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In [[mathematical logic]], the '''arithmetical hierarchy''', '''arithmetic hierarchy''' or '''Kleene-Mostowski hierarchy''' classifies certain [[Set (mathematics)|sets]] based on the complexity of formulas that define them.  Any set that receives a classification is called '''arithmetical'''.
 
The arithmetical hierarchy is important in [[recursion theory]], [[effective descriptive set theory]], and the study of formal theories such as [[Peano arithmetic]]. 
 
The [[Tarski-Kuratowski algorithm]] provides an easy way to get an upper bound on the classifications assigned to a formula and the set it defines.
 
The [[hyperarithmetical hierarchy]] and the [[analytical hierarchy]] extend the arithmetical hierarchy to classify additional formulas and sets.
 
== The arithmetical hierarchy of formulas ==
 
The arithmetical hierarchy assigns classifications to the formulas in the language of [[Peano axioms|first-order arithmetic]].  The classifications are denoted <math>\Sigma^0_n</math> and <math>\Pi^0_n</math> for natural numbers ''n'' (including 0). The Greek letters here are [[lightface]] symbols, which indicates that the formulas do not contain set parameters.
 
If a formula <math>\phi</math> is logically equivalent to a formula with only [[bounded quantifier]]s then <math>\phi</math> is assigned the classifications <math>\Sigma^0_0</math> and <math>\Pi^0_0</math>.
 
The classifications <math>\Sigma^0_n</math> and <math>\Pi^0_n</math> are defined inductively for every natural number ''n'' using the following rules:
*If <math>\phi</math> is logically equivalent to a formula of the form <math>\exists n_1 \exists n_2\cdots \exists n_k \psi</math>, where <math>\psi</math> is <math>\Pi^0_n</math>, then <math>\phi</math> is assigned the classification <math>\Sigma^0_{n+1}</math>.
*If <math>\phi</math> is logically equivalent to a formula of the form <math>\forall n_1 \forall n_2\cdots \forall n_k  \psi</math>, where <math>\psi</math> is <math>\Sigma^0_n</math>, then <math>\phi</math> is assigned the classification <math>\Pi^0_{n+1}</math>.
Also, a <math>\Sigma^0_n</math> formula is equivalent to a formula that begins with some [[existential quantifier]]s and alternates <math>n-1</math> times between series of existential and [[universal quantifier]]s; while a <math>\Pi^0_n</math> formula is equivalent to a formula that begins with some universal quantifiers and alternates similarly.
 
Because every formula is equivalent to a formula in [[prenex normal form]], every formula with no set quantifiers is assigned at least one classification. Because redundant quantifiers can be added to any formula, once a formula is assigned the classification <math>\Sigma^0_n</math> or <math>\Pi^0_n</math> it will be assigned the classifications <math>\Sigma^0_m</math> and <math>\Pi^0_m</math> for every ''m'' greater than ''n''.  The most important classification assigned to a formula is thus the one with the least ''n'', because this is enough to determine all the other classifications.
 
== The arithmetical hierarchy of sets of natural numbers ==
 
A set ''X'' of natural numbers is defined by formula &phi; in the language of [[Peano arithmetic]] if the elements of ''X'' are exactly the numbers that satisfy &phi;.  That is, for all natural numbers ''n'',
:<math>n \in X \Leftrightarrow \mathbb{N} \models \phi(\underline n),</math>
where <math>\underline n</math> is the numeral in the language of arithmetic corresponding to <math>n</math>. A set is definable in first order arithmetic if it is defined by some formula in the language of Peano arithmetic.
 
Each set ''X'' of natural numbers that is definable in first order arithmetic is assigned classifications of the form <math>\Sigma^0_n</math>, <math>\Pi^0_n</math>, and <math>\Delta^0_n</math>, where <math>n</math> is a natural number, as follows. If ''X'' is definable by a <math>\Sigma^0_n</math> formula then ''X'' is assigned the classification <math>\Sigma^0_n</math>.  If ''X'' is definable by a <math>\Pi^0_n</math> formula then ''X'' is assigned the classification <math>\Pi^0_n</math>.  If ''X'' is both <math>\Sigma^0_n</math> and <math>\Pi^0_n</math> then <math>X</math> is assigned the additional classification <math>\Delta^0_n</math>.
 
