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On Resolutions, Minimal and Virtual ON RESOLUTIONS, MINIMAL AND VIRTUAL MICHAEL LOPER FEBURARY 1, 2018 Abstract. The purpose of this paper is to introduce some possible research directions involving virtual resolutions, which were introduced in [BZES17]. However, before discussing virtual resolutions, we show why they are important. This is done by comparing them to minimal resolutions of modules over the standard graded polynomial ring over a eld. 1. Introduction In n-dimensional projective space, there is a correspondence between certain sheaves and nitely generated modules over the standard graded polynomial ring in n + 1 variables over a eld. Because of this, the geometry of varieties can be investigated through modules over a graded polynomial ring. Homological tools can be used to study the modules and therefore understand properties of the geometry of the varieties. Section2 concentrates on introducing tools such as minimal free resolutions that are useful in studying modules. We discuss invariants such as the Betti numbers of a module and men- tion how Betti numbers are able to explain properties of the modules. The Hilbert function and polynomial are dened in this section and the connection with CastelnuovoMumford regularity is described. This section lays the groundwork of attempting to generalize many of these notions to the more general toric variety case. Toric varieties are dened and discussed in Section3. In this section, the Cox ring is dened and the correspondence between subvarieties of a toric variety and certain ideals of the Cox ring is discussed. Just as subvarieties of projective space can be understood by studying modules of a polynomial ring, subvarieties of toric varieties can be better understood by studying certain modules over the Cox ring. An emphasis is placed on understanding the multigrading of the Cox ring through the group action of a torus. In a product of projective spaces or, more generally, in a toric variety, the tools introduced in Section2 do not as closely reect the geometry of subvarieties. A proposed replacement of minimal free resolutions called virtual resolutions is given in Section4. We dene virtual resolutions and give two dierent ways of constructing them. A couple of examples are also provided in this section. Finally, in Section5 possible research directions are discussed. As virtual resoutions are rather new objects of study, there are many directions to pursue. We explore some of them such as classication problems, nding new ways to construct virtual resolutions, providing new analogues to theorems from Section2, and more. 1 2 MIKE LOPER 2. Syzygies In this section we will discuss some results and techniques well known for investigating syzygies of modules in projective space n. Throughout this section, let P S = |[x0; x1; : : : ; xn] be the standard graded polynomial ring in n + 1 variables over an algebraically closed eld and let be the maximal ideal. | m = hx0; :::; xni We are able to use modules over S to investigate subvarieties of Pn because of the following theorem. Theorem 2.1. There is a bijective correspondence n fnonempty closed subvarieties of P g ! fm-saturated radical homogeneous idealsg: 2.1. Minimal Free Resolutions. Free resolutions were orginially studied by David Hilbert in the 19th century. We start by dening a minimal resolution and providing a few examples. This discussion is largely following [Eis95] and [Eis05]. Denition 2.2. Let R be a ring. A projective resolution of an R-module M is a complex '1 '2 'n 'n+1 F : F0 − F1 −· · · − Fn −−−· · · of projective R-modules Fi such that H0(F ) = M and otherwise F has no homology. We say F is a free resolution if all the Fi are free and is a graded free resolution if R is a graded ring, all the Fi are graded free modules, and the maps are all homogeneous maps of degree 0. If, for some n < 1, we have Fn+1 = 0, but Fi 6= 0 for 0 ≤ i ≤ n, then we shall say that F is a nite resolution of length n. It is straightforward to see that every nitely generated module M over S has a free resolution. To construct one, simply let F0 be the free module generated by a set of generators on M. The kernel of the map F0 ! M is called the rst syzygy module of M. Next let Fi be the free module on a set of generators of the ith syzygy of M and repeat this process. The ith syzygy module is the image of the map Fi ! Fi−1 and the elements of these syzygy modules are called syzygies. If M is a graded S-module, then dene M(a) to be the module M shifted by a. That is, M(a)n, the nth graded component of M(a) is Ma+n, the (a + n)th graded component of M. Example 2.3. Let S = |[x; y] and M = S=hx; yi = |. Following the procedure above, F0 = S. Now let K0 = ker(F0 ! M). Then K0 = hx; yi and thus F1 is free of rank 2. 