Why an euclidean map defined onto a domain $R$, so a map $\delta\colon R\smallsetminus\{0\}\to\mathbb{N}$, satisfies $\delta(a)\leq\delta(ab)$ for every $a,b\in R\smallsetminus\{0\}$?
In some books, this property does not belongs to the definition of euclidean domain, and now I have to prove it.