The Cartan Decomposition of a Complex Semisimple Lie Algebra

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1 The Cartan Decomposition of a Complex Semisimple Lie Algebra Shawn Baland University of Colorado, Boulder November 29, 2007

2 Definition Let k be a field. A k-algebra is a k-vector space A equipped with a bilinear map A A A called multiplication. Definition A Lie algebra is a k-algebra g with multiplication (x, y) [x, y] satisfying the following conditions: (i) [x, x] = 0 for all x g. (ii) [[x, y], z] + [[y, z], x] + [[z, x], y] = 0 for all x, y, z g. Condition (ii) is called the Jacobi identity.

3 Definition We define the lower central series of g recursively by g 1 = g, g n+1 = [g n, g] for n 1. Proposition Each g n is an ideal of g, and g = g 1 g 2 g 3. Definition We say that g is nilpotent if g n = 0 for some n. We say that g is abelian if g 2 = 0.

4 Definition We define the derived series of g recursively by g (0) = g, g (n+1) = [ g (n), g (n)] for n 0. Proposition Each g (n) is an ideal of g, and g = g (0) g (1) g (2). Definition A Lie algebra g is solvable if g (n) = 0 for some n. Proposition Every nilpotent Lie algebra is solvable.

5 Proposition Every finite-dimensional Lie algebra contains a unique maximal solvable ideal r. (This is called the solvable radical.) Definition A Lie algebra g is semisimple if r = 0. Definition A Lie algebra g is simple if it contains no ideals other than itself and the zero ideal. The 1-dimensional Lie algebra is called the trivial simple Lie algebra. Proposition Every non-trivial simple Lie algebra is semisimple.

6 Let M m (k) denote the ring of all n n matrices over k. We define gl n (k) to be the Lie algebra [M n (k)] formed from M n (k) via the commutator product. We denote this Lie algebra by gl n (k). Definition A representation of a Lie algebra g is a homomorphism of Lie algebras ρ : g gl n (k). Definition A g-module is a k-vector space V equipped with a left g-action g V V satisfying the following properties: (x, v) xv is linear in x and v. [x, y]v = x(yv) y(xv) for all x, y g, v V.

7 Example The vector space g forms a g-module via the action (x, y) [x, y]. We have [[x, y], z] = [x, [y, z]] [y, [x, z]] by the Jacobi identity. We call this module the adjoint module and denote the action of x on the vector y by ad x y. We have ad [x, y] = ad x ad y ad y ad x. From now on, we assume that g is a finite-dimensional Lie algebra over the field C of complex numbers.

8 Theorem (Lie s theorem) Let g be a solvable Lie algebra and V a finite-dimensional irreducible g-module. Then dim V = 1. Corollary Let g be a solvable Lie algebra and V a finite-dimensional g-module. Then a basis can be chosen for V with respect to which we obtain a matrix representation ρ of g of the form ρ(x) = for all x g. 0

9 Let V be a finite-dimensional vector space and let T : V V be a linear transformation with eigenvalues λ 1,..., λ r. The generalized eigenspace of V with respect to λ i is the set of all v V annihilated by some power of T λ i 1. We have V = V 1 V r. Each V i is invariant under the action of T. Theorem Let g be a nilpotent Lie algebra and V a finite-dimensional g-module. Then for any y g, the generalized eigenspaces of V associated with ρ(y) are all submodules of V.

10 Corollary Let g be a nilpotent Lie algebra and V a finite-dimensional indecomposable g-module. Then a basis can be chosen for V with respect to which we obtain a representation ρ of g of the form λ(x) ρ(x) = for all x g. 0 λ(x) Notice that λ : x λ(x) is a 1-dimensional representation of g.

11 Let g be a Lie algebra and V a finite-dimensional g-module. For any 1-dimensional representation λ of g, we define the set V λ = {v V ( x g)( N(x) 1) (ρ(x) λ(x)1) N(x) v = 0}. Theorem V = λ V λ, and each V λ is a submodule of V. Definition If V λ 0, then we call λ a weight of g and V λ the weight space of λ. We call V = λ V λ the weight space decomposition of V.

