The Geometric Nature of the Fundamental Lemmanadler/ceb2011.pdfF)is a locally compact group, and =...

Introduction Trace formula Endoscopy Springer fibers Hitchin fibration

The Geometric Nature of theFundamental Lemma

David NadlerNorthwestern University

Current Events Bulletin2011 Joint Mathematics Meetings

New Orleans, LA

A long strange trip

The Fundamental Lemma, first appearingin lectures of 1979, “is a precise and purelycombinatorial statement that I thoughtmust therefore of necessity yield to astraightforward analysis. This has turnedout differently than I foresaw.”

After decades of deep contributions bymany mathematicians, Ngo Bao Chaucompleted a proof in 2008 which rankedseventh on Time magazine’s Top 10Scientific Discoveries of 2009 list.

Teleportation was eighth.

A long strange trip

A difficult task

To understand the Fundamental Lemma, we must study endoscopy.

To understand endoscopy, we must study Langlands functoriality.

To understand functoriality, we must study Langlands reciprocity...

A difficult task

Linearization of subspaces

Let X be a measure space.

subspaces Y ⊂ X integral distributions Y (ϕ) =∫Y ϕ

Now can add Y1 + Y2 and scale cY subspaces.

Suppose symmetry group G acts on X preserving measure.

G -orbits Y ⊂ X G -invariant distributions Y (ϕ) =∫Y ϕ

Linearization of conjugacy classes

Now consider group G with a conjugation invariant measure.

Importance of linearization of conjugacy classes:

characters of G -representations G -invariant distributions

Given G -representation V , can form distributional character:

χV (ϕ) =

∫Gϕ(g) TrV (g)dg

(ignoring analytic technical difficulties).

χV (ϕ) =

∫Gϕ(g) TrV (g)dg

χV (ϕ) =

∫Gϕ(g) TrV (g)dg

Example: Finite groups

Specialize to finite group G . Then

G -invariant distributions = class functions

TheoremCharacters χV of irreducible G-representations V form basis forclass functions. Rescaled characters χV = χV / dim V idempotentswith respect to convolution χV ∗ χV = χV .

Interpretation

Class functions = functions on space of irreducible representations.Rescaled characters χV are characteristic functions of points.

Interpretation

Induced representations

It is difficult to construct representations.

We have trivial representation Tr and method of induction.

Given group G , and subgroup Γ ⊂ G , form “unitary induction”

IndGΓ (Tr) = L2(G/Γ)

Output: character χGΓ of induced representation IndG

Γ (Tr).

Frobenius Character Formula

G finite group, Γ ⊂ G subgroup.

Ferdinand Georg Frobenius1849–1917

Character of induced representation L2(G/Γ)

χGΓ (ϕ) =

∑γ∈Γ/Γ

aγOγ(ϕ)

Volumes of quotients of centralizers

aγ = |Γγ\Gγ |

Integrals over conjugacy classes

Oγ(ϕ) =

∫[γ]ϕ =

∑x∈Gγ\G

ϕ(x−1γx)

χGΓ (ϕ) =

∑γ∈Γ/Γ

aγOγ(ϕ)

aγ = |Γγ\Gγ |

Oγ(ϕ) =

∫[γ]ϕ =

∑x∈Gγ\G

ϕ(x−1γx)

χGΓ (ϕ) =

∑γ∈Γ/Γ

aγOγ(ϕ)

aγ = |Γγ\Gγ |

Oγ(ϕ) =

∫[γ]ϕ =

∑x∈Gγ\G

ϕ(x−1γx)

Poisson Summation Formula

R additive group, Z ⊂ R discrete subgroup.

Simeon Denis Poisson1781–1840

Character of induced representation L2(R/Z)

χRZ(ϕ) =

∑n∈Z

(Fourier analysis provides isomorphism

L2(R/Z) ' ⊕λ∈ZC〈e2πiλ〉

Hence identification of characters∑n∈Z

ϕ(n) =∑λ∈Z

ϕ(λ).)

χRZ(ϕ) =

∑n∈Z

ϕ(n) =∑λ∈Z

ϕ(λ).)

χRZ(ϕ) =

∑n∈Z

ϕ(n) =∑λ∈Z

ϕ(λ).)

Arthur-Selberg Trace Formula

Atle Selberg1917–2007

James Arthur1944–

G reductive algebraic group over number field F .Think G = GL(n) and F = Q.

