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The following pearl by Jacobson can under no circumstances be left out from the list:

Let $\mathbf{R}$ be a ring with center $\mathrm{Z}$. Let us suppose that you can find $n \in \mathbb{N}_{>1}$ such that $x^{n}-x \in \mathrm{Z}$ for every $x \in \mathbf{R}$. Then $\mathbf{R}$ is a commutative ring.

A good place to learn more about results of this kind is Herstein's Noncomutative Noncommutative rings.

show/hide this revision's text 2 added 21 characters in body; deleted 8 characters in body; deleted 11 characters in body

The following pearl by Jacobson can under no circumstances be left out from the list:

Let $\mathbf{R}$ be a ring with center $\mathrm{Z}$ \mathrm{Z}$. Let us suppose that you can find $n \in \mathbb{N}_{>1}$ such that $x^{n}-x \in \mathrm{Z}$ for every $x \in \mathbf{R}$ and a fixed natural number $n$ greater than $1$. mathbf{R}$. Then $\mathbf{R}$ is a commutative ring.

In Herstein's Noncomutative rings you can find a delightful treatment of

A good place to learn more about results of this typekind is Herstein's Noncomutative rings.

show/hide this revision's text 1 [made Community Wiki]

The following pearl by Jacobson can under no circumstances be left out from the list:

Let $\mathbf{R}$ be a ring with center $\mathrm{Z}$ such that $x^{n}-x \in \mathrm{Z}$ for every $x \in \mathbf{R}$ and a fixed natural number $n$ greater than $1$. Then $\mathbf{R}$ is a commutative ring.

In Herstein's Noncomutative rings you can find a delightful treatment of results of this type.