- Killing form
mathematics, the Killing form, named after Wilhelm Killing, is a symmetric bilinear formthat plays a basic role in the theories of Lie groups and Lie algebras. In an example of Stigler's law of eponymy, the Killing form was actually invented by Élie Cartan, whereas the Cartan matrixis due to Wilhelm Killing.
Lie algebra"g" over a field "K". Every element "x" of "g" defines the adjoint endomorphism"ad"("x") of "g" with the help of the Lie bracket, as
:"ad"("x")("y") = ["x", "y"]
Now, supposing "g" is of finite dimension, the trace of the composition of two such endomorphisms defines a
symmetric bilinear form
:B("x", "y") = trace(ad("x")ad("y")),
with values in "K", the Killing form on "g".
* The Killing form "B" is bilinear and symmetric.
* The Killing form is an invariant form, in the sense that it has the 'associativity' property
::B( ["x","y"] ,"z")=B("x", ["y","z"] ), : where [,] is the
* If "g" is a
simple Lie algebrathen 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,
::B("s"("x"),"s"("y")) = B("x","y"):for "s" in Aut(g).
Cartan criterionstates that a Lie algebra is semisimple if and only if the Killing form is non-degenerate.
* The Killing form of a
nilpotent Lie algebrais identically zero.
* If "I" and "J" are two
ideals in a Lie algebra "g" with zero intersection, then "I" and "J" are orthogonalsubspaces with respect to the Killing form.
* 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.
Given a basis "e"i of the Lie algebra g, the matrix elements of the Killing form are given by
where is the
Dynkin indexof the adjoint representation of g.
and so we can write
where the are the
structure constants of the Lie algebra. The Killing form is the simplest 2- tensorthat can be formed from the structure constants.
In the above indexed definition, we are careful to distinguish upper and lower indexes ("co-" and "contra-variant" indexes). 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, its Killing form is nondegenerate, and hence can be used as a
metric tensorto 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 indexes are completely antisymmetric.
Connection with real forms
Suppose that "g" is a
semisimple Lie algebraover 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 or -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 "g". This is a number between 0 and the dimension of "g" which is an important invariant of the real Lie algebra. In particular, a real Lie algebra "g" is called compact if the Killing form is negative definite. It is known that under the Lie correspondence, compact Lie algebras correspond to compact Lie groups.
If "g"C is a semisimple Lie algebra over the complex numbers, then there are several non-isomorphic real Lie algebras with the
complexificationis "g"C, which are called its real forms. It turns out that every complex semisimple Lie algebra admits a unique (up to isomorphism) compact real form "g". 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 sl(2,C) has two real forms, the real special linear algebra, denoted sl(2,R), and special unitary algebra, denoted su(2). The first one is noncompact, the so-called 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 SL(2,R) of 2 by 2 real matrices with the unit determinant and the special unitary group
SU(2), which is compact.
*Jurgen Fuchs, "Affine Lie Algebras and Quantum Groups", (1992) Cambridge University Press. ISBN 0-521-48412-X
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