Metric on Shift of Finite Type is Metric
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Theorem
Let $\struct {X _\mathbf A, \sigma_\mathbf A}$ be a shift of finite type.
Let $\theta \in \openint 0 1$.
Then the metric $d_\theta$ is indeed a metric on $X_\mathbf A$.
That is, $\struct {X _\mathbf A, d _\theta} $ is a metric space.
Proof
M1
Let $x\in X _\mathbf A$
Obviously, $x_i = x_i$ for all $i \in \openint {-\infty} \infty$.
Therefore:
- $\map {d _\theta} {x,x} = \theta ^\infty = 0$.
$\Box$
M3
This is clear from the definition.
$\Box$
M4
Let $x,y\in X _\mathbf A$ such that $x\ne y$.
That is:
- $\exists k\in\Z : x_k\ne y_k$.
Thus:
- $N:=\sup \set {n \in \N \cup \set \infty : x_i = y_i \text { for all } i \in \openint {-n} n}\leq \size k$
Therefore:
- $\map {d _\theta} {x,x} = \theta ^N > 0$.
$\Box$
M2
Let $x,y,z\in X _\mathbf A$.
Let:
- $N_1 := \sup \set {n \in \N \cup \set \infty : x_i = y_i \text { for all } i \in \openint {-n} n}$
and:
- $N_2 := \sup \set {n \in \N \cup \set \infty : y_i = z_i \text { for all } i \in \openint {-n} n}$
so that:
- $\map {d _\theta} {x,y} = \theta ^{N_1}$
and:
- $\map {d _\theta} {y,z} = \theta ^{N_2}$
Without loss of generality, say $N_1 \leq N_2$.
Then for all $i \in \openint {-N_1} {N_1}$:
- $x_i = y_i = z_i$
Hence:
- $\sup \set {n \in \N \cup \set \infty : x_i = z_i \text { for all } i \in \openint {-n} n} \geq N_1$
so that:
- $\map {d _\theta} {x,z} \leq \theta ^{N_1}$
Therefore:
- $\map {d _\theta} {x,z} \leq \theta ^{N_1} \leq \theta ^{N_1} + \theta ^{N_2} = \map {d _\theta} {x,y} + \map {d _\theta} {y,z}$
$\blacksquare$