TITLE: Determinant of lower triangular matrix
QUESTION [8 upvotes]: Does a lower triangular matrix have a determinant that is equal to the product of the elements in the diagonal similar to an upper triangular matrix.

REPLY [7 votes]: The matrix looks like this: $$\begin{bmatrix}
a_{1,1}&0&\cdots&\cdots&0 
\\b_{1,2}&a_{2,2}&\ddots&&\vdots
\\ \vdots&\ddots&\ddots&\ddots&\vdots
\\ \vdots&&\ddots&\ddots&0
\\ b_{1,n}&\cdots&\cdots&b_{n-1,n}&a_{n,n}
\end{bmatrix}$$
The determinant can be written as the sum of the product of the elements in the top row with their associated minors: so this determinant would be: $$a_{1,1}*\begin{bmatrix}
a_{2,2}&0&\cdots&0 
\\b_{2,3}&a_{3,3}&\ddots&\vdots
\\ \vdots&\ddots&\ddots&0
\\ b_{2,n}&\cdots&b_{n-1,n}&a_{n,n}
\end{bmatrix}$$
This action eliminates the first column, $b_{1,x}$ and we are left with a similar matrix to find the determinant of. Reducing this one likewise we are left with $a_{1,1}*a_{2,2}*$(a matrix of remaining rows and columns) and so on until we have the product $a_{1,1}*a_{2,2}*\cdots*a_{n,n}$. The product of the diagonal.