6.6. PROBLEMS 119
6 Let g be a complex semisimple Lie algebra fix a CSA and an ordering of the roots. Show that if λ is a dominant algebraically integral weight and ̟ is a fundamental weight, then degπλ+̟ > degπλ (where πµ de-notes the irreducible representation with highest weightµ). Deduce that the non-trivial representations ofgof smallest dimension are fundamental representations.
7 LetGbe a compact connected semisimple Lie group.
a. Show thatGadmits representations of arbitrarily high dimension.
b. Show that G admits (if at all) at most finitely many nonequivalent representations in any given dimension.
8 LetGbe a compact connected semisimple Lie group, fix a maximal torus and an ordering of the roots. Assume thatδ = 12P
α∈∆+α is analytically integral and denote byπδthe irreducible representation ofGwith highest weightδ. Prove thatdegπδ = 2|∆+|.
9 Check thatδ = 12P
α∈∆+α is analytically integral forSU(n)andSp(n) andSO(2n), but not forSO(2n+ 1).
10 Use the Weyl dimension formula to show that the multiplicity of the weights of the representationπ1,1 of Sp(2), lying in the inner polygon of the diagram of weights, as given in Problem 2 of Chapter 5, is2.
11 Let G be a simply-conncted compact connected simple Lie group of rank two and denote by̟1,̟2 the fundamental weights. Letπa,bdenote the irreducible representation ofGof highest weighta̟1+b̟2 (a,b∈Z).
Prove thatdegπa,bis as indicated, in the following cases:
a. SU(3): 12(a+ 1)(b+ 1)(a+b+ 2);
b. Sp(2): 16(a+ 1)(b+ 1)(a+b+ 2)(2a+b+ 3);
c. G2: 1201 (a+ 1)(b+ 1)(a+b+ 2)(2a+b+ 3)(a+ 3b+ 4)(2a+ 3b+ 5).
12 ConsiderG=SU(3)andV =Sq(C3). Identify the highest weight ofV and use the formula from Problem 11 to prove thatV is irreducible for all q≥1.
13 Consider the contraction map
c:S2(C3)⊗C3∗ →C3 given byuv⊗w∗ 7→w∗(u)v+w∗(v)u.
a. Show that this map is well defined and SU(3)-equivariant. Deduce thatkercisG-invariant.
b. Check thate21⊗e∗3 ∈kerc, where{ei}is the canonical basis ofC3and {e∗i} is the dual basis. Deduce thatπ2θ1−θ3 is a subrepresentation of kerc(in the notation of Problem 11,π2θ1−θ3 =π2,1).
6.6. PROBLEMS 121 c. Use Problem 11 to prove that
S2(C3)⊗C3∗ =π2,1⊕C3.
C H A P T E R 7
The Peter-Weyl Theorem
The Peter-Weyl Theorem is a basic result in Harmonic Analysis that essen-tially describes the regular representation of a compact topological groupG by giving an orthonormal basis of the space of square integrable functions on G, in terms of the matrix coefficients of all irreducible representations ofG. It was proved by Hermann Weyl and his student Fritz Peter in 1927, building on previous work of Frobenius and Schur for finite groups.
In this chapter, all compact topological groups are assumed Hausdorff.
7.1 Statement and proof of the theorem
Let G be a compact topological group. Fix a bi-invariant Haar measure onGof total volume1. We consider the convolution algebra(C(G),∗)of complex valued continuous functions onGwhere
f1∗f2(x) = Z
G
f1(xg−1)f2(g)dg= Z
G
f1(g)f2(g−1x)dg.
We also consider the Banach spacesL1(G),L2(G)andL∞(G), note the in-equalities
||f||1 ≤ ||f||2≤ ||f||∞,
(use Cauchy-Schwarz for the first one) and recall that L2(G) is a Hilbert space whose associated inner product with be denoted by(,). LetGˆbe the set of equivalence classes of irreducible finite-dimensional complex repre-sentations ofG. Forπ∈Gˆandu,vvectors in the underlying representation spaceVπ, we recall thematrix coefficient
fu,v(x) =hu, π(x)vi (x∈G).
