CONSTRUCTIBLE EXPONENTIAL FUNCTIONS MOTIVIC FOURIER TRANSFORM AND TRANSFER PRINCIPLE

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CONSTRUCTIBLE EXPONENTIAL FUNCTIONS, MOTIVIC FOURIER TRANSFORM AND TRANSFER PRINCIPLE RAF CLUCKERS AND FRANC¸OIS LOESER 1. Introduction In our previous work [8], we laid general foundations for motivic integration of constructible functions. One of the most salient features of motivic constructible functions is that they form a class which is stable under direct image and that motivic integrals of constructible functions depending on parameters are constructible as functions of the parameters. Though motivic constructible functions as defined in [8] encompass motivic analogues of many functions occuring in integrals over non archimedean local fields, one important class of functions was still missing in the picture, namely motivic analogues of non archimedean integrals of the type ∫ Qnp f(x)?(g(x))|dx|, with ? a (non trivial) additive character on Qp, f a p-adic constructible function and g a Qp-valued definable function on Qnp , and their parametrized versions, functions of the type ? 7?? ∫ Qnp f(x, ?)?(g(x, ?))|dx|, where ? runs over, say Qmp , and f and g are now functions on Q m+n p . Needless to say, integrals of this kind are ubiquitous in harmonic analysis over non archimedean local fields, p-adic representation Theory and the Langlands Program.

• also add constant


• constructible motivic


• fourier transform


• definable subassignments


• cell zc


• ring language


• functions


• valued field

Sujets

• Transfer
• Fourier transform
• Function
• Valuation (algebra)

Informations

• Publié par: pefav
• Nombre de lectures: 14
• Langue: English

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CONSTRUCTIBLE EXPONENTIAL FUNCTIONS, MOTIVIC FOURIER TRANSFORM AND TRANSFER PRINCIPLE

RAF CLUCKERS AND FRANC¸ OIS LOESER

1.Introduction

In our previous work [8], we laid general foundations for motivic integration of constructible functions. One of the most salient features of motivic constructible functions is that they form a class which is stable under direct image and that motivic integrals of constructible functions depending on parameters are constructible as functions of the parameters. Though motivic constructible functions as defined in [8] encompass motivic analogues of many functions occuring in integrals over non archimedean local fields, one important class of functions was still missing in the picture, namely motivic analogues of non archimedean integrals of the type Z Q n g(x))|dx| f(x)Ψ( p with Ψ a (non trivial) additive character onQ p ,fap-adic constructible function and gaQ p -valued definable function onQ n p , and their parametrized versions, functions of the type λ7→− Z pn f(x λ)Ψ(g(x λ))|dx| Q whereλruns over, sayQ pm , andfandgare now functions onQ pm+n  to. Needless say, integrals of this kind are ubiquitous in harmonic analysis over non archimedean local fields,p-adic representation Theory and the Langlands Program.

One of the purposes of the present paper is to fill this gap by extending the framework of [8] in order to include motivic analogues of exponential integrals of the above type. Once this is done one is able to develop a natural Fourier trans- form and to prove various forms of Fourier inversion. Another interesting feature of our formalism is that it makes possible to state and prove a general transfer prin- ciple for integrals over non archimedean local fields, allowing to transfer identities between functions defined by integrals over fields of characteristic zero to fields of characteristicp It, when the residual characteristic is large enough, and vice versa. should be emphasized that our statement holds for quite general functions defined by integrals depending on valued field variables. One should keep in mind that there is no meaning in comparing values of individual parameters in the integrals or the integrals themselves between characteristic zero and characteristicp. Our transfer principle, which can be considered as a wide generalization of the classical 1

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RAFCLUCKERSANDFRAN¸COISLOESER

Ax-Kochen-Ersˇovresult,shouldhaveawiderangeofapplicationstop-adic represen- tation Theory and the Langlands Program. It applies in particular to many forms of the Fundamental Lemma and to the integrals occuring in the Jacquet-Ye conjecture [24],whichhasbeenprovedbyNgoˆ[27]overfunctionsfieldsandbyJacquet[25]in general.