Note that it rarely makes sense to speak of <math>\Delta^0_n</math> formulas; the first quantifier of a formula is either existential or universal. So a <math>\Delta^0_n</math> set is not defined by a <math>\Delta^0_n</math> formula; rather, there are both <math>\Sigma^0_n</math>  and  <math>\Pi^0_n</math> formulas that define the set.
 
A parallel definition is used to define the arithmetical hierarchy on finite [[Cartesian power]]s of the natural numbers. Instead of formulas with one free variable, formulas with ''k'' free number variables are used to define the arithmetical hierarchy on sets of ''k''-[[tuple]]s of natural numbers.
 
== Relativized arithmetical hierarchies ==
 
Just as we can define what it means for a set ''X'' to be [[Recursive set|recursive]] relative to another set ''Y'' by allowing the computation defining ''X'' to consult ''Y'' as an oracle we can extend this notion to the whole arithmetic hierarchy and define what it means for ''X'' to be <math>\Sigma^0_n</math>, <math>\Delta^0_n</math> or <math>\Pi^0_n</math> in ''Y'', denoted respectively <math>\Sigma^{0,Y}_n</math> <math>\Delta^{0,Y}_n</math> and <math>\Pi^{0,Y}_n</math>. To do so, fix a set of integers ''Y'' and add a predicate for membership in ''Y'' to the language of Peano arithmetic.  We then say that ''X'' is in <math>\Sigma^{0,Y}_n</math> if it is defined by a <math>\Sigma^0_n</math> formula in this expanded language.  In other words ''X'' is <math>\Sigma^{0,Y}_n</math> if it is defined by a <math>\Sigma^{0}_n</math> formula allowed to ask questions about membership in ''Y''.  Alternatively one can view the <math>\Sigma^{0,Y}_n</math> sets as those sets that can be built starting with sets recursive in ''Y'' and alternatively [[Projection (set theory)|projecting]] and taking the [[Complement (set theory)|complements]] of these sets up to ''n'' times.
 
For example let ''Y'' be a set of integers. Let ''X'' be the set of numbers divisible by an element of Y.  Then ''X'' is defined by the formula <math>\phi(n)=\exists m \exists t (Y(m)\and m\times t = n)</math> so ''X'' is in <math>\Sigma^{0,Y}_1</math> (actually it is in <math>\Delta^{0,Y}_0</math> as well since we could bound both quantifiers by n).
 
== Arithmetic reducibility and degrees ==
 
Arithmetical reducibility is an intermediate notion between [[Turing reducibility]] and [[hyperarithmetic reducibility]].
 
A set is '''arithmetical''' (also '''arithmetic''' and '''arithmetically definable''') if it is defined by some formula in the language of Peano arithmetic.  Equivalently ''X'' is arithmetical if ''X'' is <math>\Sigma^0_n</math> or <math>\Pi^0_n</math> for some integer ''n''.  A set ''X'' '''is arithmetical in''' a set ''Y'', denoted <math>X \leq_A Y</math>, if ''X'' is definable a some formula in the language of Peano arithmetic extended by a predicate for membership in ''Y''. Equivalently, ''X'' is arithmetical in ''Y'' if ''X'' is in <math>\Sigma^{0,Y}_n</math> or <math>\Pi^{0,Y}_n</math> for some integer ''n''. A synonym for <math>X \leq_A Y</math>is: ''X'' is '''arithmetically reducible''' to ''Y''.
 
The relation <math>X \leq_A Y</math> is reflexive and transitive, and thus the relation <math>\equiv_A</math> defined by the rule
 
:<math> X \equiv_A Y \Leftrightarrow X \leq_A Y \and Y \leq_A X</math>
is an equivalence relation.  The equivalence classes of this relation are called the '''arithmetic degrees'''; they are partially ordered under <math>\leq_A</math>.
 
==The arithmetical hierarchy of subsets of Cantor and Baire space==
 
The [[Cantor space]], denoted <math>2^{\omega}</math>, is the set of all infinite sequences of 0s and 1s; the [[Baire space (set theory)|Baire space]], denoted <math>\omega^{\omega}</math> or <math>\mathcal{N}</math>, is the set of all infinite sequences of natural numbers. Note that elements of the Cantor space can be identified with sets of integers and elements of the Baire space with functions from integers to integers.
 
