Prewellordering Example Svg

In set theory, a prewellordering on a set $X$ is a preorder $\leq$ on $X$ (a transitive and strongly connected relation on $X$) that is wellfounded in the sense that the relation $x \leq y \land y \nleq x$ is wellfounded. If $\leq$ is a prewellordering on $X,$ then the relation $\sim$ defined by
$x \sim y \text x \leq y \land y \leq x$
is an equivalence relation on $X,$ and $\leq$ induces a wellordering on the quotient $X / \sim.$ The order-type of this induced wellordering is an ordinal, referred to as the length of the prewellordering.

A norm on a set $X$ is a map from $X$ into the ordinals. Every norm induces a prewellordering; if $\phi : X \to Ord$ is a norm, the associated prewellordering is given by
$x \leq y \text \phi(x) \leq \phi(y)$
Conversely, every prewellordering is induced by a unique regular norm (a norm $\phi : X \to Ord$ is regular if, for any $x \in X$ and any $\alpha < \phi(x),$ there is $y \in X$ such that $\phi(y) = \alpha$).

Prewellordering property

If $\boldsymbol$ is a pointclass of subsets of some collection $\mathcal$ of Polish spaces, $\mathcal$ closed under Cartesian product, and if $\leq$ is a prewellordering of some subset $P$ of some element $X$ of $\mathcal,$ then $\leq$ is said to be a $\boldsymbol$-prewellordering of $P$ if the relations $<^*$ and $\leq^*$ are elements of $\boldsymbol,$ where for $x, y \in X,$
# $x <^* y \text x \in P \land (y \notin P \lor (x \leq y \land y \not\leq x))$
# $x \leq^* y \text x \in P \land (y \notin P \lor x \leq y)$

$\boldsymbol$ is said to have the prewellordering property if every set in $\boldsymbol$ admits a $\boldsymbol$-prewellordering.

The prewellordering property is related to the stronger scale property; in practice, many pointclasses having the prewellordering property also have the scale property, which allows drawing stronger conclusions.

Examples

$\boldsymbol^1_1$ and $\boldsymbol^1_2$ both have the prewellordering property; this is provable in ZFC alone. Assuming sufficient large cardinals, for every $n \in \omega,$ $\boldsymbol^1_$ and $\boldsymbol^1_$
have the prewellordering property.

Consequences

Reduction

If $\boldsymbol$ is an adequate pointclass with the prewellordering property, then it also has the reduction property: For any space $X \in \mathcal$ and any sets $A, B \subseteq X,$ $A$ and $B$ both in $\boldsymbol,$ the union $A \cup B$ may be partitioned into sets $A^*, B^*,$ both in $\boldsymbol,$ such that $A^* \subseteq A$ and $B^* \subseteq B.$

Separation

If $\boldsymbol$ is an adequate pointclass whose dual pointclass has the prewellordering property, then $\boldsymbol$ has the separation property: For any space $X \in \mathcal$ and any sets $A, B \subseteq X,$ $A$ and $B$ ''disjoint'' sets both in $\boldsymbol,$ there is a set $C \subseteq X$ such that both $C$ and its complement $X \setminus C$ are in $\boldsymbol,$ with $A \subseteq C$ and $B \cap C = \varnothing.$

For example, $\boldsymbol^1_1$ has the prewellordering property, so $\boldsymbol^1_1$ has the separation property. This means that if $A$ and $B$ are disjoint analytic subsets of some Polish space $X,$ then there is a Borel subset $C$ of $X$ such that $C$ includes $A$ and is disjoint from $B.$

See also

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* – a graded poset is analogous to a prewellordering with a norm, replacing a map to the ordinals with a map to the integers
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References

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{{Order theory

Binary relations
Descriptive set theory
Order theory
Wellfoundedness