Number Fields
《Number Fields》 · 第 9 页 · Chapter 2「Number Fields and Number Rings」开篇
Another infinite class of number fields consists of the quadratic fields $\mathbb Q[\sqrt m]$, $m\in\mathbb Z$, $m$ not a perfect square. Clearly these fields have degree $2$ over $\mathbb Q$, having basis $\{1,\sqrt m\}$.