# Math4201 Topology I (Lecture 13)

## Metic spaces

### Three different metrics on $\mathbb{R}^n$

Euclidean metric:

$$
d(x,y)=\sqrt{\sum_{i=1}^n (x_i-y_i)^2}
$$

Square metric:

$$
d(x,y)=\max_{i=1}^n |x_i-y_i|
$$

Manhattan metric:

$$
d(x,y)=\sum_{i=1}^n |x_i-y_i|
$$

So to prove our proposition, we need to show that any pair of metrics $d$ and $d'$ with basis generated by balls defined

$$
\mathcal{B}=\{B_r^{(d)}(x)|x\in X,r>0,r\in \mathbb{R}\}
$$

and

$$
\mathcal{B}'=\{B_r^{(d')}(x)|x\in X,r>0,r\in \mathbb{R}\}
$$

are equivalent.

#### Proposition: The metrics induce the same topology on $\mathbb{R}^n$

The three metrics induce the same topology on $\mathbb{R}^n$, and it's the standard topology.

#### Lemma of equivalent topologies

If $\mathcal{T}$ and $\mathcal{T}'$ are two topologies on $X$, we say $\mathcal{T}$ and $\mathcal{T}'$ are equivalent to each other if and only if the following two conditions are satisfied:

1. $\forall B_1\in \mathcal{T}, \exists B_2\in \mathcal{T}'$ such that $\forall x\in B_1, \exists x\in B_2\subseteq B_1$.
2. $\forall B_2\in \mathcal{T}', \exists B_1\in \mathcal{T}$ such that $\forall x\in B_2, \exists x\in B_1\subseteq B_2$.

#### Lemma of equivalent metrics

Let $d$ and $d'$ be two metrics on $X$. If the following holds, then the metric topology associated to $d$ and $d'$ are equivalent.

1. $\forall x\in X, \forall \delta>0, \exists \epsilon>0$ such that $B_\delta(x)\subseteq B_\epsilon(x)$
2. $\forall x\in X, \forall \epsilon>0, \exists \delta>0$ such that $B_\epsilon(x)\subseteq B_\delta(x)$

<details>
<summary>Proof</summary>

To apply the lemma, we try to compute the three metrics on $\mathbb{R}^n$.

$u=(u_1,u_2,\dots,u_n), v=(v_1,v_2,\dots,v_n)\in \mathbb{R}^n$

For Euclidean metric:

$$
d(u,v)=\sqrt{\sum_{i=1}^n (u_i-v_i)^2}
$$

For square metric:

$$
\rho(u,v)=\max_{i=1}^n |u_i-v_i|
$$

For Manhattan metric:

$$
m(u,v)=\sum_{i=1}^n |u_i-v_i|
$$

**First** we will show that $d$ and $\rho$ are equivalent.

Note that

$$
\max_{i=1}^n |u_i-v_i|\leq \sqrt{\sum_{i=1}^n (u_i-v_i)^2}
$$

So $\forall u\in B_r^{(d)}(x), d(u,v)<r\implies \rho(u,v)<r$

So $u\in B_r^{(\rho)}(x)$.

$$
B_r^{(d)}(u)\subseteq B_r^{(\rho)}(u)
$$

Note that

$$
\sqrt{\sum_{i=1}^n (u_i-v_i)^2}\leq \sqrt{\sum_{i=1}^n max\{|u_i-v_i|\}^2}=\sqrt{n}\times max\{|u_i-v_i|\}
$$

So $\forall u\in B_{r/\sqrt{n}}^{(\rho)}(x), \rho(u,v)<r/\sqrt{n}\implies d(u,v)<r$

So $u\in B_r^{(d)}(x)$.

So $B_{r/\sqrt{n}}^{(\rho)}(x)\subseteq B_r^{(d)}(x)$.

> imagine two square capped circle inside

**Then we will show** that $\rho$ and $m$ are equivalent.

Observing that

$$
\max_{i=1}^n |u_i-v_i|\leq \sum_{i=1}^n |u_i-v_i|\leq n\times \max_{i=1}^n |u_i-v_i|
$$

Then we have

$$
B_{r/n}^{(\rho)}(x)\subseteq B_r^{(m)}(x)\subseteq B_n^{(\rho)}(x)
$$

> imagine two square capped a diamonds inside

**Finally**, we will show that the topology generated by the square metric is the same as the product topology on $\mathbb{R}^n$.

Recall the basis for the product topology on $\mathbb{R}^n$ with standard topology.

$$
\mathcal{B}=\{(a_1,b_1)\times (a_2,b_2)\times \cdots \times (a_n,b_n)|a_i,b_i\in \mathbb{R},a_i<b_i\}
$$

Let $x=(x_1,x_2,\dots,x_n)\in (a_1,b_1)\times (a_2,b_2)\times \cdots \times (a_n,b_n)$.

Let $\delta=\min_{i=1}^n \{|x_i-a_i|,|x_i-b_i|\}$.

Then $x\in B_\delta^{(\rho)}(x)\subseteq (a_1,b_1)\times (a_2,b_2)\times \cdots \times (a_n,b_n)$.

In the other direction, let $x\in B_r^{(\rho)}(x)$, $x=(x_1,x_2,\dots,x_n)$.

Then $B_r^{(\rho)}(x)\subseteq (x_1-r,x_1+r)\times (x_2-r,x_2+r)\times \cdots \times (x_n-r,x_n+r)$.

This is an element of $\mathcal{B}$, by combining the two directions, we have the standard topology is the same as the topology generated by the square metric.

</details>

#### Proposition of metric induced product topology

Let $(X,d),(Y,d')$ be two metric spaces with metric topology $\mathcal{T},\mathcal{T}'$. On $X\times Y$, we can define a metric $\rho$ by $\rho((x,y),(x',y'))\coloneqq \max\{d(x,x'),d'(y,y')\}$, $(x,y),(x',y')\in X\times Y$.

Then this metric topology on $X\times Y$ is the same as the product topology on $X\times Y$.

> [!NOTE]
>
> Product of metrizable topological spaces is metrizable.

