DE LIE ET POLYTOPES DES POIDS

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Cédric Lecouvey & Pierre Tarrago

Mesures centrales pour les graphes multiplicatifs, représentations d’algèbres de Lie et polytopes des poids Tome 70, n^o6 (2020), p. 2361-2407.

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MESURES CENTRALES POUR LES GRAPHES MULTIPLICATIFS, REPRÉSENTATIONS D’ALGÈBRES

DE LIE ET POLYTOPES DES POIDS

by Cédric LECOUVEY & Pierre TARRAGO

Abstract. —To each finite-dimensional representation of a simple Lie algebra is associated a multiplicative graph in the sense of Kerov and Vershik defined from the decomposition of its tensor powers into irreducible components. It was shown in [11] and [12] that the conditioning of natural random Littelmann paths to stay in their corresponding Weyl chamber is controlled by central measures on this type of graphs. Using the K-theory of associatedC^∗-algebras, Handelman [8] established a homeomorphism between the set of central measures on these multiplicative graphs and the weight polytope of the underlying representation. In the present paper, we make explicit this homeomorphism independently of Handelman’s results by using Littelmann’s path model. As a by-product we also get an explicit parametrization of the weight polytope in terms of drifts of random Littelmann paths. This explicit parametrization yields a complete description of harmonic andc-harmonic func- tions for the Littelmann path model describing the iterated tensor product of an irreducible representation.

Résumé. —Nous associons un graphe multiplicatif au sens de Vershik et Kerov à chaque représentation de dimension finie d’une algèbre de Lie simple en consi- dérant la décomposition de ses produits tensoriels successifs en représentations irréductibles. Pour chacune de ces représentations de dimension finie, il a été mon- tré en [11] et [12] que le conditionnement d’un chemin de Littelmann aléatoire à rester dans la chambre de Weyl est décrit par les mesures centrales sur le graphe multiplicatif associé. En utilisant la K-théorie des C^∗-algèbres correspondantes, Handelman a établi un homéomorphisme entre l’ensemble des mesures centrales sur un de ces graphes multiplicatifs et le polytope des poids de la représentation sous-jacente. Dans cet article, nous rendons explicite l’homéomorphisme d’Handel- man en utilisant les modèles de chemins de Littelmann. On obtient en conséquence une paramétrisation du polytope des poids en termes de dérives de chemins de Littelmann aléatoires. La paramétrisation explicite donne une description com- plète des fonctions harmoniques et c-harmoniques pour les modèles de chemins de Littelmann décrivant les itérations de produits tensoriels d’une représentation irréductible.

Keywords:représentation d’algèbre de Lie, mesure harmonique, chemin de Littelmann.

2020Mathematics Subject Classification:05E10, 17B10, 31C35.

1. Introduction

Consider a simple finite-dimensional Lie algebragof rankdoverCand its root system in R^d. Let P be the corresponding weight lattice and fix

∆ a dominant Weyl chamber. ThenP+ =P ∩∆ is the cone of dominant weights of g. Denote by S = {α1, . . . , αd} the underlying set of simple roots. To each dominant weight δ ∈ P₊ corresponds a finite-dimensional representationV(δ) ofgof highest weightδ. In [14] Littelmann associated to every representationV(δ) a setB(δ) of paths inR^dwith length 1 starting at 0 and ending in the set Πδ of weights ofV(δ). Random Littelmann paths can then be defined first by endowing B(δ) with a suitable probability distribution, next by considering random concatenations of paths inB(δ).

In [11] and [12] distributions on the setB(δ) are defined as morphisms from P toR>0. This is equivalent to associating to each simple rootα_i a realt_i in ]0,+∞[. It is then shown that these random paths and their conditioning to stay in the Weyl chamber ∆ are controlled by the representation theory ofg. In fact, one obtains particular central distributions on the set Γ^R_n^d of paths of any lengthn > 1 (obtained by concatenating n paths in B(δ)).

By central distributions we mean that the probability of a finite path only depends on its length and its end. Equivalently, we get a central measure on the set of infinite concatenations Γ^Rof paths inB(δ) (see Section 2).

Write H(R^d) for the set of central measures on Γ^R and H(∆) for the subset of H(R^d) of central measures on Γ^∆, the set of infinite trajecto- ries remaining in ∆. By Choquet’s Theorem both setsH(R^d) and H(∆) are simplices so they are essentially determined by their minimal bound- aries ∂H(R^d) and ∂H(∆). Write K(δ) for the convex hull of Πδ and set K(δ)⁺= ∆∩K(δ) . For walks in the Weyl chambers, the characterization of the sets ∂H(R^d) and ∂H(∆) has been obtained by Handelman in [8]

and [9] using an important work of Price [17, 18], by proving that they are respectively homeomorphic toK(δ) andK(δ)⁺. Nevertheless, Handel- man did not explicit the homeomorphisms. Their existence is established by considering the central measures as traces on certainC^∗-algebras and then using analytic tools. In particular, a central element of the proof is the extension of traces onC^∗-algebras using K-theory (a short explanation of these arguments is given in Section 3.4).

The goal of this paper is essentially threefold: first we make explicit both homeomorphisms by using the Weyl characters ofg(see Theorem 3.1), next we give an algebraic proof of Handelman’s results and finally we connect them with more recent works on conditioned random walks or Brownian motions, generalizations of the Pitman transform and asymptotic Young

tableaux (see [2, 4, 5, 11, 12, 13, 16, 20]). As a corollary of these results, we describe the set of harmonic andc-harmonic functions corresponding to the aforementioned random walks. Finally, we get a law of large numbers for random walks distributed according the central measures we obtain.

