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 modn 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 $\mathbb {A}$.