Cartan matrix
In mathematics, the term Cartan matrix has three meanings. All of these are named after the French mathematician Élie Cartan. In fact the Cartan matrices in the context of Lie algebras were first investigated by Wilhelm Killing, whereas the Killing form is due to Cartan.^{[citation needed]}
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Lie algebras
Group theory → Lie groups Lie groups 


A generalized Cartan matrix is a square matrix with integral entries such that
 For diagonal entries, .
 For nondiagonal entries, .
 if and only if
 can be written as , where is a diagonal matrix, and is a symmetric matrix.
For example, the Cartan matrix for G_{2} can be decomposed as such:
The third condition is not independent but is really a consequence of the first and fourth conditions.
We can always choose a D with positive diagonal entries. In that case, if S in the above decomposition is positive definite, then A is said to be a Cartan matrix.
The Cartan matrix of a simple Lie algebra is the matrix whose elements are the scalar products
(sometimes called the Cartan integers) where r_{i} are the simple roots of the algebra. The entries are integral from one of the properties of roots. The first condition follows from the definition, the second from the fact that for is a root which is a linear combination of the simple roots r_{i} and r_{j} with a positive coefficient for r_{j} and so, the coefficient for r_{i} has to be nonnegative. The third is true because orthogonality is a symmetric relation. And lastly, let and . Because the simple roots span a Euclidean space, S is positive definite.
Conversely, given a generalized Cartan matrix, one can recover its corresponding Lie algebra. (See Kac–Moody algebra for more details).
Classification
An matrix A is decomposable if there exists a nonempty proper subset such that whenever and . A is indecomposable if it is not decomposable.
Let A be an indecomposable generalized Cartan matrix. We say that A is of finite type if all of its principal minors are positive, that A is of affine type if its proper principal minors are positive and A has determinant 0, and that A is of indefinite type otherwise.
Finite type indecomposable matrices classify the finite dimensional simple Lie algebras (of types ), while affine type indecomposable matrices classify the affine Lie algebras (say over some algebraically closed field of characteristic 0).
Determinants of the Cartan matrices of the simple Lie algebras
The determinants of the Cartan matrices of the simple Lie algebras given in the following table (Along with A_{1}=B_{1}=C_{1}, B_{2}=C_{2}, D_{3}=A_{3}, D_{2}=A_{1}A_{1}, E_{5}=D_{5}, E_{4}=A_{4}, and E_{3}=A_{2}A_{1})^{[1]}
A_{n}  B_{n}  C_{n}  D_{n} n ≥ 3 
E_{n} 3 ≤ n ≤ 8 
F_{4}  G_{2} 

n+1  2  2  4  9n  1  1 
Another property of this determinant is that it is equal to the index of the associated root system, i.e. it is equal to where P, Q denote the weight lattice and root lattice, respectively.
Representations of finitedimensional algebras
In modular representation theory, and more generally in the theory of representations of finitedimensional associative algebras A that are not semisimple, a Cartan matrix is defined by considering a (finite) set of principal indecomposable modules and writing composition series for them in terms of irreducible modules, yielding a matrix of integers counting the number of occurrences of an irreducible module.
Cartan matrices in Mtheory
In Mtheory, one may consider a geometry with twocycles which intersects with each other at a finite number of points, at the limit where the area of the twocycles go to zero. At this limit, there appears a local symmetry group. The matrix of intersection numbers of a basis of the twocycles is conjectured to be the Cartan matrix of the Lie algebra of this local symmetry group.^{[2]}
This can be explained as follows. In Mtheory one has solitons which are twodimensional surfaces called membranes or 2branes. A 2brane has a tension and thus tends to shrink, but it may wrap around a twocycles which prevents it from shrinking to zero.
One may compactify one dimension which is shared by all twocycles and their intersecting points, and then take the limit where this dimension shrinks to zero, thus getting a dimensional reduction over this dimension. Then one gets type IIA string theory as a limit of Mtheory, with 2branes wrapping a twocycles now described by an open string stretched between Dbranes. There is a U(1) local symmetry group for each Dbrane, resembling the degree of freedom of moving it without changing its orientation. The limit where the twocycles have zero area is the limit where these Dbranes are on top of each other, so that one gets an enhanced local symmetry group.
Now, an open string stretched between two Dbranes represents a Lie algebra generator, and the commutator of two such generator is a third one, represented by an open string which one gets by gluing together the edges of two open strings. The latter relation between different open strings is dependent on the way 2branes may intersect in the original Mtheory, i.e. in the intersection numbers of twocycles. Thus the Lie algebra depends entirely on these intersection numbers. The precise relation to the Cartan matrix is because the latter describes the commutators of the simple roots, which are related to the twocycles in the basis that is chosen.
Note that generators in the Cartan subalgebra are represented by open strings which are stretched between a Dbrane and itself.
See also
Notes
 ^ CartanGram determinants for the simple Lie Groups Alfred C. T. Wu, J. Math. Phys. Vol. 23, No. 11, November 1982
 ^ Sen, Ashoke (1997). "A Note on Enhanced Gauge Symmetries in M and String Theory". Journal of High Energy Physics. IOP Publishing. 1997 (9): 001. doi:10.1088/11266708/1997/09/001.
References
 Fulton, William; Harris, Joe (1991). Representation theory: A first course. Graduate Texts in Mathematics. 129. SpringerVerlag. p. 334. ISBN 0387974954.
 Humphreys, James E. (1972). Introduction to Lie algebras and representation theory. Graduate Texts in Mathematics. 9. SpringerVerlag. pp. 55–56. doi:10.1007/9781461263982. ISBN 0387900527.
 Kac, Victor G. (1990). Infinite Dimensional Lie Algebras (3rd ed.). Cambridge University Press. ISBN 9780521466936..
External links
 Hazewinkel, Michiel, ed. (2001) [1994], "Cartan matrix", Encyclopedia of Mathematics, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 9781556080104
 Weisstein, Eric W. "Cartan matrix". MathWorld.