The ordinary axiomatization of [[second-order arithmetic]] uses a set-based language in which the set quantifiers can naturally be viewed as quantifying over Cantor space.  A subset of Cantor space is assigned the classification <math>\Sigma^0_n</math> if it is definable by a <math>\Sigma^0_n</math> formula.  The set is assigned the classification <math>\Pi^0_n</math> if it is definable by a <math>\Pi^0_n</math> formula.  If the set is both <math>\Sigma^0_n</math> and <math>\Pi^0_n</math> then it is given the additional classification <math>\Delta^0_n</math>. For example let <math>O\subset 2^{\omega}</math> be the set of all infinite binary strings which aren't all 0 (or equivalently the set of all non-empty sets of integers).  As <math>O=\{ X\in 2^\omega | \exists n (X(n)=1) \} </math> we see that <math>O</math> is defined by a <math>\Sigma^0_1</math> formula and hence is a <math>\Sigma^0_1</math> set. 
 
Note that while both the elements of the Cantor space (regarded as sets of integers) and subsets of the Cantor space are classified in arithmetic hierarchies but these are not the same hierarchy. In fact the relationship between the two hierarchies is interesting and non-trivial.  For instance the <math>\Pi^0_n</math> elements of the Cantor space are not (in general) the same as the elements <math>X</math> of the Cantor space so that <math>\{X\}</math> is a <math>\Pi^0_n</math> subset of the Cantor space.  However, many interesting results relate the two hierarchies.
 
There are two ways that a subset of Baire space can be classified in the arithmetical hierarchy.
*A subset of Baire space has a corresponding subset of Cantor space under the map that takes each function from <math>\omega</math> to <math>\omega</math> to the [[indicator function|characteristic function]] of its graph.   A subset of Baire space is given the classification <math>\Sigma^1_n</math>, <math>\Pi^1_n</math>, or <math>\Delta^1_n</math> if and only if the corresponding subset of Cantor space has the same classification.
*An equivalent definition of the analytical hierarchy on Baire space is given by defining the analytical hierarchy of formulas using a functional version of second-order arithmetic; then the analytical hierarchy on subsets of Cantor space can be defined from the hierarchy on Baire space.  This alternate definition gives exactly the same classifications as the first definition.
 
A parallel definition is used to define the arithmetical hierarchy on finite Cartesian powers of Baire space or Cantor space, using formulas with several free variables.  The arithmetical hierarchy can be defined on any [[effective Polish space]]; the definition is particularly simple for Cantor space and Baire space because they fit with the language of ordinary second-order arithmetic.
 
Note that we can also define the arithmetic hierarchy of subsets of the Cantor and Baire spaces relative to some set of integers. In fact boldface <math>\bold{\Sigma}^0_n</math> is just the union of <math>\Sigma^{0,Y}_n</math> for all sets of integers ''Y''.  Note that the boldface hierarchy is just the standard hierarchy of [[Borel hierarchy|Borel sets]].
 
== Extensions and variations==
 
It is possible to define the arithmetical hierarchy of formulas using a language extended with a function symbol for each [[primitive recursive function]]. This variation slightly changes the classification of some sets. 
 
A more semantic variation of the hierarchy can be defined on all finitary relations on the natural numbers; the following definition is used.  Every computable relation is defined to be <math>\Sigma^0_0</math> and <math>\Pi^0_0</math>.  The classifications <math>\Sigma^0_n</math> and <math>\Pi^0_n</math> are defined inductively with the following rules.
* If the relation <math>R(n_1,\ldots,n_l,m_1,\ldots, m_k)\,</math> is <math>\Sigma^0_n</math> then the relation <math>S(n_1,\ldots, n_l) = \forall m_1\cdots \forall m_k R(n_1,\ldots,n_l,m_1,\ldots,m_k)</math> is defined to be <math>\Pi^0_{n+1}</math>
* If the relation <math>R(n_1,\ldots,n_l,m_1,\ldots, m_k)\,</math> is <math>\Pi^0_n</math> then the relation <math>S(n_1,\ldots,n_l) = \exists m_1\cdots \exists m_k R(n_1,\ldots,n_l,m_1,\ldots,m_k)</math> is defined to be <math>\Sigma^0_{n+1}</math>
This variation slightly changes the classification of some sets. It can be extended to cover finitary relations on the natural numbers, Baire space, and Cantor space.
 
