# How do I check if a 1x1 matrix is diagonal, lower/upper triangular?

1. Nov 1, 2011

### hkBattousai

I have an A matrix with dimensions 1x1. Its the only term a11 is an arbitrary number.

For what values of a11, this A matrix is;

1. Diagonal
2. Upper triangular
3. Lower triangular

2. Nov 1, 2011

### janvdl

By definition a 1x1 matrix will be upper and lower triangular. (But not strictly; for strictly upper and lower: $a$ must be 0).

A matrix is diagonal if it is triangular and normal. Normal (for a matrix whose elements lie in the domain of real numbers) means $A \ A^T = A^T \ A$

3. Nov 2, 2011