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On tempered representations

  • David Kazhdan and Alexander Yom Din EMAIL logo
Published/Copyright: April 28, 2022

Abstract

Let G be a unimodular locally compact group. We define a property of irreducible unitary G-representations V which we call c-temperedness, and which for the trivial V boils down to Følner’s condition (equivalent to the trivial V being tempered, i.e. to G being amenable). The property of c-temperedness is a-priori stronger than the property of temperedness.

We conjecture that for semisimple groups over local fields temperedness implies c-temperedness. We check the conjecture for a special class of tempered V’s, as well as for all tempered V’s in the cases of G:=SL2() and of G=PGL2(Ω) for a non-Archimedean local field Ω of characteristic 0 and residual characteristic not 2. We also establish a weaker form of the conjecture, involving only K-finite vectors.

In the non-Archimedean case, we give a formula expressing the character of a tempered V as an appropriately-weighted conjugation-average of a matrix coefficient of V, generalising a formula of Harish-Chandra from the case when V is square-integrable.

Funding statement: Alexander Yom Din’s research was supported by the Israel Science Foundation (grant No. 1071/20). David Kazhdan’s research was partially supported by ERC grant No. 669655.

A Auxiliary claims regarding polynomial growth of exponential integrals and sums

A.1 Some notation

We denote [n]:={1,2,,n} and

0:={z:Re(z)0},D:={z:|z|1}.

Given x=(x1,,xn)0n and m=(m1,,mn)0n, we write xm:=x1m1xnmn. Given λ0n, we denote

Jλ:={1jn:Re(λj)=0}.

Given (λ,m)0n×0n, we denote d(λ,m):=jJλ(1+mj). Given J[n] and some set X, let us denote by resJ:XnXJ the natural restriction and by extJ:XJXn the natural extension by zero.

We fix a finite set 0n with the property that given j[n] there exists v such that v,ej0, where ej is the j-th standard basis vector. We denote

P<r:={x0n:v,x<r for all v}.

Given J[n], we denote by PJ0J the convex pre-compact subset

{y0J:extJ(y)P<1}.

In Section A.3 we will also use the following notations. We consider a compact space B equipped with a nowhere vanishing Radon measure db. Let us say that a function

ϕ:B×0n

is nice if it is expressible as

B×0nidB×eiB×Dnϕ,

where ei(x1,,xn):=(e-x1,,e-xn) and ϕ is continuous and holomorphic in the second variable (in the sense that when we fix the variable in B it is the restriction of a holomorphic function on a neighbourhood of Dn). Given J[n], we denote by resJϕ:B×0J the function given by

resJϕ(b,y):=ϕ(b,extJ(ei(y))).

We also write ϕ(b,+) for ϕ(b,0), etc.

A.2 Growth – The case of summation over a lattice

Lemma A.1.

Let λ:=(λ1,,λn)C0n, let m:=(m1,,mn)Z0, and let K be a compact subset of R0n. Assume that Re(λ)=0 and λ(2πi)Zn. We have

supQK|1rnx1r0nQxmerλ,x|=O(r-1)

as r+, where Q denote convex subsets.

Proof.

Let us re-order the variables, assuming that λ12πi. Let us write x=(x1,x), where x=(x2,,xn) and analogously write m, etc. Given a convex subset QK and x0n-1, let us denote by Qx0 the subset consisting of x1 for which (x1,x)Q (it is an interval). Let us enlarge K for convenience, writing it in the form K=K1×K, where K10 is a closed interval and K0n-1 is the product of closed intervals.

We have

x1r0nQxmerλ,x=x1r0n-1K(x)merλ,x(x11r0Qxx1m1erλ1x1).

We have QxK and it is elementary to see that

supRK|x11r0Rx1m1erλ1x1|=O(1)

as r+, where R denote intervals. Therefore we obtain, for some C>0 (not depending on Q) and all r1,

|1rnx1r0nQxmerλ,x|C(1rn-1x1r0n-1K(x)m)r-1.

Since the expression in brackets is clearly bounded independently of r, we are done. ∎

Lemma A.2.

Let (λ,m)C0n×Z0n. Then the limit

limr+1rd(λ,m)x0nP<rxmeλ,xdx

exists, and it is equal to 0 if resJλ(λ)2πiZJλ and otherwise equal to

(PJλyresJλ(m)𝑑y)(z0JλczresJλc(m)eresJλc(λ),z)

(the sum converging absolutely).

Proof.

Let us abbreviate J:=Jλ. Let us denote λ:=resJ(λ) and λ′′:=resJc(λ), and similarly for m. Given x′′0Jc, let us denote by P(<r)x′′0J the subset consisting of y for which extJ(ry)+extJc(x′′)P<r.

We have

x0nP<rxmeλ,x=rd(λ,m)-|J|x′′0Jc(x′′)m′′eλ′′,x′′y1r0JP(<r)x′′(y)merλ,y
=:.

Let us assume first that λ2πiJ. Then by Lemma A.1 there exists C>0 such that for all convex subsets QPJ and all r1 we have

|1r|J|y1r0JQ(y)merλ,y|Cr-1.

