  Recently, Morier-Genoud and Ovsienko introduced a $q$-analog of rational
numbers. More precisely, for an irreducible fraction $\frac{r}s>0$, they
constructed coprime polynomials ${\mathcal R}_{\frac{r}s}(q), {\mathcal
S}_{\frac{r}s}(q) \in {\mathbb Z}[q]$ with ${\mathcal R}_{\frac{r}s}(1)=r,
{\mathcal S}_{\frac{r}s}(1)=s$. Their theory has a rich background and many
applications. By definition, if $r \equiv r' \pmod{s}$, then ${\mathcal
S}_{\frac{r}s}(q)={\mathcal S}_{\frac{r'}s}(q)$. We show that $rr'=-1 \pmod{s}$
implies ${\mathcal S}_{\frac{r}s}(q)={\mathcal S}_{\frac{r'}s}(q)$, and it is
conjectured that the converse holds if $s$ is prime (and $r \not \equiv r'
\pmod{s}$). We also show that $s$ is a multiple of 3 (resp. 4) if and only if
${\mathcal S}_{\frac{r}s}(\zeta)=0$ for $\zeta=(-1+\sqrt{-3})/2$ (resp.
$\zeta=i$). We give applications to the representation theory of quivers of
type $A$ and the Jones polynomials of rational links.
