Variation of Complex Measure is Finite Measure/Lemma
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Lemma
Let $\struct {X, \Sigma}$ be a measurable space.
Let $\mu$ be a complex measure on $\struct {X, \Sigma}$.
Let $\cmod \mu$ be the variation of $\mu$.
Let $\tuple {\mu_1, \mu_2, \mu_3, \mu_4}$ be the Jordan decomposition of $\mu$.
Then:
- $\map {\cmod \mu} A < \infty$ for each $A \in \Sigma$.
Proof
From Measures in Jordan Decomposition of Complex Measure are Finite, we have:
- $\mu_1$, $\mu_2$, $\mu_3$, $\mu_4$ are finite.
So there exist real numbers $M_1, M_2, M_3, M_4 > 0$ such that:
- $\map {\mu_i} A \le M_i$
for each $A \in \Sigma$.
Taking:
- $M_1 + M_2 + M_3 + M_4 = M$
we have, from Bound for Variation of Complex Measure in terms of Jordan Decomposition:
- $\ds \sum_{i \mathop = 1}^n \cmod {\map \mu {A_i} } \le \map {\mu_1} A + \map {\mu_2} A + \map {\mu_3} A + \map {\mu_4} A \le M < \infty$
for each $A \in \Sigma$, with $M > 0$ a real number independent of $A$.
So, for each:
- $\ds S \in \set {\sum_{j \mathop = 1}^n \cmod {\map \mu {A_j} } : \set {A_1, A_2, \ldots, A_n} \in \map P A}$
we have:
- $S \le M$
for some $M \in \R$.
So, from the definition of supremum, we have:
- $\ds \sup \set {\sum_{j \mathop = 1}^n \cmod {\map \mu {A_j} } : \set {A_1, A_2, \ldots, A_n} \in \map P A} \le M$
that is:
- $\map {\cmod \mu} A \le M < \infty$
So:
- $\map {\cmod \mu} A < \infty$ for each $A \in \Sigma$.
$\blacksquare$