Our two last results seem quite disconnected from the initial algebraic setting in representation theory, and we conjecture that they still hold for a very broad class of random paths. Our approach extends that of Kerov and Vershik to which it essentially reduces whenV(δ) is the defining representation of g = sln. Nevertheless, numerous difficulties arise when considering the general case of dominant weights of any simple algebrag, which explains the involved machinery used in the proof of Handelman.

Our methods to determine ∂H(R^d) and ∂H(∆) are quite similar. So we will now give its main steps only in the case of∂H(∆).

We first need to show that the characterization of∂H(∆) is equivalent to that of the extremal harmonic functions on the growth graphG(∆) associ- ated with Γ^∆. This growth graph is rooted, graded and multiplicative: its vertices label the basisB={(s_λ, n)|V(λ) irreducible component ofV(δ)^⊗n andn >1} of a commutative algebraTb⁺_δ (here sλ is the Weyl character ofV(λ)). We then establish that the extremal nonnegative harmonic func- tions onG(∆) are in bijection with the algebra morphisms fromTb⁺_δ to R that are nonnegative onB. Next, we prove that all these morphisms are obtained by associating to each simple rootα_i, i= 1, . . . , na real in [0,1].

The difficulty here comes from the fact that two such associations can yield the same morphism. So to obtain a genuine parametrization, we need to restrict ourselves to a subset [0,1]^d_δ (see (4.5) for a precise definition) of [0,1]^dwhose combinatorial description is in terms of theδ-admissible sub- sets ofS introduced in [21]. Finally, in Proposition 6.4, we show that our set [0,1]^d_δ also parametrizes the simplex K(δ)⁺ by considering, for each d-tuple in [0,1]^d_δ, the drift of the corresponding random Littelmann path appearing in the construction of [11] and [12].

The paper is organized as follows. In Section 2, we recall some back- ground on random paths and central measures on multiplicative graphs.

We also apply the Ring Theorem of Kerov and Vershik to relate extremal harmonic functions on a multiplicative graph to nonnegative morphisms of the underlying algebra. The main result is written down in Section 3 where we also introduce the algebrasTbδ and Tb⁺_δ; a sketch of Handelman’s argu- ments is proposed at the end of Section 3. Section 4 gives the description of ∂H(R^d). Here, we define our set [0,1]^d_δ and relate it to the geometry of the polytope K(δ). The description of ∂H(∆) is deduced from that of

∂H(R^d) in Section 5. It is worth noticing that we need here (as in the result of Kerov and Vershik) a classical theorem relating polynomials with non positive roots to totally positive sequences. Another important ingredient in the proof is the use of certain plethyms of Schur and Weyl characters of g. Finally, Section 6 relates both descriptions of∂H(R^d) and∂H(∆) to the drift of random Littelmann paths. Notably it explains how the polytope K(δ) can be simply parametrized by using the set [0,1]^d_δ. A nomenclature with all recurring notations is provided at the end of the manuscript.

2. General probabilistic framework

We present here a general probabilistic model of random paths in a do- main, which is well suited to study probabilistic aspects of Littelmann paths and their asymptotics. We introduce first a discrete version of paths in a vector space.

2.1. Random paths on a lattice

Let d>0 and let Λ be a lattice ofR^d with rank d. We shall denote by

#S the cardinality of any setS.

Definition 2.1.

(1) Letn>0. A pathπonΛof lengthnis a piecewise linear function π: [0, n]→R^d withπ(0) = 0,π(i)∈Λ for all i∈ {0, . . . , n}, and π(x)∈Q^d for allxon whichπis not differentiable. The pathπ is called infinitesimal ifn= 1.

(2) An infinite path onΛis a piecewise linear functionπ: [0,+∞[→R^d withπ(0) = 0, π(i)∈Λ for all i ∈N, and π(x)∈Q^d for all xon whichπis not differentiable.

The length of the pathπon Λ is denoted byl(π)∈N∪ {+∞}. We write π.τfor the concatenation of two finite pathsπandτ. LetX be a countable set of infinitesimal paths and let Ω be a domain ofR^dsuch that 0∈Ω; from now on, the setX is fixed and is not mentioned in the various notations.

A pathπis calledX-valued ifπis the concatenation of infinitesimal paths coming from X: equivalently, π_|[i,i+1]−π(i)

∈ X for all i > 1. In the sequel, any path is always considered asX-valued. The set of infinite X -valued paths (resp. finite X-valued paths and X-valued paths of length

n>1) whose image is included in Ω is denoted by Γ^Ω (resp. by Γ^Ω_fin and Γ^Ω_n ). For x, y ∈ Λ, we denote by Γ^Ω(x, y) the set of infinitesimal paths π∈X such thatπ(1) =y−xandx+π⊂Ω, and we writex%y when

#Γ^Ω(x, y)6= 0. Finally, we denote by Γ^Ω_n(y) the set of finite paths of length nending at y.

In order to consider random infinite paths in Ω, we need to define a σ- algebra on Γ^Ω. Let τ be a finite path of length n, and let Γ^Ω(τ) be the set {π ∈ Γ^Ω|l(π) > n, π_|[0,n] = τ}. We consider the coarsest σ-algebra containing all the sets Γ^Ω(τ) for τ ∈Γ^Ω_fin. The set M1(Γ^Ω) of probability measures on Γ^Ω is considered with the initial topology with respect to the evaluation maps on the sets Γ^Ω(τ), τ ∈Γ^Ω_fin. By Tychonov’s Theorem, M₁(Γ^Ω) is a compact set with respect to this topology.