== Meaning of the notation==
 
The following meanings can be attached to the notation for the arithmetical hierarchy on formulas.
 
The subscript <math>n</math> in the symbols <math>\Sigma^0_n</math> and <math>\Pi^0_n</math> indicates the number of alternations of blocks of universal and existential number quantifiers that are used in a formula.  Moreover, the outermost block is existential in <math>\Sigma^0_n</math> formulas and universal in <math>\Pi^0_n</math> formulas.
 
The superscript <math>0</math> in the symbols <math>\Sigma^0_n</math>, <math>\Pi^0_n</math>, and <math>\Delta^0_n</math> indicates the type of the objects being quantified over. Type 0 objects are natural numbers, and objects of type <math>i+1</math> are functions that map the set of objects of  type <math>i</math> to the natural numbers. Quantification over higher type objects, such as functions from natural numbers to natural numbers, is described by a superscript greater than 0, as in the [[analytical hierarchy]]. The superscript 0 indicates quantifiers over numbers, the superscript 1 would indicate quantification over functions from numbers to numbers (type 1 objects), the superscript 2 would correspond to quantification over functions that take a type 1 object and return a number, and so on.
 
== Examples ==
 
* The <math>\Sigma^0_1</math> sets of numbers are those definable by a formula of the form <math>\exists n_1 \cdots \exists n_k \psi(n_1,\ldots,n_k,m)</math> where <math>\psi</math> has only bounded quantifiers.  These are exactly the [[recursively enumerable set]]s.
 
* The set of natural numbers that are indices for Turing machines that compute total functions is <math>\Pi^0_2</math>.  Intuitively, an index <math>e</math> falls into this set if and only if for every <math>m</math> "there is an <math>s</math> such that the Turing machine with index <math>e</math> halts on input <math>m</math> after <math>s</math> steps”.  A complete proof would show that the property displayed in quotes in the previous sentence is definable in the language of Peano arithmetic by a <math>\Sigma^0_1</math> formula.
 
* Every <math>\Sigma^0_1</math> subset of Baire space or Cantor space is an open set in the usual topology on the space.  Moreover, for any such set there is a computable enumeration of Gödel numbers of basic open sets whose union is the original set. For this reason, <math>\Sigma^0_1</math> sets are sometimes called ''effectively open''. Similarly, every <math>\Pi^0_1</math> set is closed and the <math>\Pi^0_1</math> sets are sometimes called ''effectively closed''.
 
* Every arithmetical subset of Cantor space of<sup>(or?)</sup> Baire space is a [[Borel set]].  The lightface Borel hierarchy extends the arithmetical hierarchy to include additional Borel sets.  For example, every <math>\Pi^0_2</math> subset of Cantor or Baire space is a <math>G_\delta</math> set (that is, a set which equals the intersection of countably many open sets).   Moreover, each of these open sets is <math>\Sigma^0_1</math> and the list of Gödel numbers of these open sets has a computable enumeration. If <math>\phi(X,n,m)</math> is a <math>\Sigma^0_0</math> formula with a free set variable ''X'' and free number variables <math>n,m</math> then the <math>\Pi^0_2</math> set <math>\{X \mid \forall n \exists m \phi(X,n,m)\}</math> is the intersection of the <math>\Sigma^0_1</math> sets of the form <math>\{ X \mid \exists m \phi(X,n,m)\}</math> as ''n'' ranges over the set of natural numbers.
 
== Properties ==
 
The following properties hold for the arithmetical hierarchy of sets of natural numbers and the arithmetical hierarchy of subsets of Cantor or Baire space.
 
