Let $f: X \rightarrow X'$ be a morphism of schemes, and let $\mathcal{I}^{\bullet}$ be a complex of $\mathcal{O}_X$-modules. There are two spectral sequences (well, more than that, but these are the two I care about) abutting to the hypercohomology $\mathbb{H}^n(X, \mathcal{I}^{\bullet})$. The first is the second spectral sequence of hypercohomology, which has $E_2$-term

$E_2^{p,q} = H^p(X, H^q(\mathcal{I}^{\bullet}))$

(where $H^q(\mathcal{I}^{\bullet})$ just means the $q^{th}$ cohomology object of the complex $\mathcal{I}^{\bullet}$)

and the second is the Leray spectral sequence associated with $f$, which has $E_2$-term

$E_2^{p,q} = H^p(X', R^q f_* \mathcal{I}^{\bullet})$

(Here, since the terms in the spectral sequence are $\Gamma(X', \mathcal{O}_{X'})$-modules, the abutment $\mathbb{H}^n(X, \mathcal{I}^{\bullet})$ must be viewed as a $\Gamma(X', \mathcal{O}_{X'})$-module by restricting scalars.)

Are there conditions on $X, X', f$, and/or $\mathcal{I}^{\bullet}$ under which we can say that these $E_2$-terms are the same, that is, that $H^p(X, H^q(\mathcal{I}^{\bullet})) \simeq H^p(X', R^q f_* \mathcal{I}^{\bullet})$ as $\Gamma(X', \mathcal{O}_{X'})$-modules.

In this (non-degenerate) case, it's not really a spectral sequence question at all, but a question of somehow comparing two iterated cohomology objects. In the case I care about, in which I suspect for other reasons that there will be such an isomorphism but the spectral sequences will almost surely not degenerate at $E_2$, there are lots of nice properties that can be assumed: $X$ and $X'$ are smooth and projective over a field, $f$ is smooth and projective (hence, in particular, proper and flat), and the complex $\mathcal{I}^{\bullet}$ is a complex of **injective** $\mathcal{O}_X$-modules (but not, however, an injective resolution of any single $\mathcal{O}_X$-module).

(Edited 7/2: as Karl Schwede points out, even if both degenerate at the $E_2$-term there's no reason to assume all the terms are the same.)

2more comments