The foundations of geometry
《The foundations of geometry》 · 第 362 页 · Chapter 15，Section 15.5「Solving Polynomial Equations by Radicals」，Definition 15.42
A field extension $k\subseteq K$ is ‘radical’ if there exist $u_1,\ldots,u_r\in K$ and positive integers $m_1,\ldots,m_r$ such that $k\subseteq k(u_1)\subseteq k(u_1,u_2)\subseteq\cdots\subseteq k(u_1,\ldots,u_r)=K$ and $u_i^{m_i}\in k(u_1,\ldots,u_{i-1})$ for $i=1,\ldots,r$.