Lie Algebras

234 Pages·1975·8.679 MB·English

by  Zhexian Wan

LIE
A L G E B R AS
by
ZHE-XIAN WAN
Institute of Mathematics, Academia Sinica, Peking
Translated by
C H E - Y O U NG LEE
PERGAMON PRESS
O X F O RD • NEW Y O RK . T O R O N TO . S Y D N EY . BRAUNSCHWEIG
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First edition 1975
Library of Congress Cataloging in Publication Data
Wan, Che-hsien.
Lie algebras.
(International series of monographs in pure and applied mathematics, v. 104)
Translation of Li tai shu.
Bibliography: p.
1. Lie algebras. I. Lie, Sophus, 1842-1899. II. Title.
QA252.3.W3613 1975 512'.55 74-13832
ISBN 0-08-017952-5
Printed in Hungary
PREFACE
FROM winter of 1961 to spring of 1963, the author gave a series of lectures in the seminar on
Lie groups at the Institute of Mathematics, Academia Sinica. The present book is based on
the drafts of these lectures. The contents include the classical theory of complex semisimple
Lie algebras, namely, the theory of structure, automorphisms, representations and real
forms of such Lie algebras. The purpose of the author's lectures at the Institute of Mathe-
matics was to teach the fundamentals of the theory of Lie algebras to the participants of
the seminar in order to study the modern literature on Lie groups and Lie algebras. The
main references for these lectures were "The structure of semisimple Lie algebras" by
Dynkin and the lecture notes "Theorie desalgebresde Lie et topologie desgroupesdeLie" in
Seminaire Sophus Lie. The material in Dynkin's paper is accessible to beginners but is
not complete enough; the lecture notes in Seminaire Sophus Lie contain more material but
presuppose more knowledge. While each of these references has its place, neither of them
really meets the needs of beginners. The purpose of the present book, therefore, is to supply
an elementary background to the teory of Lie algebras, together with sufficient material
to provide a reasonable overview of the subject.
Lie algebras are algebraic structures used for the study of Lie groups; they were intro-
duced by and named after S. Lie. Besides S. Lie, the important contributors to this theory
were W. Killing, E. Cartan and H. Weyl. Although discussions of Lie groups in this book
have been kept to a minimum in order to facilitate understanding, it should be pointed out
that the importance of the classical theory of Lie algebras lies in its applications to the theory
of Lie groups. It should also be mentioned that a great part of the material in this book has
been generalized to Lie algebras over algebraically closed fields of characteristic zero and
some results have been generalized to Lie algebras over arbitrary fields of characteristic
zero. In the present book, only Lie algebras over the complex numbers are considered.
This is because the theory of such Lie algebras is the most fundamental and requires only
knowledge of linear algebra to understand it.
The author wishes to thank the participants of the seminar for their suggestions and
discussions which have led to many improvements in the present book. Special thanks are
due to Gen-Dao Li, who assisted in the proof reading.
vii
CHAPTER 1
BASIC CONCEPTS
1.1. Lie algebras
Let g be a finite dimensional vector space (also called linear space) over the complex
field C and suppose that there is a binary operation [X, Y] (X, Y £ g) defined on g which
satisfies
I. [Ai Xi+2Z, Y] = k[X, F] + A[Z, Y], for all X X and Y in g
2 2 X X 2 2 U 2
and any complex numbers Xi, A.
2
II. [X, Y] = - [7, X], for all X, Y in g.
III. [X [7, Z]]+[7, [Z, X]]+[Z, [X, 7]] = 0, for all X, 7, Z in g.
9
Then g is called a Lie algebra over the complex numbers; g is also called a complex Lie
algebra or simply a Lie algebra. The operation [X, Y] is called commutation and [X, Y] is
called the commutator of Xand Y. The dimension dim g of g as a vector space is said to be
the dimension of the Lie algebra g.
Condition I states that commutation is linear with respect to the first element. Using II,
it can be proved that it is also linear with respect to the second element, i.e.
T. [X, A^i + A^] = Ai[jr, 7i] + A[X, 7], for all X, Y Y in g and complex num-
2 2 U 2
bers Ai, A.
2
Using II and III, it can be proved that
nr. [[x, n z]+[[r, z], x]+[[z xi Y] = o.
9
Ill can also be written as
III". [X, [7, Z]] = [[X, 7], Z]+[Y, [X, Z\\
Condition III is called the Jacobi identity. Finally, setting X — Y in II, we have
II'. [X, X] = 0, for any X £ g.
