# Classification of Cartan matrices

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1 Chapter 7 Classification of Cartan matrices In this chapter we describe a classification of generalised Cartan matrices This classification can be compared as the rough classification of varieties in terms of Fano varieties ( K ample, discrete moduli space), Calabi-Yau varieties (more generally K-trivial varieties, tame moduli space) and varieties of general type (K ample, big moduli space) We will indeed get the case of finite dimensional Lie algebras, of Affine Lie algebras and of Kac-Moody Lie algebras of general type In the first case, we recover semisimple Lie algebras The second class is very rich and we shall construct explicitly these algebras The last case is more obscure even if special Lie algebras can be studied 71 Finite, affine and indefinite case We will prove the decomposition into these three catgories for a more larger class of matrices than the generalised Cartan matrices We will deal with square matrices A = (a i,j ) of size n satisfying the following properties: A is indecomposable; a i,j 0; (a i,j = 0) (a j,i = 0) Let us recall the following result on systems of inequalities: Lemma 711 A system of real inequalities λ i (x j ) = j u i,jx j > 0 for i [1,m], j [1,n] has a solution if and only if there is no non trivial dependence relation i v iλ i = 0 with v i 0 for all i Proof : If there is such a non trivial relation, the system has no solution We prove the converse by induction on m Assume there is no non trivial such relation By induction, the system λ i > 0 for i < m has a solution The set of solutions is a cone C containing 0 in its closure We need to prove that this cone intersects the half space defined by λ m > 0 If it is not the case then we first prove that λ m is in the linear span of the λ i for i < m Indeed, otherwise there would exist an element x such that λ m (x) = 1 and λ i (x) = 0 for i < m But then there exists a small deformation y of x satisfying λ m (y) > 0 and λ i (y) > 0 for i < m Let us consider a relation between λ m and the λ i and write m i=1 v iλ i = 0 We take such a relation of minimal length that is the number of non vanishing v i s is as small a possible Let us denote by I the set of indices i < m such that v i 0 Consider the restriction of the cone C to the subspace 47

2 48 CHAPTER 7 CLASSIFICATION OF CARTAN MATRICES λ m = 0 If this intersection is non empty, we get as before a solution of our system by perturbing the element in the intersection If the intersection is empty, this implies that there is a relation m u i λ i = 0 i I with all u i 0 But by our minimality condition, the linear forms λ i for i I are independent Thus the restriction of these linear forms on the hyperplane λ m = 0 satisfy a unique equation This equation is given by the restriction of the relation m i=1 v iλ i = 0 Therefore that all the v i for i < m are non positive But one of the v i is positive by hypothesis thus v m > 0 and v i 0 for i < m We are done: the system has a solution has soon as it has a solution for the first m 1 inequalities Definition 712 A vector x = (x i ) will be called positive (resp non negative), this will be denoted x > 0 (resp x 0) if for all i we have x i > 0 (resp x i 0) Corollary 713 If A = (a i,j ) is an n m real matrix for which there is no x 0, x 0 such that A t x 0 (here A t is the transpose of A), then there exists v > 0 such that Av < 0 Proof : We look for a vector v = (v 1,,v m ) such that v i > 0 for all i and λ j = k a j,kv k < 0 that is λ j > 0 We know from the previous Lemma that this system has a solution if and only if there is non non trivial dependence relation between the vectors v i and λ j with non positive coefficients Assume we have such a relation, it can be written as a i v i + u j ( λ j ) = 0 j i with a i and u j non positive This leads to the equation v t a = v t A t u where a = (a i ) is non positive and u = (u j ) is non positive This gives the equation A t u = a and we get that u 0 with A t ( u) = a 0 which is impossible Lemma 714 Let A satisfy the above three properties Then Au 0 and u 0 imply that either u > 0 or u = 0 Proof : Let u a be non zero vector such that Au 0 and u 0 We have a i,i u i j ( a i,j )u j So if u i vanishes, then all the u j such that there is a sequence i 0 = i,,i k = j with a i0,i 1 a ik 1,i k 0 vanish As the matrix is indecomposable and thanks to our hypothesis on the matrix, this implies by Lemma 328 that u = 0 This is not the case so that u > 0 Here is another proof without Lemma 328 Reorder the indexes such that u i = 0 for i < s and u i > 0 for i s Then we get that a i,j = 0 = a j,i for i < s and j s The matrix is decomposable except if u > 0 We may now prove the classification of our matrices into three disjoint categories Theorem 715 Let A be a real n n-matrix satisfying the above three conditions Then we have the following alternative for both A and A t : det(a) 0, there exists u > 0 such that Au > 0 and Av 0 implies that v > 0 or v = 0