2.2. Central random paths

Definition 2.2. — A probability measure Pon Γ^Ω is called central if there is a functionp: Λ×N→R⁺ such that

P Γ^Ω(π)

=p(y, n),

for allπ∈Γ^Ω_n(y)withy∈Λ, n>0. A random path inΓ^Ωis called central if the induced measure is central.

Similarly, we could have defined central measure on Γ^Ω_finby similar means.

It is then easily seen that any central measure supported on Γfin(Ω) is a convex combination of uniform measures on the sets Γ^Ω_n(y) with y ∈ Λ and n > 1. Therefore, the main interesting phenomena arise for central measures supported on Γ^Ω. The set of central measures supported on Γ^Ω is denoted byH(Ω).

LetP∈ H(Ω). Then, by Definition 2.2 there exists a functionp: Λ×N→ R⁺ such that

P(Γ^Ω(π)) =p(y, n),

for allπ∈Γ^Ω_n(y) withy∈Λ andn>1. Letx∈Λ, and suppose thatπ is a finite path in Γ^Ω_n(x). A path τ of length n+ 1 ending aty ∈Λ satisfies τ_|[0,n] = π if and only if τ_|[0,n] = π and τ[n,n+1] is an infinitesimal path joiningxto y. Therefore, Γ^Ω(π) can be decomposed as

Γ^Ω(π) =a

y∈Λ

a

τ∈Γ^Ω(x,y)

Γ^Ω(π.τ).

Thus,

P Γ^Ω(π)

=X

y∈Λ

X

τ∈Γ^Ω(x,y)

P Γ^Ω(π.τ) ,

which translates into the relation

(2.1) p(x, n) =X

y∈Λ

#Γ^Ω(x, y)p(y, n+ 1)

forx∈Λ∩Ω such that Γ^Ω_n(x)6=∅. Describing the set of solutions to (2.1) is very complicated in general. It is known however thatH(Ω) is a convex subset ofM1(Γ^Ω) and even a Choquet simplex.

Definition 2.3. — The minimal boundary ofΓ^Ωis the unique subset

∂H(Ω)⊂ H(Ω), such that any central measureP0inH(Ω)admits a unique integral representation

P0= Z

∂H(Ω)

Pdµ(P), whereµis a probability measure on the set∂H(Ω).

2.3. Central measures and Doob’s conditioning

We establish here some connections between central measures on random paths and random walks on lattices. Indeed, any random path π ∈ Γ^Ω following a central measureP∈ H(Ω) yields a random walkZ = (π(0) = 0, π(1), . . .) on the lattice Λ∩Ω. The family of Markov kernels (Qn)n>0 of Z can be explicitly given from the function p: Λ×N→R⁺ associated to the central measureP. Indeed one can show that

Qn(x, y) =1_p(x,n)6=0#Γ^Ω(x, y)p(y, n+ 1) p(x, n) . By the equalityp(x, n) =P

y∈Λ#Γ^Ω(x, y)p(y, n+ 1),Qn is a well-defined Markov kernel. Note that this random walk is generally not homogeneous in time, since the kernelQ_n depends onnthroughp.

Doob’s conditioning is a standard way to produce random walks on Λ∩Ω coming from central measures. LetZ be the random walk on R^d starting at 0 and with Markov kernel

P(Z_n+1=y|Z_n =x) =Γ^R(x, y)

#X

for any x, y ∈ Λ. Note that this random walk actually comes from the central measurePwhose value is

P(Γ^R(π)) = 1 (#X)ⁿ for allπ∈Γ^R_n^d withn>0.

Definition 2.4. — Let c > 0. A function h : Ω∩Λ → R⁺ is a c- harmonic function for the random path Z killed when exiting Ω if and only if

h(x) = 1 c(#X)

X

π∈X x+π⊂Ω

h(x+π(1)) = 1 c(#X)

X

y∈Λ∩Ω

#Γ^Ω(x, y)h(y).

Let c >0 and assume that his ac-harmonic function for Z. Then, the Doob conditioningZ^h ofZ in Ω is given by the Markov kernel

P(Z_n+1^h =y|Z_n^h=x) =1h(x)>0

1 c(#X)

#Γ^Ω(x, y)h(y) h(x) ,

fory∈Λ∩Ω. The random walkZ^his well-defined becausehisc-harmonic and is time homogeneous. This random walk comes from the central mea- sureP^h whose value is

P^h(Γ^Ω(π)) = 1

c(#X) ⁿ

h(y)

forπ∈Γ^Ω_n(y) withn>0 andy∈Λ∩Ω.

Conversely, suppose thatPis a central measure with an associated func- tionpsuch thatp(x, n) =aⁿh(x) for some function h: Λ∩Ω→R⁺ and a >0. Then, (2.1) yields

aⁿh(x) =X

y∈Λ

#Γ^Ω(x, y)aⁿ⁺¹h(y), which is equivalent to the relation h(x) = aP

y∈Λ#Γ^Ω(x, y)h(y). Thus, the functionhis (a#X)⁻¹-harmonic.

Hence, the set ofc-harmonic functions forZ is homeomorphic to the set of central measuresP∈ H(Ω) whose associated functions phave the form p(x, n) =

1

#X·c

n

h(x) withh: Λ∩Ω→R⁺.

For any real c >0, denote byHc(Ω) the set of central measures coming from c-harmonic functions. A quick computation shows that the random walkZ induced by a central measurePis time homogeneous if and only if P∈ Hc(Ω) for somec >0.

