Entropy of Measure-Preserving Transformation with respect to Finite Sub-Sigma Algebra is Well-Defined
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Theorem
Let $\struct {X, \BB, \mu}$ be a probability space.
Let $T: X \to X$ be a $\mu$-preserving transformation.
Let $\AA \subseteq \BB$ be a finite sub-$\sigma$-algebra.
Then the entropy of $T$ with respect to $\AA$:
- $\ds \map h {T, \AA} := \lim_{n \mathop \to \infty} \frac 1 n \map H {\bigvee_{k \mathop = 0}^{n - 1} T^{-k} \AA}$
is well-defined.
Proof
Let:
- $\ds a_n := \map H {\bigvee_{k \mathop = 0}^{n - 1} T^{-k} \AA}$
We need to show that the limit:
- $\ds \lim_{n \mathop \to \infty} \frac {a_n} n$
exists.
In view of Fekete's Subadditive Lemma, it suffices to show the subadditivity of $\sequence {a_n}$.
To this end, let $m, n \ge 1$.
Then:
\(\ds a_{m + n}\) | \(=\) | \(\ds \map H {\bigvee_{k \mathop = 0}^{m - 1} T^{-k} \AA \vee \bigvee_{k \mathop = m}^{m + n - 1} T^{-k} \AA }\) | ||||||||||||
\(\ds \) | \(\le\) | \(\ds \map H {\bigvee_{k \mathop = 0}^{m - 1} T^{-k} \AA} + \map H {\bigvee_{k \mathop = m}^{m + n - 1} T^{-k} \AA }\) | Conditional Entropy of Join as Sum/Corollary 5 | |||||||||||
\(\ds \) | \(=\) | \(\ds \map H {\bigvee_{k \mathop = 0}^{m - 1} T^{-k} \AA} + \map H {\bigvee_{k \mathop = 0}^{n - 1} T^{-k} \AA }\) | Entropy of Finite Sub-Sigma-Algebra is Transformation Invariant | |||||||||||
\(\ds \) | \(=\) | \(\ds a_m + a_n\) |
$\blacksquare$