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Motives and Motivic L-Functions

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Motives and Motivic L-Functions Motives and Motivic L-functions Christopher Elliott December 6, 2010 1 Introduction This report aims to be an exposition of the theory of L-functions from the motivic point of view. The classical theory of pure motives provides a category consisting of `universal cohomology theories' for smooth projective varieties defined over { for instance { number fields. Attached to every motive we can define a function which is holomorphic on a subdomain of C which at least conjecturally satisfies similar properties to the Riemann zeta function: for instance meromorphic continuation and functional equation. The properties of this function are expected to encode deep arithmetic properties of the underlying variety. In particular, we will discuss a conjecture of Deligne and its more general forms due to Beilinson and others, that relates certain \special" values of the L-function { values at integer points { to a subtle arithmetic invariant called the regulator, vastly generalising the analytic class number formula. Finally I aim to describe potentially fruitful ways in which these exciting conjectures might be rephrased or reinterpreted, at least to make things clearer to me. There are several sources from which I have drawn ideas and material. In particular, the book [1] is a very good exposition of the theory of pure motives. For the material on L-functions, and in particular the Deligne and Beilinson conjectures, I referred to the surveys [9], [11] and [4] heavily. In particular, my approach to the Deligne conjecture follows the survey [11] of Schneider closely. Finally, when I motivate the definition of the motivic L-function I use the ideas and motivation in the introduction to the note [5] of Kim heavily. These notes were written for a seminar topics class at Northwestern university directed by David Nadler. I gave a series of lectures on motives and motivic L-functions with Clemens Koppensteiner and Marc Hoyois. 2 Pure Motives 2.1 The Weil Conjectures The following extremely well-known conjectures of Andr´eWeil on the number of points on varieties over finite fields will serve as the starting point for all of the theory that will follow: Conjecture 2.1. Let X be a smooth projective variety over Fq of dimension n. We define the zeta function of X by 1 ! X Nm ζ(X; s) = exp (q−s)m m m=0 −s where Nm denotes the number of Fqm points of X. Write t = q . Then 1. The zeta function can be written as a rational function of t. Furthermore P (t)P (t) ··· P (t) ζ(X; s) = 1 3 2d−1 P0(t)P2(t) ··· P2d(t) where Pi(t) 2 Z(t), 2 Section 2 Pure Motives 2. It satisfies a functional equation of the form ζ(X; s) = ±q(n=2−s)χζ(X; n − s) where χ = χ(X) is the Euler characteristic. 3. (The Riemann hypothesis) We can factor Pi(t) as nj Y Pi(t) = (q − αijt); j=0 i=2 i and jαijj = q . Thus the zeroes of Pi lie on the critical line <s = 2 . The zeta function is a generating function for the number of points of X over finite extensions Fqn of Fq. We'll see how these zeta functions fit into a more general picture later on. This particular viewpoint is the main focus of this report, with the Weil conjectures as one motivating aspect, justifying how studying these meromorphic functions might tell about concrete geometric facts like the number of points over various finite fields (i.e. solving congruences). The case where X is an elliptic curve was proved by Weil himself (he also proved the conjecture for all curves and abelian varieties), and is particularly easy. The reason is that we have a nice linearisation of the curve to use { the Tate module T`(E) for a prime ` 6= p { which allows us to reduce to an argument of linear algebra. This method is essentially cohomological. In general, associated to a variety X we can produce a number of cohomology theories, which one should think of as linearisations of X, that remember as much of its structure as they can. One might imagine a simple proof of the Weil conjectures in their full generality proceeding as follows: produce a sufficiently nice cohomology theory for smooth projective varieties, so that one has in particular a Lefschetz trace formula, then mimic Weil's proof for the elliptic curve. In fact, once the theory of ´etalecohomology was invented and sufficiently well developed, this was sufficient for Deligne to prove the Weil conjectures in full generality, though not quite with the beautiful proof invisioned by Grothendieck: the Riemann hypothesis proved particularly tricky, and needed more subtle methods. 