ALGEBRAIC GEOMETRY NOTES 1. Conventions and Notation Fix A
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Arxiv:1708.06494V1 [Math.AG] 22 Aug 2017 Proof
CLOSED POINTS ON SCHEMES JUSTIN CHEN Abstract. This brief note gives a survey on results relating to existence of closed points on schemes, including an elementary topological characterization of the schemes with (at least one) closed point. X Let X be a topological space. For a subset S ⊆ X, let S = S denote the closure of S in X. Recall that a topological space is sober if every irreducible closed subset has a unique generic point. The following is well-known: Proposition 1. Let X be a Noetherian sober topological space, and x ∈ X. Then {x} contains a closed point of X. Proof. If {x} = {x} then x is a closed point. Otherwise there exists x1 ∈ {x}\{x}, so {x} ⊇ {x1}. If x1 is not a closed point, then continuing in this way gives a descending chain of closed subsets {x} ⊇ {x1} ⊇ {x2} ⊇ ... which stabilizes to a closed subset Y since X is Noetherian. Then Y is the closure of any of its points, i.e. every point of Y is generic, so Y is irreducible. Since X is sober, Y is a singleton consisting of a closed point. Since schemes are sober, this shows in particular that any scheme whose under- lying topological space is Noetherian (e.g. any Noetherian scheme) has a closed point. In general, it is of basic importance to know that a scheme has closed points (or not). For instance, recall that every affine scheme has a closed point (indeed, this is equivalent to the axiom of choice). In this direction, one can give a simple topological characterization of the schemes with closed points. -
3 Lecture 3: Spectral Spaces and Constructible Sets
3 Lecture 3: Spectral spaces and constructible sets 3.1 Introduction We want to analyze quasi-compactness properties of the valuation spectrum of a commutative ring, and to do so a digression on constructible sets is needed, especially to define the notion of constructibility in the absence of noetherian hypotheses. (This is crucial, since perfectoid spaces will not satisfy any kind of noetherian condition in general.) The reason that this generality is introduced in [EGA] is for the purpose of proving openness and closedness results on the locus of fibers satisfying reasonable properties, without imposing noetherian assumptions on the base. One first proves constructibility results on the base, often by deducing it from constructibility on the source and applying Chevalley’s theorem on images of constructible sets (which is valid for finitely presented morphisms), and then uses specialization criteria for constructible sets to be open. For our purposes, the role of constructibility will be quite different, resting on the interesting “constructible topology” that is introduced in [EGA, IV1, 1.9.11, 1.9.12] but not actually used later in [EGA]. This lecture is organized as follows. We first deal with the constructible topology on topological spaces. We discuss useful characterizations of constructibility in the case of spectral spaces, aiming for a criterion of Hochster (see Theorem 3.3.9) which will be our tool to show that Spv(A) is spectral, our ultimate goal. Notational convention. From now on, we shall write everywhere (except in some definitions) “qc” for “quasi-compact”, “qs” for “quasi-separated”, and “qcqs” for “quasi-compact and quasi-separated”. -
Asymptotic Behavior of the Length of Local Cohomology
ASYMPTOTIC BEHAVIOR OF THE LENGTH OF LOCAL COHOMOLOGY STEVEN DALE CUTKOSKY, HUY TAI` HA,` HEMA SRINIVASAN, AND EMANOIL THEODORESCU Abstract. Let k be a field of characteristic 0, R = k[x1, . , xd] be a polynomial ring, and m its maximal homogeneous ideal. Let I ⊂ R be a homogeneous ideal in R. In this paper, we show that λ(H0 (R/In)) λ(Extd (R/In,R(−d))) lim m = lim R n→∞ nd n→∞ nd e(I) always exists. This limit has been shown to be for m-primary ideals I in a local Cohen Macaulay d! ring [Ki, Th, Th2], where e(I) denotes the multiplicity of I. But we find that this limit may not be rational in general. We give an example for which the limit is an irrational number thereby showing that the lengths of these extention modules may not have polynomial growth. Introduction Let R = k[x1, . , xd] be a polynomial ring over a field k, with graded maximal ideal m, and I ⊂ R d n a proper homogeneous ideal. We investigate the asymptotic growth of λ(ExtR(R/I ,R)) as a function of n. When R is a local Gorenstein ring and I is an m-primary ideal, then this is easily seen to be equal to λ(R/In) and hence is a polynomial in n. A theorem of Theodorescu and Kirby [Ki, Th, Th2] extends this to m-primary ideals in local Cohen Macaulay rings R. We consider homogeneous ideals in a polynomial ring which are not m-primary and show that a limit exists asymptotically although it can be irrational. -
What Is a Generic Point?
