If $D$ is a division ring, then the ring $M_n(D)$ of $n \times n$matrices with entries in $D$ is a simple ring.

The Weyl algebra$k\langle x, y\rangle/(x y - y x - 1)$ over a field $k$ is a simple ring. (In different language: this is the ring of differential operators with polynomial coefficients in one variable $t$, obtained as the image of the ring homomorphism from the noncommutative polynomial ring$k \langle x, y \rangle$ to the ring of $k$-linear endomorphisms $Vect(k[t], k[t])$ that sends $x$ to the derivative operator $\frac{d}{d t}$ and $y$ to the multiplication operator $t \cdot -$.) An explanation of why this is simple may be found here at Qiaochu Yuan’s blog.

Last revised on December 11, 2017 at 11:10:54.
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