Consider the ring $R_1 = \left\{ \begin{pmatrix} a & b \\ 0 & c \end{pmatrix} \mid a, c \in \mathbb{Z}, b \in \mathbb{Q} \right\}$. We want to show that this ring is right Noetherian but not left Noetherian.
To show that $R_1$ is right Noetherian, consider an
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