It is easily seen thatH_c(Ω) is a convex subset ofH(Ω) and we denote by

∂Hc(Ω) the set of extreme points ofHc(Ω). To the best of our knowledge,

there is no general proof that∂Hc(Ω) =∂H(Ω)∩ Hc(Ω); in particular, an answer to the following problem yields a nice description of∂H(Ω).

Problem. — Do we have the decomposition∂H(Ω) =tc>0∂Hc(Ω)?

In our case of study, this equality is proven by explicitly describing both sets (see Section 3.3). As we shall explain in the following we do need here to consider the closure oftc>0∂Hc(Ω).

2.4. Central measures on multiplicative graphs

We investigate here the general solution of (2.1) when the path concate- nation on Γ^Ω_fin encodes the multiplicative structure of an algebra. Let us first give a brief overview of the general theory in the setting of graded graphs before applying it to our situation.

So consider a rooted graded graph G = {∗} t`

n>1Gn where Gn is the set of vertices in level n (G0 = {∗} by convention). For any n > 0, we can only have directed weighted arrows between vertices λ∈ Gn and µ∈ Gn+1 with weighte(λ, µ). Such a graph is called multiplicative if there exists a commutative algebra A and an injective map ι : G → A such that ι(λ)ι(∗) =P

λ%µe(λ, µ)ι(µ). Here λ%µ means we consider all the neighborsµof the vertexλ. We suppose that the graph is connected, which means that for all µ∈ G, the number of paths between the root and µ is positive. The weighte(π) of a pathπ between the root and a vertex µ is the product of all the weights of the edges ofπ.

Let K be the positive cone spanned by ι(G), and let A_G be the unital subalgebra ofA generated by K. Denote by Mult(A_G)⁺ ⊂A^∗_G the set of multiplicative functions onAG which are nonnegative onK and equal to 1 onι(∗). Note thatι:G →A_G induces a mapι^∗:A^∗_G →F(G,R) such that ι^∗(φ) =φ◦ιfor any linear mapφ:AG →R. Now denote byH(G) the set of functionsp:G →R⁺ such that

(2.2)







p(∗) = 1, p(λ) = X

λ%µ

e(λ, µ)p(µ) for anyλ∈ G.

We can characterize the set∂H(G) of extremal points inH(G):

Proposition 2.5. — Suppose thatK.K⊂K. Then, the mapι^∗ yields an homeomorphism betweenMult⁺(A_G)and the set ∂H(G).

The proof of this proposition is an application of the Ring Theorem of Kerov and Vershik.

Theorem 2.6 ([7, Section 8.4]). — LetB be a unital commutative al- gebra overRandK⊂Ba convex cone satisfying the following conditions:

• K−K=B (K generatesB).

• K.K⊂K (K is stable by multiplication).

• Kis spanned by a countable set of elements.

• For alla∈B, there exists >0such that 1−a∈K.

If Ldenotes the convex set of linear forms onB which are nonnegative onK and map1B to1, thenφis an extreme point ofL if and only ifφis multiplicative (meaning thatφ(ab) =φ(a)φ(b)for alla, b∈B).

We give now the proof of Proposition 2.5.

Proof. — LetB =A_G/hι(∗) = 1iand let pr :A_G →B be the canonical projection; denote byKe the projection of the cone R⁺Id +K in B. Since K.K⊂K and{1, K} spansA_G,K.e Ke ⊂Ke andKe spansB. SinceG has a countable set of vertices,Ke is spanned by a countable set of elements. Note that there is a bijection between the elements ofH(G) and the linear forms onB which are nonnegative on Ke and equal to 1 on 1: indeed h∈ H(G) if and only ifh(µ) = P

µ%νe(µ, ν)h(ν). Thus, for f ∈A^∗_G, ι^∗(f)∈ H(G) if and only iff(ι(∗)ι(µ)) =f(ι(µ)); equivalently, this means thatf factors throughB. The fact ι^∗(f) is nonnegative on G is then equivalent to the fact it is nonnegative on K. We also have [ιe ^∗(f)](∗) = 1 if and only if f(pr◦ι(∗)) =f(1) = 1.

Leta∈B, and let us show that there existssuch that 1−a∈K. Sincee Ke −Ke =B, and 1−b ∈Ke for all b∈ −K, we can suppose without losse of generality thata∈K. It is thus enough to prove that fore µ∈ G, there existssuch that 1−pr◦ι(µ)∈K. Suppose thatµhas rankn. Since the graph is connected, there exists a pathπ₀of weighte(π₀) between∗andµ.

By iteration of the relation coming from the multiplicative structure ofG, ι(∗)ⁿ=P

ν∈G, rk(µ)=n(P

π:∗→µe(π))ι(ν). Thusι(∗)ⁿ−e(π₀)ι(µ) belongs to K. Since pr(ι(∗)ⁿ) = 1, 1−e(π₀) pr◦ι(µ) belongs toK. Therefore, we cane apply Theorem 2.6 to (B,K), which yields that the extreme linear mapse among the set of linear maps onB which are nonnegative onKe and equal to 1 on 1 are the multiplicative ones. Since there is a bijection between multiplicative maps onB which are nonnegative on Ke and multiplicative maps onA_G which are nonnegative onKand equal to 1 onι(∗), the proof

is complete.

In order to apply the previous result to central random paths on Λ∩Ω, we need to relate Γ^Ωto a graded graph.

Definition 2.7. — The growth graph ofΓ^Ωis the rooted graded graph G(Ω) with

• set of vertices of rank n: the set G_n(Ω) = {(x, n) ∈ Λ∩Ω×N, Γ^Ω_n(x)6=∅},

• a directed weighted edge between (x, n)∈ G_n(Ω) and (y, n+ 1)∈ Gn+1(Ω)with weight#Γ^Ω(x, y).

