Equivalence of Definitions of Oscillation of Real Function at Point

Theorem
Let $X$ and $Y$ be real sets.

Let $f: X \to Y$ be a real function.

Let $x \in X$.

Then the following definitions of Oscillation at a Point are equivalent:

Definition 3
In the definitions above, the oscillation of $f$ on a non-empty set $A \subseteq X$ is defined as:


 * $\displaystyle \omega_f \left({A}\right) := \sup_{y, z \mathop \in A} \left\lvert{f \left({y}\right) - f \left({z}\right)}\right\rvert$

where the supremum is taken in the extended real numbers $\overline \R$.

Definitions 1 and 3 are equivalent
We reformulate Definition 1 into Definition 1' by:
 * substituting the definition of $\omega_f \left({U \cap X}\right)$ into the definition of $\omega_f \left({x}\right)$

Definition 1':
 * $\displaystyle \omega_f \left({x}\right) := \inf_{U \mathop \in \mathcal N_x} \left({\sup_{y, z \mathop \in U \cap X} \left\lvert{f \left({y}\right) - f \left({z}\right)}\right\rvert}\right)$

We reformulate Definition 3 into Definition 3' by:
 * substituting the definition of $\omega_f \left({\left({x - h \,.\,.\, x + h}\right) \cap X}\right)$ into the definition of $\omega_f \left({x}\right)$

Definition 3':
 * $\displaystyle \omega_f \left({x}\right) := \lim_{h \to 0^+} \left({\sup_{y, z \mathop \in \left({x - h \,.\,.\, x + h}\right) \cap X} \left\lvert{f \left({y}\right) - f \left({z}\right)}\right\rvert}\right)$

The theorem text of Oscillation at a Point equals Limit of Oscillation on a Set also contains two definitions of Oscillation at a Point for a real function $f$.

We call them definitions a and b.

Definition a:

Let $\mathcal N_x$ be the set of open subset neighborhoods of $x$.


 * $\omega_f \left({x}\right) = \displaystyle \inf \left\{{\omega_f \left({I}\right): I \in \mathcal N_x}\right\}$

where:
 * $\omega_f \left({I}\right) = \displaystyle \sup \left\{{\left\vert{f \left({y}\right) - f \left({z}\right)}\right\vert: y, z \in I \cap X}\right\}$

Definition b:
 * $\omega_f \left({x}\right) = \displaystyle \lim_{h \to 0^+} \omega_f \left({\left({x - h \,.\,.\, x + h}\right)}\right)$

We reformulate Definition a into Definition a' by:
 * changing set conditions into sup tags
 * renaming parameter set $I$ to $U$
 * substituting the definition of $\omega_f \left({I}\right)$ into the definition of $\omega_f \left({x}\right)$

Definition a':
 * $\displaystyle \omega_f \left({x}\right) = \inf_{U \mathop \in \mathcal N_x} \left({\sup_{y, z \mathop \in U \cap X} \left\lvert{f \left({y}\right) - f \left({z}\right)}\right\rvert}\right)$

We reformulate Definition b into Definition b' by:
 * substituting the definition of $\omega_f \left({\left({x - h \,.\,.\, x + h}\right)}\right)$ (by using the definition of $\omega_f \left({I}\right)$) into the definition of $\omega_f \left({x}\right)$
 * changing set condition into sup tag

Definition b':
 * $\displaystyle \omega_f \left({x}\right) = \lim_{h \to 0^+} \left({\sup_{y, z \mathop \in \left({x - h \,.\,.\, x + h}\right) \cap X} \left\lvert{f \left({y}\right) - f \left({z}\right)}\right\rvert}\right)$

We observe that definitions 1' and a' are the same, so they are equivalent.

Therefore:
 * definitions 1 and a are equivalent

Also, definitions 3' and b' are the same, so they are equivalent.

Therefore:
 * definitions 3 and b are equivalent

We have:
 * definitions 1 and a are equivalent
 * definitions a and b are equivalent by Oscillation at a Point equals Limit of Oscillation on a Set
 * definitions b and 3 are equivalent

Therefore:
 * definitions 1 and 3 are equivalent