\begin{definition}[Definition:Apotome]
Let $a, b \in \set {x \in \R_{>0} : x^2 \in \Q}$ be two rationally expressible numbers such that $a > b$.
Then $a - b$ is an '''apotome''' {{iff}}:
:$(1): \quad \dfrac a b \notin \Q$
:$(2): \quad \paren {\dfrac a b}^2 \in \Q$
where $\Q$ denotes the set of rational numbers.
{{:Euclid:Proposition/X/73}}
\end{definition}