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Canonical basis

In mathematics, a canonical basis is a basis of an algebraic structure that is canonical in a sense that depends on the precise context:

Representation theory

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 .

One can show that there exists at most one canonical basis for each precanonical structure. A sufficient condition for existence is that the polynomials defined by satisfy and .

A canonical basis induces an isomorphism from to .

Hecke algebras

Let be a Coxeter group. The corresponding Iwahori-Hecke algebra has the standard basis , the group is partially ordered by the Bruhat order which is interval finite and has a dualization operation defined by . This is a precanonical structure on that satisfies the sufficient condition above and the corresponding canonical basis of is the Kazhdan–Lusztig basis

with being the Kazhdan–Lusztig polynomials.

Linear algebra

If we are given an n × n matrix and wish to find a matrix in Jordan normal form, similar to , we are interested only in sets of linearly independent generalized eigenvectors. A matrix in Jordan normal form is an "almost diagonal matrix," that is, as close to diagonal as possible. A diagonal matrix is a special case of a matrix in Jordan normal form. An ordinary eigenvector is a special case of a generalized eigenvector.

Every n × n matrix possesses n linearly independent generalized eigenvectors. Generalized eigenvectors corresponding to distinct eigenvalues are linearly independent. If is an eigenvalue of of algebraic multiplicity , then will have linearly independent generalized eigenvectors corresponding to .

For any given n × n matrix , there are infinitely many ways to pick the n linearly independent generalized eigenvectors. If they are chosen in a particularly judicious manner, we can use these vectors to show that is similar to a matrix in Jordan normal form. In particular,

Definition: A set of n linearly independent generalized eigenvectors is a canonical basis if it is composed entirely of Jordan chains.

Thus, once we have determined that a generalized eigenvector of rank m is in a canonical basis, it follows that the m − 1 vectors that are in the Jordan chain generated by are also in the canonical basis.

Computation

Let be an eigenvalue of of algebraic multiplicity . First, find the ranks (matrix ranks) of the matrices . The integer is determined to be the first integer for which has rank (n being the number of rows or columns of , that is, is n × n).

Now define

The variable designates the number of linearly independent generalized eigenvectors of rank k (generalized eigenvector rank; see generalized eigenvector) corresponding to the eigenvalue that will appear in a canonical basis for . Note that

Once we have determined the number of generalized eigenvectors of each rank that a canonical basis has, we can obtain the vectors explicitly (see generalized eigenvector).

Example

This example illustrates a canonical basis with two Jordan chains. Unfortunately, it is a little difficult to construct an interesting example of low order. The matrix

has eigenvalues and with algebraic multiplicities and , but geometric multiplicities and .

For we have

has rank 5,
has rank 4,
has rank 3,
has rank 2.

Therefore

Thus, a canonical basis for will have, corresponding to one generalized eigenvector each of ranks 4, 3, 2 and 1.

For we have

has rank 5,
has rank 4.

Therefore

Thus, a canonical basis for will have, corresponding to one generalized eigenvector each of ranks 2 and 1.

A canonical basis for is

is the ordinary eigenvector associated with . and are generalized eigenvectors associated with . is the ordinary eigenvector associated with . is a generalized eigenvector associated with .

A matrix in Jordan normal form, similar to is obtained as follows:

where the matrix is a generalized modal matrix for and .

See also

Notes

References