The following are examples of Lie algebras.
I
2 LIE ALGEBRAS [1.1
EXAMPLE 1. Let g be a finite dimensional vector space over C;if for any X and Y in g,
[X, Y] is defined to be the zero vector, then I, II and HI certainly hold, g thus becomes a Lie
algebra; it is also called an abelian Lie algebra.
In general, if two elements X, Y of a Lie algebra g satisfy [X, Y] = 0, then we say that X
and Y commute.
EXAMPLE 2. Let V be a three-dimensional vector space over C and ei e, e form a basis
3 9 2 3
of V. For any two elements
3
x = xie\+xe+xe
2 2 3 3
and y = y&l-ye+ye,
2 2 3 3
define [x, y] = (xy - xy)e+{xy - xy)e+(xy - xji>?,
2 3 3 2 x 3 x 1 z 2 2 2 2 3
then V becomes a Lie algebra.
3
EXAMPLE 3. Let g3 be the collection of all 3 x3 skew symmetric matrices, g can be con-
3
sidered as a vector space over C. If for any X,Y £ g, [X, Y] is defined to be XY— YX, then
3
g becomes a Lie algebra.
3
We can choose a basis consisting of the elements
"0 0 0" " 0 0 1" "0 -1 0"
0 0 -1 , M = 0 0 0 , M = 1 0 0
2 3
.0 1 0. .-1 0 0. .0 0 0.
then [Mu M] = Ms, [M, M] = Mj, [M, Mi] = M. An element Xin g can be writ-
2 2 3 3 2 3
ten as
" 0
- *3 *2
X = 0 x\M\ + xM+xM.
2 2 3 3
0.
- - *2 Xi
If
* 0 -y* yi
F = 0 -yi y\Mx+yM+yM,
2 2 3 3
yi 0.
then [X, Y] = (xy - xy)M!+(x yi - xi j)M+(pay* - xji)M.
2 3 3 2 3< 3 2 2 3
Therefore, the mapping from V to g defined by
3 3
x = xe+xe X — xiM±+xM+xM
2 2 3 3 2 2 3 3
is one to one and satisfies
(1) if x — X, y — 7, then any A, fi 6 C, Xx+ \iy fiY;
(2) if x X, y F, then [x, j] - [X, 7].
That is, V and g have the same algebraic structure.
3 3
1.1] BASIC CONCEPTS 3
In general, a one-one mapping from a Lie algebra gi onto a Lie algebra g is called an
2
isomorphism if it satisfies:
(1) if X - Y X F, then for any A, /x <E C, XXi+iaX - AFi+^Fa;
x l9 2 2 2
(2) if JTx - Fi, X - F, then [*k, X] - [F F].
2 2 2 i5 2
We also say that gi and g are isomorphic and write g ^ g. In particular, an isomorphism
2 x 2
from g onto itself is called an automorphism.
One of the fundamental problems of the theory of Lie algebras is to determine all non-
isomorphic Lie algebras.
Let g be an r-dimensional Lie algebra with basis Xu ..., X. If
r
[XXj] = £ c$X,
h k
k=l
then the commutator of any two elements can be obtained by using the r3 constants i.e. if
X=±hX Y=£PLJXJ
h
then
[XY] = f ^jclX. (1.1)
9 k
The r3 constants c*- (i, j\ k = 1,2,..., r) are called structure constants. It is not hard to see
that a set of structure constants satisfies:
(1) 4 = -c£, l^Ufk^r.
r
(2) £ (4<4 + +4.4,) = 0, 1 «s i, j, k*sr.
Conversely, let g be an r-dimensional vector space and (i,j,k = 1, ..., r) be r3 constants
satisfying (1) and (2). If a basis X\ ..., X of g is chosen and the commutator of two ele-
9 r
ments X = Y!i=i anc* ^ = Lj=i PJXJ a re defined by (1.1), then g becomes a Lie algebra.
Obviously, if two Lie algebras are isomorphic, then with respect to a suitable basis, they
have the same structure constants. Conversely, Lie algebras with the same set of structure
constants are isomorphic. Structure constants are basis dependent, if Fi,.. .,Fis another
r
basis and
[YYj]= f c*Y, U/ J< r.
i9 k
F, = £ a\X l^i^r where det(^j) ^ 0,
j9
then £ c\fal = £ a\a)c[ 1 < ij I < r. (1.2)
k t9 9
4 LIE ALGEBRAS [1.1
Therefore, two Lie algebras are isomorphic if and only if their structure constants and
satisfy the equations (1.2) where (a{) is a non-singular matrix.