* The collections <math>\Pi^0_n</math> and <math>\Sigma^0_n</math> are closed under finite [[union (set theory)|union]]s and finite [[intersection (set theory)|intersection]]s of their respective elements.
* A set is <math>\Sigma^0_n</math> if and only if its complement is <math>\Pi^0_n</math>.  A set is <math>\Delta^0_n</math> if and only if the set is both <math>\Sigma^0_n</math> and <math>\Pi^0_n</math>, in which case its complement will also be <math>\Delta^0_n</math>.
* The inclusions <math>\Delta^0_n \subsetneq \Pi^0_n</math> and <math>\Delta^0_n \subsetneq \Sigma^0_n</math> hold for <math>n \geq 1</math>.
* The inclusions <math>\Pi^0_n \subsetneq \Pi^0_{n+1}</math> and <math>\Sigma^0_n \subsetneq \Sigma^0_{n+1}</math> hold for all <math>n</math> and the inclusion <math>\Sigma^0_n \cup \Pi^0_n \subsetneq \Delta^0_{n+1}</math> holds for <math>n \geq 1</math>. Thus the hierarchy does not collapse.
 
== Relation to Turing machines ==
 
The Turing computable sets of natural numbers are exactly the sets at level <math>\Delta^0_1</math> of the arithmetical hierarchy. The recursively enumerable sets are exactly the sets at level <math>\Sigma^0_1</math>.
 
No [[oracle machine]]  is capable of solving its own [[halting problem]] (a variation of Turing's proof applies).  The halting problem for a <math>\Delta^{0,Y}_n</math> oracle in fact sits in <math>\Sigma^{0,Y}_{n+1}</math>.
 
[[Post's theorem]] establishes a close connection between the arithmetical hierarchy of sets of natural numbers and the [[Turing degree]]s. In particular, it establishes the following facts  for all ''n'' ≥ 1:
* The set <math>\emptyset^{(n)}</math> (the ''n''th [[Turing jump]] of the empty set) is [[Many-one reduction|many-one complete]] in <math>\Sigma^0_n</math>.
* The set <math>\mathbb{N} \setminus \emptyset^{(n)}</math> is many-one complete in <math>\Pi^0_n</math>.
* The set <math>\emptyset^{(n-1)}</math> is [[Turing complete set|Turing complete]] in <math>\Delta^0_n</math>.
 
The [[polynomial hierarchy]] is a "feasible resource-bounded" version of the arithmetical hierarchy in which polynomial length bounds are placed on the numbers involved (or, equivalently, polynomial time bounds are placed on the Turing machines involved). It gives a finer classification of some sets of natural numbers that are at level <math>\Delta^0_1</math> of the arithmetical hierarchy.
 
== See also ==
* [[Interpretability logic]]
* [[Hierarchy (mathematics)]]
 
== References ==
*{{citation|first=Giorgie|last=Japaridze|title=The logic of arithmetical hierarchy|journal=Annals of Pure and Applied Logic|volume=66|issue=2|year=1994|pages=89–112|doi=10.1016/0168-0072(94)90063-9 | zbl=0804.03045 }}.
*{{citation|last=Moschovakis|first=Yiannis N. | authorlink=Yiannis N. Moschovakis | title=Descriptive Set Theory | publisher=North Holland | year=1980 | isbn=0-444-70199-0 | zbl=0433.03025 | series=Studies in Logic and the Foundations of Mathematics | volume=100 }}.
*{{citation | last=Nies | first=André | title=Computability and randomness | series=Oxford Logic Guides | volume=51 | location=Oxford | publisher=Oxford University Press | year=2009 | isbn=978-0-19-923076-1 | zbl=1169.03034 }}.
*{{citation|last=Rogers|first=H., jr | title= Theory of recursive functions and effective computability|publisher=McGraw-Hill | year=1967 | zbl=0183.01401 | location=Maidenhead }}.
 
{{ComplexityClasses}}
 
{{DEFAULTSORT:Arithmetical Hierarchy}}
[[Category:Mathematical logic hierarchies]]
[[Category:Computability theory]]
[[Category:Effective descriptive set theory]]
[[Category:Hierarchy]]
[[Category:Complexity classes]]

Revision as of 10:21, 29 January 2014

In mathematical logic, the arithmetical hierarchy, arithmetic hierarchy or Kleene-Mostowski hierarchy classifies certain sets based on the complexity of formulas that define them. Any set that receives a classification is called arithmetical.