Therefore

||Crd(λ,m)-1x′′0Jc(x′′)m′′eRe(λ′′),x′′,

giving the desired.

Now we assume that λ2πiJ. It is not hard to see that

limr+1r|J|y1r0JP(<r)x′′(y)m=PJ(y)m𝑑y.

Hence we have (by dominated convergence)

limr+1rd(λ,m)=x′′0Jc(x′′)m′′eλ′′,x′′PJ(y)m𝑑y.

Claim A.3.

Let p1, let {(λ(),m())}[p]C0n×Z0n be a collection of pairwise different couples and let {c()}[p]C{0} be a collection of non-zero scalars. Denote

d:=max[p]d(2Re(λ()),2m()).

The limit

limr+1rdx0nP<r|[p]c()xm()eλ(),x|2

exists and is strictly positive.

Proof.

Let us break the integrand into a sum following

|[p]A|2=1,2[p]A1A2¯.

Using Lemma A.2, we see the that resulting limit breaks down as a sum, over (1,2)[p]2, of limits which exist, so the only thing to check is that the resulting limit is non-zero. It is easily seen that the limit at the (1,2) place is zero unless d(λ(1),m(1))=d, d(λ(2),m(2))=d, Jλ(1)=Jλ(2) and resJ(1)(λ(2))-resJ(1)(λ(1))2πiJ. We thus can reduce to the case when, for a given J[n], we have Jλ()=J for all [p], we have d(λ(),m())=d for all [p], and we have resJ(λ(2))-resJ(λ(1))2πiJ for all 1,2[p]. We then obtain, using Lemma A.2, that our overall limit equals

z0JcPJ|[p]c()yresJ(m())zresJc(m())eresJc(λ()),z|2𝑑y.

It is therefore enough to check that

[p]c()yresJ(m())zresJc(m())eresJc(λ()),z,

a function in (z,y)0Jc×PJ, is not identically zero. By the local linear independence of powers of y, we can further assume that resJ(m()) is independent of [p], and want to check that

[p]c()zresJc(m())eresJc(λ()),z,

a function in z0Jc, is not identically zero. Notice that, by our assumptions, the elements in the collection {(resJc(λ()),resJc(m()))}[p] are pairwise different. Thus the non-vanishing of our sum is clear (by linear algebra of generalized eigenvectors of shift operators on Jc). ∎

A.3 Growth – The case of an integral

Lemma A.4.

Let λ:=(λ1,,λn)C0n, let m:=(m1,,mn)Z0, and let K be a compact subset of R0n. Assume that Re(λ)=0 and λ0. We have

supQK|Qxmerλ,x𝑑x|=O(r-1)

as r+, where Q denote convex subsets.

Proof.

Let us re-order the variables, assuming that λ10. Let us write x=(x1,x), where x=(x2,,xn) and analogously write m, etc. Given a convex subset QK and x0n-1, let us denote by Qx0 the subset consisting of x1 for which (x1,x)Q (it is an interval). Let us enlarge K for convenience, writing it in the form K=K1×K, where K10 is a closed interval and K0n-1 is the product of closed intervals.

Using Fubini’s theorem,

Qxmerλ,x𝑑x=K(x)merλ,x(Qxx1m1erλ1x1𝑑x1)𝑑x.

We have QxK and it is elementary to see that

supRK|Rx1m1erλ1x1𝑑x1|=O(r-1)

as r+, where R denote intervals. Therefore we obtain, for some C>0 and all r1,

|Qxmerλ,x𝑑x|C(K(x)m𝑑x)r-1,

as desired. ∎

Lemma A.5.

Let (λ,m)C0n×Z0n and let ϕ:B×R0nC be a nice function. Then the limit

limr+1rd(λ,m)BP<rxmeλ,xϕ(b,x)𝑑x𝑑b

exists, and it is equal to 0 if resJλλ0 and otherwise equal to

(PJλyresJλ(m)𝑑y)(B0JλczresJλc(m)eresJλc(λ),zresJλcϕ(b,z)𝑑z𝑑b)

(the double integral converging absolutely).

Proof.

Let us re-order the variables, assuming that J:=Jλ=[k]. Write x=(x,x′′), where x consists of the first k components and x′′ consists of the last k components. Let us write analogously m,λ, etc.

First, let us notice that if k0, we can write

ϕ(b,x)=e-x1ϕ0(b,x)+ϕ1(b,x),

where ϕ0,ϕ1:B×0n are nice functions and ϕ1 does not depend on x1. Dealing with e-x1ϕ0(b,x) instead of ϕ(b,x) makes us consider λ with smaller set Jλ and thus (λ,m) with a smaller d(λ,m) and from this, reasoning inductively, we see that we can assume that ϕ only depends on (b,x′′). Let us write ϕ′′:=resJcϕ.