12 Since each V λ is the direct sum of the indecomposable components giving rise to λ, it follows that a basis can be chosen for V λ with respect to which a representation ρ of g on V λ has the form λ(x) ρ(x) = 0 λ(x) for all x g.

13 Theorem (Engel s theorem) A Lie algebra g is nilpotent if and only if ad x : g g is nilpotent for all x g. Corollary A Lie algebra g is nilpotent if and only if g has a basis with respect to which the adjoint representation of g has the form 0 ρ(x) = for all x g. 0 0

14 Notice that if h is a subalgebra of a Lie algebra g, then h induces an h-module structure on g via the adjoint action. If g is semisimple and h is chosen carefully, then h induces weight space decomposition of g that tells us a lot about a Lie algebra s structure. Definition Let h be a subalgebra of a Lie algebra g. We define the normalizer of h to be the set N(h) = {x g [h, x] h for all h h}.

15 Proposition N(h) is a subalgebra of g, h is an ideal of N(h), and N(h) is the largest subalgebra of g containing h as an ideal. Proof. If x, y N(h), h h, then [h, [x, y]] = [[y, h], x] + [[h, x], y] h by the Jacobi identity, and so N(h) is a subalgebra of g. h is clearly an ideal of N(h). If h is an ideal of m, then [h, m] h so that m N(h).

16 Definition A subalgebra h of g is a Cartan subalgebra of g if h is nilpotent and N(h) = h. Definition Let x g. The null component g 0,x of g with respect to x is the generalized eigenspace of ad x : g g, that is, g 0,x = {y g (ad x) n y = 0 for some n 1}. Definition An element x g is regular if dim g 0,x is as small as possible. Any Lie algebra will certainly contain regular elements.

17 Theorem If x is a regular element of g, then g 0,x is a Cartan subalgebra of g. Definition A derivation of a Lie algebra g is a linear map D : g g satisfying D[x, y] = [Dx, y] + [x, Dy] for all x, y g. Proposition ad x is a derivation for all x g. Proof. adx [y, z] = [x, [y, z]] = [[x, y], z]+[y, [x, z]] = [adx y, z]+[y, adx z].

18 The automorphisms of g form a group Aut(g) under composition of maps. Proposition If D is a nilpotent derivation of g, then exp(d) is an automorphism of g. Definition An inner automorphism of g is an automorphism of the form exp(ad x) for x g with ad x nilpotent. The inner automorphism group is the subgroup Inn(g) of Aut(g) generated by all inner automorphisms. Proposition Inn(g) is a normal subgroup of Aut(g).

19 Definition Two subalgebras h, k are conjugate in g if there exists a φ Inn(g) such that φ(h) = k. Theorem Any two Cartan subalgebras of g are conjugate. Let g be a Lie algebra and h a Cartan subalgebra of g. Since h is nilpotent, the adjoint action of h on g induces a weight space decomposition g = λ g λ where g λ = {x g ( h h)( n 1) (ad h λ(h)1) n x = 0}. Proposition h = g 0.

20 Proof. Since h is nilpotent, we can choose a basis of g with respect to which ad x is represented by a strict upper-triangular matrix for all x h. This follows from the corollary to Engel s theorem. Each such matrix has eigenvalue zero, and so h g 0. Now suppose h g 0 and let m/h be an irreducible h-submodule of g 0 /h. By Lie s theorem, we have dim m/h = 1. The 1-dimensional representation induced by m/h must be the zero map since h is nilpotent. Hence [h, m] h, and so m N(h). This contradicts the fact that h = N(h).

21 Thus we obtain a decomposition of g of the form g = h g λ1 g λr λ 1,..., λ r 0. Definition A 1-dimensional representation λ of h is called a root of g with respect to h if λ 0 and g λ 0. We denote the set of roots of g with respect to h by Φ. Thus g = h g α. α Φ We call this decomposition the Cartan decomposition of g with respect to h. Each g α is called the root space of α.

