what can be reached by flat degeneration of (globally) complete intersection? - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-22T06:34:20Z http://mathoverflow.net/feeds/question/97865 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/97865/what-can-be-reached-by-flat-degeneration-of-globally-complete-intersection what can be reached by flat degeneration of (globally) complete intersection? Dmitry Kerner 2012-05-24T19:51:26Z 2012-06-20T16:55:01Z <p>Let $X\subset\mathbb{P}^n$ be a (globally) complete intersection, let $(X_t)_{t\in\mathbb{C}^1}$ be a flat family, with $X_1=X$. Which types of schemes can we get as $X_0$?</p> <p>Or, conversely, which (embedded, projective) schemes deform to complete intersections?</p> <p>e.g. some non-ACM schemes (not arithmetically Cohen-Macaulay) deform to C.I.'s. Does every pure-dimensional subscheme of $\mathbb{P}^n$ deform to a C.I.? Two immediate obstructions are the degree and the arithmetic genus. Any other necessary conditions?</p> <p>(Just for housekeeping, a somewhat <a href="http://mathoverflow.net/questions/17110/complete-intersections-and-flat-families" rel="nofollow">related question</a> on deformations of C.I.:)</p> <p>upd: I meant the family over the germ $(\mathbb{C}^1,0)$, so there are no complications with the geometry of the base.</p> http://mathoverflow.net/questions/97865/what-can-be-reached-by-flat-degeneration-of-globally-complete-intersection/97885#97885 Answer by Will Sawin for what can be reached by flat degeneration of (globally) complete intersection? Will Sawin 2012-05-25T00:08:20Z 2012-06-20T16:55:01Z <p>Anything with the same Hilbert polynomial as a globally complete intersection. This is true because two fibers of the same flat projective family have the same Hilbert polynomial, and because the Hilbert scheme is connected. EDIT: Since the base of the flat family is required to be irreducible, the projective scheme must be in the same irreducible component of the Hilbert scheme as a complete intersection. I don't know any nice characterization of when this is the case.</p> <p>The Hilbert polynomial of a complete intersection is easy to calculate using the Koszul complex. The coefficients will be some set of polynomials in the degrees of the hypersurfaces you completely intersect. You can set them equal to the actual coefficients of the Hilbert polynomial and check if the system of equations has a positive integer solution.</p> <p>Computationally, you can just check all possible factorizations of the degree.</p> <p>For curves, degree, arithmetic genus, and dimension of the embedding are the only invariants needed. The last one is important. For instance, a genus $5$, degree $8$ curve is the complete intersection of three degree $2$ hypersurfaces in $\mathbb P^4$, but not the complete intersection of any pair of hypersurfaces in $\mathbb P^3$.</p>