User:Caliburn/s/mt/Equality Almost Everywhere is Equivalence Relation/Lebesgue Space
Theorem
Let $\struct {X, \Sigma, \mu}$ be a measure space and $p \in \closedint 1 \infty$.
Let $\map {\LL^p} {X, \Sigma, \mu}$ be the Lebesgue $p$-space on $\struct {X, \Sigma, \mu}$.
Let $\sim_\mu$ be the $\mu$-almost-everywhere equality relation on $\map {\LL^p} {X, \Sigma, \mu}$.
Then $\sim_\mu$ is an equivalence relation.
Proof
$\sim_\mu$ is Reflexive
We first show that $\sim_\mu$ is reflexive.
Let $f \in \map {\LL^p} {X, \Sigma, \mu}$.
We have that:
- $\norm {f - f}_p = \norm 0_p = 0$
from P-Seminorm of Function Zero iff A.E. Zero.
So $f \sim_\mu f$.
So $\mu_\sim$ is reflexive.
$\Box$
$\sim_\mu$ is Symmetric
We now show that $\sim_\mu$ is symmetric.
Let $f, g \in \map {\LL^p} {X, \Sigma, \mu}$.
Then:
- $\norm {f - g}_p = \size {-1} \norm {g - f}_p = \norm {g - f}_p$
from property $(\text N 2)$ of the seminorm $\norm \cdot_p$.
So:
- $\norm {f - g}_p = 0$ if and only if $\norm {g - f}_p = 0$
so:
- $f \sim_\mu g$ if and only if $g \sim_\mu f$.
So $\sim_\mu$ is symmetric.
$\Box$
$\sim_\mu$ is Transitive
We finally show that $\sim_\mu$ is transitive.
Let $f, g, h \in \map {\LL^p} {X, \Sigma, \mu}$ be such that $f \sim_\mu g$ and $g \sim_\mu h$.
Then:
- $\norm {f - g}_p = 0$ and $\norm {g - h}_p = 0$.
Then:
\(\ds \norm {f - h}_p\) | \(=\) | \(\ds \norm {\paren {f - g} - \paren {g - h} }_p\) | ||||||||||||
\(\ds \) | \(\le\) | \(\ds \norm {f - g}_p + \norm {g - h}_p\) | Minkowski's Inequality on Lebesgue Space | |||||||||||
\(\ds \) | \(=\) | \(\ds 0\) |