The actual problem looks like-
Let, $(X,d)$ be a metric space where $X\ne\emptyset$. Define $d_1$ and
$d_2$ on $X\times X$ by $d_1(x,y)=\frac{d(x,y)}{1+d(x,y)}$,
$d_2(x,y)=\min\{1, d(x,y)\}\quad\forall(x,y)\in X\times X.$(It is easy
to verify $d_1,d_2$ are two metrics on $X$.) Show that, $d_1, d_2$ are
equivalent metrics on $X$.
Firstly, I'll use the standard theorem to prove two metrics on a non-empty set are equivalent-
Two metrics $d_1$ and $d_2$ on a non-empty set $X$ are equivalent if and only if for each $x\in X$ and each open ball $B_{d_i} (x,r)$ in $(X,d_i)$(for some $r>0$), there is an $s>0$ such that $B_{d_j} (x,s)\subseteq B_{d_i} (x,r)$, for $i, j=1,2$ and $i\ne j$.
Now, to solve the problem, I break it into to parts- If I can show that both of $d_1,d_2$ are equivalent with the metric $d$, then we are done.
I'm able to show $d$ and $d_2$ are equivalent in the following way-
Let, $x\in X$ and $B_{d_2} (x,r)$ be an open ball in $(X, d_2)$ for some $r>0$. Then $B_{d} (x,r)\subseteq B_{d_2} (x,r)$.
Again, if we take an open ball $B_{d} (x,t)$ in $(X,d)$ for some $t>0$. Then $B_{d_2} (x,s)\subseteq B_{d} (x,t)$, where $s=\min \{1,t\}$.
But how to show $d_1$ and $d$ are equivalent metrics in $X$?
Can anyone assist me to prove $d_1$ and $d$ are equivalent metrics on $X$? Thanks for your help in advance.
Best Answer
Note that $d_1(x,y)\leq d(x,y)$ for all $x,y\in X$, since $1+d(x,y)\geq 1$. Thus, it follows that $B_{\varepsilon}^d(x)\subseteq B_{\varepsilon}^{d_1}(x)$ for all $x\in X$ and $\varepsilon>0$. Hence, if $U\subseteq X$ is $d_1$-open, it is also $d$-open.
Conversely, suppose that $U$ is $d$-open and let $x\in U$. By definition of an open set in a metric space, there exists $\varepsilon>0$ such that $B_{\varepsilon}^d(x)\subseteq U$. Now let $\delta=\min\{\varepsilon/2,1/2\}$. If $d_1(x,y)<\delta$ then $d(x,y)<\varepsilon$, because: $$d(x,y)=\frac{d_1(x,y)}{1-d_1(x,y)}\leq 2d_1(x,y)< \epsilon$$ Hence, $B_{\delta}^{d_1}(x)\subseteq B_{\varepsilon}^d(x)\subseteq U$, which implies that $U$ is $d_1$-open.
Thus, by the above, $(X,d)$ and $(X,d_1)$ are topologically equivalent.
Since you have shown that $(X,d)$ and $(X,d_2)$ are topologically equivalent, it follows from the fact that topological equivalence is a equivalence relation that $(X,d_1)$ and $(X,d_2)$ are topologically equivalent.