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The Arithmetical Hierarchy in the Setting of 1 !1 - computability Arithmetical Hierarchy in !1 Computable infinitary formulas The Arithmetical Hierarchy in the Setting of !1 - Computability Jesse Johnson Department of Mathematics University of Notre Dame 2011 ASL North American Meeting – March 26, 2011 Johnson The Arithmetical Hierarchy in the Setting of !1 - Computability !1 - computability Arithmetical Hierarchy in !1 Computable infinitary formulas A.H. in !1 - computability Joint work with Jacob Carson, Julia Knight, Karen Lange, Charles McCoy, John Wallbaum. The Arithmetical hierarchy in the setting of !1 - computability, preprint. Continuation of work from N. Greenberg and J. F. Knight, Computable structure theory in the setting of !1. Johnson The Arithmetical Hierarchy in the Setting of !1 - Computability ! - computability 1 Introductory definitions Arithmetical Hierarchy in ! 1 Indicies and the jump Computable infinitary formulas Two definitions for the arithmetical hierarchy We will give two definitions for the arithmetical hierarchy in the setting of !1 - computability. The first will resemble the definition of the effective Borel Hierarchy. The second will resemble the standard definition of the hyper-arithmetical hierarchy. Johnson The Arithmetical Hierarchy in the Setting of !1 - Computability ! - computability 1 Introductory definitions Arithmetical Hierarchy in ! 1 Indicies and the jump Computable infinitary formulas !1 - computability Definition Suppose R is a relation of countable arity α. R is computably enumerable if the set of ordinal codes for sequences in R is definable by a Σ1 formula in (L!1 ; ∈). R is computable if it is both c.e. and co-c.e. Johnson The Arithmetical Hierarchy in the Setting of !1 - Computability ! - computability 1 Introductory definitions Arithmetical Hierarchy in ! 1 Indicies and the jump Computable infinitary formulas Working in !1 We assume that P(!) ⊆ L!1 . Results of Gödel give a computable 1-1 function g from the countable ordinals onto L!1 , such that the relation g(α) ∈ g(β) is computable. So, computing in !1 is essentially the same as computing in L!1 . Johnson The Arithmetical Hierarchy in the Setting of !1 - Computability ! - computability 1 Introductory definitions Arithmetical Hierarchy in ! 1 Indicies and the jump Computable infinitary formulas Indices for c.e. sets As in the standard setting, we have a c.e. set of codes for Σ1 definitions. We write Wα for the c.e. set with index α. All these definitions relativize in the natural way. Johnson The Arithmetical Hierarchy in the Setting of !1 - Computability ! - computability 1 Introductory definitions Arithmetical Hierarchy in ! 1 Indicies and the jump Computable infinitary formulas The jump Definition We define the halting set as K = {α ∶ α ∈ Wα}. ′ X For a arbitrary set X, X = {α ∶ α ∈ Wα }. X(0) = X. X(α+1) = (X(α))′. For limit λ, X(λ) is the set of codes for pairs (β; x) such that β < λ and x ∈ X(β). 0 n−1 We write ∆n for ∅ for 1 ≤ n < !. 0 α We write ∆α for ∅ for α ≥ !. Johnson The Arithmetical Hierarchy in the Setting of !1 - Computability ! - computability 1 Two definitions for the arithmetical hierarchy Arithmetical Hierarchy in ! 1 Comparing the two definitions Computable infinitary formulas First definition for the arithmetical hierarchy Our first definition of the arithmetical hierarchy resembles the definition of the effective Borel hierarchy. Definition Let R be a relation. 0 0 R is Σ0 and Π0 if it is computable. 0 0 R is Σ1 if it is c.e.; R is Π1 if the complementary relation, ¬ R, is c.e. 0 For countable α > 1, R is Σα if it is a c.e. union of relations, 0 0 0 each of which is Πβ for some β < α; R is Πα if ¬ R is Σα. Johnson The Arithmetical Hierarchy in the Setting of !1 - Computability ! - computability 1 Two definitions for the arithmetical hierarchy Arithmetical Hierarchy in ! 1 Comparing the two definitions Computable infinitary formulas 0 0 Indices for Σα and Πα sets 0 0 For α ≥ 1, we may assign indices for the Σα and Πα sets in the natural way. For α = 1, we write (Σ; 1; γ) as the index for the c.e. set with index γ. The set with index (Π; 1; γ) is the complement. For α > 1, the set with index (Σ; α; γ) is the union of sets with indices in Wγ of the form (Π; β; δ) for some β < α and some countable δ. The set with index (Π; α; γ) is the complement. Johnson The Arithmetical Hierarchy in the Setting of !1 - Computability ! - computability 1 Two definitions for the arithmetical hierarchy Arithmetical Hierarchy in ! 1 Comparing the two definitions Computable infinitary formulas Second definition for the arithmetical hierarchy Our second definition for the arithmetical hierarchy resembles the standard definition for the hyper-arithmetical hierarchy. Definition Let R be a relation. 0 0 R is Σ0 and Π0 if it is computable. 