Quantum torus algebra and quivers

Definition 3.1 (Quantum torus algebra) A seed i is a triple (I,I0,B,D) where I is a finite set,I0 ⊂ Iis a subset called the frozen subset, B = (bi j)i,j∈I a skew-symmetrizableQ-valued matrix called theexchange matrix, andD=di ag(di)i∈Iis a diagonal matrix such thatD B= −B^TDis skew-symmetric.

Letqbe a formal parameter. We define thequantum torus algebraXiassociated to the seedito be an associative algebra overC(q^d), whered =mini∈I(di), generated by{Xi}i∈Isubject to the relations

XiXj =q_i^{−2bi j}XjXi, i,j∈I (3.1) whereqi :=q^di. The generatorsXiare called thequantum cluster variables, and they are said to befrozenifi∈I0. We calldithemultipliersof the variablesXi. We denote byTithe non-commutative field of fraction ofXi.

The structure of the quantum torus algebraXiassociated to a seedican be conve-niently encoded in a quiver:

Definition 3.2 (Quiver associated toi) We associate to each seedia generalized quiver Qⁱ=(Q0, w)with verticesQ0labeled byI, and for each pairi,j ∈Q0a weight

wi j :=dibi j = −wj i. (3.2) We will draw arrows fromi−−→^{wi j} jifwi j >0. We will call an isomorphismπ :SQ0

from a finite setSanexternal labelof the quiverQ.

We will use squares to denote frozen nodesi ∈ I0and circles otherwise. In the sequel, whenqi =qsorql given by Definition2.3, we will distinguish the arrows by thick or thin arrows instead of writing the weights. We will also use dashed lines to denote arrows with weightwi j = ¹₂, which only occurs between frozen nodes (Fig.1).

We introduce the following notations which will be useful throughout the paper:

Definition 3.3 We denote by X_i^m1

1 ,...,i^mn_n :=q^CX^m_i ¹

1 . . .X^m_i ⁿ

n , (3.3)

whereCis the unique rational number such that q^CX_i^m1

1 . . .X_i^mn

n =q^−CX_i^mn

n . . .X_i^m1

1 . Explicitly, ifXiXj =q^{ci j}XjXi, then

C = −1 2

p>q

mpmqcipiq. (3.4)

If we introduce a∗-structure such thatq^∗ =q⁻¹andX_i^∗ = Xi (and positive), then the expressionX_i^m1

1 ,...,i_n^mn is also (positive) self-adjoint.

We also denote by

X(i1, . . . ,in):=

n k=1

Xi₁,...,i_k. (3.5)

Definition 3.4 A permutation of a seedσ : i−→iis a bijectionσ : I−→Isuch that

σ (I0)=I₀, b_{i j} =b_σ(i)σ(j),

d_i=d_σ(i). It induces an isomorphismσ^∗:Ti −→Tiby

σ^∗(X_σ(i)):=Xi, whereX_σ(i)denotes the quantum cluster variables ofTi. 3.2 Quantum cluster mutation

Next we define the cluster mutations of a seed and its quiver, and the quantum cluster mutations for the algebra. Here we will use the notion that keeps the indexingIof the seeds, which ensures the consistency of the relationμ²_k=Id.

Definition 3.5 (Cluster mutation) Given a pair of seeds i = (I,I0,B,D), i = (I,I₀,B,D)withI = I,I0 = I₀, and an element k ∈ I\I0, acluster mutation in direction kis an isomorphismμk:i−→isuch thatμk(i)=ifor alli ∈I, and

b_{i j} =

−bi j ifi =kor j =k,

bi j+^{bi k|bk j|+|}₂^{bi k|bk j} otherwise, (3.6)

d_i=di. (3.7)

q

Then the quiver mutation Qⁱ −→ Qⁱ corresponding to the mutation μk can be performed by:

(1) reverse all the arrows incident to the vertexk;

(2) for each pair of arrowsi −−→^{wi k} kandk−−→^{wk j} j, update the arrowi ^{wi j+}

wi kwk j

−−−−−−−→dk j. (3) delete any arrows with weightwi j =0.