AF adeles of F of all hypothetical“Laurent series expansions” of elements in the

form of p-adic and real numbers.

Then G = G(AF ) is a locally compact group,and Γ = G(F ) ⊂ G(AF ) is a discrete subgroup.

χGΓ (ϕ) =

∑γ∈Γ/Γ

aγOγ(ϕ) + · · ·

Upshot: character involves integrals overconjugacy classes in real and p-adic Lie groups.

James Arthur1944–

χGΓ (ϕ) =

∑γ∈Γ/Γ

aγOγ(ϕ) + · · ·

James Arthur1944–

χGΓ (ϕ) =

∑γ∈Γ/Γ

aγOγ(ϕ) + · · ·

James Arthur1944–

χGΓ (ϕ) =

∑γ∈Γ/Γ

aγOγ(ϕ) + · · ·

James Arthur1944–

χGΓ (ϕ) =

∑γ∈Γ/Γ

aγOγ(ϕ) + · · ·

James Arthur1944–

χGΓ (ϕ) =

∑γ∈Γ/Γ

aγOγ(ϕ) + · · ·

Conjugacy classes

For simplicity, let’s consider the Lie algebra sl(2,R).

Orbits of SL(2,R) acting onits Lie algebra sl(2,R) ' R3.

Three types of orbits under conjugation:

• hyperbolic: det < 0.

• nilpotent: det = 0.

• elliptic: det > 0.

We will focus on the two elliptic orbitsOA+ ,OA− ⊂ s(2,R) through the matrices

[0 1−1 0

]A− =

[0 −11 0

Conjugacy classes

[0 1−1 0

]A− =

[0 −11 0

Conjugacy classes

[0 1−1 0

]A− =

[0 −11 0

Stable conjugacy

Over the complex numbers C, the matrices

[0 1−1 0

]A− =

[0 −11 0

]are both conjugate to the matrix

[i 00 −i

]∈ sl(2,C).

One says that A+ and A− are stably conjugate.

Stable conjugacy

Over the complex numbers C, the matrices

[0 1−1 0

]A− =

[0 −11 0

]are both conjugate to the matrix

[i 00 −i

]∈ sl(2,C).

One says that A+ and A− are stably conjugate.

Invariant polynomials

Said another way, the two elliptic orbits

OA+ ,OA− ⊂ s(2,R)

coalesce into a single conjugacy class

OA ⊂ s(2,C)

cut out by the invariant polynomial

det = 1

stable conjugacy classes ←→ invariant polynomials

Invariant polynomials

Said another way, the two elliptic orbits

OA+ ,OA− ⊂ s(2,R)

coalesce into a single conjugacy class

OA ⊂ s(2,C)

cut out by the invariant polynomial

det = 1

stable conjugacy classes ←→ invariant polynomials

Linearization of adjoint orbits

Consider the distributions given byintegrating over the elliptic orbits

OA+(ϕ) =

∫OA+

ϕ OA−(ϕ) =

∫OA−

with respect to an invariant measure.

Alternative basis

The distributions OA+ ,OA− span atwo-dimensional complex vector space.

It admits the alternative basis

Ost = OA+ +OA−

Otw = OA+ −OA−

Here st stands for stable andtw stands for twisted.

Alternative basis

Ost = OA+ +OA−

Otw = OA+ −OA−

Alternative basis

Ost = OA+ +OA−

Otw = OA+ −OA−

Stable distributions

Stable distribution

Ost = OA+ +OA−

is integral over union of orbits

OA+ t OA− .

Algebraic variety defined by invariant polynomial

det = 1.

Stable distribution is object of

algebraic geometry (finite mathematics)

rather than harmonic analysis (continuous mathematics).

Stable distribution

Ost = OA+ +OA−

OA+ t OA− .

det = 1.

Stable distribution

Ost = OA+ +OA−

OA+ t OA− .

det = 1.

Twisted distributions

What to do with twisted distribution

Otw = OA+ −OA−?

Distinguishes between

OA+ and OA−

though no invariant polynomial separates them.

Twisted distribution appears to be noncanonical: exchanging terms

OA+ ←→ OA−

induces sign changeOtw ←→ −Otw .

Otw = OA+ −OA−?

OA+ and OA−

OA+ ←→ OA−

Otw = OA+ −OA−?

