摘要:

We classify for whicha,b,c, andkthere exists complexeswith α injective and β surjective. Furthermore we show that when it exists, we may assume that α degenerates inPkin expected codimension, which isb-a-c+ 1 We give applications to instanton bundles, space curves, and surfaces in .P4.

ISSN:0092-7872    

DOI:10.1080/00927870008827171

出版商:Gordon and Breach Science Publishers Ltd.    

年代:2000

数据来源: Taylor

ISSN:0092-7872    

DOI:10.1080/00927870008827172

出版商:Gordon and Breach Science Publishers Ltd.    

年代:2000

数据来源: Taylor

摘要:

LetFbe a normalized rank 2 reflexive sheaf on P3with Chern classesc1,c2,c3. Let α be the least integer such that 0≠H0F(α) and β be the smallest integer such thatH0F(n) has sections whose zero scheme is a curve for alln≥ β . We show that ifT0is the largest root of the cubic polynomialthen β ≥T0-α-c1-1. There are applications to the smallest degree of a surface containing a curves which are the zero schemes of sections ofH0F(α).

ISSN:0092-7872    

DOI:10.1080/00927870008827173

出版商:Gordon and Breach Science Publishers Ltd.    

年代:2000

数据来源: Taylor

ISSN:0092-7872    

DOI:10.1080/00927870008827174

出版商:Gordon and Breach Science Publishers Ltd.    

年代:2000

数据来源: Taylor

摘要:

Some special cases are proved in which localization commutes with tight closure.

ISSN:0092-7872    

DOI:10.1080/00927870008827175

出版商:Gordon and Breach Science Publishers Ltd.    

年代:2000

数据来源: Taylor

ISSN:0092-7872    

DOI:10.1080/00927870008827176

出版商:Gordon and Breach Science Publishers Ltd.    

年代:2000

数据来源: Taylor

ISSN:0092-7872    

DOI:10.1080/00927870008827177

出版商:Gordon and Breach Science Publishers Ltd.    

年代:2000

数据来源: Taylor

摘要:

For flat modulesMover a ringAwe study the similarities between the three statements,dimk(P) (k(P)⊗AM=dfor all prime idealsPofA, the Ap-module Mpis free of rankdfor all prime idealsPofA, andMis a locally free J4-module of rank d. We have particularly emphasized the case when there is an>l-algebraB, essentially of finite type, andMis a finitely generatedB-module.

ISSN:0092-7872    

DOI:10.1080/00927870008827178

出版商:Gordon and Breach Science Publishers Ltd.    

年代:2000

数据来源: Taylor

摘要:

We give examples of subcanonical subvarieties of codimen-sion 3 in projective ra-space which are not Pfaffian, i.e. defined by the ideal sheaf of submaximal Pfaffians of an alternating map of vector bun-dles. This gives a negative answer to a question asked by Okonek [29]. Walter [36] had previously shown that a very large majority of sub-canonical subschemes of codimension 3 in Pnare Pfaffian, but he left open the question whether the exceptional non-Pfaffian cases actually occur. We give non-Pfaffian examples of the principal types allowed by his theorem, including (Enriques) surfaces in P5in characteristic 2 and a smooth 4-fold in. These examples are based on our previous work [14] showing that any strongly subcanonical subscheme of codimension 3 of a Noetherian scheme can be realize as a locus of degenerate intersection of a pair of Langrangian (maximal isotropic)subbundles of a twisted orthogonal bundle

ISSN:0092-7872    

DOI:10.1080/00927870008827179

出版商:Gordon and Breach Science Publishers Ltd.    

年代:2000

数据来源: Taylor

摘要:

We study in detail locally Cohen-Macaulay curves in P4which are contained in a double plane 2H, thus completing the classification of curves lying on surfaces of degree two. We describe the irreducible components of the Hilbert schemesHd,g(2H) of lo-cally Cohen-Macaulay curves in 2Hof degreedand arithmetic genusg, and we show thatHd,g(2H) is connected. We also discuss the Rao module of these curves and liaison and biliaison equiva-lence classes.

ISSN:0092-7872    

DOI:10.1080/00927870008827180

出版商:Gordon and Breach Science Publishers Ltd.    

年代:2000

数据来源: Taylor