22 Proposition If λ and µ are 1-dimensional representations of h, then [g λ, g µ ] g λ+µ. Proof. Let y g λ and z g µ. If x h, then (adx λ(x)1 µ(x)1) n [y, z] = n i=0 ( ) n [(ad x λ(x)1) i y, (ad x µ(x)1) n i z ]. i Hence (ad x λ(x)1 µ(x)1) n [y, z] = 0 if n is sufficiently large. Corollary If α, β Φ are roots of g with respect to h, then [g α, g β ] g α+β if α + β Φ [g α, g β ] h if β = α [g α, g β ] = 0 if α + β 0 and α + β / Φ.

23 Proposition Let α Φ. Given any β Φ, there exists a number r Q, depending on α and β, such that β = rα on the subspace [g α, g α ] of h. Proof. If α is not a weight of g with respect to h, then g α = 0, and the proof is trivial. So assume α is a weight. Then since α 0, we must have α Φ. For i Z, we consider the function iα + β : h C. Since Φ is finite, there exist integers p and q with p 0 and q 0 such that pα + β,..., β,..., qα + β are all in Φ but (p + 1)α + β and (q + 1)α + β are not in Φ.

24 If either (p + 1)α + β = 0 or (q + 1)α + β = 0, then the result is obvious. So assume (p + 1)α + β 0 and (q + 1)α + β 0. Let m be the subspace of g given by m = g pα+β g qα+β. Let x = [y, z] with y g α and z g α. We have ad y g qα+β g (q+1)α+β. Because (q + 1)α + β 0 and (q + 1)α + β / Φ, we must have g (q+1)α+β = 0. Thus ad y m m. By a similar argument, we have ad z m m, and so ad x m = (ad y ad z ad z ad y)m m.

25 We calculate the trace tr m (ad x). Since x h, each weight space g iα+β is invariant under ad x. Thus q tr m (ad x) = tr giα+β (ad x). i= p Now ad x acts on g iα+β via a matrix of the form (iα + β)(x). 0 (iα + β)(x) Thus tr giα+β (ad x) = dim g iα+β (iα + β)(x).

26 It follows that q tr m (ad x) = dim g iα+β (iα + β)(x) i= p = q i dim g iα+β α(x) + i= p i= p q dim g iα+β β(x). But we also have tr m (adx) = tr m (ady adz adz ady) = tr m (ady adz) tr m (adz ady) = 0. Hence q q i dim g iα+β α(x) + dim g iα+β β(x) = 0. i= p i= p We know that dim g iα+β > 0 for all p i q. Thus ) β(x) = ( q ( q i= p i dim g iα+β ) α(x). i= p dim g iα+β

27 Definition We define the Killing form of g to be the bilinear form g g C given by x, y = tr(ad x ad y). Proposition (i) The Killing form is symmetric, i.e., x, y = y, x for all x, y g. (ii) The Killing form is invariant, i.e., [x, y], z = x, [y, z] for all x, y, z g. Proposition Let a be an ideal of g and let x, y a. Then x, y a = x, y g. Hence the killing form of g restricted to a is the Killing form of a.

28 Proof. We choose a basis of a and extend it to a basis of g. With respect to this basis, ad x : g g is represented by a matrix of the form since x a. A 1 A Similarly, ad y : g g is represented by a matrix of the form B 1 B

29 Thus ad x ad y : g g is represented by the matrix A 1B 1 A 1 B Hence tr a (ad x ad y) = tr(a 1 B 1 ) = tr g (ad x ad y), and so x, y a = x, y g. Proposition If a is an ideal of g, then a is an ideal of g. Proof. If [x, y] [a, g] with x a and y g, then for all z a, we have [x, y], z = x, [y, z] = 0. In particular, g is an ideal of g.

30 Definition The Killing form of g is nondegenerate if g = 0. The Killing form of g is identically zero if g = g. Proposition Let g be a Lie algebra such that g 0 and g 2 = g. Let h be a Cartan subalgebra of g. Then there exists an x h such that x, x 0. Proof. Let g = g λ be the Cartan decomposition of g. Then [ g 2 = [g, g] = g λ, ] g λ = λ, g µ ]. λ λ λ,µ[g We have [g λ, g µ ] g λ+µ. Thus [g λ, g λ ] h, while [g λ, g µ ] is contained in the complement of h in g if µ λ.

