Filters equal Ideals in Dual Ordered Set

Theorem
Let $L_1 = \struct {S, \preceq_1}$ be an ordered set.

Let $L_2 = \struct {S, \preceq_2}$ be a dual ordered set of $L_1$.

Then $\map {\mathit {Filt} } {L_1} = \map {\mathit {Ids} } {L_2}$

where
 * $\map {\mathit {Filt} } {L_1}$ denotes the set of all filters of $L_1$,
 * $\map {\mathit {Ids} } {L_2}$ denotes the set of all ideals of $L_2$.

Proof
Let $x$ be a set.

By definition of $\mathit {Filt}$:
 * $x \in \map {\mathit {Filt} } {L_1} \iff x$ is a filter on $L_1$.

By Filter is Ideal in Dual Ordered Set:
 * $x \in \map {\mathit {Filt} } {L_1} \iff x$ is an ideal in $L_2$.

By definition of $\mathit{Ids}$:
 * $x \in \map {\mathit {Filt} } {L_1} \iff x \in \map {\mathit {Ids} } {L_2}$

Hence by definition of set equality:
 * $\map {\mathit {Filt} } {L_1} = \map {\mathit {Ids} } {L_2}$