Definition 3.6 (Quantum cluster mutation) The cluster mutation in directionk,μk : i−→i, induces an isomorphismμ^q_k :Ti −→Ticalled thequantum cluster mutation, defined by

μ^q_k(Xi)=

⎧⎪

⎪⎪

⎪⎪

⎪⎪

⎪⎨

⎪⎪

⎪⎪

⎪⎪

⎪⎪

⎩

X⁻_k¹ ifi =k,

Xi

|bki| r=1

(1+q_i^2r−1Xk) ifi =kandbki ≤0,

Xi bki

r=1

(1+q_i^2r−1X⁻_k¹)⁻¹ ifi =kandbki ≥0,

(3.8)

where we denote by Xi the quantum cluster variables corresponding toXi with exchange matrixB, i.e.b_ki = −bki for everyi ∈I.

The quantum cluster mutationμ^q_k can be written as a composition of two homo-morphisms

μ^q_k =μ^#k◦μk, (3.9)

whereμ_k:Ti −→Tiis a monomial transformation defined by

μk(Xi):=

⎧⎪

⎨

⎪⎩

X_k⁻¹ ifi=k,

Xi ifi=kandbki≤0, q_i^{bi kbki}XiX^b_k^ki ifi =kandbki ≥0,

(3.10)

andμ^#_k :Ti−→Tiis a conjugation by thequantum dilogarithm function

μ^#_k:= Adqk(X_k), (3.11)

where^q(x)is given by a formal power series inx:

^q(x):=

∞ r=0

(1+q^2r+1x)⁻¹. (3.12)

In the remaining of the paper, however, we will use the notation

gbk(x):=^qk(x)⁻¹, (3.13)

g_b^∗_k(x):=g_b⁻_k¹(x)=^qk(x) (3.14) instead, in accordance to the universalR-operator formula given in [23]. The various identities ofgb(x)that are needed in this paper are summarized in “Appendix A”.

Remark 3.7 We remark that in the split real setting where|q| = 1 is given byq = e^π√−1b2,gb(x)is the notation for the non-compact quantum dilogarithm, which plays a central role in the theory of positive representation, various quantum Teichmüller theories [10,28] and non-rational conformal field theories [3,38,39]. It is composed by two commuting copies, associated to the so-calledFaddeev’s modular double, of the compact quantum dilogarithm^q(x)[6,8], and it is a unitary operator whenxis positive self-adjoint.

In this paper however, we are only interested in the formal algebraic theory, hence one may consider only the compact part and think of the correspondence

gb(x)∼^q(x)⁻¹=^∞

r=0

(1+q_k^2r+1x)=E x p_q−2

− u q−q⁻¹

, (3.15)

where

E x pq(x):=

k≥0

x^k

(k)q!, (3.16)

(k)q:= 1−q^k

1−q . (3.17)

The use of the notationgb(x)suggests that the theory of the current paper can be nat-urally applied to the case of the non-compact split real setting, where all the algebraic relations are satisfied, and naturally the positivity and self-adjointness of the operators are automatically taken care into account, which makes the choice extremely natural.

The following version of the useful Lemma from [41, Lemma 1.1] is rewritten in the notation of the current paper:

Lemma 3.8 Letμi₁, . . . , μi_kbe a sequence of mutations, and denote the intermediate seeds byij :=μi_j. . . μi₁(i). Then the induced quantum cluster mutationμ^q_i1. . . μ^q_ik : Ti_k −→Tican be written as

μ^q_i1. . . μ^q_ik =k◦Mk, (3.18) where Mk :Tik −→Tiandk :Ti−→Tiare given by

Mk :=μi₁μi₂. . . μi_k, (3.19) k :=Adg^∗

bi1(X_i

1)Ad

g^∗

bi2(M1(X_iⁱ¹

2)). . .Ad

g_bi^∗

k(Mk−1(X_ik^ik−1)), (3.20) and Xⁱ_i^j denotes the corresponding quantum cluster variables of the algebraXi_j.

q

3.3 Amalgamation

We also recall the procedure ofamalgamationof two quivers [9]:

Definition 3.9 Let Q1 := Qⁱ¹ and Q2 := Qⁱ² be a pair of quivers associated to the seedi1 =(I¹,I¹₀,B¹,D¹),i2 = (I²,I²₀,B²,D²)and with edge weightsw¹, w² respectively, and letJ1⊂I¹₀,J2⊂I²₀be certain subsets of the frozen nodes ofQ1and Q2respectively. Assume there exists a bijectionφ:J1−→J2such thatd_φ(i)=difor i ∈J1. Then theamalgamationofQ1andQ2alongφis a new quiverQconstructed as follows:

(1) The vertices of Qare given by Q1∪_φ Q2 by identifying verticesi ∈ Q1 and φ(i)∈ Q2and assigned with the same weightdi,

(2) The frozen nodes ofQare given by(I¹₀\J1)(I²₀\J2), i.e. we “defroze” the vertices that are glued.