31 Since g = g 2, we must have h = λ [g λ, g λ ] summed over all weights λ such that λ is also a weight. Thus h = [h, h] + α [g α, g α ] summed over all roots α such that α is also a root. Note that g is not nilpotent since g 2 = g 0. But we know that h is nilpotent, and so h g. Thus there exists at least one root β Φ. Now β is a 1-dimensional representation of h, and so β vanishes on [h, h]. But β does not vanish on h since β 0.

32 Using the above decomposition of h, we see there exists some root α Φ such that α Φ and β does not vanish on [g α, g α ]. Choose an x [g α, g α ] such that β(x) 0. Then x, x = tr(ad x ad x) = λ dim g λ (λ(x)) 2 since ad x is represented on g λ by a matrix of the form λ(x). 0 λ(x) For each λ, there exists an r λ,α Q such that λ(x) = r λ,α α(x).

33 Thus ( ) x, x = dim g λ rλ,α 2 α(x) 2. λ Now β(x) = r β,α α(x) and β(x) 0. Thus r β,α 0 and α(x) 0. It follows that x, x 0. Theorem If the Killing form of g is identically zero, then g is solvable. Proof. We proceed by induction on dim g. If dim g = 1, then g is clearly solvable. So assume dim g > 1. By the contrapositive of the last proposition, we see that g g 2. Now g 2 is an ideal of g, so the Killing form of g 2 is the restriction of the Killing form of g.

34 Hence the Killing form of g 2 is identically zero. It follows by induction that g 2 is solvable. We also have ( g/g 2) 2 = 0, and so g/g 2 is solvable. Thus g is solvable. Theorem (Cartan s criterion) A Lie algebra g is semisimple if and only if the Killing form of g is nondegenerate. Proof. We prove the contrapositive. If the Killing form of g is degenerate, then g 0. We know that g is an ideal, and thus the Killing form of g is identically zero. This implies g is solvable by the last theorem.

35 Thus g has a nonzero solvable ideal, and so g is not semisimple. Now suppose g is not semisimple. Then the solvable radical r of g is nonzero. Consider the chain of subspaces r = r (0) r (1) r (2) r (k 1) r (k) = 0. Each subspace r (i) is an ideal of g since the product of two ideals is an ideal. Let a = r (k 1). Then a is a nonzero ideal such that a 2 = 0. We choose a basis of a and extend it to a basis of g.

36 Let x a, y g. With respect to our chosen basis, ad x is represented by a matrix of the form 0 A 0 0 since a 2 = 0 and a is an ideal of g, and ad y is represented by a matrix of the form B 1 B 2. 0 B 3 Thus ad x ad y is represented by the matrix 0 AB

37 Hence x, y = tr(ad x ad y) = 0. This holds for all x a, y g, and so a g. Thus g 0, and so the Killing form of g is degenerate. From now on, we assume that g is a semisimple Lie algebra. Proposition If µ λ, then g λ and g µ are orthogonal with respect to the Killing form. Proof. Let x g λ, y g µ. For every weight space g ν, we have ad x ad y g ν g λ+µ+ν.

38 We choose a basis of g adapted to the Cartan decomposition. With respect to such a basis, ad x ad y is represented by a block matrix of the form 0 0 since λ + µ + ν ν. It follows that x, y = tr(ad x ad y) = 0, and so g λ is orthogonal to g µ.

39 Proposition If α is a root of g with respect to h, then α is also a root. Proof. Recall that α is a root if α 0 and g α 0. Suppose α is not a root. Then since α 0, we must have g α = 0. This implies that g α is orthogonal to all g λ, and thus g α g. But g is semisimple, and so g = 0 by Cartan s criterion. Thus g α = 0, contradicting the fact that α is a root.

