# Definition:Triangular Matrix/Lower Triangular Matrix

## Definition

A lower triangular matrix is a matrix in which all the upper triangular elements are zero.

That is, all the non-zero elements are on the main diagonal or in the lower triangle.

That is, $\mathbf L$ is lower triangular if and only if:

$\forall a_{i j} \in \mathbf U: i < j \implies a_{i j} = 0$

## Also defined as

Some sources define a lower triangular matrix only as a square matrix.

## Examples

### Lower Triangular Matrix with fewer Rows than Columns

A lower triangular matrix of order $m \times n$ such that $m < n$:

$\mathbf L = \begin{bmatrix} a_{1 1} & 0 & 0 & \cdots & 0 & 0 & \cdots & 0 & 0 \\ a_{2 1} & a_{2 2} & 0 & \cdots & 0 & 0 & \cdots & 0 & 0 \\ a_{3 1} & a_{3 2} & a_{3 3} & \cdots & 0 & 0 & \cdots & 0 & 0 \\ \vdots & \vdots & \vdots & \ddots & \vdots & \vdots & \ddots & \vdots & \vdots \\ a_{m - 1, 1} & a_{m - 1, 2} & a_{m - 1, 3} & \cdots & a_{m - 1, m - 1} & 0 & \cdots & 0 & 0 \\ a_{m 1} & a_{m 2} & a_{m 3} & \cdots & a_{m - 1, m} & a_{m m} & \cdots & 0 & 0 \\ \end{bmatrix}$

### Lower Triangular Matrix with more Rows than Columns

A lower triangular matrix of order $m \times n$ such that $m > n$:

$\mathbf L = \begin{bmatrix} a_{1 1} & 0 & 0 & \cdots & 0 & 0 \\ a_{2 1} & a_{2 2} & 0 & \cdots & 0 & 0 \\ a_{3 1} & a_{3 2} & a_{3 3} & \cdots & 0 & 0 \\ \vdots & \vdots & \vdots & \ddots & \vdots & \vdots \\ a_{n - 1, 1} & a_{n - 1, 2} & a_{n - 1, 3} & \cdots & a_{n - 1, n - 1} & 0 \\ a_{n 1} & a_{n 2} & a_{n 3} & \cdots & a_{n, n - 1} & a_{n n} \\ a_{n + 1, 1} & a_{n + 1, 2} & a_{n + 1, 3} & \cdots & a_{n + 1, n - 1} & a_{n + 1, n} \\ \vdots & \vdots & \vdots & \ddots & \vdots & \vdots \\ a_{m - 1, 1} & a_{m - 1, 2} & a_{m - 1, 3} & \cdots & a_{m - 1, n - 1} & a_{m - 1, n} \\ a_{m 1} & a_{m 2} & a_{m 3} & \cdots & a_{m, n - 1} & a_{m n} \\ \end{bmatrix}$

### Square Lower Triangular Matrix

$\mathbf L = \begin{bmatrix} a_{1 1} & 0 & 0 & \cdots & 0 & 0 \\ a_{2 1} & a_{2 2} & 0 & \cdots & 0 & 0 \\ a_{3 1} & a_{3 2} & a_{3 3} & \cdots & 0 & 0 \\ \vdots & \vdots & \vdots & \ddots & \vdots & \vdots \\ a_{n - 1, 1} & a_{n - 1, 2} & a_{n - 1, 3} & \cdots & a_{n - 1, n - 1} & 0 \\ a_{n 1} & a_{n 2} & a_{n 3} & \cdots & a_{n, n - 1} & a_{n n} \\ \end{bmatrix}$