty^{0,\ast}$ and $E_\infty^{0,\ast}=k[x_1,...,x_n]$ is a polynomial algebra with homogeneous $x_i$. Then </p> <blockquote> <p>$\qquad\qquad H^\ast(C) = E_\infty^{\ast,0}\otimes_k k[X_1,...,X_n] \cong E_\infty$ </p> </blockquote> <p>The proof is straightforward. As an application let $$B\mathbb{Z}/2 \to E \to B$$ be a fibration with path-connected base such that $\Pi_1(B)$ acts trivially on $H^\ast(B\mathbb{Z}/2;\mathbb{F}_2)$. Write ${H^\ast(B\mathbb{Z}/2;\mathbb{F}_2) =\mathbb{F}_2[z]}$ and let $h$ be minimal with</p> <p>$d_{2^{h}+1}(z^{2^h})=0$. Assume moreover that $y_i := Sq^{2^i}\cdots Sq^1(d_2z)$ $(i=0,...,h-1)$ is a regular sequence in $H^\ast(B;\mathbb{F}_2)$. Then </p> <blockquote> <p>$\qquad H^\ast(E;\mathbb{F}_2)=\mathbb{F}_2[x] \otimes H^\ast(B;\mathbb{F}_2)/(y_1,...,y_{h-1}),\quad \deg(x)=2^h$. </p> </blockquote> <p>This was used by Quillen in his computation of the cohomology of the extra-special 2-groups and the Spinor groups. </p>