(3) The weightswof the edges ofQare given by

wi j =

⎧⎪

⎨

⎪⎩

0 ifi ∈I^k\Jkand j∈I^2−k\J2−kfork=1,2, w_{i j}^k ifi ∈I^k\Jkor j ∈I^k\Jkfork=1,2, w_{i j}¹ +w_φ(²_i)φ(j) if i,j ∈J1.

Amalgamation of a pair of quiver induces an embedding X −→ X1⊗X2 of the corresponding quantum clusterX-tori by

Xi →

⎧⎪

⎨

⎪⎩

Xi⊗1 ifi ∈ Q1\J1, 1⊗Xi ifi ∈ Q2\J2, Xi⊗X_φ(i) otherwise.

(3.21)

Visually this is just gluing two quivers together along the chosen frozen nodes, such that the weights of the corresponding arrows among those nodes are added.

4 From positive representations to quantum group embedding In this section, we recall the explicit structure of positive representations of estab-lished in the previous works, and from their explicit expressions we provide the main construction of this paper, where we embedUq(g)into certain quantum torus algebra Dg.

4.1 Positive representationsPofUq(g_R)

In [11,21,22], a special class of representations called thepositive representationsis constructed for split real quantum groupsUq(g_R)(and its modular double, which is not needed in this paper). HereUq(g_R)is a Hopf∗-algebra, defined to be the real form ofUq(g)equipped with in addition the star structure

e^∗_i =ei, f_i^∗=fi, K_i^∗=Ki, (4.1) and necessarily |qi| = 1 for everyi ∈ I, whence we let qi := e^π√−1bi2 ∈ Cfor bi ∈R.

Remark 4.1 In the setting of positive representations, we assumed theqi’s are not root of unity for simplicity, such that the quantum dilogarithm functiongb(x)consists only of simple zeros and poles, and the intertwiners involvinggb(x)are well-defined. In the remainder of the paper however, we will treatqi as formal variables and hence such assumption can be dropped.

Theorem 4.2 (Positive representations)There exists a family of irreducible represen-tationsP_λofUq(g_R)parametrized by theR_≥0-span of the cone of dominant weights, λ∈ P_R⁺⊂h^∗_R, or equivalently byλ:=(λ1, . . . , λn)∈Rⁿ_≥0where n=r ank(g), such that

• For each reduced wordi∈R, the generatorsei,fi,Kiare represented by positive essentially self-adjoint operators acting on L²(R^N),

• Each generators can be represented by a sum of monomials generated by the positive operators

{e^±πbixi,e^±2πbipi}i=1,...,N,

where pi = _2π^√1_−1∂^∂_xi are the momentum operators such that[pi,xi] = _2π^√1₋₁, and each monomials are positive essentially self-adjoint. These expressions depend on the choice of reduced wordi∈R.

• There exists a unitary equivalencebetween positive representations correspond-ing to different reduced words, hence the representation does not depend on the choice of reduced expression ofw0.

In the theory of positive representations of split real quantum groups, the represen-tation carries a real structure and the operators are represented by unbounded positive operators on certain Hilbert spaces. However in this paper, we will only be dealing with the representations formally, so all the generators and relations are treated on the algebraic level only. In particular, if we define formally

U_i^±1=e^±πbixi, V_i^±1=e^±2πbipi, (4.2) then algebraically we have fori =1, . . . ,N:

UiVi =qiViUi,

UiVj =VjUi, i= j. (4.3)

As a corollary, if we just consider the quantum torus algebraC[Tq]generated by the elements{U_i^±1,V_i^±1}i=1,...,N subjected to (4.3), then the irreducibility ofPλimplies that

q

Corollary 4.3 [21,22]The positive representations give an embedding ofUq(g)into C[Tq], generalizing the Feigin’s homomorphismUq(b₋)−→C[Tq].

