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Demazure Formulas for Weight Polytopes

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Demazure Formulas for Weight Polytopes Demazure Formulas for Weight Polytopes Mark A. Walton Department of Physics & Astronomy, University of Lethbridge Lethbridge, Alberta, Canada T1K 3M4 [email protected] Abstract. The characters of simple Lie algebras are naturally decom- posed into lattice polytope sums. The Brion formula for those polytope sums is remarkably similar to the Weyl character formula. Here we start to investigate if other character formulas have analogs for lattice poly- tope sums, by focusing on the Demazure character formulas. Using De- mazure operators, we write expressions for the lattice sums of the weight polytopes of rank-2 simple Lie algebras, and the rank-3 algebra A3. Keywords: lattice polytopes, simple Lie algebras, characters, Demazure character formula 1 Introduction The Brion formula [3, 4] for lattice polytope sums is remarkably similar to the Weyl formula for characters of simple Lie algebras [6, 11, 15]. As a consequence, the expansion of Weyl characters in terms of lattice polytope sums is natural and useful [6,11{13,15,16]. This polytope expansion of Lie characters is highly remi- niscent of the early work of Antoine and Speiser [2] and the recursive formulas found by Kass [8]. Here we explore further the relation between Lie characters and lattice poly- tope sums. Other formulas exist for the characters - might corresponding formu- las describe lattice polytope sums?1 We focus on the Demazure character formulas [1,5,7], and use the Demazure operators involved to write expressions for the lattice polytope sums. So far, we have obtained results for all rank-2 simple Lie algebras, and for the rank-3 algebra A3. We hope these first formulas will help lead to general Demazure formulas for the lattice polytope sums relevant to Lie characters, and perhaps others. arXiv:2104.02701v1 [math-ph] 6 Apr 2021 How might such a formula be useful? Starting with the Demazure charac- ter formulas, Littelmann was able to derive a generalization for all simple Lie algebras of the famous Littlewood-Richardson rule for Ar tensor-product decom- positions [10]. In a similar way, formulas of the Demazure type might lead to efficient, general computational methods for lattice polytopes. Physical applications should also be possible. An attempt to apply the De- mazure character formula to Wess-Zumino-Witten conformal field theories was 1 This question was already asked in [15]. 2 M.A. Walton made in [14], and it has already been used in the study of solvable lattice mod- els [9]. In the following section, we review the initial motivation for the present work, the similarity between the Weyl character formula and the Brion lattice-polytope sum formula, and the polytope expansion that exploits it. Section 3 is a quick review of the Demazure character formulas. Our new results are presented in Section 4: expressions involving Demazure operators for lattice-polytope sums for rank-2 simple Lie algebras, and A3. The final section is a short conclusion. 2 Polytope expansion of Lie characters Let Xr denote a simple Lie algebra of rank r, so that X = A; B; C; D; E; F; or G. The sets of fundamental weights and simple roots will be written as F := i f Λ j i 2 f1; 2; : : : ; rg g and S := f αi j i 2 f1; 2; : : : ; rg g, respectively. The weight and root lattices are P := Z F and Q := Z S, respectively. The set of integrable weights is P+ := N0 F , and we write R (R+) for the set of (positive) roots of Xr. 2.1 Weyl character formula Consider an irreducible representation L(λ) of Xr of highest weight λ 2 P+. The formal character of L(λ) is defined to be X µ chλ = multλ(µ) e : (1) µ2P (λ) Here P (λ) is the set of weights of the representation L(λ), and multλ(µ) is the multiplicity of weight µ in L(λ). The formal exponentials of weights obey eµ eν = eµ+ν . If we write eµ(σ) =: ehµ,σi ; (2) where hµ, σi is the inner product of weights µ and σ, the formal exponential eµ simply stands for ehµ,σi before a choice of weight σ is made. A choice of σ fixes a conjugacy class of elements in the Lie group exp(Xr). The formal character then becomes a true character chλ(σ), the trace, in the representation of highest weight λ, of elements of exp(Xr) in the conjugacy class labelled by σ. The celebrated Weyl character formula is P (det w) ew.