Note thatfu,vis really a matrix coefficient ofπ∗. We havefu,v ∈L2(G).
7.1.1 Proposition (Schur orthogonality relations) We have:
123
a. Ifπ∈Gˆandu,v,u′,v′∈Vπ, then (fu,v, fu′,v′) = 1
dimVπhu, u′ihv, v′i. b. Ifπ,π′∈Gˆare inequivalent, andu,v∈Vπ,u′,v′ ∈Vπ′, then
(fu,v, fu′,v′) = 0.
Thecharacterofπ∈Gˆ is the smooth function
χπ :G→C, χπ(x) = traceπ(x).
We recall that equivalent representations have the same character and that χπ(gxg−1) =χπ(x). We also haveχπ⊕π′ =χπ +χπ′ andχπ⊗π′ =χπχπ′. A consequence of the Schur orthogonality relations is:
7.1.2 Corollary Forπ,π′ ∈ Gˆ, (χπ, χπ′) is1if π andπ′ are equivalent and0 otherwise. It follows that two representations of Gare equivalent if and only if they have the same character.
Let {πk} be a maximal set of mutually nonequivalent irreducible uni-tary finite dimensional representations ofGacting on spacesVk of dimen-sion dk. For each indexk, we choose an orthonormal basis {ek1, . . . , ekd
k} of Vk and consider the matrix coefficients πijk(x) = fek
i,ekj(x). The Schur orthogonality relations say that
{d1/2k πkij}i,j,k
is an orthonormal set inL2(G). SinceC(G)is dense inL2(G), the following theorem implies that this is an orthonormal basis ofL2(G); in particular,Gˆ is at most countable (note that the theorem says that thesetis dense, not its span).
7.1.3 Theorem (Peter-Weyl, first version) LetGbe a compact topological group.
Then the set of matrix coefficients of representations inGˆis dense inC(G)in the L∞-norm.
Ifϕ∈C(G)andf ∈L1(G), setTϕf =ϕ∗f. Plainly, we have:
7.1.4 Lemma Ifϕ ∈ C(G)andf ∈ L1(G), then||Tϕf||∞ ≤ ||ϕ||∞||f||1. In particular,Tϕis a bounded operator onL1(G).
7.1.5 Lemma Ifϕ∈C(G)andf ∈L2(G), thenTϕis a compact bounded oper-ator onL2(G)with norm bounded by||ϕ||∞. If, in addition,ϕ(x−1) =ϕ(x)for x∈G, thenTϕis self-adjoint.
7.1. STATEMENT AND PROOF OF THE THEOREM 125 Proof. We have
||Tϕf||2 ≤ ||Tϕf||∞≤ ||ϕ||∞||f||1 ≤ ||ϕ||∞||f||2.
To prove compactness of Tϕ, we need to show that it takes bounded se-quences to sese-quences with convergent subsese-quences. Let (fn) be a se-quence bounded inL2(G). Then it is bounded inL1(G), say||fn|| ≤ M for someM > 0 and all n, and it follows from Lemma 7.1.4 that (Tϕfn) is (equi)bounded inL∞(G). Since convergence inL2-norm is implied by convergence inL∞-norm, the result will follow from the Arzelà-Ascoli the-orem if we can show that this sequence is also equicontinuous. Compact-ness of G implies that ϕ is uniformly continuous; this means that given ǫ >0there exists a neighborhoodU of1such that|ϕ(gx)−ϕ(x)|< ǫ/M for g∈U. Now
|Tϕfn(gx)−Tϕfn(x)| = Z
G
ϕ(gxh−1)−ϕ(xh−1)
fn(h)dh
≤ Z
G
ϕ(gxh−1)−ϕ(xh−1)|fn(h)|dh
< ǫ M||fn||1
≤ǫ,
proving equicontinuity of(Tϕfn). The last assertion is easy.
