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あれ,位数4の単位的環って同型を除いて$\mathbb Z/4\mathbb Z,\mathbb F_4,$
$$\left\{\begin{pmatrix}
a&b
\\0&a
\end{pmatrix}\mathrel{}\middle|\mathrel{}a,b\in\mathbb{Z}/2\mathbb{Z}\right\},$$$$\left\{\begin{pmatrix}
a&0
\\0&b
\end{pmatrix}\mathrel{}\middle|\mathrel{}a,b\in\mathbb{Z}/2\mathbb{Z}\right\}$$
の他ある? 今のところ全部可換

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$$\left\{\begin{pmatrix} a&0 \\0&b \end{pmatrix}\mathrel{}\middle|\mathrel{}a,b\in\mathbb{Z}/2\mathbb{Z}\right\}$$
は$(\mathbb Z/2\mathbb Z)^2$と同型か

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