40 Proposition The Killing form of g remains nondegenerate on restriction to h. Proof. Let x h and suppose x, y = 0 for all y h. We also have x, y = 0 for all y g α where α Φ. Thus x, y = 0 for all y g, and so x g. But g = 0 since g is semisimple, and so x = 0. Theorem The Cartan subalgebras of a semisimple Lie algebra are abelian.

41 Proof. Let h be a Cartan subalgebra of g. For all x [h, h], y h, we have x, y = tr(ad x ad y) = λ dim g λ λ(x)λ(y) since ad x ad y is represented on g λ by a matrix of the form λ(x)λ(y). 0 λ(x)λ(y) But λ is a 1-dimensional representation of h, and so λ vanishes on [h, h]. Thus λ(x) = 0.

42 It follows that x, y = 0 for all y h. Since the Killing form of g restricted to h is nondegenerate, this implies x = 0. Hence [h, h] = 0, and so h is abelian. Let h = Hom(h, C) be the dual space of h. We have dim h = dim h. We define a map h h using the Killing form of g. Given h h, we define h h by h (x) = h, x for all x h. Lemma The map h h is an isomorphism of vector spaces between h and h.

43 Notice that Φ is a finite subset of h. Because the map h h is bijective, we know that for each α Φ, there exists a unique element h α h such that h α (x) = α(x) for all x h, that is, α(x) = h α, x for all x h. Proposition The vectors h α for α Φ span h. Proof. Suppose the vectors h α are contained in a proper subspace of h. Then the annihilator of this subspace is nonzero. Thus there exists a nonzero x h such that x (h α) = 0 for all α Φ, that is, h α, x = 0. Hence α(x) = 0 for all α Φ.

44 Let y h. Then x, y = tr(ad x ad y) = λ dim g λ λ(x)λ(y) = 0 since λ(x) = 0 for all weights λ. Thus x, y = 0 for all y h. Since the Killing form of g restricted to h is nondegenerate, this implies x = 0, a contradiction. Proposition h α [g α, g α ] for all α Φ. Proof. We know that g α is an h-module. Since all irreducible h-modules are 1-dimensional, g α contains a 1-dimensional h-submodule Ce α. We have [x, e α ] = α(x)e α for all x h.

45 Let y g α. Then [e α, y] [g α, g α ] h. I claim that [e α, y] = e α, y h α. We define the element z = [e α, y] e α, y h α h. Let x h. Then x, z = x, [e α, y] e α, y x, h α = [x, e α ], y e α, y α(x) = α(x) e α, y e α, y α(x) = 0. Thus x, z = 0 for all x h. Since the Killing form of g restricted to h is nondegenerate, this implies z = 0. Hence [e α, y] = e α, y h α for all y g α.

46 Now there exists a y g α such that e α, y 0. For otherwise e α would be orthogonal to g α, and thus to the whole of g. This would imply e α g. But g = 0 since g is semisimple, and so e α = 0, a contradiction. Choosing a y g α such that e α, y 0, we have Proposition h α, h α 0 for all α Φ. h α = 1 e α, y [e α, y] [g α, g α ].

47 Proof. Suppose h α, h α = 0 for some α Φ. Let β be any element of Φ. There exists an r β,α Q such that β = r β,α α on [g α, g α ]. Now h α [g α, g α ]. Thus β (h α) = r β,α α (h α), that is, h β, h α = rβ,α h α, h α = 0. This holds for all β Φ. But the vectors h α for α Φ span h, and so x, h α = 0 for all x h. Since the Killing form of g restricted to h is nondegenerate, this implies h α = 0. Thus α = 0, contradicting the fact that α Φ.

48 Theorem dim g α = 1 for all α Φ. Proof. Choose a 1-dimensional h-submodule Ce α of g α. We can find an e α g α such that [e α, e α ] = h α. Consider the subspace m of g given by m = Ce α Ch α g α g 2α. There are only finitely-many summands of m since Φ is finite. Thus there are only finitely-many non-negative integers r such that g rα = 0.

49 Observe that ad e α m m because [e α, e α ] = 0, [e α, h α] = α (h α) e α, [e α, y] = e α, y h α for all y g α. and ad e α g rα g (r 1)α for all r 2. Similarly, ad e α m m because [e α, e α ] = h α, [e α, h α] = α (h α) e α, and ad e α g rα g (r+1)α for all r 1.

