Notations.

The set of non-negative integer numbers, integer numbers and rational numbers are denoted respectively ${\mathbb{N}}$ , ${\mathbb{Z}}$ and ${\mathbb{Q}}$ , as usual. For an integer $n \in {\mbox{${\mathbb{Z}}$}}$ the set of the multiples of $n$ (all of them: positive, null or negative) is denoted by $n{\mbox{${\mathbb{Z}}$}}$ . For an integer $n \in {\mbox{${\mathbb{Z}}$}}$ the residue class of the binary relation $(a,b) \longmapsto a \equiv b \mod{n}$ is denoted by ${\mbox{${\mathbb{Z}}$}}/n{\mbox{${\mathbb{Z}}$}}$ , as usual. For a ring ${\mathbb{A}}$ , we denote by ${\mbox{${\mathbb{A}}$}}[x]$ the ring of univariate polynomials in $x$ with coefficients in ${\mathbb{A}}$ .