# Open Ball in Cartesian Product under Chebyshev Distance

## Theorem

Let $M_1 = \struct {A_1, d_1}, M_2 = \struct {A_2, d_2}, \ldots, M_n = \struct {A_n, d_n}$ be metric spaces.

Let $\displaystyle \AA = \prod_{i \mathop = 1}^n A_i$ be the cartesian product of $A_1, A_2, \ldots, A_n$.

Let $d_\infty: \AA \times \AA \to \R$ be the Chebyshev distance on $\AA$:

$\displaystyle \map {d_\infty} {x, y} = \max_{i \mathop = 1}^n \set {\map {d_i} {x_i, y_i} }$

where $x = \tuple {x_1, x_2, \ldots, x_n}, y = \tuple {y_1, y_2, \ldots, y_n} \in \AA$.

Let $a = \tuple {a_1, a_2, \ldots, a_n} \in \AA$.

Let $\epsilon \in \R_{>0}$.

Let $\map {B_\epsilon} {a; d_\infty}$ be the open $\epsilon$-ball of $a$ in $M = \struct {\AA, d_\infty}$.

Then:

$\displaystyle \map {B_\epsilon} {a; d_\infty} = \prod_{i \mathop = 1}^n \map {B_\epsilon} {a_i; d_i}$

## Proof

Let $\epsilon \in \R_{>0}$.

Let $x = \tuple {x_1, x_2, \ldots, x_n} \in \AA$.

Then:

 $\displaystyle x$ $\in$ $\displaystyle \map {B_\epsilon} {a; d_\infty}$ $\displaystyle \leadsto \ \$ $\displaystyle \map {d_\infty} {x, a}$ $<$ $\displaystyle \epsilon$ Definition of Open $\epsilon$-Ball $\displaystyle \leadsto \ \$ $\displaystyle \max_{i \mathop = 1}^n \set {\map {d_i} {x_i, a_i} }$ $<$ $\displaystyle \epsilon$ Definition of Chebyshev Distance $\displaystyle \leadsto \ \$ $\, \displaystyle \forall i \in \set {1, 2, \ldots, n} \,$ $\displaystyle \map {d_i} {x_i, a_i}$ $<$ $\displaystyle \epsilon$ Definition of Maximum Element $\displaystyle \leadsto \ \$ $\, \displaystyle \forall i \in \set {1, 2, \ldots, n} \,$ $\displaystyle x_i$ $\in$ $\displaystyle \map {B_\epsilon} {a_i; d_i}$ Definition of Open $\epsilon$-Ball $\displaystyle \leadsto \ \$ $\displaystyle x$ $\in$ $\displaystyle \prod_{i \mathop = 1}^n \map {B_\epsilon} {a_i; d_i}$ Definition of Finite Cartesian Product $\displaystyle \leadsto \ \$ $\displaystyle \map {B_\epsilon} {a; d_\infty}$ $\subseteq$ $\displaystyle \prod_{i \mathop = 1}^n \map {B_\epsilon} {a_i; d_i}$ Definition of Subset

And then:

 $\displaystyle x$ $\in$ $\displaystyle \prod_{i \mathop = 1}^n \map {B_\epsilon} {a_i; d_i}$ $\displaystyle \leadsto \ \$ $\, \displaystyle \forall i \in \set {1, 2, \ldots, n} \,$ $\displaystyle x_i$ $\in$ $\displaystyle \map {B_\epsilon} {a_i; d_i}$ Definition of Finite Cartesian Product $\displaystyle \leadsto \ \$ $\, \displaystyle \forall i \in \set {1, 2, \ldots, n} \,$ $\displaystyle \map {d_i} {x_i, a_i}$ $<$ $\displaystyle \epsilon$ Definition of Open $\epsilon$-Ball $\displaystyle \leadsto \ \$ $\displaystyle \max_{i \mathop = 1}^n \set {\map {d_i} {x_i, a_i} }$ $<$ $\displaystyle \epsilon$ Definition of Maximum Element $\displaystyle \leadsto \ \$ $\displaystyle \map {d_\infty} {x, a}$ $<$ $\displaystyle \epsilon$ Definition of Chebyshev Distance $\displaystyle \leadsto \ \$ $\displaystyle x$ $\in$ $\displaystyle \map {B_\epsilon} {a; d_\infty}$ Definition of Open $\epsilon$-Ball $\displaystyle \leadsto \ \$ $\displaystyle \prod_{i \mathop = 1}^n \map {B_\epsilon} {a_i; d_i}$ $\subseteq$ $\displaystyle \map {B_\epsilon} {a; d_\infty}$ Definition of Subset

The result follows by definition of set equality.

$\blacksquare$