Generic Point. Eric Brussel, Emory University We define and prove the existence of generic points of schemes, and prove that the irreducible components of any scheme correspond bijectively to the scheme's generic points, and every open subset of an irreducible scheme contains that scheme's unique generic point. All of this material is standard, and [Liu] is a great reference. Let X be a scheme. Recall X is irreducible if its underlying topological space is irre- ducible. A (nonempty) topological space is irreducible if it is not the union of two proper distinct closed subsets. Equivalently, if the intersection of any two nonempty open subsets is nonempty. Equivalently, if every nonempty open subset is dense. Since X is a scheme, there can exist points that are not closed. If x 2 X, we write fxg for the closure of x in X. This scheme is irreducible, since an open subset of fxg that doesn't contain x also doesn't contain any point of the closure of x, since the compliment of an open set is closed. Therefore every open subset of fxg contains x, and is (therefore) dense in fxg. Definition. ([Liu, 2.4.10]) A point x of X specializes to a point y of X if y 2 fxg. A point ξ 2 X is a generic point of X if ξ is the only point of X that specializes to ξ. Ring theoretic interpretation. If X = Spec A is an affine scheme for a ring A, so that every point x corresponds to a unique prime ideal px ⊂ A, then x specializes to y if and only if px ⊂ py, and a point ξ is generic if and only if pξ is minimal among prime ideals of A. -
Arxiv:1703.06832V3 [Math.AC]
REGULARITY OF FI-MODULES AND LOCAL COHOMOLOGY ROHIT NAGPAL, STEVEN V SAM, AND ANDREW SNOWDEN Abstract. We resolve a conjecture of Ramos and Li that relates the regularity of an FI- module to its local cohomology groups. This is an analogue of the familiar relationship between regularity and local cohomology in commutative algebra. 1. Introduction Let S be a standard-graded polynomial ring in finitely many variables over a field k, and let M be a non-zero finitely generated graded S-module. It is a classical fact in commutative algebra that the following two quantities are equal (see [Ei, §4B]): S • The minimum integer α such that Tori (M, k) is supported in degrees ≤ α + i for all i. i • The minimum integer β such that Hm(M) is supported in degrees ≤ β − i for all i. i Here Hm is local cohomology at the irrelevant ideal m. The quantity α = β is called the (Castelnuovo–Mumford) regularity of M, and is one of the most important numerical invariants of M. In this paper, we establish the analog of the α = β identity for FI-modules. To state our result precisely, we must recall some definitions. Let FI be the category of finite sets and injections. Fix a commutative noetherian ring k. An FI-module over k is a functor from FI to the category of k-modules. We write ModFI for the category of FI-modules. We refer to [CEF] for a general introduction to FI-modules. Let M be an FI-module. Define Tor0(M) to be the FI-module that assigns to S the quotient of M(S) by the sum of the images of the M(T ), as T varies over all proper subsets of S. -
DENINGER COHOMOLOGY THEORIES Readers Who Know What the Standard Conjectures Are Should Skip to Section 0.6. 0.1. Schemes. We
DENINGER COHOMOLOGY THEORIES TAYLOR DUPUY Abstract. A brief explanation of Denninger's cohomological formalism which gives a conditional proof Riemann Hypothesis. These notes are based on a talk given in the University of New Mexico Geometry Seminar in Spring 2012. The notes are in the same spirit of Osserman and Ile's surveys of the Weil conjectures [Oss08] [Ile04]. Readers who know what the standard conjectures are should skip to section 0.6. 0.1. Schemes. We will use the following notation: CRing = Category of Commutative Rings with Unit; SchZ = Category of Schemes over Z; 2 Recall that there is a contravariant functor which assigns to every ring a space (scheme) CRing Sch A Spec A 2 Where Spec(A) = f primes ideals of A not including A where the closed sets are generated by the sets of the form V (f) = fP 2 Spec(A) : f(P) = 0g; f 2 A: By \f(P ) = 000 we means f ≡ 0 mod P . If X = Spec(A) we let jXj := closed points of X = maximal ideals of A i.e. x 2 jXj if and only if fxg = fxg. The overline here denote the closure of the set in the topology and a singleton in Spec(A) being closed is equivalent to x being a maximal ideal. 1 Another word for a closed point is a geometric point. If a point is not closed it is called generic, and the set of generic points are in one-to-one correspondence with closed subspaces where the associated closed subspace associated to a generic point x is fxg. -
Automorphisms in Birational and Affine Geometry
