# mx.nd.linalg.extracttrian¶

## Description¶

Extracts a triangular sub-matrix from a square matrix. Input is a tensor A of dimension n >= 2.

If n=2, then A represents a single square matrix from which a triangular sub-matrix is extracted as a 1-dimensional tensor.

If n>2, then A represents a batch of square matrices on the trailing two dimensions. The extracted triangular sub-matrices are returned as an n-1-dimensional tensor.

The offset and lower parameters determine the triangle to be extracted:

• When offset = 0 either the lower or upper triangle with respect to the main diagonal is extracted depending on the value of parameter lower.

• When offset = k > 0 the upper triangle with respect to the k-th diagonal above the main diagonal is extracted.

• When offset = k < 0 the lower triangle with respect to the k-th diagonal below the main diagonal is extracted.

Note

The operator supports float32 and float64 data types only.

Example:

Single triagonal extraction
A = [[1.0, 2.0],
[3.0, 4.0]]

extracttrian(A) = [1.0, 3.0, 4.0]
extracttrian(A, lower=False) = [1.0, 2.0, 4.0]
extracttrian(A, 1) = [2.0]
extracttrian(A, -1) = [3.0]

Batch triagonal extraction
A = [[[1.0, 2.0],
[3.0, 4.0]],
[[5.0, 6.0],
[7.0, 8.0]]]

extracttrian(A) = [[1.0, 3.0, 4.0],
[5.0, 7.0, 8.0]]


## Arguments¶

Argument

Description

A

NDArray-or-Symbol.

Tensor of square matrices

offset

int, optional, default=’0’.

Offset of the diagonal versus the main diagonal. 0 corresponds to the main diagonal, a negative/positive value to diagonals below/above the main diagonal.

lower

boolean, optional, default=1.

Refer to the lower triangular matrix if lower=true, refer to the upper otherwise. Only relevant when offset=0

## Value¶

out The result mx.ndarray