3 71 FINITE, AFFINE AND INDEFINITE CASE 49 Corank(A) = 1, there exists u > 0 such that Au = 0 and Av 0 implies that Av = 0 there exists u > 0 such that Au < 0 and (Av 0 and v 0) implies that v = 0 A matrix of the first type is called a finite type matrix, of the second case, a affine type matrix and of the third one, a indefinite type matrix Proof : Remark that the third case is disjoint from the first two because in the first two there is no u > 0 such that Au < 0 (set v = u and apply the result on Av 0) Furthermore because of the rank condition on the matrix the first two cases are disjoint We consider the following convex cone: K A = {u / Au 0} But the previous Lemma, this cone intersects the cone {u 0} only in {0} or in its interior {u > 0} We thus have the inclusion K A {u 0} {0} {u > 0} Assume this intersection is not reduced to the point 0 Then we have the alternative: K A is contained in {u > 0} {0} (and thus does not contain any linear subspace) K A = {u / Au = 0} is a 1-dimensional line Indeed, we assume that K A meets the cone {u > 0} in some point say u But if there is an element v of K A outside this cone, because of the convexity of K A, we know that the interval [u,v] is contained in K A and this interval has to meet the boundary of {u 0} This is only possible on the 0 element thus v has to be in the half line generated by u and the whole line through u is contained in K A We are in the second case Assume there is an element w K A not in that line Then because K A is a convex cone we get that the whole half plane generated by w and the line trough u is contained in K A but this half plane will meet the boundary of the cone {u 0} outside 0 which is impossible Because u and u are in K A, this implies that Au = 0 The first case is equivalent to the finite type case If Av 0, then v K A and v > 0 or v = 0 because of the inclusion of K A in {u > 0} {0} Because K A does not contain any linear subspace, A has to be non degenerate In particular A is surjective thus there exists a vector u with Au > 0 This u satisfies u > 0 or u = 0 The last case is not possible since otherwise we would have Au = 0 The second case is equivalent to the affine case: we know that there is an element u > 0 such that Au = 0 and the kernel of A is K A and of dimension 1 and we get the condition on Av 0 Furthermore, in the finite (resp affine) case, we see that there is no v > 0 such that Av < 0 By Corollary 713, this implies that there is an element u in the cone {u 0} different from 0 and in K A t In particular A t is again of finite (resp affine type), we distinguish the two cases thanks to the rank of the matrix In the last case, we know that there is no u 0 with u 0 such that A t u 0 Lemma 713 tells us that there exists an element v > 0 such that Av < 0 If Aw 0 and w 0 we know that w = 0 Corollary 716 Let A be as in the previous Theorem, then A is of finite (resp affine, resp indefinite) type if and only if there exists and element u > 0 such that Au > 0 (resp Au = 0, resp Au < 0)