Introduce the right-regular representation R of G on L2(G) by the rule (Rgf)(x) =f(xg). Similarly, theleft-regular representationLofGonL2(G) is given by(Lgf)(x) =f(g−1x). Both actions ofGonL2(G)are continuous, in the sense thatG×L2(G) → L2(G)is continuous,1and preserve theL2 -norm.
7.1.6 Lemma Ifϕ∈C(G)andλ∈C, theλ-eigenspaceVλ ={f ∈L2(G)|Tϕf = λf}is invariant under the right-regular representation ofG.
Proof. In factTϕ commutes with the right-regular representation, as is
easily checked.
Proof of Theorem 7.1.3. Letf ∈ C(G). We shall prove that for anyǫ > 0 there exists a matrix coefficientf′of an irreducible representation such that
||f −f′||∞ < ǫ. Compactness ofGimplies thatf is uniformly continuous, so||Rgf −f||∞ < ǫ/2if g ∈ U for some neighborhoodU of 1. Choose a continuousϕ: G→ [0,+∞)with support contained inU, total integral 1 and satisfyingϕ(g−1) =ϕ(g)for allg. One easily checks that
(7.1.7) ||Tϕf−f||∞< ǫ 2.
1Need a check!
Owing to Lemma 7.1.5,Tϕis a compact self-adjoint operator onL2(G). By the spectral theorem, there is Hilbert direct sum decomposition ofL2(G) =
⊕λVλ, whereVλis theλ-eigenspace ofTϕ, anddimVλ <∞ifλ6= 0. Letfλ be the component off inVλ, and chooseq >0such thatP
0<|λ|<q|fλ|2 <
ǫ2/(4||ϕ||2∞). Then
(7.1.8) || X
0<|λ|<q
fλ||1 < ǫ/(2||ϕ||∞).
Finally, putf′′ = P
|λ|≥qfλ andf′ = Tϕ(f′′). By using (7.1.7), (7.1.8) and Lemma 7.1.4, we can write
||f−f′||∞ ≤ ||f−Tϕf||∞+||Tϕ(f −f′′)||∞
< ǫ
2 +||ϕ||∞
ǫ 2||ϕ||∞
= ǫ.
There remains only to identifyf′as a matrix coefficient of some irreducible finite-dimensional representation ofG. Note thatf′,f′′lie inP
|λ|≥qVλ, and the latter is a finite-dimensional (since the eigenvalues of a compact self-adjoint operator on a Hilbert space can only accumulate at0),R-invariant subspace ofL2(G), so the result follows from the following lemma.
7.1.9 Lemma Iff ∈L2(G)lies in a finite-dimensional,R-invariant subspace of L2(G), thenf is a matrix coefficient of some irreducible finite-dimensional repre-sentation ofG.
Proof. Let V ⊂ L2(G) be a R-irreducible subspace containing f. The linear functionalu∈V 7→ u(1)∈Ccan be represented asu(1) =hu, vifor somev∈V. In particular,f(x) =Rxf(1) =hRxf, vi=hv, Rxfiis a matrix
coefficient.
7.1.10 Theorem (Peter-Weyl, second version) LetGbe a compact topological group. Then there is aG×G-equivariant isometric identification (Hilbert space direct sum)
L2(G) =M
π∈Gˆ
Vπ⊗Vπ∗, induced byu⊗v∗ 7→√
dimVπfu,vforu,v∈Vπ, whereG×Gacts onL2(G)by the left- and right-regular representation.
Proof. Note that
fπ(g)u,v(x) =hπ(g)u, π(x)vi=hu, π(g−1x)vi= (Lgfu,v)(x)
for allπ ∈ G,ˆ u, v ∈ Vπ andg, x ∈ G. Similarly, fu,π(g)v = Rgfu,v. This proves the equivariance of the map. Its isometric property is derived from
the Schur orthogonality relations.
7.2. HIGHEST WEIGHT CLASSIFICATION 127