Killing form
Group theory → Lie groups Lie groups 


In mathematics, the Killing form, named after Wilhelm Killing, is a symmetric bilinear form that plays a basic role in the theories of Lie groups and Lie algebras.
History and name
The Killing form was essentially introduced into Lie algebra theory by Élie Cartan (1894) in his thesis. The name "Killing form" first appeared in a paper of Armand Borel in 1951, but he stated in 2001 that he doesn't remember why he chose it. Borel admits that the name seems to be a misnomer, and that it would be more correct to call it the "Cartan form".^{[1]} Wilhelm Killing had noted that the coefficients of the characteristic equation of a regular semisimple element of a Lie algebra is invariant under the adjoint group, from which it follows that the Killing form (i.e. the degree 2 coefficient) is invariant, but he did not make much use of this fact. A basic result Cartan made use of was Cartan's criterion, which states that the Killing form is nondegenerate if and only if the Lie algebra is a direct sum of simple Lie algebras.^{[1]}
Definition
Consider a Lie algebra g over a field K. Every element x of g defines the adjoint endomorphism ad(x) (also written as ad_{x}) of g with the help of the Lie bracket, as
Now, supposing g is of finite dimension, the trace of the composition of two such endomorphisms defines a symmetric bilinear form
with values in K, the Killing form on g.
Properties
 The Killing form B is bilinear and symmetric.
 The Killing form is an invariant form, in the sense that it has the 'associativity' property
 where [ , ] is the Lie bracket.
 If g is a simple Lie algebra then any invariant symmetric bilinear form on g is a scalar multiple of the Killing form.
 The Killing form is also invariant under automorphisms s of the algebra g, that is,
 for s in Aut(g).
 The Cartan criterion states that a Lie algebra is semisimple if and only if the Killing form is nondegenerate.
 The Killing form of a nilpotent Lie algebra is identically zero.
 If I, J are two ideals in a Lie algebra g with zero intersection, then I and J are orthogonal subspaces with respect to the Killing form.
 The orthogonal complement with respect to B of an ideal is again an ideal.^{[2]}
 If a given Lie algebra g is a direct sum of its ideals I_{1},...,I_{n}, then the Killing form of g is the direct sum of the Killing forms of the individual summands.
Matrix elements
Given a basis e^{i} of the Lie algebra g, the matrix elements of the Killing form are given by
where I_{ad} is the Dynkin index of the adjoint representation of g. Here
in Einstein summation notation, where the c^{ij}_{k} are the structure coefficients of the Lie algebra. The index k functions as column index and the index n as row index in the matrix ad(e_{i})ad(e_{j}). Taking the trace amounts to putting k = n and summing, and so we can write
The Killing form is the simplest 2tensor that can be formed from the structure constants.
In the above indexed definition, we are careful to distinguish upper and lower indices (co and contravariant indices). This is because, in many cases, the Killing form can be used as a metric tensor on a manifold, in which case the distinction becomes an important one for the transformation properties of tensors. When the Lie algebra is semisimple over a zerocharacteristic field, its Killing form is nondegenerate, and hence can be used as a metric tensor to raise and lower indexes. In this case, it is always possible to choose a basis for g such that the structure constants with all upper indices are completely antisymmetric.
The Killing form for some Lie algebras g are (for X, Y in g viewed in their fundamental n by n (2n by 2n) representations):
g  B(X, Y) 

gl(n, R)  2n tr(XY) − 2 tr(X)tr(Y) 
sl(n, R)  2n tr(XY) 
su(n)  2n tr(XY) 
so(n, R)  (n−2) tr(XY) 
so(n)  (n−2) tr(XY) 
sp(2n, R)  (4n+2) tr(XY) 
sp(2n, C)  (4n+2) tr(XY) 
Connection with real forms
Suppose that is a semisimple Lie algebra over the field of real numbers . By Cartan's criterion, the Killing form is nondegenerate, and can be diagonalized in a suitable basis with the diagonal entries ±1. By Sylvester's law of inertia, the number of positive entries is an invariant of the bilinear form, i.e. it does not depend on the choice of the diagonalizing basis, and is called the index of the Lie algebra . This is a number between 0 and the dimension of which is an important invariant of the real Lie algebra. In particular, a real Lie algebra is called compact if the Killing form is negative definite. Note that this is one of two inequivalent definitions commonly used for compactness of a Lie algebra; the other states that a Lie algebra is compact if it corresponds to a compact Lie group. The definition of compactness in terms of negative definiteness of the Killing form is more restrictive, since using this definition it can be shown that under the Lie correspondence, compact Lie algebras correspond to compact Lie groups.
If is a semisimple Lie algebra over the complex numbers, then there are several nonisomorphic real Lie algebras whose complexification is , which are called its real forms. It turns out that every complex semisimple Lie algebra admits a unique (up to isomorphism) compact real form . The real forms of a given complex semisimple Lie algebra are frequently labeled by the positive index of inertia of their Killing form.
For example, the complex special linear algebra has two real forms, the real special linear algebra, denoted , and the special unitary algebra, denoted . The first one is noncompact, the socalled split real form, and its Killing form has signature (2, 1). The second one is the compact real form and its Killing form is negative definite, i.e. has signature (0, 3). The corresponding Lie groups are the noncompact group of 2 × 2 real matrices with the unit determinant and the special unitary group , which is compact.
See also
Notes
 1 2 Borel, p.5
 ↑ Fulton, William; Harris, Joe (1991). Representation theory. A first course. Graduate Texts in Mathematics, Readings in Mathematics. 129. New York: SpringerVerlag. ISBN 9780387974958. MR 1153249, ISBN 9780387975276. See page 207.
References
 Borel, Armand (2001). Essays in the history of Lie groups and algebraic groups. History of Mathematics, Vol 21. American Mathematical Society and the London Mathematical Society.
 Daniel Bump, Lie Groups (2004), Graduate Texts In Mathematics, 225, SpringerVerlag. ISBN 9780387211541
 Cartan, Élie (1894), Sur la structure des groupes de transformations finis et continus, Thesis, Nony
 Jurgen Fuchs, Affine Lie Algebras and Quantum Groups, (1992) Cambridge University Press. ISBN 052148412X
 Fulton, William; Harris, Joe (1991). Representation theory. A first course. Graduate Texts in Mathematics, Readings in Mathematics. 129. New York: SpringerVerlag. ISBN 9780387974958. MR 1153249, ISBN 9780387975276.
 Hazewinkel, Michiel, ed. (2001), "Killing form", Encyclopedia of Mathematics, Springer, ISBN 9781556080104