Geometric motivic integration


 Dwain Joseph
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1 Université Lille 1 Modnet Workshop 2008
2 Introduction Motivation: padic integration Kontsevich invented motivic integration to strengthen the following result by Batyrev. Theorem (Batyrev) If two complex CalabiYau varieties are birationally equivalent, then they have the same Betti numbers. Batyrev proved this result using padic integration and the Weil Conjectures. Kontsevich observed that Batyrev s proof could be geometrized, avoiding the passage to finite fields and yielding a stronger result: equality of Hodge numbers. The key was to replace the padic integers by C[[t]], and padic integration by motivic integration.
3 Kontsevich presented these ideas at a famous Lecture at Orsay in 1995, but never published them. The theory was developed and generalized in the following directions: Denef and Loeser developed a theory of geometric motivic integration on arbitrary algebraic varieties over a field of characteristic zero. They also created a theory of arithmetic motivic integration, with good specialization properties to padic integrals in a general setting, using the model theory of pseudofinite fields. The motivic integral appears here as a universal integral, specializing to the padic ones for almost all p.
4 Loeser and Sebag constructed a theory of motivic integration on formal schemes and rigid varieties, working over an arbitrary complete discretely valued field with perfect residue field. Cluckers and Loeser built a very general framework for motivic integration theories, based on model theory. We will only discuss the socalled naïve geometric motivic integration on smooth algebraic varieties.
5 Outline Motivation: padic integration 1 Motivation: padic integration 2
6 Basic references: J. Denef & F. Loeser, Geometry on arc spaces of algebraic varieties, European Congress of Mathematics, Vol. 1 (Barcelona, 2000), Progr. Math. 201, 2001 W. Veys,, motivic integration and stringy invariants, Advanced Studies in Pure Mathematics 43, Proc. of Singularity Theory and its applications, Sapporo, september 2003 (2006)
7 padic integration Motivation: padic integration We fix a prime number p. For any integer n 0, we consider the compact group Z n p = (Z p ) n with its unique Haar measure µ such that µ(z n p) = 1.
8 Definition We consider, for each m, n 0, the natural projection π m : Z n p (Z p /p m+1 ) n A cylinder C in Z n p is a subset of the form (π m ) 1 (C m ), for some m 0 and some subset C m of (Z p /p m+1 ) n.
9 Lemma For any cylinder C, the series (p n(m+1) π m (C) ) m 0 is constant for m 0, and its limit is equal to the Haar measure of C. More precisely, if we choose m 0 0 such that C = (π m0 ) 1 (π m0 (C)), then for m m 0 p n(m+1) π m (C) = p n(m 0+1) π m0 (C) = µ(c)
10 Proof. For m m 0, C can be written as a disjoint union C = (a + (p m+1 Z p ) n ) a π m(c) so by translation invariance of the Haar measure and the fact that (p m+1 Z p ) n has measure p (m+1)n, we see that the measure of C equals p (m+1)n π m (C)
11 Motivation: padic integration If we identify Z p with the ring of Witt vectors W (F p ), then the map π m simply corresponds to the truncation map W (F p ) W m+1 (F p ) : (a 0, a 1,...) (a 0, a 1,..., a m )
12 The idea behind the theory of motivic integration is to make a similar construction, replacing W (F p ) by the ring of formal power series k[[t]] over some field k, and the map π m by the truncation map k[[t]] k[t]/(t m+1 ) : m a i t i a i t i i 0 i=0 The problem is to give meaning to the expression π m (C) if C is a cylinder in k[[t]] n for infinite fields k, and to find a candidate to replace p in the formula µ(c) = p n(m+1) π m (C) )
13 But interpreting the coefficients of a power series as affine coordinates, the set (k[[t]]/(t m+1 )) n gets the structure of the set of kpoints on an affine space A (m+1)n k, and if we restrict to cylinders C such that π m (C) is constructible in A (m+1)n k, we can use the Grothendieck ring of varieties as a universal way to count points on constructible subsets of an algebraic variety. The cardinality p of F p is replaced by the number of points on the affine line A 1 k ; this is the Lefschetz motive L.
14 The price to pay is that we leave classical integration theory since our value ring will be an abstract gadget (the Grothendieck ring of varieties) instead of R.
15 k any field; kvariety= separated reduced kscheme of finite type. Definition (Grothendieck ring of kvarieties) K 0 (Var k ) abelian group generators: isomorphism classes [X ] of kvarieties X relations: [X ] = [X \ Y ] + [Y ] for Y closed in X ( scissor relations ) Ring multiplication: [X 1 ] [X 2 ] = [(X 1 k X 2 ) red ] L = [A 1 k ] M k = K 0 (Var k )[L 1 ] for X not reduced: [X ] := [X red ]
16 If X is a kvariety and C a constructible subset of X, then C can be written as a disjoint union of locally closed subsets (subvarieties) of X and this yields a welldefined class [C] in K 0 (Var k ). If a morphism of kvarieties Y X is a Zariskilocally trivial fibration with fiber F, then [Y ] = [X ] [F ] in K 0 (Var k ). Indeed, using the scissor relations and Noetherian induction we can reduce to the case where the fibration is trivial.