The arithmetical hierarchy is important in recursion theory, effective descriptive set theory, and the study of formal theories such as Peano arithmetic.

The Tarski-Kuratowski algorithm provides an easy way to get an upper bound on the classifications assigned to a formula and the set it defines.

The hyperarithmetical hierarchy and the analytical hierarchy extend the arithmetical hierarchy to classify additional formulas and sets.

The arithmetical hierarchy of formulas

The arithmetical hierarchy assigns classifications to the formulas in the language of first-order arithmetic. The classifications are denoted Σn0 and Πn0 for natural numbers n (including 0). The Greek letters here are lightface symbols, which indicates that the formulas do not contain set parameters.

If a formula ϕ is logically equivalent to a formula with only bounded quantifiers then ϕ is assigned the classifications Σ00 and Π00.

The classifications Σn0 and Πn0 are defined inductively for every natural number n using the following rules:

Also, a Σn0 formula is equivalent to a formula that begins with some existential quantifiers and alternates n1 times between series of existential and universal quantifiers; while a Πn0 formula is equivalent to a formula that begins with some universal quantifiers and alternates similarly.

Because every formula is equivalent to a formula in prenex normal form, every formula with no set quantifiers is assigned at least one classification. Because redundant quantifiers can be added to any formula, once a formula is assigned the classification Σn0 or Πn0 it will be assigned the classifications Σm0 and Πm0 for every m greater than n. The most important classification assigned to a formula is thus the one with the least n, because this is enough to determine all the other classifications.

The arithmetical hierarchy of sets of natural numbers

A set X of natural numbers is defined by formula φ in the language of Peano arithmetic if the elements of X are exactly the numbers that satisfy φ. That is, for all natural numbers n,

nXϕ(n_),

where n_ is the numeral in the language of arithmetic corresponding to n. A set is definable in first order arithmetic if it is defined by some formula in the language of Peano arithmetic.

Each set X of natural numbers that is definable in first order arithmetic is assigned classifications of the form Σn0, Πn0, and Δn0, where n is a natural number, as follows. If X is definable by a Σn0 formula then X is assigned the classification Σn0. If X is definable by a Πn0 formula then X is assigned the classification Πn0. If X is both Σn0 and Πn0 then X is assigned the additional classification Δn0.

Note that it rarely makes sense to speak of Δn0 formulas; the first quantifier of a formula is either existential or universal. So a Δn0 set is not defined by a Δn0 formula; rather, there are both Σn0 and Πn0 formulas that define the set.

A parallel definition is used to define the arithmetical hierarchy on finite Cartesian powers of the natural numbers. Instead of formulas with one free variable, formulas with k free number variables are used to define the arithmetical hierarchy on sets of k-tuples of natural numbers.

Relativized arithmetical hierarchies

Just as we can define what it means for a set X to be recursive relative to another set Y by allowing the computation defining X to consult Y as an oracle we can extend this notion to the whole arithmetic hierarchy and define what it means for X to be Σn0, Δn0 or Πn0 in Y, denoted respectively Σn0,Y Δn0,Y and Πn0,Y. To do so, fix a set of integers Y and add a predicate for membership in Y to the language of Peano arithmetic. We then say that X is in Σn0,Y if it is defined by a Σn0 formula in this expanded language. In other words X is Σn0,Y if it is defined by a Σn0 formula allowed to ask questions about membership in Y. Alternatively one can view the Σn0,Y sets as those sets that can be built starting with sets recursive in Y and alternatively projecting and taking the complements of these sets up to n times.

For example let Y be a set of integers. Let X be the set of numbers divisible by an element of Y. Then X is defined by the formula ϕ(n)=mt(Y(m)m×t=n) so X is in Σ10,Y (actually it is in Δ00,Y as well since we could bound both quantifiers by n).

Arithmetic reducibility and degrees

Arithmetical reducibility is an intermediate notion between Turing reducibility and hyperarithmetic reducibility.

A set is arithmetical (also arithmetic and arithmetically definable) if it is defined by some formula in the language of Peano arithmetic. Equivalently X is arithmetical if X is Σn0 or Πn0 for some integer n. A set X is arithmetical in a set Y, denoted XAY, if X is definable a some formula in the language of Peano arithmetic extended by a predicate for membership in Y. Equivalently, X is arithmetical in Y if X is in Σn0,Y or Πn0,Y for some integer n. A synonym for XAYis: X is arithmetically reducible to Y.

