A more general form of Grauert's Theorem on Higher Direct Image Sheaves? - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-19T04:23:12Z http://mathoverflow.net/feeds/question/116092 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/116092/a-more-general-form-of-grauerts-theorem-on-higher-direct-image-sheaves A more general form of Grauert's Theorem on Higher Direct Image Sheaves? HNuer 2012-12-11T15:12:27Z 2013-01-09T21:35:06Z <p>When Grauert's Theorem is presented in Hartshorne, the statement goes as follows:</p> <p>Let $f:X\rightarrow Y$ be a projective morphism of noetherian schemes, and $\mathcal F$ a coherent sheaf on $X$, flat over $Y$. Assume that $Y$ is integral and for some $i$, the function $h^i(y,\mathcal F)=\dim_{k(y)} H^i(X_y,\mathcal F_y)$ is constant on $Y$. Then $R^if_*(\mathcal F)$ is locally free on $Y$ and for every $y$ the natural map </p> <p>$$R^if_*(\mathcal F)\otimes k(y)\rightarrow H^i(X_y,\mathcal F_y).$$</p> <p>I was wondering if the assumption that $Y$ be integral can be removed. Certainly the proof in Hartshorne uses the integrality condition.</p> <p>I know that the projective requirement can also be made more general in allowing proper morphisms. I have seen a statement to this effect (in allowing properness and any Noetherian scheme $Y$ as a base) in Nitsure's notes "Construction of Hilbert and Quot schemes" (Part 2 of "FGA:Explained"), and indeed part (3) of Theorem 5.10 there says precisely the statement above with properness replacing projective and without the requirement that the base be integral, but I was wondering if that was accurate. The reference given for the proof is the above result and proof in Hartshorne, which doesn't cover the general case.</p> <p>In short, I would like to know whether this generalization (1) is indeed true, (2) if Hartshorne's proof can be easily modified, and (3) if not, is there a good reference (EGA is acceptable, but not preferred).</p> http://mathoverflow.net/questions/116092/a-more-general-form-of-grauerts-theorem-on-higher-direct-image-sheaves/116196#116196 Answer by jlk for A more general form of Grauert's Theorem on Higher Direct Image Sheaves? jlk 2012-12-12T20:28:17Z 2012-12-12T20:28:17Z <p>Did you look at Corollary 8.3.6.5 of Illusie's article in the "FGA:Explained" volume? </p> http://mathoverflow.net/questions/116092/a-more-general-form-of-grauerts-theorem-on-higher-direct-image-sheaves/118482#118482 Answer by Angelo for A more general form of Grauert's Theorem on Higher Direct Image Sheaves? Angelo 2013-01-09T21:35:06Z 2013-01-09T21:35:06Z <p>There is a very clear exposition of the base change theory in Mumford's book on abelian varieties.</p>