Remark 4.4 In [21,22], we showed that one can shift the generators ei,fi by some appropriate Ki factors such that the “modified quantum group”Uq(g)embeds into the “true” quantum torus algebraC U^±_i ¹,V^±_i ¹ with the relationsUiVi = q_i²ViUi

instead.

4.2 Explicit construction ofP

The positive representationsP_λwere computed explicitly for all types ofg. Let us first recall some notations used in [21,22].

Definition 4.5 We denote bypu=_2π^√1_−1∂^∂_u and

e(u):=e^πbu, [u] :=q¹²e(u)+q⁻¹²e(−u), (4.4) so that whenever[p,u] = _2π^√1₋₁,

[u]e(−2p):=(q¹²e^πbu+q⁻¹²e^−πbu)e^−2πbp

=e^πbu−2πbp+e^{−2πbu−2πbp}

=e(u−2p)+e(−u−2p) is self-adjoint.

Definition 4.6 (Notation) Leti = (i1, . . . ,iN) ∈ Rbe a reduced word forw0. We associate toia set ofNvariables indexed in two ways:

• u^k_i denotes thek-th variables from the left²inicorresponding to the root indexi.

• vjdenotes the j-th variable from the left ini, i.e. corresponding toij, andij is the root index corresponding tovj.

• We denote the corresponding momentum operators as p^k_i and pj respectively if no confusion arises.

• v(i,k)denotes the index such thatu^k_i =vv(i,k).

Example 4.7 For type A3, leti = (1,2,1,3,2,1). Then the 6 variables are ordered as:

(u¹₁,u¹₂,u²₁,u¹₃,u²₂,u³₁)=(v1, v2, v3, v4, v5, v6).

Definition 4.8 By abuse of notation, we denote by

[us +ul]e(−2ps−2pl):=e^{πbs(−us−2ps)+πbl(−ul−2pl)}+e^{πbs(us−2ps)+πbl(ul−2pl)}, (4.5)

2 This differs from the previous notation used in [21,22] where the variables read from the right. This version will be more convenient in this paper.

wheneverus(resp.ul) is a linear combination of the variablesu^k_i and the parameters λi, corresponding to short (resp. long) root indexi. Similarly ps (resp.pl) are linear combinations of the momentum variablesp^k_i corresponding to short (resp. long) root indexi. This applies to all simpleg, with the convention given in Definition2.3.

Now we can summarize the construction of the positive representations as follows:

Theorem 4.9 Given a reduced wordi∈R, the positive representationP_λL²(R^N) ofUq(g_R)is parametrized byλ=(λi)∈Rⁿ_≥0, and the generators are represented in the form

fi :=f_i¹+f_i²+ · · · +f_iⁿⁱ, (4.6)

Ki :=e

⎛

⎝2λi− N

j=1

aij,ivj

⎞

⎠, (4.7)

where

f_i^k=

⎡

⎣−

v(i,k) j=1

aij,ivj+u_i^k+2λi

⎤

⎦e(2p^k_i) (4.8)

=e

⎛

⎝−

v(i,k) j=1

ai_j,ivj +u^k_i +2λi +2p_i^k

⎞

⎠+e

⎛

⎝^v(i,k)

j=1

ai_j,ivj−u^k_i +2λi+2p_i^k

⎞

⎠

=:f_i^k,−+f_i^k,+ (4.9)

splitting according to Definition4.8.

The representation ofej is explicitly written case by case. In general, if j =iN, then

ej = [vN]e(−pN), (4.10) Otherwise

ej =◦ [vN]e(−2pN)◦⁻¹, (4.11) where we recallis the unitary transformation, expressed in terms of quantum dilog-arithms, that relatesP_λto another representations corresponding to a reduced word i∈Rwith i_N = j .

Theorem 4.10 Each generatorej is expressed as a Laurant polynomial in Ui and Vi

[(cf.(4.2)], with a uniqueinitial termof the form[u_i^k]e(−2p^k_i + · · ·)for some index i and k. One determines this initial term by applying the transformation(4.11)and tracing the changes of the corresponding initial term by the following rules:

q

• if j=iN, then from(4.10)the initial term is[vi_N]by definition.

• If we have a change of word(. . .i,j,i, . . .) ←→ (. . . , j,i,j, . . .), inducing a change of variables

(. . . ,u^k_i,u^l_j,u^k_i⁺¹, . . .)←→(. . . ,u^l_j,u^k_i,u^l_j⁺¹, . . .),

then the initial term changes from[u^k_i] ←→ [u^l_j⁺¹].

