# triangular matrix

## 1 Triangular Matrix

Let $n$ be a positive integer.

An upper triangular matrix is of the form:

 $\begin{bmatrix}a_{11}&a_{12}&a_{13}&\cdots&a_{1n}\\ 0&a_{22}&a_{23}&\cdots&a_{2n}\\ 0&0&a_{33}&\cdots&a_{3n}\\ \vdots&\vdots&\vdots&\ddots&\vdots\\ 0&0&0&\cdots&a_{nn}\end{bmatrix}$

An upper triangular matrix is sometimes also called right triangular.

A lower triangular matrix is of the form:

 $\begin{bmatrix}a_{11}&0&0&\cdots&0\\ a_{21}&a_{22}&0&\cdots&0\\ a_{31}&a_{32}&a_{33}&\cdots&0\\ \vdots&\vdots&\vdots&\ddots&\vdots\\ a_{n1}&a_{n2}&a_{n3}&\cdots&a_{nn}\end{bmatrix}$

A lower triangular matrix is sometimes also called left triangular.

Note that upper triangular matrices and lower triangular matrices must be square matrices.

A triangular matrix is a matrix that is an upper triangular matrix or lower triangular matrix. Note that some matrices, such as the identity matrix, are both upper and lower triangular. A matrix is upper and lower triangular simultaneously if and only if it is a diagonal matrix.

Triangular matrices allow numerous algorithmic shortcuts in many situations. For example, if $A$ is an $n\times n$ triangular matrix, the equation $Ax=b$ can be solved for $x$ in at most $n^{2}$ operations.

In fact, triangular matrices are so useful that much computational linear algebra begins with factoring (or decomposing) a general matrix or matrices into triangular form. Some matrix factorization methods are the Cholesky factorization and the LU-factorization. Even including the factorization step, enough later operations are typically avoided to yield an overall time savings.

## 2 Properties

Triangular matrices have the following properties ( “triangular” with either “upper” or “lower” uniformly):

• The inverse of a triangular matrix is a triangular matrix.

• The product of two triangular matrices is a triangular matrix.

• The determinant of a triangular matrix is the product of the diagonal elements.

• The eigenvalues of a triangular matrix are the diagonal elements.

The last two properties follow easily from the cofactor expansion of the triangular matrix.

 Title triangular matrix Canonical name TriangularMatrix Date of creation 2013-03-22 12:11:40 Last modified on 2013-03-22 12:11:40 Owner Wkbj79 (1863) Last modified by Wkbj79 (1863) Numerical id 13 Author Wkbj79 (1863) Entry type Definition Classification msc 65-00 Classification msc 15-00 Defines upper triangular Defines lower triangular Defines upper triangular matrix Defines lower triangular matrix Defines right triangular Defines right triangular matrix Defines left triangular Defines left triangular matrix