17 By its very definition, the Grothendieck ring is a universal additive and multiplicative invariant of kvarieties. specialization morphisms of rings χ top : K 0 (Var k ) Z (Euler characteristic) k = C : K 0 (Var k ) Z (number of rational points) k finite So in a certain sense, taking the class [X ] of a kvariety X is the most general way to count points on X.
18 Motivation: padic integration Let X be a variety over k. For each integer n 0, we consider the functor F n : (k alg) (Sets) : A X (A[t]/(t n+1 )) Proposition The functor F n is representable by a separated kscheme of finite type L n (X ), called the nth jet scheme of X. If X is affine, then so is L n (X ).
19 Note: Saying that L n (X ) represents the functor F n simply means the following: for any kalgebra A, there exists a bijection φ A n : L n (X )(A) F n (A) = X (A[t]/t n+1 ) such that for any morphism of kalgebras h : A B, the square L n (X )(A) φ A n X (A[t]/t n+1 ) L n (X )(B) φ B n X (B[t]/t n+1 ) commutes. This property uniquely determines the kscheme L n (X ).
20 Idea of proof: We only consider the affine case X = Spec k[x 1,..., x r ]/(f 1,..., f l ) i.e. X is the closed subvariety of A r k defined by the equations f 1 =... = f l = 0. Consider a tuple of variables (a 1,0,..., a 1,n, a 2,0,..., a r,n ) and the system of congruences f j ( for j = 1,..., l. n a 1,i t i,..., i=0 n a r,i t i ) 0 mod t n+1 i=0
21 If we replace the variables a i,j by elements of a kalgebra A, these congruences express exactly that the tuple (a 1, a 1,n t n,..., a r, a r,n t n ) A r k (A[t]/tn+1 ) lies in X (A[t]/(t n+1 )).
22 Developing, for each j, f j ( n a 1,i t i,..., i=0 n a r,i t i ) into a polynomial in t and putting the coefficient of t i equal to 0 for i = 0,..., n yields a system of l(n + 1) polynomial equations over k in the variables a i,j, and these define the scheme L n (X ) as a closed subscheme of A r(n+1) k. i=0
23 Example: Let X be the closed subvariety of A 2 k = Spec k[x, y] defined by the equation x 2 y 3 = 0. Then a point of L 2 (X ) with coordinates in some kalgebra A is a couple with x 0,..., y 2 A such that (x 0 + x 1 t + x 2 t 2, y 0 + y 1 t + y 2 t 2 ) (x 0 + x 1 t + x 2 t 2 ) 2 (y 0 + y 1 t + y 2 t 2 ) 3 0 mod t 3
24 (x 0 + x 1 t + x 2 t 2 ) 2 (y 0 + y 1 t + y 2 t 2 ) 3 0 mod t 3 Working this out, we get the equations (x 0 ) 2 (y 0 ) 3 = 0 2x 0 x 1 3(y 0 ) 2 y 1 = 0 (x 1 ) 2 + 2x 0 x 2 3y 0 (y 1 ) 2 3(y 0 ) 2 y 2 = 0 and if we view x 0,..., y 2 as affine coordinates, these equations define L 2 (X ) as a closed subscheme of A 6 k.
25 For any m n and any kalgebra A, the truncation map A[t]/t m+1 A[t]/t n+1 defines a natural transformation of functors F m F n, so by Yoneda s Lemma we get a natural truncation morphism of kschemes π m n : L m (X ) L n (X ) This is the unique morphism such that for any kalgebra A, the square X (A[t]/t m+1 ) X (A[t]/t n+1 ) φ A m φ A n commutes. L m (X )(A) π m n L n (X )(A)
26 Since the schemes L n (X ) are affine for affine X and L n (.) takes open covers to open covers, the morphisms πn m are affine for any kvariety X, and we can take the projective limit in the category of kschemes. L(X ) = lim n L n (X )
27 The scheme L(X ) is called the arc scheme of X. It is not Noetherian, in general. It comes with natural projection morphisms π n : L(X ) L n (X ) For any field F over k, we have a natural bijection L(X )(F ) = X (F [[t]]) and the points of these sets are called F valued arcs on X. The morphism π n maps an arc to its truncation modulo t n+1.
28 So by definition, a F valued arc is a morphism ψ : Spec F [[t]] X i.e. a point of X with coordinates in F [[t]]. The image of the closed point of Spec F [[t]] is called the origin of the arc and denoted by ψ(0); it is obtained by putting t = 0. An arc should be seen as an infinitesimal disc on X with origin at ψ(0).
29 We have L 0 (X ) = X and L 1 (X ) is the tangent scheme of X. The truncation morphism π 0 : L(X ) X maps an arc ψ to its origin ψ(0). A morphism of kvarieties h : Y X induces morphisms L(h) : L(Y ) L(X ) L n (h) : L n (Y ) L n (X ) commuting with the truncation maps.