0 0 R is Σ1 if it is c.e.; R is Π1 if ¬ R, is c.e. 0 0 0 For α > 1, R is Σα if it is c.e. relative to ∆α; R is Πα if ¬ R 0 is Σα. 0 0 We assign indices for the Σα and Πα sets in the same way. Johnson The Arithmetical Hierarchy in the Setting of !1 - Computability ! - computability 1 Two definitions for the arithmetical hierarchy Arithmetical Hierarchy in ! 1 Comparing the two definitions Computable infinitary formulas Comparing the two definitions The two definitions agree at finite levels, but disagree at level ! and beyond. Under the first definition, membership of an element into a 0 Σα set occurs if and only if that element is a member of 0 one of the lower Πβ sets. 0 So membership into a Σα set uses information from a single lower level. Under the second definition, membership of an element 0 0 into a Σα set may use a ∆α oracle to get information from all lower levels simultaneously. Johnson The Arithmetical Hierarchy in the Setting of !1 - Computability ! - computability 1 Two definitions for the arithmetical hierarchy Arithmetical Hierarchy in ! 1 Comparing the two definitions Computable infinitary formulas The two definitions disagree at level ! Proposition 0 There is a set S that is ∆! under the second definition, but is 0 not Σ! under the first definition. Johnson The Arithmetical Hierarchy in the Setting of !1 - Computability ! - computability 1 Two definitions for the arithmetical hierarchy Arithmetical Hierarchy in ! 1 Comparing the two definitions Computable infinitary formulas Proof of the proposition Proof. Define S such that α ∈ S iff α is not in the set with index (Σ; !; α) under the first definition. 0 For each n, α, let Sα,n be the union of the Σn sets with indices in Wα of the form (Π; k; β) with k < n. The union of these sets over all n will be the set with index (Σ; !; α). 0 A ∆! oracle can determine whether α ∈ Sn,α for all n. So S 0 is ∆! under the second definition. 0 However, S cannot be one of the Σ! sets under the first defintion. Johnson The Arithmetical Hierarchy in the Setting of !1 - Computability !1 - computability Definitions Arithmetical Hierarchy in !1 Main theorem Computable infinitary formulas Conclusion Computable infinitary formulas The first definition of the computable infinitary formulas corresponds to the first definition of the arithmetical hierarchy. Definition Let L be a predicate language with computable symbols. We consider L-formulas '(x) with a countable tuple of variables x. '(x) is computable Σ0 and computable Π0 if it is a quantifier-free formula of L!1;!. For α > 0, '(x) is computable Σα if ' ≡ (∃u) i(u; x), c:e: where each i is computable Πβ for some β < α. '(x) is computable Πα if ' ≡ (∀u) i(u; x), where each c:e: i is computable Σβ for some β < α. Johnson The Arithmetical Hierarchy in the Setting of !1 - Computability !1 - computability Definitions Arithmetical Hierarchy in !1 Main theorem Computable infinitary formulas Conclusion Computable infinitary formulas The second definition of the computable infinitary formulas corresponds to the second definition of the arithmetical hierarchy. Definition '(x) is computable Σ0 and computable Π0 if it is a quantifier-free formula of L!1;!. For α > 0, '(x) is computable Σα if ' ≡ (∃u) i(u; x), c:e: where each i is a countable conjunction of formulas, each computable Πβ for some β < α. '(x) is computable Πα if ' ≡ (∀u) i(u; x), where each c:e: i is a countable disjunction of formulas, each computable Σβ for some β < α. Johnson The Arithmetical Hierarchy in the Setting of !1 - Computability !1 - computability Definitions Arithmetical Hierarchy in !1 Main theorem Computable infinitary formulas Conclusion Proposition on computable infinitary formulas Using either one of the definitions for the computable infinitary formulas, the following proposition holds and is proved by induction on α. Proposition Let A be an L-structure, and let '(x) be a computable Σα (computable Πα) L-formula. Then the relation defined by '(x) 0 0 in A is Σα (Πα) relative to A. Johnson The Arithmetical Hierarchy in the Setting of !1 - Computability !1 - computability Definitions Arithmetical Hierarchy in !1 Main theorem Computable infinitary formulas Conclusion Relatively intrinsically arithmetical relations Definition Let A be a computable structure, and let R be a relation on A. 0 We say that R is relatively intrinsically Σα on A if for all 0 isomorphisms F from A onto a copy B, F(R) is Σα(B). Johnson The Arithmetical Hierarchy in the Setting of !1 - Computability !1 - computability Definitions Arithmetical Hierarchy in !1 Main theorem Computable infinitary formulas Conclusion Main theorem We now present our main theorem. Theorem Let 1 ≤ α < !1. For a relation R on a computable structure A, the following are equivalent: 1 0 R is relatively intrinsically Σα on A.
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