Let us perform a change of variables y:=1rx. Let P(<r)0n denote the transform of P<r under this changes of variables (i.e. (x,x′′)P<r if and only if (y,x′′)P(<r)). We obtain

BP<rxmeλ,xϕ(b,x)𝑑x𝑑b
=rdBP(<r)(y)merλ,y(x′′)m′′eλ′′,x′′ϕ′′(b,x′′)dydx′′db=:.

Given x′′0Jc, let us denote by P(<r)x′′0J the set

{y0J:(y,x′′)P(<r)}.

Notice that P(<r1)x′′P(<r2)x′′ for r1<r2 and rP(<r)x′′=PJ. Using Fubini’s theorem,

=rdB0Jc(x′′)m′′eλ′′,x′′ϕ′′(b,x′′)(P(<r)x′′(y)merλ,y𝑑y)𝑑x′′𝑑b.

If λ0, by Lemma A.4 there exists C>0 such that for all convex subsets QPJ and all r1 we have

|Q(y)merλ,y𝑑y|Cr-1.

We have therefore

||Crd-1B0Jc(x′′)m′′eRe(λ′′),x′′|ϕ′′(b,x′′)|𝑑x′′𝑑b

and thus indeed the desired limit is equal to 0.

Now we assume λ=0. Using Lebesgue’s dominated convergence theorem, we have

limr+1rd=limr+B0Jc(x′′)m′′eλ′′,x′′ϕ′′(b,x′′)(P(<r)x′′(y)m𝑑y)𝑑x′′𝑑b
=B0Jc(x′′)m′′eλ′′,x′′ϕ′′(b,x′′)(PJ(y)m𝑑y)𝑑x′′𝑑b,

as desired. ∎

Claim A.6.

Let {(λ(),m())}[p]C0n×Z0n be a collection of pairwise different couples. Let {ϕ()}[p] be a collection of nice functions B×R0nC such that for every [p] the function bϕ()(b,+) on B is not identically zero. Denote

d:=max[p]d(2Re(λ()),2m()).

The following limit exists and is strictly positive:

limr+1rdBP<r|[p]xm()eλ(),xϕ()(b,x)|2𝑑x𝑑b.

Proof.

Let us break the integrand into a sum following

|[p]A|2=1,2[p]A1A2¯.

Using Lemma A.5, we see the that resulting limit breaks down as a sum, over (1,2)[p]2, of limits which exist, so the only thing to check is that the resulting limit is non-zero. It is easily seen that the limit at the (1,2) place is zero unless d(λ(1),m(1))=d, d(λ(2),m(2))=d, Jλ(1)=Jλ(2) and resJ(1)(λ(1))=resJ(1)(λ(2)). We thus can reduce to the case when, for a given J[n], we have Jλ()=J for all [p], we have d(λ(),m())=d for all [p], and we have resJ(λ(1))=resJ(λ(2)) for all 1,2[p]. We then obtain, using Lemma A.5, that our overall limit equals

B0JcPJ|[p]yresJ(m())zresJc(m())eresJc(λ()),zresJcϕ()(b,z)|2𝑑y𝑑z𝑑b.

It is therefore enough to check that

[p]yresJ(m())zresJc(m())eresJc(λ()),zresJcϕ()(b,z),

a function in (b,z,y)B×0Jc×PJ, is not identically zero. By the local linear independence of powers of y, we can further assume that resJ(m()) is independent of [p], and want to check that

[p]zresJc(m())eresJc(λ()),zresJcϕ()(b,z),

a function in (b,z)B×0Jc, is not identically zero. Notice that, by our assumptions, the elements in the collection {(resJc(λ()),resJc(m()))}[p] are pairwise different and for every [p], the function bϕ()(b,extJc(+)) on B is not identically zero. Considering the partial order on Jc given by μ1μ2 if μ2-μ10Jc, we can pick [p] for which resJc(λ()) is maximal among the {resJc(λ())}[p]. We can then pick bB such that ϕ()(b,extJc(+))0. We then boil down to Lemma A.7 that follows. ∎

In the end of the proof of Claim A.6 we have used the following:

Lemma A.7.

Let {(λ(),m())}[p]Cn×Z0n be a collection of pairwise different couples. Let {ϕ()}[p] be a collection of nice functions R0nC (so here B={1}). Suppose that ϕ()(+)0 for some [p] for which λ() is maximal among the {λ()}[p] with respect to the partial order λ1λ2 if λ2-λ1Z0n. Then the function

x[p]xm()eλ(),xϕ()(x)

on R0n is not identically zero.

Proof.

We omit the proof of the lemma – one develops the ϕ() into power series in e-x1,,e-xn and uses separation by generalized eigenvalues of the partial differentiation operators x1,,xn. ∎

Acknowledgements

We would like to thank Pavel Etingof for great help with the proof of Claim 7.3 in the case G=SL2(), which was present in a prior draft of the paper, before we encountered the work [6]. We would like to thank Vincent Lafforgue for a very useful discussion. We thank Erez Lapid for useful correspondence.

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Received: 2022-02-11
Published Online: 2022-04-28
Published in Print: 2022-07-01

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