- Killing form
In

mathematics , the**Killing form**, named afterWilhelm Killing , is asymmetric bilinear form that plays a basic role in the theories ofLie group s andLie algebra s. In an example ofStigler's law of eponymy , the Killing form was actually invented byÉlie Cartan , whereas theCartan matrix is due to Wilhelm Killing.**Definition**Consider a

Lie algebra "g" over a field "K". Every element "x" of "g" defines theadjoint 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".**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

::B( ["x","y"] ,"z")=B("x", ["y","z"] ), : 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

automorphism s "s" of the algebra "g", that is,::B("s"("x"),"s"("y")) = B("x","y"):for "s" in Aut(g).

* The

Cartan criterion states that a Lie algebra is semisimple if and only if the Killing form is non-degenerate.* The Killing form of a

nilpotent Lie algebra is identically zero.* If "I" and "J" are two

ideal s in a Lie algebra "g" with zero intersection, then "I" and "J" areorthogonal subspaces 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.**Matrix elements**Given a basis "e"

^{i}of the Lie algebra g, the matrix elements of the Killing form are given by:$B^\{ij\}=\; tr\; (\; extrm\{ad\}(e^i)circ\; extrm\{ad\}(e^j))\; /\; I\_\{ad\}$

where $\{I\}\_\{ad\}$ is the

Dynkin index of the adjoint representation of g.Here

:$left(\; extrm\{ad\}(e^i)\; circ\; extrm\{ad\}(e^j)\; ight)(e^k)=\; [e^i,\; [e^j,\; e^k]\; =\; \{c^\{im\_\{n\}\; \{c^\{jk\_\{m\}\; e^n$

and so we can write

:$B^\{ij\}\; =\; frac\{1\}\{I\_\{\; extrm\{ad\}\; \{c^\{im\_\{n\}\; \{c^\{jn\_\{m\}$

where the $\{c^\{ij\_\{k\}$ are the

structure constant s of the Lie algebra. The Killing form is the simplest 2-tensor that 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 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 indexes are completely antisymmetric.**Connection with real forms**Suppose that "g" 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 or -1. BySylvester'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 isnegative definite . It is known that under theLie correspondence , compact Lie algebras correspond tocompact Lie group s.If "g"

_{C}is a semisimple Lie algebra over the complex numbers, then there are several non-isomorphic real Lie algebras with thecomplexification is "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 groupSU(2) , which is compact.**ee also***

Casimir invariant **References***Fulton-Harris

*Jurgen Fuchs, "Affine Lie Algebras and Quantum Groups", (1992) Cambridge University Press. ISBN 0-521-48412-X

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2010.*

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