4 50 CHAPTER 7 CLASSIFICATION OF CARTAN MATRICES 72 Finite and affine cases 721 First results Definition 721 A matrix (a i,j ) i,j I with I [1,n] is called a principal submatrix of A = (a i,j ) i,j [1,n] The determinant of a principal submatrix is a principal minor Lemma 722 Let A be indecomposable of finite or affine type Then any proper principal submatrix of A decomposes into a direct sum of matrices of finite type Proof : Let I [1, n] and let A I be the associated principal submatrix If u = (u i ) i [1,n], denote by u I the vector (u i ) i I We know by hypothesis that there exists a vector u > 0 such that Au 0 Consider the subproduct A I u I Because all the a i,j for i j are non positive, this implies that (Au) I A I u I with equality if and only if for all i I and all j I we have a i,j = 0 We thus have u I > 0 and A I u I 0 which implies that A I is of affine or finite type If it is of affine type then A I u I 0 implies A I u I = 0 and in particular A I u I = (Au) I This means that I = [1,n] or A is decomposable Lemma 723 A symmetric matrix A is of finite (resp affine) type if and only if A is positive definite (resp positive semidefinite of corank 1) Proof : Assume A is positive semidefinite, then if there exists u > 0 with Au < 0, then u t Au < 0 which is impossible In particular A is of finite of affine type The rank condition distinguishes the two cases Conversely, if A is of finite or affine type, then there exists an u > 0 such that Au 0 For λ > 0, we have (A + λi)u > 0 thus A is of finite type and hence non degenerate Therefore the eigenvalues of A are non negative and the result follows with the rank condition Lemma 724 Let A = (a i,j ) i,j [1,n] be a matrix of finite or affine type such that a i,i = 2 and a i,j a j,i 1 Then A is symmetrisable Moreover if there exists a sequence i 1,i 2,i 3,,i k of indices with k 3 such that a i1,i 2 a i2,i 3 a ik 1,i k a ik,i 1 0, then A is of the form where the u i are some positive numbers 2 u 1 0 u 1 n u u n 1 u n 0 u 1 n 1 2 Proof : Let us prove that the second part of the Lemma implies the first one Indeed, if there is no such sequence, then the conditions of Proposition 613 are satisfies (all products a i1,i 2 a i2,i 3 a ik 1,i k a ik,i 1 vanish) and the matrix is symmetrisable) Consider such a sequence of indices with a i1,i 2 a i2,i 3 a ik 1,i k a ik,i 1 0 We may reorder the indices and suppose that i j = j so that we have a principal submatrix of A of the form 2 b 1 b k b 1 2 B = 2 b k 1 b k b k 1 2

5 72 FINITE AND AFFINE CASES 51 where the are non positive elements and the b i and b i are positive We know that this matrix B is of affine or finite type so that there exists a vector u > 0 such that Bu 0 Let U be the diagonal matrix with the coefficients of u on the diagonal Replacing B by U t BU, we may assume that u t = (1,,1) The condition Bu 0 gives us conditions j B i,ju j 0 and by sum of all these conditions and the fact that the in B are non positive we get 2k k (b i + b i) 0 i=1 But because b i b i 1, we get that b i + b i 2 (the roots of X2 (b i + b i )X + b ib i are real positive thus the discriminant (b i + b i )2 4b i b i is non negative and b i + b i 2) But this, together with the previous inequality, implies that for all i, we have b i = b i = 1 and all the in B vanish But then det(b) = 0 and thus A = B (because non proper principal submatrix is affine Conjugating with U gives the matrix of the lemma To summarise properties of generalised Cartan matrices (as for classical Cartan matrices), it is usefull to define the associated Dynkin diagram D(A) Definition 725 The Dynkin diagram D(A) of a generalised Cartan matrix A is the graph whose vertices are indexed by the row of the matrix (ie by [1,n]) and whose edges are described as follows if a i,j a j,i 4 and a i,j a j,i, the vertices i and j are connected by a i,j lines equipped with an arrow pointing toward i if a i,j > 1; if a i,j a j,i > 4, the vertices i and j are connected by a line colored by the ordered pair of integers a i,j, a j,i The matrix A is indecomposable if and only if D(A) is connected and the matrix A is determined by the Dynkin diagram D(A) modulo permutation of the indices We say that the Dynkin diagramm D(A) is of finite, affine or indefinite type if A is Proposition 726 Let A be any indecomposable generalised Cartan matrix, then we have: (ı) the matrix A is of finite type if and only if all its principal minors are positive (ıı) The matrix A is of affine type if and only if all its proper principal minors are positive and det(a) = 0 (ııı) If A is of finite or affine type, then any proper subdiagram of D(A) is an union of Dynkin diagrams of finite type (ıv) If A is of finite type or affine type and if D(A) contains a cycle of length at least 3, then D(A) is the following cycle: (v) The matrix A is of affine type if and only if there exists a vector δ > 0 with Aδ = 0 Such a vector is unique up to scalar multiple Proof : Let us prove (ı) and (ıı) Let A I be a principal submatrix of A, it is of finite or affine type and is a generalised Cartan matrix We know from Lemma 724 that any generalised Cartan matrix of finite or affine type is symmetrisable Let D I be a diagonal matrix with positive diagonal coefficients such that B I = D I A I is symmetric The matrix B I has the same type as A I and by Lemma 723