2 Let '1 : S (−1) ! S be dened by '1(a; b) = ax + by. Notice that this is a degree zero map. K1 = ker('1) = h(y; −x)i is generated by a single element so F2 has rank 1. In order for '2 to be a degree zero map, F2 = S(−2) and '2 : 1 7! (y; −x). This map is injective so K3 = 0 which means that F3 = 0 so our free resolution has length 2. The free resolution is 2 y3 hx yi 4−x5 F : S −−−− S2(−1) −−−− S(−2) − 0; where the maps over the arrows are denoted by the corresponding matrices. This particular example is the Koszul complex of length 2. ON RESOLUTIONS, MINIMAL AND VIRTUAL 3 It is not true in general that free resolutions are unique. Of course, we could always take id the direct sum of a complex F with the shifted trivial complex 0 S − S 0 to yield a new free resolution. If S is a graded ring (such as a polynomial ring), there is however, one smallest free resolution, called the minimal resolution, in the following sense. If F and G are two minimal resolutions of M, then there is a graded isomorphism of complexes F ! G that induces the identity on M. Denition 2.4. A free resolution F of an S-module M is minimal if the image of each 'n : Fn ! Fn−1 is contained in mFn−1. In other words, a free resolution F is minimal if all the maps of the complex are zero. | ⊗S F In fact, it turns out that every free resolution contains the minimal resolution as a direct summand. Looking at Example 2.3 above, it is easy to see that has all zero maps F ⊗S | (because all entries in the matrices are positively graded) so F is the minimal resolution of |. Next is a slightly more interesting example of a minimal free resolution. Example 2.5. Let S = |[x; y]. Let m be the maximal ideal hx; yi in S. Following the exact same procedure as in Example 2.3, a resolution of m2 = hx2; xy; y2i is 2 y 03 6 7 6 7 6−x y7 hx2 xy y2i 4 0 −x5 S −−−−−−−−− S(−2)3 −−−−−−−− S(−3)2 − 0: Notice that all entries of the matrices are contained in m so this resolution is indeed minimal. Up until this point in this paper, all examples have had nite minimal free resolutions. It turns out that for nitely generated modules over S, this will always be the case. Theorem 2.6 (Hilbert Syzygy Theorem). If , then every nitely generated S = |[x0; :::; xn] graded S-module has a nite graded resolution of length ≤ n + 1, by nitely generated free modules. 2.2. The Hilbert Function. Hilbert also dened the Hilbert function which is a way to measure invariants of a module. Denition 2.7. Let be a nitely generated module over . Recall the M S = |[x0; x1; :::; xn] degree d component of M be denoted by Md. The Hilbert function of M is the numerical function HM (d) := dim|(Md): Let F be a nite free resolution of M of length r. Since each 'i is a degree zero map, extracting the dth degree component of M and the Fi, there is an exact sequence of vector spaces Fd :(F0)d − (F1)d − · · · − (Fr)d − 0: 4 MIKE LOPER So the Hilbert function can be computed as r (2.7.1) X i HM (d) = (−1) dim|(Fi)d: i=0 Example 2.8. Revisiting Example 2.5 where S = |[x; y] and M = S=m2, notice that the zero degree component of is , so . Moving to the degree 1 component M M0 = | HM (0) = 1 of , since span , then . Finally, for so M M1 = |hx; yi HM (1) = 2 Mn = 0 n ≥ 2 HM (n) = 0 for n ≥ 2. This is summarized below: 8 1 for d = 0; <> HM (d) = 2 for d = 1; :>0 for d ≥ 2: In particular, in Example 2.8, the Hilbert function stabilized to zero. Does this always happen? It turns out the answer is no. There is good news, though. Hilbert discovered that the innite values of the Hilbert function can be encoded in a polynomial. Theorem 2.9. If M is a nitely generated graded module over S, then for d 0, HM (d) agrees with a polynomial of degree ≤ n + 1. Denition 2.10. The polynomial in Theorem 2.9, denoted PM (d), is called the Hilbert polynomial of M. 2.3. Betti Numbers. Because minimal free resolutions are unique up to graded isomor- phism, the twists of the free modules at each step in the resolution are independent of the choice of minimal free resolution.
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