It is readily seen that the weighted sum on paths in G(Ω) between the root (0,0) and (y, n) is equal to #Γ^Ω_n(y). Moreover, the sets H(Ω) and H(G(Ω)) are canonically homeomorphic through the equivalence be- tween (2.1) and (2.2).

3. Littelmann paths in Weyl chambers

We describe a class of random paths coming from the representation theory of semi-simple Lie algebras.

3.1. Background

We consider a simple Lie group G over C and its Lie algebra g. Let R⊂V be the set of roots ofgregarded as a finite subset of the Euclidean vector space V with scalar product h ·,· i. We fixR₊ a subset of positive roots and S = {α1, α2, . . . , αd} ⊂ R+ a basis of simple roots in R. The Weyl group ofgis denoted by W. This is the Coxeter group generated by the reflectionss_αi associated to the simple roots. Thus for anyx∈V and anyα∈S, we have

(3.1) s_α(x) =x−2hα, xi

hα, αiα Denote by`the length function onW defined fromS.

Write P for the weight lattice of g and ω1, . . . , ωd for its fundamental weights so that we have

P =

d

M

i=1

Zωi.

Let us denote by6the dominant order onP such that γ6γ⁰ if and only ifγ⁰−γis a sum of simple roots. Let ∆ be the fundamental Weyl chamber

of gwith respect to S, which corresponds to the positive orthant on the weight spaceLd

i=1Rωi. The cone of dominant weights is then P⁺=P∩∆ =

d

M

i=1

Z>0ωi.

WriteQ⁺ the subset of P spanned by linear combinations of the simple roots with nonnegative coefficients. We denote byR[P] the ring group of P over R with basis {e^β|β ∈ P}, and by R[Q⁺] the subalgebra of R[P] generated byQ⁺. Then

R^W[P] ={u∈R[P]|w(u) =u, w∈W}

is the character ring of g. To each λ ∈ P⁺ corresponds a simple finite- dimensional representation ofgwe denote byV(λ). The Weyl character of V(λ) is

sλ=X

γ∈P

Kλ,γe^γ

where K_λ,γ is the dimension of the weight spaceγ in V(λ). For t∈R^d_>0

andγ ∈P with γ=Pd

i=1γiωi, set t^γ =Q

16i6dexp(γilog(ti)), with the conventiont^γ = +∞when there exists 16i6dsuch thatti= 0 andγi<

0. It is then possible to evaluatesλ ont∈(R⁺)^d assλ(t) =P

γ∈PKλ,γt^γ. Hence, with our convention,sλ(t) = +∞as soon as sometivanishes, since for anyλ∈P⁺ and any 16i6d, there existsγ∈P such that K_λ,γ 6= 0 andγi<0. For µ>λ, denote bySλ,µthe function

(3.2) Sλ,µ=e^−µsλ=X

γ∈P

Kλ,γe^γ−µ

where for any γ such that K_λ,γ > 0, γ−µ is a linear combination of the simple roots with nonpositive coefficients; forµ=λ, we simply write S_λ, instead of S_λ,λ. By setting T_i = e^−αi we thus obtain that S_λ,µ = Sλ,µ(T1, . . . , Td) is polynomial in the variablesT1, . . . , Tdwith nonnegative integer coefficients. Recall also the Weyl dimension formula

dim(V(λ)) = Y

α∈R+

(λ+ρ, α) (ρ, α) , whereρ= ¹₂P

α∈R+α. In particular, dim(V(λ)) is polynomial in the coor- dinates ofλon the basis of fundamental weights.

3.2. Random Littelmann paths

Now, fix a dominant weightδ∈P⁺and denote by Π_δthe set of weights of the irreducible representationV(δ). LetPδbe the sublattice ofP generated by Πδ. This defines subalgebras

R[Pδ] ={e^β|β∈Pδ} ⊂R[P]

and R^W[Pδ] ={u∈R[Pδ]|w(u) =u} ⊂R^W[P].

Finally writeT_δ⁺ the subset ofP⁺ of weightsλsuch thatV(λ) appears as an irreducible component in a tensor powerV(δ)^⊗n, n>0. Givenλandµ inT_δ⁺,we clearly haveλ+µinT_δ⁺.

Now letAδ be the subalgebra ofR^W[P] generated by the Weyl character s_λwithλ∈T_δ⁺. We have the inclusions

Aδ ⊂R^W[Pδ]⊂R[Pδ]⊂R[P].

We denote byK(δ) the convex hull of the set Π_δ:K(δ) is a polytope whose extreme points are the elementsw(δ) forw∈W. The intersection ofK(δ) with the Weyl chamber ∆ is denoted by K(δ)⁺. By Littelmann’s paths theory, there is a setB(δ) ={πi}_16i6dimV(δ) of infinitesimal paths onPδ, with the following properties:

• πi(1)∈Πδ for all 16i6dimV(δ),

• the multiplicity of the weightµin V(δ)^⊗n is equal to #Γ^R_n^d(µ),

• the multiplicity of the irreducible representation V(ν) in V(µ)⊗ V(δ) is equal to #Γ^∆(µ, ν) and the multiplicity of the irreducible representationV(ν) inV(δ)^⊗nis equal to #Γ^∆_n(ν) for allµ, ν∈P⁺ andn>0.

The set of infinite paths we are interested in is the set of infinite paths starting at 0 with set of infinitesimal pathsB(δ).