OA+ and OA−

OA+ ←→ OA−

Therein lies our salvation

Twisted distribution is integral over union of orbits

OA+ t OA−

against nontrivial character

κ(+) = 1 κ(−) = −1

OA+ t OA−

κ(+) = 1 κ(−) = −1

OA+ t OA−

κ(+) = 1 κ(−) = −1

Interpretation via Fourier analysis

Alternative basis

Ost = OA+ +OA−

Otw = OA+ −OA−

results from Fourier analysis on set of orbits{OA+ ,OA−

What is the Fundamental Lemma all about?

Basic ideaLanglands’s theory of endoscopy, and the Fundamental Lemma atits heart, confirms that one can systematically express

twisted distributions in terms of stable distributions

nonconstant Fourier modes in terms of constant Fourier modes

Example continued

Endoscopy relates twisted distribution

Otw = OA+ −OA−

to stable distribution on Lie algebra so(2,R) ' R of subgroup

SO(2,R) ⊂ SL(2,R)

stabilizing matrices

[0 1−1 0

]A− =

[0 −11 0

]Outside of bookkeeping, this is empty of content since SO(2,R) isabelian, and so its orbits in so(2,R) are single points.

Example continued

Endoscopy relates twisted distribution

Otw = OA+ −OA−

to stable distribution on Lie algebra so(2,R) ' R of subgroup

SO(2,R) ⊂ SL(2,R)

stabilizing matrices

[0 1−1 0

]A− =

[0 −11 0

]Outside of bookkeeping, this is empty of content since SO(2,R) isabelian, and so its orbits in so(2,R) are single points.

Why is the Fundamental Lemma difficult?

General theory of endoscopy is deep and elaborate.

Key challenge

Extraordinary difficulty of the Fundamental Lemma, and also itsmystical power, emanates from fact that sought-after stabledistributions live on so-called endoscopic groups H with littleapparent geometric relation to original group G .

Why is the Fundamental Lemma difficult?

General theory of endoscopy is deep and elaborate.

Key challenge

Extraordinary difficulty of the Fundamental Lemma, and also itsmystical power, emanates from fact that sought-after stabledistributions live on so-called endoscopic groups H with littleapparent geometric relation to original group G .

Endoscopic groups

To find relation between group G and endoscopic group H, onemust pass to Langlands dual groups

“noncommutative Pontryagin dual group” of “geometric characters”

There one finds H∨ is naturally subgroup of G∨.

Example

Consider the symplectic group G = Sp(2n).

The special orthogonal group H = SO(2n) is not a subgroup.

But H∨ = SO(2n) is a subgroup of G∨ = SO(2n + 1).

Endoscopy gives precise relationship

twisted distributions on Sp(2n) stable distributions on SO(2n)

Endoscopic groups

Example

Endoscopic groups

Example

Endoscopic groups

Example

Endoscopic groups

Example

Endoscopic groups

Example

Low rank example

Foreground: roots of the group G = Sp(4) with roots of theendoscopic group H = SO(4) highlighted.

Background: roots of the Langlands dual group G∨ = SO(5) withroots of the subgroup H∨ = SO(4) highlighted.

Low rank example

Foreground: roots of the group G = Sp(4) with roots of theendoscopic group H = SO(4) highlighted.

Background: roots of the Langlands dual group G∨ = SO(5) withroots of the subgroup H∨ = SO(4) highlighted.

Real versus p-adic Lie groups

Detailed conjectures organizing the intricacies of the transfer ofdistributions first appear in Langlands’s joint work with Shelstad.

General setting needed for applications to number theory andharmonic analysis: p-adic and real Lie groups (algebraic groupsover local fields).

For real Lie groups, Shelstad rapidly proved the conjectures.

What became known as the Fundamental Lemma is the most basicconjecture for p-adic groups.

Useful to have picture of real Lie groups in mind. Langlands andShelstad: “if it were not that [transfer factors] had been proved toexist over the real field, it would have been difficult to maintainconfidence in the possibility of transfer or in the usefulness ofendoscopy.”

From p-adic groups to loop groups

Dictionary between arithmetic and geometry.

One-dimensional Arithmetic

• Number field F

• Rational numbers Q• p-adic field

• p-adic group

One-dimensional Geometry

• Smooth projective curve X

• Projective line P1

• Formal disk D

• Loop group LG

Theorem (Waldspurger)

To prove Fundamental Lemma, it suffices to prove its analogue inthe geometric setting.