6 52 CHAPTER 7 CLASSIFICATION OF CARTAN MATRICES this implies that B I is positive and even positive definite if A I is proper or if A is of finite type In particular in all these cases we have det(b I ) > 0 and thus det(a I ) > 0 Conversely, if all principal minors of A are positive, then assume there exists a vector u > 0 with Au < 0 We get inequalities of the form j a i,ju j < 0 Now we eliminate the variables u i for i > 1 in the equation i a 1,iu i < 0 For this we remove a 1,i /2 times the inequality j a i,ju j < 0 Because a 1,i leq0 for i > 1 we end up with an inequality of the form λu 1 < 0 Furthermore, we have det(a) = λ det(a ) where A is the submatrix of A defined by the indices [2, n] Because all the proper minors are positive, we get that λ > 0 in the first case and λ = 0 in the second case In both case the inequalities u 1 > 0 and λu 1 < 0 are impossible The matrix A is thus of finite or affine type and the cases are described by the rank (ııı) This is a direct consequence of Lemma 722 (ıv) If D(A) contains a cycle of length at least 3, this implies that the condition: there exists a sequence i 1,i 2,i 3,, i k of indices with k 3 such that a i1,i 2 a i2,i 3 a ik 1,i k a ik,i 1 0, of Lemma 724 is satisfied The matrix A is thus given by Lemma 724 Furthermore, in the case of generalised Cartan matrices, if u i and u 1 i are positive integers, this implies that u i = 1 The associated Dynkin diagram is the desired cycle (v) This is a direct consequence of Theorem Examples of finite and affine type matrices In this subsection we give examples of generalised Cartan matrices of finite and affine type and we describe their Dynkin diagrams Proposition 727 (ı) The following matrices are generalised Cartan matrices of finite type Their associated Dynkin diagrams are given in Table A n = B n = Cartan matrices

7 72 FINITE AND AFFINE CASES C n = = Bn t D n = E 6 = E 7 = E 8 = F 4 = G 2 = ( )

8 54 CHAPTER 7 CLASSIFICATION OF CARTAN MATRICES Dynkin diagrams A n B n C n D n : E 6 E 7 E 8 F 4 G 2 : Table 1 (ıı) The following matrices are generalised Cartan matrices of affine type Their associated Dynkin diagrams are given in Table 21, Table 22 and Table 23 Furthermore the smallest vector δ with positive integers values such that Aδ = 0 is given by δ = i a ie i where the a i are the coefficients in the vertices and the e i are the vector of the canonical basis The node 0 is represented by a square We will often forget the tilde in the sequel and denote A 1 n instead of Ã1 n for example The exponent 4 on the arrow of Ã2 2 mens that we have a quadruple arrow

9 72 FINITE AND AFFINE CASES 55 ( ) 2 2 Ã 1 1 = Ã n = B n = Ct n = Cartan matrices Order D n 1 = Ẽ6 1 =