• If we have a change of word(. . . ,i,j,i,j, . . .)←→(. . . , j,i,j,i, . . .), inducing a change of variables

(. . . ,u^k_i,u^l_j,u^k_i⁺¹,u^l_j⁺¹. . .)←→(. . . ,u^l_j,u^k_i,u^l_j⁺¹,u^k_i⁺¹. . .),

then the initial term changes from[u^l_j] ←→ [u^l_j⁺¹].

• In type G2, for the change of word (2,1,2,1,2,1) ←→ (1,2,1,2,1,2), the initial term fore1is[u³₁] ←→ [u¹₁]and initial term fore2is[u¹₂] ←→ [u³₂].

From the explicit expression (4.8), we have Proposition 4.11 If we writefias

fi =f_i^ni,−+f_i^ni−1,−+ · · · +f_i^1,−+f_i^1,++f_i^2,++ · · · +f_i^ni,+,

then each term q_i⁻²-commute (i.e. A B=q_i⁻²B A) with all the terms on the right, and each term q_i⁻²-commute with K_i⁻¹.

Remark 4.12 Feigin’s homomorphismUq(b₋)−→C[Tq]is given by the expression ofKi and half offi:

f_i:=f_i^1,++f_i^2,++ · · · +f_i^ni,+

only, so the expression of Theorem4.9is really a “double” of Feigin’s homomorphism.

4.3 Embedding ofUq(g)into quantum torus algebraDg

Now we are ready to construct the quantum torus algebraDgin the flavor of [41] that will provide a clear description of the embedding of the generators of the quantum groupUq(g).

Definition 4.13 Define 2N+2nvariables indexed by

S= {f_i⁻ⁿⁱ, . . . , f_iⁿⁱ}i∈I∪ {e_i⁰}i∈I {1, . . . ,2N+2n}

as follows: For eachi ∈ I, we take the consecutive “ratio” of the monomial terms of fi as:

X_f^k

i :=

⎧⎪

⎪⎪

⎪⎪

⎪⎪

⎨

⎪⎪

⎪⎪

⎪⎪

⎪⎩

f_i^ni,− k= −ni, qif_i^k,−(f_i^k−1,−)⁻¹ k<0, qif_i^1,+(f_i^1,−)⁻¹ k=0, qif_i^k+1,+(f_i^k,+)⁻¹ k>0, qiK_i⁻¹(f_i^ni,+)⁻¹ k=ni.

(4.12)

Let the initial term ofei be

[vn]e(−2pn)=e(vn−2pn)+e(−vn−2pn)

=:e_i^n,−+eⁿ_i^,+

as in Theorem4.10. Then we define X_e0

i :=qieⁿ_i^,+(eⁿ_i^,−)⁻¹=e(−2vn). (4.13) We note that theqi factors are chosen such that eachXkis self-adjoint. Moreover, since allXkare expressed formally as a monomial of{U_i^±1,V_i^±1}as in (4.2), we have XjXk=q⁻_j^{2bj k}XkXj (4.14) for some skew-symmetrizable exchange matrixB=(bj k)andqj :=qi if j = f_i^k or e⁰_i. By abuse of notation, we will use the same variables for the definition below:

Definition 4.14 We define the quantum torus algebraDgto be the algebra generated by theN+2nvariables

X_f−ni

i , . . . ,X_fni

i ,X_e⁰

i, i =1, . . . ,n subject to the relations (4.14).

The correspondingDg-quiver is associated to the seed(S,S0,B,D)with frozen nodes

S0= {f_i⁻ⁿⁱ}i∈I ∪ {f_iⁿⁱ}i∈I∪ {e_i⁰}i∈I

The multiplier D=di ag(dj)j∈S is defined bydj :=di if j = f_i^kore_i⁰, anddi for i ∈I is given as in (2.5).

Now we can state our first main result of the paper.

Theorem 4.15 We have an embedding of algebra

ι:D_g→Dg, (4.15)

q

which induces an embedding of the quantum group into a quotient ofDg

Uq(g) →Dg/ ι(Ki)ι(Ki)=1. (4.16) Proof By construction from (4.12), we can write (cf. Definition3.3)

fi =X_f−ni

i +qiX_f−ni

i

Xf_i⁻ⁿⁱ⁺¹+ · · · +q_iⁿⁱ⁻¹X_f−ni i . . .X

f_iⁿⁱ⁻¹

=X(f_i⁻ⁿⁱ,f_i⁻ⁿⁱ⁺¹, . . . , f_iⁿⁱ⁻¹) K_i=X_f−ni

i ,...,f_iⁿⁱ.