3.3. Statements of the results

We recall that we consider the space of probability measures on each Γ^∆ with the initial topology with respect to the evaluation maps on the cylinders Γ^∆(τ),τ ∈Γ^∆_fin. We give an algebraic proof of the identification of the minimal boundaries for random paths in Γ^Rand Γ^∆with the topological spacesK(δ) andK(δ)⁺, respectively. In both cases, the homeomorphism can be made explicit by the introduction of a natural parametrization

t:K(δ)−→[0,1]^d×W

of K(δ) such that t(K(δ)⁺) ⊂ [0,1]^d×IdW (this parametrization is ex- plained in Section 5). Form ∈K(δ), we set t(m) = (tm, wm). The main result of the paper is summarized in the following theorem:

Theorem 3.1.

(1) The map

P:

(K(δ)−→∂H(R^d) m7−→Pm

withPm(Γ^R(π)) = ^t

nδ−wm(λ) m

S_δ(t_m) forn>0andπ∈Γ^R_n^d(λ)is a homeo- morphism between the set of extremal measures∂H(R^d)andK(δ).

(2) The map

P⁺:

(K(δ)⁺−→∂H(∆) m7−→P⁺m

withP⁺m(Γ^∆(π)) = ^S_S^λ,nδ(tm)

δ(tm)ⁿ forn>0andπ∈Γ^∆_n(λ)is a homeo- morphism between the set of extremal measures∂H(∆)andK(δ)⁺. It is easy to see that the measuresPm andP⁺m are indeed central. Note moreover that for m ∈ K(δ)⁺, Littelmann’s theory yields that for π ∈ Γ^∆_n(y),

X

π∈Γ˜ ^∆_n+1,˜π_|[0,n]=π

P⁺m(Γ^∆(eπ))

= 1

Sδ(tm)ⁿ⁺¹

X

µ∈B(δ),π.µ∈Γ^∆_n+1(y)

Sπ(n)+µ(1),nδ+x(t_m)

= 1

S_δ(t_m)ⁿ⁺¹S_π(n),nδ(tm)Sδ(tm)

=Sπ(n),nδ(tm)

Sδ(tm)ⁿ =P⁺m(Γ^∆(π)),

so thatP⁺mis a well defined probability measure on Γ^∆. The main point of the result is to prove thatPandP⁺ are bijective.

Remark 3.2. — In typeAd, whenδ=ω1is the first fundamental weight, V(δ) can be regarded as the defining representation ofsl_d+1 or more con- veniently, ofgl_d+1. The set∂H(∆) is then homeomorphic to

K(δ)⁺=

(p1, . . . , pn+1)∈R^d+1

p1>· · ·>pn+1>0 andp1+· · ·+pn+1= 1

and we recover the finite-dimensional version of the Thoma simplex.

As a corollary of Theorem 3.1, we get the complete characterization of c-harmonic measures killed when exiting ∆. Define the function bsδ :

∂H(∆)→R⁺∪ {∞} bysb_δ(P⁺m) =s_δ(t_m).

Corollary 3.3. — Forc >0, the set∂Hc(∆)is equal to bs_δ⁻¹({cdimV(δ)}).

In particular,

• H1(∆) =∂H1(∆)is a singleton corresponding toP⁺0,

• and forc <1,Hc(∆) =∅.

This corollary gives a positive answer to the our Problem 2.3. We prove Corollary 3.3 in Section 6.3. We discuss here a possible generalization of the latter result. LetX be an arbitrary multiset of infinitesimal paths (or alternatively a weight set of such paths). Set Z(t) :=b P

π∈Xt^π(1) and z=Zb(1). Finally, fix a coneC centered at 0 and denote by K_C the set of elementst∈R^d such thatP

π∈Xt^π(1)π(1)∈ C.

A functionf isc-harmonic for these paths if and only if f(x) = X

π∈X x+π⊂C

1

cz wt(π)f(x+π(1)).

We can use the same notation as in the case whereX=B(δ) is the a set of Littelmann paths associated toδ. Then, we conjecture that the following general result holds:

Conjecture 3.4. — For c > 0, the set ∂Hc(∆) is homeomorphic to Zb⁻¹({cz})∩K_C. In particular,

• foru= min_KCZ,b H_u/z(∆)is a singleton.

• and forc < u/z, Hc(∆) =∅.

This conjecture is a generalization of the conjecture of Raschel [19, Con- jecture 1] for two dimensional random walks with bounded increments, which asserts that such a random walk admits a unique harmonic function killed on the boundary of a quarter plane. This special situation can be seen in the above conjecture, in which case the minimum ofZbis exactlyz.

3.4. The approach of Handelman and Price

The existence of the homeomorphisms of Theorem 3.1 can also be de- duced from the main results of [8, 9], themselves based on fundamental re- sults of [17, 18]. We review here their approach, and the reader could read the aforementioned articles and references therein for a detailed proof.

Letndenote the dimension ofV(δ), and consider the adjoint representa- tionρ:G→GL(Mn(C)) which is defined byρ(g)(M) =uδ(g)M uδ(g)⁻¹, whereu_δ is the irreducible representation associated withδ. Form the in- finite tensor productA:=N

Mn(C) as an inductive limit of the sequence of finite-dimensionalC^∗-algebras (Mn(C)^⊗k)k>1, whereMn(C)^⊗k embeds inMn(C)^⊗k+1 with the mapX 7→X⊗Idn. We can canonically associate a structure of C^∗-algebra to this inductive limit of C^∗-algebras. Then, G acts continuously on eachM_n(C)^⊗kand onAwith the mapeρ(g) :=Nρ(g) (which means thatgacts asρ(g) on each component of the tensor product), and we can therefore consider theC^∗-algebraA^δ (resp.A^δ_k) of elements of A(resp.Mn(C)^⊗k) fixed byρ. The algebraA^δ is the inductive limit of the finite-dimensional C^∗-algebras (A^δ_k)k>1, and the Bratteli diagram of this inductive limit is exactly the growth graph of Γ^∆. Therefore, the set of central measures on Γ^∆ is in bijection with the set of traces onA^δ.