10 56 CHAPTER 7 CLASSIFICATION OF CARTAN MATRICES Ẽ7 1 = Ẽ8 1 = F 4 1 = G 1 2 = Cartan matrices Order 2 ( ) 2 4 Ã 2 2 = 1 2 Ã 2 2n =

11 72 FINITE AND AFFINE CASES Ã 2 2n 1 = D n+1 2 = Ẽ6 2 = Cartan matrices Order 3 D 3 4 = Dynkin diagrams Ã Ã n B 1 n

12 58 CHAPTER 7 CLASSIFICATION OF CARTAN MATRICES C n D 1 n Ẽ Ẽ Ẽ F G Table 21

13 72 FINITE AND AFFINE CASES 59 Ã Ã 2 2n Ã 2 2n D n F Table 22 D Table 23 Remark 728 (ı) One may ask: where do these matrices come from For the finite type case, these are the Cartan matrices of simple Lie algebras We will see later that there is a natural construction associating to any Lie algebra g a Lie algebra t which is essentially the loop algebra Lg = g C[t,t 1 ] This construction with g a simple Lie algebra give rise to the affine matrices of order 1 described in the proposition When taking a finite order automorphism of a simple Lie algebra and performing this construction twisted by the automorphism, we get the order 2 and 3 cases (ıı) One may remark that, in the finite type case, the matrix is symmetric if and only if the Dynkin diagram has only simple edges This case is called the simply laced case Remark also that even if the matrices F 4 and G 2 are not symmetric, their transpose give rise to the same Lie algebra, we only need to reorder the simple roots However the matrices of type B n and C n are exchanged by transposition Things are a little more complicated for affine type matrices because the order may change while transposing the matrix but we still have that the transpose of an affine type matrix if of affine type

14 60 CHAPTER 7 CLASSIFICATION OF CARTAN MATRICES We get the following equalities: t B1 n = Ã2 2n 1, t C1 n = D 2 n+1, t F 1 4 = Ẽ2 6 and t G1 2 = D 3 4 the other matrices are symmetric or their transpose give rise to the same matrix (and thus the same Lie algebra g(a)) after reordering the indices Proof : To prove this proposition, we only need to check that the vector δ given by δ = i a i e i satisfies Aδ = 0 for all the affine type matrices and then to prove that the finite type matrices (or Dynkin diagrams) are principal proper submatrices of matrix of affine type This is done case by case Remark however that in the order 1 case, the vector δ is the sum θ +α 0 of the longest root of the finite root system and the simple added root 723 Classification of finite and affine type matrices We may now prove the following classification Theorem: Theorem 729 All the indecomposable generalised Cartan matrices of finite or affine type are those given in Proposition 727 Proof : We proceed by induction on the number of vertices of the Dynkin diagram The only matrix of size 1 1 being the matrix A = (2) of type A 1 It is also clear that the rank 2 finite and affine type indecomposable generalised Cartan matrices are given by matrices of type A 2, B 2, C 2, G 2, A 1 1 and A 2 2 We will also need that the following matrices are not of finite or affine type: , and But the vectors v = (2,5, 7), v = (4, 5,7) and v = (7,15,7) are such that v > 0 and Av < 0 proving the result For the induction, we proceed as follows: a diagram D of finite or affine type with at least three vertices is obtained from a smaller diagram D of finite type (and at least two vertices) by adding a vertex (by Lemma 722) Furthermore, the diagram D has to be such that when removing any vertex, we obtain a finite type diagram Furthermore, thanks to Proposition 726 we may assume that D has no cycle Starting with type A n, we may add a vertex with simple edge to root 1 and root 2 (or the last two roots) to root 3 (or n 2) if and only if n 8 and also to root 4 if n 7 We get diagrams of type A n+1, D n+1, E 6, E 7, Ẽ7 1, E 8 or Ẽ1 9 We may now add a vertex with double edge to root 1 (or n) or to root 2 if n = 3 We get diagrams of type B n+1, C n+1, B 3 1 and Ã2 5 We may now add a vertex with triple edge to root 1 (or n) if n = 2 We get diagrams of type G 1 2 and D 4 3 Starting with type B n or C n, we may add a vertex with simple edge to root 1 and root 2, we may also add it to root n if and only if n 4 We get diagrams of type B n+1, C n+1, B1 n, Ã 2 2n 1, F 4, F 1 4 or Ẽ2 6 We may now add a vertex with double edge to root 1 We get diagrams of type C n, 1 Ã 2 2n and D n+1 2 We may not add a vertex with triple edge

