In mathematics, the term Cartan matrix has two meanings. Both of these are named after Elie Cartan.

Lie algebras

A generalized Cartan matrix is a square matrix A = (a_ij) with integral entries such that

1. For diagonal entries, a_ii = 2.
2. For non-diagonal entries, .
3. a_ij = 0 if and only if a_ji = 0
4. A can be written as DS, where D is a diagonal matrix, and S is a symmetric matrix.

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

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 S_ij = 2(r_i,r_j). Because the simple roots span a Euclidean space, S is positive definite.

Representations of finite-dimensional algebras

In modular representation theory, and more generally in the theory of representations of finite-dimensional 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 projective modules, yielding a matrix of integers counting the number of occurrences of a projective module.