Doing the same construction for the restriction of the representation δ to the maximal torusT ⊂G, we get another sequence of finite dimensional C^∗-algebras (A^T_k)_k>1, whose inductive limit is denoted by A^T. Similarly, the Bratteli diagram ofA^T is exactly the growth graph of Γ^R, and the set of central measures on Γ^R is in bijection with the set of traces onA^T.

Note that we have the natural inclusion of C^∗-algebras A^δ ⊂A^T. The main result of [8] is that any extremal trace onA^δ extends to an extremal trace onA^T. To prove this, the author uses the bijection between the set of traces on an approximately finiteC^∗-algebraAand the set of states on its associated dimension groupK₀(A). Let us quickly explain the nature of K0(A): a dimension group is a group with a notion of positive cone. By con- sidering equivalence classes of projections on the ∗-algebra L

k>1Mk(A), one can canonically associate a dimension groupK0(A) to eachC^∗-algebra A; this dimension group is always a ring in our case. An important fact is that an inclusion ofC^∗-algebras induces an inclusion of the associated di- mension groups, and therefore the problem reduces to extend any state on K₀(A^δ) to a state onK₀(A^T). Handelman managed to prove this in [8], and the main ingredient of the proof is the non-trivial property thatK0(A^T) is a finitely generatedK₀(A^δ)-module.

Once proven that any trace onA^δextends to a trace onA^T, the problem amounts to describe the set of traces on A^T. In [9], the author achieves this by proving that the set of faithful traces onA^T is in bijection with the interior ofK(δ). Then, the identification of the set of faithful traces onA^δ with the interior ofK(δ)⁺ is done thanks to a result of [18], which asserts that the Weyl group W acts transitively on the set of traces extending a

particular faithful trace onA^T. Finally, the case of non-faithful traces is done by considering parabolic subgroups ofG.

3.5. The extended algebra of characters

Our proof of Theorem 3.1 will mainly use algebraic properties of the representations of the Lie algebra g. We define the extended algebra of charactersAbδ as follows:

• Abδ is isomorphic to Aδ ×R[T] as a vector space; for x∈ Aδ, we simply denote by (x, n) the element (x, Tⁿ). A basis ofAb_δ is given by the setB={(sλ, n)}_n>0,λ∈T+

δ and the multiplicative structure ofAb_δ is defined onB by the product

(sλ, n)×(sµ, m) = (sλsµ, n+m).

• We also denote byTb⁺_δ the subalgebra ofAbδspanned by{(sλ, n)|λ∈ T_δ,n⁺ }whereT_δ,n⁺ is the set of dominant weightsλsuch thatV(λ) is an irreducible component ofV(δ)^⊗n.

Likewise, we define the extended algebra of weights Pbδ as follows

• Pbδ is isomorphic to R[Pδ]×R[T] as a vector space. A basis of Pbδ

is given by the set {(e^γ, n)|n > 0, γ ∈ Pδ}. The multiplicative structure ofPb_δ is defined by the product

(e^γ, n)×(e^γ0, m) = (e^γ+γ0, n+m).

Write T_δ,n for the set of weightsγ appearing with nonzero multiplicity in the representationV(δ)^⊗n. We shall also need the algebraTbδ defined as follows

• Tbδ is the subalgebra ofPbδ spanned by the elements{(e^γ, n)|n>1, γ∈T_δ,n}.

Note that the inclusionAδ⊂R[Pδ] translates naturally into the inclusion Abδ ⊂Pbδ andTb⁺_δ ⊂Tbδ.

We can write the multiset of weights of δ in Tb_δ as Π_δ ={(e^γ1,1), . . . , (e^γN,1)}where each weight appears a number of times equal to its multi- plicity. For any k = 0, . . . , N, let ek(X1, . . . , XN) be thek-th elementary symmetric function in the variables X₁, . . . , X_N. Define the polynomial Φ(X)∈Tbδ[X] by

Φ(X) = Y

γ∈Πδ

(X+ (e^γ,1)).

Proposition 3.5. — We have

(3.3) Φ(X) =

N

X

k=0

(ek(e^γ1, . . . , e^γN), k)X^N−k

and for anyk= 0, . . . , N,the expression (ek(e^γ1, . . . , e^γN), k) decomposes as a sum of elements (sλ, n) ∈ Tb⁺_δ with positive integer coefficients. In particular, we haveΦ(X)∈Tb⁺_δ[X].

Proof. — Recall that e_k((e^γ1,1), . . . ,(e^γN,1)) is the k-th elementary symmetric function in minus the roots of the polynomial Φ. Hence,ek(e^γ1, . . . , e^γN) is the plethysm of the elementary symmetric function e_k by s_δ. This means that

e_k(e^γ1, . . . , e^γN) = char

k

^V(δ)

!

is the character of thek-th exterior power of the representationV(δ). Since Vk

V(δ) is a submodule of V(δ)^⊗k, its character indeed decomposes as a sum of characters in{sλ|V(λ)∈V(δ)^⊗k}with positive integer coefficients.

Corollary 3.6. — Tb⁺_δ is integrally closed inTbδ.