51 Thus α (h α) (1 dim g α 2 dim g 2α ) = 0. Now α (h α) = h α, h α 0, and so 1 dim g α 2 dim g 2α = 0. This can happen only if dim g α = 1 and dim g rα = 0 for all r 2. Now α Φ if and only if α Φ. Thus dim g α = 1 for all α Φ. Note that while all of the root spaces g α are 1-dimensional, the space g 0 = h need not be 1-dimensional.

52 Proposition If α Φ and rα Φ where r Z, then r = 1 or r = 1. Proof. From the above, we have dim g rα = 0 for all r 2, that is, rα is not a root. Now rα Φ if and only if rα Φ. Thus only α and α can be roots. We are now ready to examine some stronger properties of the set Φ of roots.

53 Let α, β Φ be roots such that β α and β α. Then β is not an integer multiple of α. There do, however, exist integers p 0, q 0 such that the elements pα + β,..., α + β, β, α + β,..., qα + β all lie in Φ but (p + 1)α + β and (q + 1)α + β do not. The set of roots pα + β,..., qα + β is called the α-chain of roots through β.

54 Proposition Let α, β be roots such that β α and β α. Let pα + β,..., β,..., qα + β be the α-chain of roots through β. Then h α, h β h α, h α = p q. Proof. Consider the subspace m of g given by m = g pα+β g qα+β. Recall that h α = [e α, e α ] [g α, g α ].

55 Now β is not an integer multiple of α, and so (p + 1)α + β 0 and (q + 1)α + β 0. We have ad e α g qα+β g (q+1)α+β. Because (q + 1)α + β 0 and (q + 1)α + β / Φ, we must have g (q+1)α+β = 0. Thus ad e α m m. By a similar argument, we have ad e α m m, and so ad h α m = (ad e α ad e α ad e α ad e α )m m. We calculate the trace of ad h α on m in two different ways. We have tr m (ad h α) = since dim g iα+β = 1. q (iα + β) (h α) i= p

56 Second, we have tr m (ad h α) = tr m (ad e α ad e α ) tr m (ad e α ad e α ) = 0. Thus that is, ( q(q + 1) 2 q (iα + β) (h α) = 0, i= p Since p + q + 1 0, this yields ) p(p + 1) α (h 2 α) + (p + q + 1)β (h α) = 0. (q p) h 2 α, h α + h α, h β = 0.

57 Hence h α, h β since h α, h α 0. Corollary h α, h α = p q If α Φ and ζα Φ where ζ C, then ζ = 1 or ζ = 1. Proof. Suppose ζ ±1 and let β = ζα. Then β (h α) = ζα (h α), that is, h α, h β = ζ h α, h α. From the previous proposition, this yields h α, h β 2ζ = 2 = p q. h α, h α

58 Hence 2ζ Z. If ζ Z, then ζ = ±1. Thus ζ / Z. It follows that p q is odd. The α-chain of roots through β is ( ) ( ) p + q p q α,..., β = α,..., 2 2 ( p + q 2 ) α. Since p q is odd and consecutive roots differ by α, we see that all roots in the α-chain are odd multiples of 1 2 α. Also, p q 0, and so p and q cannot both be zero. Thus p + q 0. Because the first and last roots are negatives of one another, 1 2α must lie in the α-chain. Thus 1 2 α Φ. But α Φ, and so 2 ( 1 2 α) Φ, a contradiction.

59 Proposition h α, h β Q for all α, β Φ. Proof. We already know that h α, h β C. We also have h α, h β 2 Z. h α, h α Thus h α,h β h α,h α Q. It is therefore sufficient to show that h α, h α Q. We have h α, h α = tr (ad h α ad h α) = h α, h β 2. β Φ (β (h α)) 2 = β Φ

60 Dividing by h α, h α 2, this yields 1 = ( h α, h 2 β Z. h α, h α β Φ h α, h α ) Hence h α, h α Q, completing the proof.

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