Finally, we give the following example.
EXAMPLE 4. Let $l(n, C) be the set of all nXn matrices. It is known that with respect to
matrix addition and scalar multiplication, gl(ft, C) forms an «2-dimensional vector space.
Now for any X, Y e gl(«, C), define
[X,F] = XY-YX;
then gl(n, C) forms a Lie algebra.
gl(«, C) can also be considered as the set of all linear transformations of an ^-dimensional
vector space V; then it is usually denoted by gl(F). We will sometimes adopt the first
viewpoint and sometimes the second viewpoint.
1.2. Subalgebras, ideals and quotient algebras
Let g be a Lie algebra and m, tt be subsets of g. Denote by trt+rt the linear subspace
spanned by elements of the form M+N(M £ m, N £ tt) and by [m, n] the subspace spanned
by elements of the form [M, N] (M £ m, N £ tt). If m, ttti, tn, tt, p are subspaces of g, then
2
(1) [mi+m, n] Q [ttti, n] + [m, n];
2 2
(2) [m, n] = [n, m];
(3) [m, [n, p]] Q [tt, [p, m]]+[p [ttt, tt]].
9
A subspace f) of g is said to be a subalgebra if [% ij] Q i), i.e. X, F £ I) implies that
[X, Y] £ f). A subspace f) of g is said to be an ideal if [g, tj] Q g, i.e. X £ g and Y 6 t) implies
that [X, Y] g f). An ideal is a subalgebra. If and f) are ideals, then +f) and I)i n ^2
2 2
are also ideals.
Subalgebras of gl(«, C) are called matrix Lie algebras or /m^ar Lie algebras.
If f) is an ideal of g, then the quotient space g/t), which consists of all cosets (congruence
classes mod Ij), is defined. For X £ g, denote the congruence class containing Xby X. Define
[X, F] = [XjF].
It can be proved that this definition is independent of the choices of Zand Y. The quotient
space Q/i) thus becomes a Lie algebra; this algebra is called the quotient algebra of g with
respect to t).
If g is a Lie algebra and i) is an ideal of g, then it can be proved that the mapping
X- X
satisfies the conditions:
(1) if AT - X, F - Fthen for any A, ^ £ C, AX-f-^F - AX+/xF;
(2) if X - X, F - Fthen [X, F] - [X, 7].
In general, a mapping
X- Xi
1.2] BASIC CONCEPTS 5
from a Lie algebra g to a Lie algebra g is said to be a homomorphism if it satisfies
3
(1) if X -> X Y - Y then for any 2, C C, AX+^7 - AXi + //7r,
u x
(2) MX - X Y 7 then [X, 7] - [X 7].
u l9 l9 3
If this mapping is onto, then gi is said to be a homomorphic image of g.
THEOREM 1. The mapping X X from QtoQ/h is a homomorphism (called canonical homo-
morphism ) and g/f) is a homomorphic image g. Conversely\ iff is a homomorphism from g onto
gi, with kernel f), then f) is an ideal of g and the mapping J defined by
is an isomorphism from g/t) onto Qi(f is called the canonical isomorphism induced byf).
Proof It is only necessary to prove the second part of the theorem. We first prove that 1)
is an ideal of g. Let 1 , 7 ^, i.e./(X) =f(Y) = 0, then
/(X+7) = /(X)+/(7) = 0+0 = 0,
/(AX) = A/(X) = A-0 = 0 for any A £ C.
Therefore X+ 7 g f) and IX £ f) and t) is a subspace of g. If X 6 g and 7 £ f), then
f([X, Y]) = [/(X),/(7)] = [/(X), 0] = 0.
Therefore [X, 7] € I) and t) is an ideal of g.
We now verify that the definition of / does not depend on choices of elements in the
congruence classes. Suppose X, 7 belong to the same class, i.e. X = 7, then X— 7 = H 6 f).
Thus
/(X-7)=/(#) = 0
and/(X) =/(7), therefore/(X) = /(7).
Finally, we show that/is an isomorphism. Let X, 7 £ g/f), then
/(X+ 7) = f(X+ 7) = /(X)+/(7) = /(X)+/(7),
/(AX)=/(AX)= A/(X) = A/(X), for A € C,
/ ( [* F] = / ( [* 7]) = t/(X),/(7)] = [/(X),/(7)].
Thus/is a homomorphism. If /(X) = /(7), then
/(X-7) =/(X)-/(7) = /(X)-/(7) = 0.