Later proof by Cluckers, Hales, and Loeser via model theory:

local arithmetic and geometry are equivalent

• Number field F

• p-adic group

• Formal disk D

• Loop group LG

• Number field F

• p-adic group

• Formal disk D

• Loop group LG

• Number field F

• p-adic group

• Formal disk D

• Loop group LG

Loop Grassmannians

Orbital integrals of Fundamental Lemma in geometric setting areequivalent to counting points in subvarieties of Grassmannians.

DefinitionLet LG be loop group. Let L+G ⊂ LG be subgroup of arcs.Loop Grassmannian GrG is homogenous space LG/L+G .

Why Grassmannian? ∞/2-dim subspaces of ∞-dim vector space.

Geometric analogue of affine building.

Loop Grassmannians

Geometric cousin of affine building.

Affine Springer fibers

Now the subvarieties...

DefinitionLet ξ be element of Lie algebra of LG .Affine Springer fiber Xξ ⊂ GrG is fixed-point locus of ξ.

Example for ξ diagonal with distinct eigenvalues.

Basic structure of affine Springer fibers

• Xξ is finite-dimensional increasing union of projective varieties.

• Xξ/Λξ quotient by symmetry lattice is projective variety.

From point counts to cohomology

Grothendieck1928–

Lefschetz1884–1972

Trace formula: count points inalgebraic variety by calculating

traces of Galois symmetriesacting on topological cohomology.

Now can stand on the shoulders of giants: Kazhdan-Lusztig,Goresky-MacPherson, Beilinson-Bernstein-Deligne-Gabber,...

Challenge: cohomology of affine Springer fibers quantifiably toocomplicated to calculate in any combinatorially explicit form.

Grothendieck1928–

Springer theory

T.A. Springer1926–

Problem: cohomology of fixed-points of vector fields onflag varieties.

Trivial case: when vector field is generic, for examplesum of linearly independent commuting vector fields.

General solution: “analytically continue” solution fromgeneric locus to all vector fields.

Springer theory

Compactified Jacobians

Laumon and Ngo

Beautiful insight

Affine Springer fibers modulo natural symmetriesparametrize generalized line bundles on curves.

Deformations to simpler curves providedeformations to simpler affine Springer fibers.

Striking consequence

Fundamental Lemma for unitary groups!

Laumon and Ngo

Beautiful insight

Laumon and Ngo

Beautiful insight

Hitchin fibers

Meanwhile in a galaxy far, far away...

Nigel Hitchin1946–

Hitchin fibrationX smooth projective curve (Riemann surface).

Hitchin moduli MG (X ) parametrizes G-bundleon X together with twisted endomorphism.

Base AG (X ) parametrizes possible eigenvalues oftwisted endomorphism (spectral curve).

Integrable system MG (X )→ AG (X ) assignscharacteristic polynomial of endomorphism.

Fibers parametrize generalized line bundles on spectral curves.

Hitchin fibration organizes deformations of affine Springer fibersinto a proper finite-dimensional algebraic family.

Hitchin fibers

Not so surprising...

Fundamental Lemma involves distributions on conjugacy classes

adjoint quotient G/G

Hitchin moduli space parametrizes twisted maps

curve X −→ adjoint quotient g/G

Furthermore, stable conjugacy classes involve invariant polynomials

adjoint quotient G/G −→ possible eigenvalues T/W

Hitchin fibration parametrizes twisted maps

curve X −→ {adjoint quotient g/G −→ possible eigenvalues t/W }

Global curve organizes local group theory!

Ngo’s Support Theorem

Main new technical input to proof of Fundamental Lemma. Precisedescription of the cohomology of the fibers of an integrable systemin terms of its generic fibers.

Toy model: consider a family of irreducible curves

f : M → S , with M and S smooth.

Over a Zariski open locus S0 ⊂ S , the fibers

Ms = f −1(s), s ∈ S0

are topologically equivalent curves, hence their cohomologiesH∗(Ms) form a local system of vector spaces

H → S0

Exercise: the cohomology of any fiber can be recovered from H.

Ms = f −1(s), s ∈ S0

H → S0

Ms = f −1(s), s ∈ S0

H → S0

Ms = f −1(s), s ∈ S0

H → S0

Ms = f −1(s), s ∈ S0

H → S0

Ms = f −1(s), s ∈ S0

H → S0

Family of plane cubics

y 2 = x3 + ax + bsingular at (a, b) = (0, 0)

Thank you for listening!

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