2 hours ago, by
Balarka Sen @GustavoMontano We have the two metric space $(X, d)$ and $(X, \bar{d} = d/(1+d))$. We want to prove that the open sets of the two are the same. Consider the open sphere $S_r(x_0)$ on $(X, d)$. $r > d(x, x_0) > d(x, x_0)/(1+d(x, x_0)) = \bar{d}(x, x_0)$ so we get an open ball $S'_r(x_0)$ in $(X, \bar{d})$. Conversely, $S_r(x_0)$ be the ball in $(X, \bar{d})$. Then $\bar{d}(x, x_0) < r$ implies $d(x, x_0) < \frac{r}{1-r}$.Pick $r_1 = r/(1-r)$ which gets you a ball $S_{r_1}(x_0)$ in $(X, \bar{d})$