2.2 Cohomology Theories Although cohomology groups have a similar use in algebraic geometry to their use in topology: as linearisations of geometric objects, allowing one to attack geometric problems by methods of linear algebra, the situation is quite difficult for the following reason. In the topological setting there is essentially only one possible choice of a cohomology theory, at least if one imposes certain reasonable restrictions on what functors are allowed. This is the content of the Eilenberg-Steenrod theorem. In the algebro-geometric setting however, things are nowhere near this nice. Indeed, we have a sensible notion of a `reasonable' cohomology theory: what is called a Weil cohomology theory. One can define these over various different base fields. The problems arise in part due to the non-existence of a Weil cohomology theory over the rational numbers. This makes it quite difficult to compare, for instance, cohomology theories with C coefficients to those with Q` coefficients. Slightly more formally, though omitting many details, Definition 2.2. A Weil cohomology theory is a contravariant functor H∗ from the category of smooth projective varieties over a field k, to graded vector spaces over a field K where K has characteristic zero, satisfying a number of conditions: 1. dim H2(P1) = 1 2. H∗ preserves the monoidal structure, where ⊗ on the category of smooth projective varieties is given by the usual product. This automatically makes H∗(X) into a commutative algebra, because the diagonal embedding of varieties automatically gives a variety X the structure of a coalgebra. We get a K¨unneth formula H∗(X × Y ) =∼ H∗(X) ⊗ H∗(Y ) for free. 3 Section 2 Pure Motives 3. We have Poincar´eduality: loosely, ignoring technical issues of weights, this is a perfect pairing Hi(X) × H2d−i(X) ! K; where d = dim X. 4. There is a cycle class map, which assigns a cohomology class of degree i to an algebraic cycle of codimension i. Loosely, this is a homomorphism of rings for each X ∗ 2∗ clX : CH (X) ! H (X) where CH∗(X) is the Chow ring of algebraic cycles modulo rational equivalence, with the intersection product. These homomorphisms should be compatible with the induced morphisms coming from morphisms f : X ! Y There are a number of ways to prove that there is no Weil cohomology theory when K = Q that underlies the other Weil cohomology theories. One could for instance give the explicit counterexample of a supersingular elliptic curve. This is described in Milne's expository article [8]. The most important examples of Weil cohomology theories for our purposes are: ∗ • If K = C, the de Rham cohomology HdR(X(C)), where X(C) is given the analytic topology. ∗ • If K = Q`, the `-adic topology H´et(X; Q`). This is well behaved whenever char k 6= `, where k is the field of definition of X. ∗ • If char k = 0, given an embedding ν : k ,! C, the singular cohomology H ((X ×k,ν C)(C);K) for any subfield K of C. Again we give the complex points the analytic topology. Comparison theorems between these rings and others are not easy to come by. For a truly unified approach we would need some kind of universal cohomology theory. Since there is no genuine universal Weil cohomology theory, to produce such a thing we will need a larger target abelian category. This is exactly the role the category of pure motives plays. 2.3 The construction As before, assume X is smooth and projective over K any field. We will enlarge the category of smooth projective varieties over K, first by producing extra morphisms from the intersection theory of subvarieties of X (correspon- dences), then splitting idempotents, then inverting the Lefschetz motive h2(P1) to ensure Poincar´eduality works. We will only sketch the construction here. For full details, see the book [1] by Yves Andr´e. So, first of all, let Var(K) denote the category of smooth projective varieties over K. For X in this category, let C∗(X) denote the group of algebraic cycles in X, graded by codimension. There are a number of sources where these notions of intersection theory are explained, but I found [7] and [12] section III to be particularly helpful. This comes equipped with an intersection product and there are a number of equivalence relations we can impose on cycles to make this into a genuine multiplication on the group, compatible with the grading. Choosing rational equivalence we produce the Chow ring, but this will turn out to be too coarse for our needs. For now we will denote ∗ by C∼(X) the ring of cycles produced by choosing some adequate equivalence relation ∼. ∗ We enlarge the hom sets of Var(K) by putting HomCorr(X; Y ) = C∼(X × Y ). Note that this includes all former morphisms, as their graphs describe an algebraic cycle. One defines composition by pushing forward to the triple product X × Y × Z, applying the intersection product, then pulling back to X × Z, and this produces a category Corr(K) which contains all the data of algebraic cycles that we might need for the cycle class map to exist.
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