Proof. — LetTb⁺_δ denote the integral closure ofTb⁺_δ inTbδ. We haveTb⁺_δ ⊂ Tb_δ by definition. Conversely, sinceTb⁺_δ is a ring andTb_δ is generated by the monomials (e^γ,1) with γ ∈ Πδ, it suffices to prove that each such (e^γ,1) belongs to Tb⁺_δ. But −(e^γ,1) is a root of Φ(X) which is, by the previous proposition, a monic polynomial with coefficients inTb⁺_δ. Therefore−(e^γ,1) and (e^γ,1) are integers overTb⁺_δ and thus belong toTb⁺_δ.

4. Minimal boundary of Γ^R

4.1. Algebraic description of the growth graph

LetG(R^d) be the growth graph of Γ^RandG(∆) be the one of Γ^∆. Namely, the set Gn(R^d) of vertices of rank n of the graph G(R^d) are pairs (γ, n) whereγis a weight ofPδ such that Γ^R_n^d(γ)6=∅, and the weight of the edge between (γ, n) and (γ⁰, n+ 1) is #Γ^R(γ, γ⁰). From the graph embedding of Section 2.4, the set of extreme central measures on Γ^R is in bijection with the set of extreme points of the convex set ∂H(G(R^d)) of nonnega- tive harmonic functionsp : G(R^d) → R⁺ with p(0,0) = 1 and the same

holds forG(∆). An important feature ofG(R^d) is that this graded graph is multiplicative: it is related to the algebraTb_δ as follows.

Proposition 4.1. — G(R^d) is a multiplicative graph associated with the algebraPb_δ with the injective map

ι:











G(R^d)−→Pbδ

(γ, n)7−→(e^γ, n), n>1 7−→(sδ,1),

andι(G(R^d)) =Tbδ. In particular,∂H(G(R^d))is homeomorphic toMult(Tbδ)⁺ through the map

ι^∗:

(Mult(Tb_δ)⁺−→∂H(G(R^d)) f 7−→f◦ι.

Proof. — Since #Γ^R(γ, γ⁰) = K_δ,γ⁰_−γ, the following equality holds for (γ, n)∈ Gn(R^d):

ι(γ, n)ι(∗) = (e^γ, n) X

κ∈Πδ

K_δ,κe^κ,1

!

= X

κ∈Πδ

K_δ,κ(e^γ+κ, n+ 1)

= X

γ⁰∈Pδ

γ⁰−γ∈Πδ

K_δ,γ⁰_−γ(e^γ0, n+ 1)

= X

γ⁰∈Pδ

#Γ^R(γ, γ⁰)ι(γ⁰, n+ 1).

Thus,G(R^d) is a multiplicative graph associated withPbδ through the map ι. Note that by construction, the sub-algebra of Pbδ generated by the ele- ments{ι(γ, n)}_(γ,n)∈G(Rd) is preciselyTb_δ: the last part of the proposition

is deduced from Proposition 2.5.

4.2. Characterization of the multiplicative maps onTbδ

The set of extreme central measures on G(R^d) is thus given by the set of positive morphisms fromTb_δ to Rwhich take the value 1 on (s_δ,1). We will prove in this subsection the following result:

Proposition 4.2. — Letf ∈Mult(Tb_δ)⁺. There exists a multiplicative mapφ:R[Q⁺]→R⁺ and an elementw∈W such that

f(e^γ, n) = 1

φ(S_δ)ⁿφ(e^nδ−w(γ)), for all(e^γ, n)∈Tb_δ.

Note that the elementφ(e^nδ−w(γ)) is well-defined: indeed, if (e^γ, n)∈Tbδ, then the weightγ appears in the representationV(δ)^⊗n and w(γ) is thus smaller thannδwith respect to the roots order relative to the set of simple rootsS. Therefore,nδ−w(γ)∈Q⁺.

Letf be a multiplicative map onTbδ. Sincef is multiplicative andTbδ is generated by the setΠe_δ:={(e^γ,1), γ∈Π_δ},f is completely determined by its values onΠe_δ. We can also extend naturally the action ofW by setting w.(e^γ, n) = (e^w(γ), n). We suppose from now on thatf ∈Mult(Tbδ)⁺. Let

(4.1) Mf = X

γ∈Π_δ

Kδ,γf(γ,1)γ.

The vectorMf belongs toR^d, thus there existsw∈W such thatw(Mf)∈

∆. Replacingf by f ◦w⁻¹ gives another multiplicative map on Tb_δ such that

M_f◦w⁻¹ = X

γ∈Πδ

Kδ,γ(f◦w⁻¹)(e^γ,1)γ∈∆ and we havef = (f◦w⁻¹)◦w.

Lemma 4.3. — Assume that Mf ∈ ∆ and let α ∈ S. For all γ ∈ Πδ

such that

f(e^γ,1) = 0−→f(e^γ−α,1) = 0.

In particular,f(e^δ,1)6= 0.

Proof. — It is a classical result in the representation theory ofgthat for any weightγ ∈ Πδ and any simple root α∈S such thatγ−α /∈Πδ, we must havehγ, αi60. Also if both γ and γ−αbelong to Πδ but γ−2α does not, one hashγ−α, αi<0.

Now letα∈S, and suppose that there existsγ∈Π_δ such that γ−α∈ Πδ, f(e^γ,1) = 0 andf(e^γ−α,1)6= 0. Ifγ⁰ is another weight of Πδ such that f(e^γ0,1)6= 0, then necessarilyγ⁰−α6∈Πδ: indeed, ifγ⁰−α∈Πδ, then

f(e^γ0−α,1)f(e^γ,1) =f(e^γ+γ0−α,2) =f(e^γ−α,1)f(e^γ0,1)6= 0,