λ ch = w2W : (3) λ Q (1 − e−α) α2R+ Here W is the Weyl group of the simple Lie algebra Xr, and w.λ = w(λ + ρ) − ρ denotes the shifted action of Weyl-group element w 2 W on the weight λ, with Pr i ρ = i=1 Λ . Demazure Formulas for Weight Polytopes 3 The Weyl invariance of the character can be made manifest: X wλ Y −wα −1 chλ = e (1 − e ) : (4) w2W α2R+ Here now β 2β β −1 1 + e + e + : : : ; β 2 R−; (1 − e ) = −β −2β (5) −e − e − : : : ; β 2 R+: The rule-of-thumb is: expand in powers of eβ, with β a negative root. 2.2 Brion lattice-polytope sum formula A polytope is the convex hull of finitely many points in Rd. A polytope's vertices are such a set of points with minimum cardinality. A lattice polytope has all its vertices in an integral lattice in Rd. The (formal) lattice polytope sum is the sum of terms e φ over the lattice points φ in the polytope. Brion [3, 4] found a general formula for these lattice-polytope sums. Let the weight polytope Ptλ be the polytope with vertices given by the Weyl orbit W λ. Consider the lattice-polytope sum X µ Bλ := e ; (6) µ2(λ+Q)\Ptλ where the relevant lattice is the λ-shifted root lattice λ + Q of the algebra Xr. Applied to a weight polytope, the Brion formula yields X wλ Y −wα −1 Bλ = e (1 − e ) : (7) w2W α2S Here S denotes the set of simple roots of Xr. 2.3 Polytope expansion The Brion formula (7) is remarkably similar to the Weyl character formula, as written in (4) [6,15]. It is therefore natural, and fruitful, to consider the polytope expansion of Lie characters [6, 15, 16]: X chλ = polytλ(µ)Bµ : (8) µ≤λ The polytope multiplicities polytλ(µ) are defined in analogy with weight multi- plicities multλ(µ). We do not consider the polytope epansion further in this note. Instead we focus on the striking relation described above between characters and polytope sums. 4 M.A. Walton 3 Demazure character formulas Do other character formulas point to the existence of new formulas for the lattice sums of weight polytopes? More general polytopes? In particular, do the Demazure formulas for Lie characters indicate the exis- tence of Demazure-type formulas for the lattice sums of weight polytopes? Let us first sketch the Demazure character formulas. The Weyl group W is generated by the primitive reflections ri in weight space across the hyperplanes normal to the corresponding simple roots αi: _ ri λ = λ − (λ · αi ) αi ; (9) _ here αi = 2αi=(αi · αi) is the simple co-root. Define Demazure operators for every simple root αi 2 S: −αi 1 − e ri Dα =: Di = ; (10) i 1 − e−αi λ riλ where ri(e ) = e : Then e λ + e λ−αi + e λ−2αi + ::: + e riλ; λ · α_ ≥ 0 ; D e λ = i (11) i λ+αi λ+2αi ri(λ+αi) _ −e − e − ::: − e ; λ · αi < 0 : For every Weyl-group element w 2 W a Demazure operator Dw can be defined. First, identify Dri := Di, and then use any reduced decomposition of w, replacing the factors rj in the reduced decomposition with Dj. The resulting operator Dw must be independent of which reduced decomposition is used. As a result, the Demazure operators obey relations encoded in the Coxeter-Dynkin diagrams of Xr. For example, consider the longest element wL of the Weyl group of A2: wL = r1r2r1 = r2r1r2. The associated Demazure operator DwL can be written two ways, so that D1D2D1 = D2D1D2: If wL is the longest element of the Weyl group W , then the Demazure char- acter formula is λ chλ = DwL e : (12) Also, define Di =: 1 + di, and then dw for all w 2 W by reduced decom- positions. Then X λ chλ = dw e : (13) w2W Demazure operators can also be defined for every positive root β 2 R+: 1 − e−β r(β) D(β) := ; (14) 1 − e−β where r(β)λ := λ − (λ · β_)β and r(β)(eλ) = er(β)λ : (15) Demazure Formulas for Weight Polytopes 5 Operators d(β) = D(β) − 1, e−β [1 − r(β)] d(β) := ; (16) 1 − e−β are also defined for all positive roots β 2 R+. 4 Lattice-polytope formulas of Demazure type In the hopes of helping lead to a more general result, we take a direct approach here, and write formulas for low-rank weight-polytope sums that involve the Demazure operators. We report only preliminary new results, formulas for all rank-2 cases, and for one of rank 3, related to the Lie algebra A3. But before treating ranks 2 and 3, let us first dispense with the unique rank- 1 algebra, A1. In this case, the character and weight-polytope lattice sum are identical, λ λ−α1 λ−2α1 −λ chλ = Bλ = e + e + e + ::: + e : (17) The Demazure character formulas therefore apply to Bλ.
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