An m times n upper triangular matrix is one whose entries below the main diagonal are zeros, as is shown in the matrix to the right. when is a square upper triangular matrix invertible? justify your answer.

A square matrix is invertible when its determinant is non-zero. An upper-triangular square matrix will have a non-zero determinant if there are no zeros on the diagonal.

The determinant is the sum of the values in a row or column multiplied by the determinant of the corresponding cofactor matrix. When all matrix entries below the diagonal are zero, the determinant becomes the product of diagonal terms.