In mathematics, a canonical basis is a basis of an algebraic structure that is canonical in a sense that depends on the precise context: In a coordinate space, and more generally in a free module, it refers to the standard basis defined by the Kronecker delta. In a polynomial ring, it refers to its standard basis given by the monomials, . For finite extension fields, it means the polynomial basis. In linear algebra, it refers to a set of n linearly independent generalized eigenvectors of an n×n matrix , if the set is composed entirely of Jordan chains. In representation theory, it refers to the basis of the quantum groups introduced by Lusztig. The canonical basis for the irreducible representations of a quantized enveloping algebra of type and also for the plus part of that algebra was introduced by Lusztig by two methods: an algebraic one (using a braid group action and PBW bases) and a topological one (using intersection cohomology). Specializing the parameter to yields a canonical basis for the irreducible representations of the corresponding simple Lie algebra, which was not known earlier. Specializing the parameter to yields something like a shadow of a basis. This shadow (but not the basis itself) for the case of irreducible representations was considered independently by Kashiwara; it is sometimes called the crystal basis. The definition of the canonical basis was extended to the Kac-Moody setting by Kashiwara (by an algebraic method) and by Lusztig (by a topological method). There is a general concept underlying these bases: Consider the ring of integral Laurent polynomials with its two subrings and the automorphism defined by . A precanonical structure on a free -module consists of A standard basis of , An interval finite partial order on , that is, is finite for all , A dualization operation, that is, a bijection of order two that is -semilinear and will be denoted by as well. If a precanonical structure is given, then one can define the submodule of . A canonical basis of the precanonical structure is then a -basis of that satisfies: and for all .