Springer Proceedings in Mathematics & Statistics Ivan Cheltsov Ciro Ciliberto Hubert Flenner James McKernan Yuri G. Prokhorov Mikhail Zaidenberg Editors Automorphisms in Birational and A ne Geometry Levico Terme, Italy, October 2012 Springer Proceedings in Mathematics & Statistics Vo lu m e 7 9 For further volumes: http://www.springer.com/series/10533 Springer Proceedings in Mathematics & Statistics This book series features volumes composed of selected contributions from workshops and conferences in all areas of current research in mathematics and statistics, including OR and optimization. In addition to an overall evaluation of the interest, scientific quality, and timeliness of each proposal at the hands of the publisher, individual contributions are all refereed to the high quality standards of leading journals in the field. Thus, this series provides the research community with well-edited, authoritative reports on developments in the most exciting areas of mathematical and statistical research today. Ivan Cheltsov • Ciro Ciliberto • Hubert Flenner • James McKernan • Yuri G. Prokhorov • Mikhail Zaidenberg Editors Automorphisms in Birational and Affine Geometry Levico Terme, Italy, October 2012 123 Editors Ivan Cheltsov Ciro Ciliberto School of Mathematics Department of Mathematics University of Edinburgh University of Rome Tor Vergata Edinburgh, United Kingdom Rome, Italy Hubert Flenner James McKernan Faculty of Mathematics Department of Mathematics Ruhr University Bochum University of California San Diego Bochum, Germany La Jolla, -
Lectures on Local Cohomology
Contemporary Mathematics Lectures on Local Cohomology Craig Huneke and Appendix 1 by Amelia Taylor Abstract. This article is based on five lectures the author gave during the summer school, In- teractions between Homotopy Theory and Algebra, from July 26–August 6, 2004, held at the University of Chicago, organized by Lucho Avramov, Dan Christensen, Bill Dwyer, Mike Mandell, and Brooke Shipley. These notes introduce basic concepts concerning local cohomology, and use them to build a proof of a theorem Grothendieck concerning the connectedness of the spectrum of certain rings. Several applications are given, including a theorem of Fulton and Hansen concern- ing the connectedness of intersections of algebraic varieties. In an appendix written by Amelia Taylor, an another application is given to prove a theorem of Kalkbrenner and Sturmfels about the reduced initial ideals of prime ideals. Contents 1. Introduction 1 2. Local Cohomology 3 3. Injective Modules over Noetherian Rings and Matlis Duality 10 4. Cohen-Macaulay and Gorenstein rings 16 d 5. Vanishing Theorems and the Structure of Hm(R) 22 6. Vanishing Theorems II 26 7. Appendix 1: Using local cohomology to prove a result of Kalkbrenner and Sturmfels 32 8. Appendix 2: Bass numbers and Gorenstein Rings 37 References 41 1. Introduction Local cohomology was introduced by Grothendieck in the early 1960s, in part to answer a conjecture of Pierre Samuel about when certain types of commutative rings are unique factorization 2000 Mathematics Subject Classification. Primary 13C11, 13D45, 13H10. Key words and phrases. local cohomology, Gorenstein ring, initial ideal. The first author was supported in part by a grant from the National Science Foundation, DMS-0244405. -
Chapter 1: Lecture 11
Chapter 1: Lecture 11 1. The Spectrum of a Ring & the Zariski Topology Definition 1.1. Let A be a ring. For I an ideal of A, define V (I)={P ∈ Spec(A) | I ⊆ P }. Proposition 1.2. Let A be a ring. Let Λ be a set of indices and let Il denote ideals of A. Then (1) V (0) = Spec(R),V(R)=∅; (2) ∩l∈ΛV (Il)=V (Pl∈Λ Il); k k (3) ∪l=1V (Il)=V (∩l=1Il); Then the family of all sets of the form V (I) with I ideal in A defines a topology on Spec(A) where, by definition, each V (I) is a closed set. We will call this topology the Zariski topology on Spec(A). Let Max(A) be the set of maximal ideals in A. Since Max(A) ⊆ Spec(A) we see that Max(A) inherits the Zariski topology. n Now, let k denote an algebraically closed fied. Let Y be an algebraic set in Ak .Ifwe n consider the Zariski topology on Y ⊆ Ak , then points of Y correspond to maximal ideals in A that contain I(Y ). That is, there is a natural homeomorphism between Y and Max(k[Y ]). Example 1.3. Let Y = {(x, y) | x2 = y3}⊂k2 with k algebraically closed. Then points in Y k[x, y] k[x, y] correspond to Max( ) ⊆ Spec( ). The latter set has a Zariski topology, and (x2 − y3) (x2 − y3) the restriction of the Zariski topology to the set of maximal ideals gives a topological space homeomorphic to Y (with the Zariski topology). -
18.726 Algebraic Geometry Spring 2009
