Wadge hierarchy

In descriptive set theory, within mathematics, Wadge degrees are levels of complexity for sets of reals. Sets are compared by continuous reductions. The Wadge hierarchy is the structure of Wadge degrees. These concepts are named after William W. Wadge.
Wadge degrees.
Suppose formula_1 and formula_2 are subsets of Baire space ωω. Then formula_1 is Wadge reducible to formula_2 or formula_1 ≤W formula_2 if there is a continuous function formula_7 on ωω with formula_8. The Wadge order is the preorder or quasiorder on the subsets of Baire space. Equivalence classes of sets under this preorder are called Wadge degrees, the degree of a set formula_1 is denoted by [formula_1]W. The set of Wadge degrees ordered by the Wadge order is called the Wadge hierarchy.
Properties of Wadge degrees include their consistency with measures of complexity stated in terms of definability. For example, if formula_1 ≤W formula_2 and formula_2 is a countable intersection of open sets, then so is formula_1. The same works for all levels of the Borel hierarchy and the difference hierarchy. The Wadge hierarchy plays an important role in models of the axiom of determinacy. Further interest in Wadge degrees comes from computer science, where some papers have suggested Wadge degrees are relevant to algorithmic complexity.
Wadge and Lipschitz games.
The Wadge game is a simple infinite game discovered by William Wadge (pronounced "wage"). It is used to investigate the notion of continuous reduction for subsets of Baire space. Wadge had analyzed the structure of the Wadge hierarchy for Baire space with games by 1972, but published these results only much later in his PhD thesis. In the Wadge game formula_15, player I and player II each in turn play integers that may depend on those played before. The outcome of the game is determined by checking whether the sequences "x" and "y" generated by players I and II are contained in the sets "A" and "B", respectively. Player II wins if the outcome is the same for both players, i.e. formula_16 is in formula_1 if and only if formula_18 is in formula_2. Player I wins if the outcome is different. Sometimes this is also called the "Lipschitz game", and the variant where player II has the option to pass (but has to play infinitely often) is called the Wadge game.
Suppose for a moment that the game is determined. If player I has a winning strategy, then this defines a continuous (even Lipschitz) map reducing formula_2 to the complement of formula_1, and if on the other hand player II has a winning strategy then you have a reduction of formula_1 to formula_2. For example, suppose that player II has a winning strategy. Map every sequence "x" to the sequence "y" that player II plays in formula_15 if player I plays the sequence "x", and player II follows his or her winning strategy. This defines a continuous map "f" with the property that "x" is in formula_1 if and only if "f"("x") is in formula_2.
Wadge's lemma states that under the axiom of determinacy (AD), for any two subsets formula_27 of Baire space, formula_1 ≤W formula_2 or formula_2 ≤W ωω–formula_1. The assertion that the Wadge lemma holds for sets in Γ is the "semilinear ordering principle" for Γ or SLO(Γ). Any defines a linear order on the equivalence classes modulo complements. Wadge's lemma can be applied locally to any pointclass Γ, for example the Borel sets, Δ1n sets, Σ1n sets, or Π1n sets. It follows from determinacy of differences of sets in Γ. Since Borel determinacy is proved in ZFC, ZFC implies Wadge's lemma for Borel sets.
Structure of the Wadge hierarchy.
Martin and Monk proved in 1973 that AD implies the Wadge order for Baire space is well founded. Hence under AD, the Wadge classes modulo complements form a wellorder. The Wadge rank of a set formula_1 is the order type of the set of Wadge degrees modulo complements strictly below [formula_1]W. The length of the Wadge hierarchy has been shown to be Θ. Wadge also proved that the length of the Wadge hierarchy restricted to the Borel sets is φω1(1) (or φω1(2) depending on the notation), where φ"γ" is the "γ"th Veblen function to the base ω1 (instead of the usual ω).
As for the Wadge lemma, this holds for any pointclass Γ, assuming the axiom of determinacy. If we associate with each set formula_1 the collection of all sets strictly below formula_1 on the Wadge hierarchy, this forms a pointclass. Equivalently, for each ordinal "α" ≤ θ the collection W"α" of sets that show up before stage "α" is a pointclass. Conversely, every pointclass is equal to some formula_36"α". A pointclass is said to be "self-dual" if it is closed under complementation. It can be shown that W"α" is self-dual if and only if "α" is either 0, an even successor ordinal, or a limit ordinal of countable cofinality.
Other notions of degree.
Similar notions of reduction and degree arise by replacing the continuous functions by any class of functions "F" that contains the identity function and is closed under composition. Write formula_1 ≤F formula_2 if formula_8 for some function formula_7 in "F". Any such class of functions again determines a preorder on the subsets of Baire space. Degrees given by Lipschitz functions are called "Lipschitz degrees", and degrees from Borel functions "Borel–Wadge degrees".