Let us now review the content of the paper in more detail. In section 3 we en- large our Grothendieck rings in order to add exponentials. In fact it is useful to consider not only exponentials of functions with values in the valued field, but also exponentials of functions with values in the residue field. This is performed in a formal way by replacing the category RDef S considered in [8] - consisting of certain objectsX→S- by a larger category RDef e S xp consisting of the sameX→Sto- gether with functionsgandξonXvalues in the valued field, resp.with  residue the field. We define a Grothendieck ringK 0 (RDef e S xp ) generated by classes of objects (X g ξto add some new relations to the Here we have ) modulo certain relations. classical ones already considered in [8]. WhenX→Sis the identity, the class of (X g (0), resp.X0 ξ), corresponds to the exponential ofg the, resp. exponential ofξ defines the ring. OneC(S) exp of motivic exponential functions onSby tensoring K 0 (RDef e S xp ) with the ringP(S) of constructible Presburger functions onS are. We then able to state our main results on integration of exponential functions in section 4. In particular we show that integrals with parameters of functions inC exp still lie inC exp construct integrals of exponential functions in relative first directly . We dimension 1 in section 5 and then perform the general construction in section 6. As was the case in [8], extensive use is made of the Denef-Pas cell decomposition Theorem. Though some parts of our constructions and proofs are quite similar to what we performed in [8], or sometimes even follow directly from [8], other require new ideas and additional work specific to the exponential setting. As a first applica- tion, we develop in section 7 the fundamentals of a motivic Fourier transform. More precisely, there are two Fourier transforms, the first one over residue field variables and the second one, which is more interesting, over valued field variables. Calculus with our valued field Fourier transform is completely similar to the usual one. Using convolution, we define motivic Schwartz-Bruhat functions, and we show that the valued field Fourier transform is involutive on motivic Schwartz-Bruhat functions. We finally deduce Fourier inversion for integrable functions with integrable Fourier transform. In the following section 8 we move to thep-adic setting, defining the p-adic analogue ofC(S) exp and proving stability under integration with parameters of thesep-adic constructible exponential functions. Such a result is the natural extension to the exponential context of Denef’s fundamental result on stability of p-adic constructible functions under integration with respect to parameters. This result of Denef greatly influenced our work [8] and the present one. It has been later generalized to the subanalytic case by the first author in [4] and [5]. In section 9, we close the circle by showing that motivic integration of constructible exponential functions commutes with specialization to the corresponding non archimedean ones, when the residue characteristic is large enough. Finally, we end the paper by proving

CONSTRUCTIBLE EXPONENTIAL FUNCTIONS

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our fundamental transfer Theorem, a form of which was already stated in [9] when there is no exponential. Let us note that in their recent paper [22] Hrushovski and Kazhdan have also considered integrals of exponentials.

Some of the results in this paper have been announced in [10].

2.Preliminaries

2.1.Definable subassignments and constructible functions.Let us start by recalling briefly some definitions and constructions from [8], cf. also [6], [7]. We fix a fieldkzero and we consider for any fieldof characteristic Kcontainingkthe field of Laurent seriesK((t)) endowed with its natural valuation (2.1.1) ord :K((t)) × −→Z

and with the angular component mapping

(2.1.2) ac :K((t))−→K defined by ac(x) =xt −ord(x) modtifx6= 0 and ac(0) = 0. We use the Denef-Pas languageL DP,P which is a 3-sorted language (2.1.3) (L Val L Res L Ord ordac) with sorts corresponding respectively to valued field, residue field and value group variables. The languagesL Val andL Res are equal to the ring languageL Rings = {+−∙01}, and forL Ord we take the Presburger language (2.1.4)L PR ={+−01≤} ∪ {≡ n |n∈N > n1} with≡ n the equivalence relation modulon. Symbols ord and ac will be interpreted respectively as valuation and angular component, so that (K((t)) KZ) is a structure forL DP,P . We Res, shall also add constant symbols in the Val, resp. sort, for every element ofk((t)), resp.k. Letϕbe a formula in the languageL DP,P with respectivelym,nandrfree variables in the various sorts. For everyKinF k , the category of fields containing k, we denote byh ϕ (K) the subset of (2.1.5)h[m n r](K) :=K((t)) m ×K n ×Z r