Therefore X— 7 € 1} and X = 7. This proves that/is a one-one correspondence, hence it is
an isomorphism. This completes the proof of Theorem 1.
To explain the concepts introduced above, we give the following examples.
EXAMPLE 5. All trace zero matrices of gl(«, C) form a subalgebra; denote it by A_
n 1
In fact, A_ is an ideal of gl(«, C), for if X, 7 6 gl(«, C), then
n x
7>[X, 7] = Tr(XY-YX) = 0,
therefore [X, 7] € A-i-
6 LIE ALGEBRAS [1.2
All scalar matrices of gt(«, C) form a one-dimensional subalgebra, which is also an ideal
of gl(w, C); for if XI is a scalar matrix, then for any X 6 gt(«, C), we have
[X, A/] = X-XI-XLX^ 0.
All diagonal matrices of gt(«, C) form an n-dimensional abelian subalgebra; denote it by
b(«, C). The set of trace zero diagonal matrices form an (n— l)-dimensional abelian sub-
algebra of A_
n v
EXAMPLE 6. Let M be an nXn matrix. The set of all complex matrices satisfying
XM+MX' = 0
form a linear Lie algebra. In fact, from XM+MX1 = 0 and YM+MY' — 0, we have
[X, Y]M+M[X, Y\ = {XY-YX)M+M(XY-YXy
= XYM— YXM+ MY'X'—MX'Y' = -XMY'+YMX'-YMX'+XMY' = 0.
Denote this Lie algebra by g(fl, Af, C). It is easy to see that if Mi and M% are congruent
then g(«, M C) and g(«, M, C) are isomorphic.
u 2
The following are important examples of g(«, M, C):
When M is non-singular and symmetric, we obtain the orthogonal algebra. Since any
complex non-singular symmetric matrix is congruent to the identity matrix, the orthogonal
algebra (with respect to some M) can be considered as consisting of all skew-symmetric
matrices. Furthermore, a complex non-singular symmetric matrix is either congruent to
[I o]' i f"= 2 w i s e v e n'
or to
'1 0 0'
0 0 I if n — 2m+l is odd.
m
.0 I 0.
m
Therefore, there are two series of orthogonal Lie algebras. Denote them by B (n = 2m -f-1)
m
and D (n = 2m), respectively.
m
If M is non-singular and skew-symmetric, then n must be an even number n — 2m\ any
such matrix is congruent to
[ 0 I]
m
l-Im 0\>
the corresponding algebra is called the symplectic algebra and is denoted by C.
m
The Lie algebras A, B, C, D are called the classical Lie algebras.
n n n n
1.3] BASIC CONCEPTS 7
1.3. Simple algebras
Let g be a Lie algebra; obviously, g and {0} are ideals of g. If g does not have any other
ideals, then it is said to be a simple Lie algebra.
Obviously, one-dimensional Lie algebras are simple and any abelian Lie algebra of
dimension greater than one is not simple. Therefore, except for the one-dimensional Lie
algebra, simple Lie algebras are not abelian.
THEOREM 2. The algebras A (n 1), B (n s> 1), C (n ^ 1) and D (n s> 3) are simple
n n n n
Lie algebras.
Proof. We will separately consider the structure formulas of A, B, C and D.
n n n n
(A) Let m = «+ l.AllmXm matrices of trace zero form the Lie algebra A, the dimension
n n
is n2+2n. Let
5
then the set of all H ... (Ya^i — 0) forms an ^-dimensional abelian subalgebra f). Let
Xu Xm
E denote the matrix with one at the ith row and fcth column and zeros elsewhere and
iU
Hx-h = Eu—Ekk, (i k),
f
then I) and al\E _ (i k, i, k = 1,2, ..., m) span A. X—X (i ^ k, i, k = 1, ..., m)
Xi Xk n i lc
are called roots of A. If n ^ 2, then any root of A can be obtained by adding roots of A
n n n
to a fixed root. The structure formulas of A are
n
[H,H] = 0, for any fli,ff € -|
1 2 2
[HAL,.. .AM, E«] = <xEx, for any root a,
[E«, 2?_J = H for any root a, I (1.3)
X
„ „ , f 0, if a+/3 is not a root, f
r
[+E b if a+8 is a root. J
x+
(B„) Letm = 2«+l. If
"1 0 0"
0 0 /„
.0 /„ 0.
then B consists of all mXm matrices X satisfying
n
XS+SX' = 0