Given a reduced wordi=(i1, . . . ,iN)∈R, ifi =iN, then one computes explicitly ei = [u¹_i]e(−2p_i¹)

=e^{πbiu1i−2πbip1i} +e^{πbiu1i−2πbip1i}

=X_fni

i +qiX_fni

i X_e0

i

=X(f_iⁿⁱ,e_i⁰), Ki =X_fni

i ,e_i⁰,f_i⁻ⁿⁱ.

Otherwise, from the construction of positive representation, each mutation of the reduced expression of w0 correspond to a unitary transformation given by the quantum dilogarithm function with an argument given by a consecutive difference of thef_iⁿ’s in the corresponding mutated quiver. (This is described in detail in Sect.7.) Henceei will be expressed as a sum of monomials, each of which is expressed as a product of Xf_iⁿ and the ratios between the initial term, which is given by X_e0

i. The explicit expression is given in the next section.

Furthermore, the unitary transformationhas the properties that for any reduced wordi∈R, ifejis expressed as a sum

ei =Xi1+ · · · +Xi1,...,ik, then the leading termXi1 =X_fni

i and the ending term satisfies Xi₁,...,i_kX_f−ni

i =q_i⁻²X_f−ni

i

Xi₁,...,i_k.

The unitary transformation , while inducing a change of variables given by a linear transformation, will preserve the monomialKi. Hence from the relation

[ei,fi] =(qi −q_i⁻¹)(K_i−Ki),

we see that the term X

i1,...,ik,f_i⁻ⁿⁱ does not vanish. Since we already have K_i = X_f−ni

i ,...,f_iⁿⁱ, we must have

Ki =X_i

1,...,ik,f_i⁻ⁿⁱ, hence giving the desired homomorphism ofD_gintoDg.

To see thatιis an embedding, we first note that the positive representationP_λis a faithful irreducible representation coming from the quantization of the induced repre-sentation of the left regular reprerepre-sentation. Then by choosing explicitly the parameters λsuch that

2√

−1λi ∈Qi+biN, Qi :=bi+b_i⁻¹, (4.17) we recover every finite dimensional highest weight irreducible representation for the compactquantum groupUq(g). (see [25], where this fact has been utilized to calculate the eigenvalues of the positive Casimir operators.) In particular, all the PBW basis cannot be identically zero in the representation, hence the homomorphismιis indeed an embedding.

Finally, we note that the monomial ι(Ki)ι(K_i)lies in the center of the algebra Dg. Hence taking the quotient with ι(Ki)ι(K_i) = 1,i ∈ I we obtain the desired

embedding ofUq(g)as well.

Remark 4.16 This result is stronger than the embedding given by Corollary4.3since we do not require to take inverses of the generators of Dg, i.e. it is a polynomial embedding.

Remark 4.17 Note that by the Cartan involution, one can also define another embed-ding

ι^w :Dg−→Dg, ei →ι(fi), fi →ι(ei), Ki →ι(Ki), K_i→ι(Ki).

This interchanges the expressions of the explicit embeddings ofeiandfiin the quantum torus algebraDg.

Remark 4.18 In [41], the proof of the injectivity ofιfor typeAnis explicitly checked on the PBW basis. The expression relating the PBW exponents to those of the q-tori generators turns out to involve some combinaq-torialhive-typeconditions from the work of Knutson–Tao [33]. It will be interesting to see explicitly analogues of such combinatorics in other types.

q

5 Construction of theDg-quiver

In the previous section, we show the embedding of the quantum groups into a quan-tum torus algebraDg, where theq-commutation relations are encoded in the exchange matrixB. In this section, let us describe the general construction of the quiver associ-ated to theDgalgebra forgof all types in more details.

5.1 Relation among cluster variables First we have the obvious relations.

Lemma 5.1 X_f0

i and X_e0

i mutually commute with each other.

Proof By Definition 4.13, the formal expression of all the X_f0

i ’s and X_e0

i’s do not contain any momentum operatorse(2p), hence they commute with each other.