MIT OpenCourseWare http://ocw.mit.edu 18.726 Algebraic Geometry Spring 2009 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms. 18.726: Algebraic Geometry (K.S. Kedlaya, MIT, Spring 2009) More properties of schemes (updated 9 Mar 09) I’ve now spent a fair bit of time discussing properties of morphisms of schemes. How ever, there are a few properties of individual schemes themselves that merit some discussion (especially for those of you interested in arithmetic applications); here are some of them. 1 Reduced schemes I already mentioned the notion of a reduced scheme. An affine scheme X = Spec(A) is reduced if A is a reduced ring (i.e., A has no nonzero nilpotent elements). This occurs if and only if each stalk Ap is reduced. We say X is reduced if it is covered by reduced affine schemes. Lemma. Let X be a scheme. The following are equivalent. (a) X is reduced. (b) For every open affine subsheme U = Spec(R) of X, R is reduced. (c) For each x 2 X, OX;x is reduced. Proof. A previous exercise. Recall that any closed subset Z of a scheme X supports a unique reduced closed sub- scheme, defined by the ideal sheaf I which on an open affine U = Spec(A) is defined by the intersection of the prime ideals p 2 Z \ U. See Hartshorne, Example 3.2.6. 2 Connected schemes A nonempty scheme is connected if its underlying topological space is connected, i.e., cannot be written as a disjoint union of two open sets. -
Degq Algebraic Geometry
degQ Algebraic Geometry Harpreet Singh Bedi [email protected] 15 Aug 2019 Abstract Elementary Algebraic Geometry can be described as study of zeros of polynomials with integer degrees, this idea can be naturally carried over to ‘polynomials’ with rational degree. This paper explores affine varieties, tangent space and projective space for such polynomials and notes the differences and similarities between rational and integer degrees. The line bundles O (n),n Q are also constructed and their Cechˇ cohomology computed. ∈ Contents 1 Rational Degree 3 1.1 Rational degree via Direct Limit . ............. 4 2 Affine Algebraic Sets 5 2.1 Ideal ........................................... .......... 7 2.2 Nullstellensatz ................................. ............... 8 3 Noether Normalization 9 4 Rational functions and Morphisms 10 4.1 FiniteFields ....................................... .......... 11 4.1.1 degZ[1/p] viaDirectLimit .................................... 11 arXiv:2003.12586v1 [math.GM] 24 Mar 2020 5 Projective Geometry 13 5.1 GradedRingsandHomogeneousIdeals. ................ 13 6 Projective Nullstellensatz 14 6.1 Affinecone........................................ .......... 14 6.2 StandardAffineCharts .............................. ............. 15 7 Schemes 15 1 8 Proj and Twisting Sheaves O (n) 16 8.1 O (1) ................................................... .. 17 8.2 ComputingCohomology ............................... ........... 17 8.3 KunnethFormula .................................. ............ 21 9 Tangent Space 22 9.1 JacobianCriterion................................. -
AN INTRODUCTION to AFFINE SCHEMES Contents 1. Sheaves in General 1 2. the Structure Sheaf and Affine Schemes 3 3. Affine N-Space
AN INTRODUCTION TO AFFINE SCHEMES BROOKE ULLERY Abstract. This paper gives a basic introduction to modern algebraic geom- etry. The goal of this paper is to present the basic concepts of algebraic geometry, in particular affine schemes and sheaf theory, in such a way that they are more accessible to a student with a background in commutative al- gebra and basic algebraic curves or classical algebraic geometry. This paper is based on introductions to the subject by Robin Hartshorne, Qing Liu, and David Eisenbud and Joe Harris, but provides more rudimentary explanations as well as original proofs and numerous original examples. Contents 1. Sheaves in General 1 2. The Structure Sheaf and Affine Schemes 3 3. Affine n-Space Over Algebraically Closed Fields 5 4. Affine n-space Over Non-Algebraically Closed Fields 8 5. The Gluing Construction 9 6. Conclusion 11 7. Acknowledgements 11 References 11 1. Sheaves in General Before we discuss schemes, we must introduce the notion of a sheaf, without which we could not even define a scheme. Definitions 1.1. Let X be a topological space. A presheaf F of commutative rings on X has the following properties: (1) For each open set U ⊆ X, F (U) is a commutative ring whose elements are called the sections of F over U, (2) F (;) is the zero ring, and (3) for every inclusion U ⊆ V ⊆ X such that U and V are open in X, there is a restriction map resV;U : F (V ) ! F (U) such that (a) resV;U is a homomorphism of rings, (b) resU;U is the identity map, and (c) for all open U ⊆ V ⊆ W ⊆ X; resV;U ◦ resW;V = resW;U : Date: July 26, 2009.