consisting of points satisfyingϕ call the assignment. WeK7→h ϕ (K) a definable subassignment and we define a category Def k whose objects are definable subassign- ments. ForZin Def k , a pointxofZis by definition a tuplex= (x 0  K) such that x 0 is inZ(K) andKis in Field k . For a pointx= (x 0  K) ofZ, we writek(x) =K and we callk(x) the residue field ofx. More generally forSin Def k , we denote by Def S the category of objects of Def k overS denote by RDef. We S the subcategory of Def S consisting of definable subassignments ofS×h[0 n0], for variablen, and byK 0 (RDef S ) the corresponding Grothendieck ring.

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We consider the ring (2.1.6)A=Z h LL −1   1− 1 L −i  i>0 i  whereLis a symbol, and the subringP(S) of the ring of functions from the set of points ofStoAgenerated by constant functions, definable functionsS→Zand functions of the formL β withβdefinableS→Z. IfYis a definable subassignment ofS, we denote by1 Y the function inP(S) with value 1 onYand 0 outside. We denote byP 0 (S) the subring ofP(Sby such functions and by the) generated constant functionL. There is a morphismP 0 (S)→K 0 (RDef S ) sending1 Y to the class ofYand sendingLto the class ofh[010]. Finally we define the ring of constructible motivic functions onSby

(2.1.7)C(S) :=K 0 (RDef S )⊗ P 0 (S) P(S). To any algebraic subvarietyZofA k m ((t)) we assign the definable subassignmenth Z ofh[m00] given byh Z (K) =Z(K((t))). The Zariski closure of a subassignmentSof h[m00] is the intersectionWof all algebraic subvarietiesZofA m k((t)) such thatS⊂ h Z  set dim. WeS:= dimW generally, if. MoreSis a subassignment ofh[m n r], we define dimSto be dimp(S) withpthe projectionh[m n r]→h[m00]. One proves, using results of [30] and [20], that two isomorphic objects in Def k have the same dimension. For every non negative integerd, we denote byC ≤d (S) the ideal of C(S) generated by the characteristic functions1 Z of definable subassignmentsZof Swith dimZ≤d. We setC(S) =⊕ d C d (S) withC d (S) :=C ≤d (S)/C ≤d−1 (S). In [8], we defined, forka field of characteristic zero,Sin Def k , andZin Def S , a graded subgroup I S C(Z) ofC(Z) together with pushforward morphisms

(2.1.8)f ! : I S C(Z)−→I S C(Y) for every morphismf:Z→Yin Def S . WhenSis the final objecth[000] and fis the morphismZ→S, the morphismf ! corresponds to motivic integration and we denote it byµ. Finally, fix Λ in Def k . Replacing dimension by relative dimension, we defined relative analoguesC(Z→Λ) ofC(Z) forZ→Λ in Def Λ and extended the above constructions to this relative setting. In particular we constructed a morphism

(2.1.9)µ Λ : I Λ C(Z→Λ)→−C(Λ) = I Λ C(Λ→Λ) which corresponds to motivic integration along the fibers of the morphismZ→Λ.

2.2.Cell decomposition.now recall the definition of cells given in [8], whichWe is a slight generalization of the one in [30]. LetCbe a definable subassignment in Def k . Letα,ξ, andcbe definable mor- phismsα:C→Z,ξ:C→h G m,k , andc:C→h[100]. The cellZ C,α,ξ,c with basisC, orderα, centerc, and angular componentξis the definable subassignment ofC[100] defined by ord(z−c(y)) =α(y), and ac(z−c(y)) =ξ(y), whereylies inCandzinh[10 if0]. Similarly,cis a definable morphismc:C→h[100],