# Thread: M_n Matrix ring

1. ## M_n Matrix ring

Let $M_n(A)$ be the ring of matrixes with elements from the ring $A$. $T_n(A) \subset M_n(A)$ the set of matrixes in which all elements below the diagonal are 0. $T'_n(A) \subset T_n(A)$ the set of matrixes with the elements in the principal diagonal are 0. $D_n(A)$ the set of diagonal matrixes.

Prove that,

$T'_n(A) \cong T_n(A)/D_n(A)$

2. Originally Posted by roporte
Let $M_n(A)$ be the ring of matrixes with elements from the ring $A$. $T_n(A) \subset M_n(A)$ the set of matrixes in which all elements below the diagonal are 0. $T'_n(A) \subset T_n(A)$ the set of matrixes with the elements in the principal diagonal are 0. $D_n(A)$ the set of diagonal matrixes.

Prove that,

$T'_n(A) \cong T_n(A)/D_n(A)$
I do it for $n=3$ it will be clear.

Define the mapping $\begin{bmatrix} a & b & c \\ 0 & d & e \\ 0 & 0 & f \end{bmatrix} \mapsto \begin{bmatrix} 0 & b & c \\ 0 & 0 & e \\ 0 & 0 & 0 \end{bmatrix}$

Then the kernel of this mapping ( $\theta: T_n(A) \to T'_n(A)$) is $D_n$ and range is $T'_n(A)$.

By the fundamental isomorphism theorem,
$T'_n(A) \simeq T_n(A)/D_n(A)$