Let $F\to E\to B$ a fibration and $\{E_{r}^{\ast,\ast},d_{r}\}$ the Leray-Serre Spectral sequence converging to $H^{\ast}(E;R),$ such that $$E_{2}^{p,q}=H^{p}(B;\mathcal{H}^{q}(F;R))$$ is the cohomology of $B$ with local coefficients in the cohomology of the fiber $F.$ We know that when the action of $\pi_{1}(B)$ induced by fibration on the cohomology of $F$ is trivial then the system of local coefficients is simple. My question: Can anything be said (assuming that $R=\mathbb{Z}_{2}$ and $\pi_{1}(B)=\mathbb{Z}_{2}$) when the action is trivial except for ONE element of the cohomology? To be more specific, there is only one element $c\in H^{1}(F;R)$ such that $\mu(g,c)=gc\neq c,$ for $g\in \mathbb{Z}_{2}=\langle g\rangle$, where $\mu:\pi_{1}(B)\times H^{\ast}(F)\to H^{\ast}(F)$ denotes the induced action.
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1$\begingroup$ Surely there are $2$ elements on which the action is non-trivial ($c$ and $gc$)? Anyway, I would suggest if you know the cohomology of $F$ as a $\pi_1(B)=\mathbb{Z}_2$-module, then you have a good chance of computing the $E_2$-term (using the definition on cohomology with local coefficients in $H^q(F)$ as the cohomology of the cochain complex $\operatorname{Hom}_{\pi_1(B)}(C_*(\tilde{B}),H^q(F))$, where $\tilde{B}$ is the universal cover of $B$). $\endgroup$– Mark GrantCommented Apr 3, 2020 at 14:32
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