Next we have the following observation:

Lemma 5.2 Recall that F_i^k,±is defined in(4.8). We have

F_i^k,±F^l_j^,±=q_i^{∓ai j}F^l_j^,±F_i^k,± (5.1) ifv(i,k) < v(j,l).

Proof Let us consider the + case, while the − case is similar. By definition, if v(i,k) < v(j,l), then there are no terms of u^l_j appearing in F_i^k,+, hencee(2p^l_j) in F^l_j^,+ commutes with everything in F_i^k,+, while e(2p^k_i)from F_i^k,+ q-commutes withe(· · · +ai ju^k_i + · · ·)fromF^l_j^,+giving the factorq_i^{−ai j}. Thus one can derive the commutation relation directly between the variablesX_f^k andX_fl i

j. First, by construction we have X_f^k

i X_f^l

i =q_i⁻²X_f^l

i X_f^k

i (5.2)

wheneverl=k+1 and commute otherwise.

Corollary 5.3 Assume i = j,k,l≥0andv(i,k) < v(j,l), we have:

X_fk

i X_fl

j =q_i^CX_fl

jX_fk

i , (5.3)

where

C=

⎧⎪

⎪⎪

⎨

⎪⎪

⎪⎩

2ai j v(i,k) < v(j,l) < v(i,k+1) < v(j,l+1), 0 v(i,k) < v(j,l) < v(j,l+1) < v(i,k+1), 0 v(i,k) < v(i,k+1) < v(j,l) < v(j,l+1), ai j k=ni and l=nj,

where in the inequalities we let the boundaries bev(i,0):= −∞andv(i,ni):= +∞. Next, we observe that by construction, the cluster variables X_fk

i and X_fl

j with

k,l ≤ 0 have exactly the commutation relations opposite to (5.3), while ifk,l = 0 have different signs they commute with each other.

Finally, the cluster variablesX_e0

i is given bye(−2u^k_j)where[u^k_j]is the initial term of the positive representation ofei which is determined explicitly by Theorem4.10.

Then we have

X_e0

iX_fk

j =q_i²X_fk

j X_e0 i, X_e0

i X_fk−1

j =q_i⁻²X_fk−1

j X_e0

i, k=1.

Combining the above relations amongXk, this completes the description of theDg -quiver. From Figs.4,5,6,7,8,9,10,11,12,13, we can also conclude:

Corollary 5.4 For specific choices ofw0, theDg-quiver is plannar whengis of type An,Bn,Cn,F4and G2, i.e. when the Dynkin diagram has no branches.

5.2 Elementary quiver associated to simple reflections

With the above observations, a more conceptual way of constructing theDg quiver motivated by [34] is as follows. We define the following quiver:

Definition 5.5 Theelementary quiver Q^k_i associated to thek-th simple reflectionsi

with root indexiof the reduced expression ofw0, i.e. to the variableu^k_i, is constructed by the frozen nodes

f_i^k−1 di f_i^k and for every j ∈ Iwithai j <0 we have in addition

f_i^k−1 f_i^k

f^l_j

for the uniquelwithv(j,l) < v(i,k) < v(j,l+1), and the nodes f_i^a have weight di.

Recall from Definition3.2that the weight of the dashed arrows arew= ¹₂. For example, in typeA3, an elementary quiver associated tos2is drawn as in Fig.2, where we indicate the corresponding simple reflection.

Proposition 5.6 The subquiver of theDg-quiver generated by f_iⁿ with n≥0 corre-sponding to the reduced wordi=(i1, . . . ,iN)∈Ris given by amalgamation of the

q

Fig. 2 Elementary quiver in type A3

f₂^a f₂^a+1

f₁^b

f₃^c s2

elementary quivers Q^k_i in the same order asialong vertices with the same indices, ignoring the arrows between f_i⁰’s.

The subquiver of theDg-quiver generated by f_iⁿ for all n corresponding to the reduced word(i1, . . . ,iN)∈Ris obtained by amalgamation of the elementary quivers Q^±_i ^kin the same order asiialong vertices with the same indices, where Q⁻_i^k are the elementary quivers corresponding to the opposite wordi=(iN, . . . ,i1)of the opposite nodes f_i^−k.

Proof This follows directly from Corollary5.3.

TheDgquiver is then obtained by the above amalgamation together with additional arrows connecting the nodese⁰_i.