15 72 FINITE AND AFFINE CASES 61 In type E, it is clear that we may only add vertices with simple edges Furthermore, these vertices can be add only to end vertices of the diagram We get diagrams of type Ẽ1 6, E 7, Ẽ1 7 E 8 and Ẽ1 8 For F 4, we may only add simple vertices to the end vertices of the diagram We get diagrams of type Ẽ2 6 and F 4 1 Finally, for G 2, We end with a characterisation of Kac-Moody Lie algebras g(a) isomorphic to simple finite dimensional Lie algebras Proposition 7210 The following conditions are equivalent: (ı) The matrix A is a generalised Cartan matrix of finite type (ıı) The matrix A is symmetrisable and the bilinear form (, ) h is positive definite (ııı) The Weyl group W is finite (ıv) The root system is finite (v) The Lie algebra g(a) is simple and finite dimensional (vı) There exists a root θ such that for any simple root α i, we have θ + α i Such a root is called a highest root Proof : (ı) (ıı) We know by Lemma 724 that the matrix A is symmetrisable (so that (, ) h is well defined) and we also know that the matrix B = D 1 A defining the bilinear form is of finite type (because we have u > 0 such that Au > 0 thus Bu = D 1 Au > 0 because D can be chosen positive) Now Lemma 723 gives that the form is positive definite (ıı) (ııı) Follows from Proposition 5211 and is an equivalence (ııı) (ıv) Follows from Theorem 552 and is an equivalence (ıv) (v) The bilinear form (, ) h is positive definite and in particular A is regular thus by Proposition 329 we get that g(a) is simple and because all the weight spaces are of finite dimension, the Lie algebra g(a) is of finite dimension (v) (vı) We take θ a root of maximal height and the result follows (vı) (ı) Let θ be a root such that for all simple root α i we have θ + α i Consider the g (i) -submodule of g(a) generated by an element x in g θ It is of finite dimension and by the sl 2 theory, we obtain that x is a highest weight vector of non negative weight α i,θ In particular we have α i,θ 0 for all i This implies that the matrix A is of affine or finite type If it was of affine type we would have α i,θ = 0 for all i Furthermore, if θ was negative, the condition on θ would imply that x = 0 thus θ > 0 and because x is non zero, there exists an index i such that [f i,x] 0 ie θ α i But α i, θ = 0 is the highest weight (and thus the lowest weight) of the g (i) -module generated by x, this is a contradiction

16 62 CHAPTER 7 CLASSIFICATION OF CARTAN MATRICES

17 Bibliography [Bo54] Bourbaki, N Groupes et algèbres de Lie 4,5,6, Hermann 1954 [Hu72] [Hu90] [Ku02] [PK83] Humphreys, JE Introduction to Lie algebras and representation theory GTM 9, Berlin- Heidelberg-New York, Springer, 1972 Humphreys, JE Reflection groups and Coxeter groups Cambridge Studies in Advanced Mathematics, 29 Cambridge University Press, Cambridge, 1990 Kumar, S Kac-Moody groups, their flag varieties and representation theory Progress in Mathematics, 204 Birkhäuser Boston, Inc, Boston, MA, 2002 Peterson, DH and Kac, VG Infinite flag varieties and conjugacy theorems, Proc Natl Acad Sci USA, 80 (1983), pp [Sp98] Springer, TA Linear algebraic groups Second edition Progress in Mathematics, 9 Birkhäuser Boston, Inc, Boston, MA, 1998 Nicolas Perrin, HCM, Universität Bonn 63

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