30 If X is smooth over k, of pure dimension d, then the morphisms πn m are Zariskilocally trivial fibrations with fiber A d(m n) k (use étale charts and the fact that A[t]/(t m+1 ) A[t]/(t n+1 ) is a nilpotent immersion for any kalgebra A). Intuitively, arcs are local objects on X and any smooth variety of pure dimension d looks locally like an open subvariety of A d k ; but an element of L n (A d k )(A) is simply a dtuple of elements in A[t]/t n+1.
31 If X is singular, the spaces L n (X ) and L(X ) are still quite mysterious. They contain a lot of information on the singularities of X.
32 Example: Let us go back to our previous example, where X A 2 k was given by the equation x 2 y 3 = 0. For any kalgebra A, a point on L(X ) with coordinates in A is given by a couple (x(t) = x 0 + x 1 t + x 2 t , y(t) = y 0 + y 1 t + y 2 t ) with x i, y i A, such that x(t) 2 y(t) 3 = 0. Working this out yields an infinite number of polynomial equations in the variables x i, y i and these realize L(X ) as a closed subscheme of the infinitedimensional affine space A k = Spec k[x 0, y 0, x 1, y 1,...].
33 The truncation map π n : L(X ) L n (X ) sends (x(t), y(t)) to (x x n t n, y y n t n ) and (if A is a field) the origin of (x(t), y(t)) is simply the point (x 0, y 0 ) on X.
34 Note: If k has characteristic zero, one can give an elegant construction of the schemes L n (X ) and L(X ) using differential algebra. Assume that X is affine, say given by polynomial equations f 1 (x 1,..., x r ) =... = f l (x 1,..., x r ) in affine rspace A r k = Spec k[x 1,..., x r ]. Consider the kalgebra B = k[y 1,0,..., y r,0, y 1,1,...] and the unique kderivation δ : B B mapping y i,j to y i,j+1 for each i, j.
35 Then L(X ) is isomorphic to the closed subscheme of Spec B defined by the equations δ (i) (f q (y 1,0,..., y r,0 )) = 0 for q = 1,..., l and i N. The point with coordinates y i,j corresponds to the arc ( j 0 y 1,j j! tj,..., y r,j j! tj ) j 0
36 Copying the notion of cylinder and the description of its Haar measure, we can define a motivic measure on a class of subsets of the arc space L(X ). From now on, we assume that X is smooth over k, of pure dimension d.
37 Definition A cylinder in L(X ) is a subset C of the form (π m ) 1 (C m ), with m 0 and C m a constructible subset of L m (X ). Note that the set of cylinders in L(X ) is a Boolean algebra, i.e. it is closed under complements, finite unions and finite intersections.
38 DefinitionLemma Let C be a cylinder in L(X ), and choose m 0 such that C = (π m ) 1 (C m ) with C m constructible in L m (X ). The value µ(c) := [π m (C)]L d(m+1) M k does not depend on m, and is called the motivic measure µ(c) of C. This follows immediately from the fact that the truncation morphisms π n m are Zariskilocally trivial fibrations with fiber A d(n m) k.
39 Example If C = L(X ), then µ(c) = L d [X ]. The normalization factor L d is added in accordance with the padic case, where the ring of integers gets measure one (rather then the cardinality of the residue field). Note that the motivic measure µ is additive w.r.t. finite disjoint unions.
40 In the general theory of motivic integration, one constructs a much larger class of measurable sets and one defines the motivic measure via approximation by cylinders. This necessitates replacing M k by a certain dimensional completion. We will not consider this generalization here.
41 Definition We say that a function α : L(X ) N { } is integrable if its image is finite, and if α 1 (i) is a cylinder for each i N. We define the motivic integral of α by L α = µ(α 1 (i))l i M k L(X ) i N
42 The central and most powerful tool in the theory of motivic integration is the change of variables formula. For its precise statement, we need some auxiliary notation. For any kvariety Y, any ideal sheaf J on Y and any arc ψ : Spec F [[t]] Y on Y, we define the order of J at ψ by ord t J (ψ) = min{ord t ψ f f J ψ(0) } where ord t is the tadic valuation. In this way, we obtain a function whose fibers over N are cylinders. ord t J : L(Y ) N { }
43 Theorem (Change of variables formula) Let h : Y X be a proper birational morphism, with Y smooth over k, and denote by Jac h the Jacobian ideal of h. Let α be an integrable function on L(X ), and assume that ord t Jac h takes only finitely many values on each fiber of α L(h) over N. Then L α = L ((α L(h))+ordtJac h) in M k. L(X ) L(Y )
44 The very basic idea behind the change of variables formula is the following: if we denote by V the closed subscheme of Y defined by the Jacobian ideal Jac h, and by U its image under h, then the morphism h : Y V X U is an isomorphism. Combined with the properness of h, this implies that L(h) : L(Y ) L(V ) L(X ) L(U) is a bijection; but L(V ) and L(Y ) have measure zero in L(Y ), resp. L(X ) (w.r.t. a certain more refined motivic measure) so it is reasonable to expect that there exists a change of variables formula.
45 The jet spaces L n (Y ), however, are contracted under the morphism L n (h) : L n (Y ) L n (X ) and this affects the motivic measure of cylinders. The contraction factor is measured by the Jacobian.
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