6 Explicit embedding ofUq(g)

In this section, we apply the construction of the previous section for each type ofg and display graphically the embeddingUq(g) → Dgexplicitly as certain paths on a quiver diagram. In particular one can write down immediately a Heisenberg-type representation ofUq(g)just by looking at the quiver diagram.

In the previous section, we constructed the Dg-quivers as amalgamation of the elementary quivers Q^k_i together with the arrows joining the nodes e⁰_i. In particular, they can be presented in a way that is symmetric along a vertical axis, where the arrows are flipped over. It turns out that this can be expressed as an amalgamation of a pair ofbasic quiversassociated tog, and that these basic quivers are mutation equivalent to the cluster structure of framedG-local system associated to the disk with 3 marked points, recently discovered by [34,35]. We will determine and describe the basic quivers in Sect.8.

By Theorem4.15, the action of Ki (resp.K_i) are obtained by multiplyingX_f−ni

(resp.X_fni i

i )to the last term ofei(resp.fi), hence we will omit it from the description below for simplicity.

Definition 6.1 (Eiand Fi-path) Sincefi =X(f_i⁻ⁿⁱ, . . . , f_iⁿⁱ⁻¹), we will call the path of the quiver given by the nodes

f_i⁻ⁿⁱ −→ f_i⁻ⁿⁱ⁺¹−→ · · · −→ f_iⁿⁱ⁻¹−→ f_iⁿⁱ anFi-path.

On the other hand, ifei =X(m1,m2, . . . ,mk)(or similar variants in typeCn,E8, F4andG2), we call the path of the quiver given by the nodes

m1−→m2, . . .−→mk −→ f_i⁻ⁿⁱ

anEi-path. From the relations[ei,fi] =(qi−q_i⁻¹)(K_i−Ki), one can derive the fact that the path always begin withm1= f_iⁿⁱ.

Remark 6.2 In [41], theEi-paths andFi-paths are also known as theVi-paths andi -paths respectively in typeAn, which describe the corresponding shapes of the paths, see Fig.4.

6.1 Toy example: typeA2

In this section, we illustrate the method of recovering theDg-quiver. Let g = sl₃, and recall the notation from Definition 4.5. For simplicity we label the variables (u¹₁,u¹₂,u²₁)below by(u, v, w).

Proposition 6.3 [21]The positive representationP_λofUqq(sl(3,R))with parameters λ = (λ1, λ2) ∈ R²_≥0, corresponding to the reduced word i = (1,2,1)acting on

f(u, v, w)∈ L²(R³), is given by e1= [w]e(−2p_w),

e2= [u]e(−2pu−2p_v+2p_w)+ [v−w]e(−2p_v), f1= [−u−2λ1]e(2pu)+ [−2u+v−w−2λ1]e(2p_w), f2= [u−v−2λ2]e(2p_v),

K1=e(−2u+v−2w−2λ1), K2=e(u−2v+w−2λ2).

In the expanded form, we have

e1=e(w−2p_w)+e(−w−2p_w),

e2=e(u−2pu−2p_v+2p_w)+e(v−w−2p_v) +e(−v+w−2p_v)+e(−u−2pu−2p_v+2p_w), f1=e(−2u+v−w−2λ2+2p_w)+e(−u−2λ2+2pu)

+e(u+2λ2+2pu)+e(2u−v+w+2λ2+2p_w), f2=e(u−v−2λ1+2p_v)+e(−u+v+2λ1+2p_v).

We recover the following cluster variables following Definition 4.13 by taking successive ratios of the fi generators, which by definition are positive self-adjoint monomials:

q

X_f−2

1 =e(−2u+v−w−2λ2+2p_w), X_f−1

1 =e(u−v+w+2pu−2p_w), X_f⁰

1 =e(2u+4λ2), X_f¹

1 =e(u−v+w−2pu+2p_w), X_f²

1 =e(w−2p_w), X_f−1

2 =e(u−v−2λ1+2p_v), X_f⁰

2 =e(−2u+2v+4λ1), X_f¹

2 =e(v−w−2p_v).

Taking the ratio of the initial terms of theei generators (i.e. the first and last terms in

Taking the ratio of the initial terms of theei generators (i.e. the first and last terms in

Dans le document R -matrix Clusterrealizationof U ( g ) andfactorizationsoftheuniversal (Page 11-0)