Convergent matrix

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In linear algebra, a convergent matrix is a matrix that converges to the zero matrix under matrix exponentiation.

Contents

Background

When successive powers of a matrix T become small (that is, when all of the entries of T approach zero, upon raising T to successive powers), the matrix T converges to the zero matrix. A regular splitting of a non-singular matrix A results in a convergent matrix T. A semi-convergent splitting of a matrix A results in a semi-convergent matrix T. A general iterative method converges for every initial vector if T is convergent, and under certain conditions if T is semi-convergent.

Definition

We call an n×n matrix T a convergent matrix if

for each i = 1, 2, ..., n and j = 1, 2, ..., n. [1] [2] [3]

Example

Let

Computing successive powers of T, we obtain

and, in general,

Since

and

T is a convergent matrix. Note that ρ(T) = 1/4, where ρ(T) represents the spectral radius of T, since 1/4 is the only eigenvalue of T.

Characterizations

Let T be an n×n matrix. The following properties are equivalent to T being a convergent matrix:

  1. for some natural norm;
  2. for all natural norms;
  3. ;
  4. for every x. [4] [5] [6] [7]

Iterative methods

A general iterative method involves a process that converts the system of linear equations

into an equivalent system of the form

for some matrix T and vector c. After the initial vector x(0) is selected, the sequence of approximate solution vectors is generated by computing

for each k 0. [8] [9] For any initial vector x(0), the sequence defined by ( 4 ), for each k 0 and c 0, converges to the unique solution of ( 3 ) if and only if ρ(T) < 1, that is, T is a convergent matrix. [10] [11]

Regular splitting

A matrix splitting is an expression which represents a given matrix as a sum or difference of matrices. In the system of linear equations ( 2 ) above, with A non-singular, the matrix A can be split, that is, written as a difference

so that ( 2 ) can be re-written as ( 4 ) above. The expression ( 5 ) is a regular splitting of A if and only if B10 and C0, that is, B1 and C have only nonnegative entries. If the splitting ( 5 ) is a regular splitting of the matrix A and A10, then ρ(T) < 1 and T is a convergent matrix. Hence the method ( 4 ) converges. [12] [13]

Semi-convergent matrix

We call an n×n matrix T a semi-convergent matrix if the limit

exists. [14] If A is possibly singular but ( 2 ) is consistent, that is, b is in the range of A, then the sequence defined by ( 4 ) converges to a solution to ( 2 ) for every x(0) if and only if T is semi-convergent. In this case, the splitting ( 5 ) is called a semi-convergent splitting of A. [15]

See also

Notes

References