《Algebra》
· 第 233 页 · Chapter V，Section 1「Field Extensions」，Theorem 1.3 后的定义段
If $L$ and $M$ are subfields of a field $F$, the composite of $L$ and $M$ in $F$, denoted $LM$, is the subfield generated by the set $L\cup M$. An immediate consequence of this definition is that $LM=L(M)=M(L)$.