The relation XAY is reflexive and transitive, and thus the relation A defined by the rule

XAYXAYYAX

is an equivalence relation. The equivalence classes of this relation are called the arithmetic degrees; they are partially ordered under A.

The arithmetical hierarchy of subsets of Cantor and Baire space

The Cantor space, denoted 2ω, is the set of all infinite sequences of 0s and 1s; the Baire space, denoted ωω or 𝒩, is the set of all infinite sequences of natural numbers. Note that elements of the Cantor space can be identified with sets of integers and elements of the Baire space with functions from integers to integers.

The ordinary axiomatization of second-order arithmetic uses a set-based language in which the set quantifiers can naturally be viewed as quantifying over Cantor space. A subset of Cantor space is assigned the classification Σn0 if it is definable by a Σn0 formula. The set is assigned the classification Πn0 if it is definable by a Πn0 formula. If the set is both Σn0 and Πn0 then it is given the additional classification Δn0. For example let O2ω be the set of all infinite binary strings which aren't all 0 (or equivalently the set of all non-empty sets of integers). As O={X2ω|n(X(n)=1)} we see that O is defined by a Σ10 formula and hence is a Σ10 set.

Note that while both the elements of the Cantor space (regarded as sets of integers) and subsets of the Cantor space are classified in arithmetic hierarchies but these are not the same hierarchy. In fact the relationship between the two hierarchies is interesting and non-trivial. For instance the Πn0 elements of the Cantor space are not (in general) the same as the elements X of the Cantor space so that {X} is a Πn0 subset of the Cantor space. However, many interesting results relate the two hierarchies.

There are two ways that a subset of Baire space can be classified in the arithmetical hierarchy.

  • A subset of Baire space has a corresponding subset of Cantor space under the map that takes each function from ω to ω to the characteristic function of its graph. A subset of Baire space is given the classification Σn1, Πn1, or Δn1 if and only if the corresponding subset of Cantor space has the same classification.
  • An equivalent definition of the analytical hierarchy on Baire space is given by defining the analytical hierarchy of formulas using a functional version of second-order arithmetic; then the analytical hierarchy on subsets of Cantor space can be defined from the hierarchy on Baire space. This alternate definition gives exactly the same classifications as the first definition.

A parallel definition is used to define the arithmetical hierarchy on finite Cartesian powers of Baire space or Cantor space, using formulas with several free variables. The arithmetical hierarchy can be defined on any effective Polish space; the definition is particularly simple for Cantor space and Baire space because they fit with the language of ordinary second-order arithmetic.

Note that we can also define the arithmetic hierarchy of subsets of the Cantor and Baire spaces relative to some set of integers. In fact boldface Σn0 is just the union of Σn0,Y for all sets of integers Y. Note that the boldface hierarchy is just the standard hierarchy of Borel sets.

Extensions and variations

It is possible to define the arithmetical hierarchy of formulas using a language extended with a function symbol for each primitive recursive function. This variation slightly changes the classification of some sets.

A more semantic variation of the hierarchy can be defined on all finitary relations on the natural numbers; the following definition is used. Every computable relation is defined to be Σ00 and Π00. The classifications Σn0 and Πn0 are defined inductively with the following rules.

This variation slightly changes the classification of some sets. It can be extended to cover finitary relations on the natural numbers, Baire space, and Cantor space.

Meaning of the notation

The following meanings can be attached to the notation for the arithmetical hierarchy on formulas.

The subscript n in the symbols Σn0 and Πn0 indicates the number of alternations of blocks of universal and existential number quantifiers that are used in a formula. Moreover, the outermost block is existential in Σn0 formulas and universal in Πn0 formulas.

The superscript 0 in the symbols Σn0, Πn0, and Δn0 indicates the type of the objects being quantified over. Type 0 objects are natural numbers, and objects of type i+1 are functions that map the set of objects of type i to the natural numbers. Quantification over higher type objects, such as functions from natural numbers to natural numbers, is described by a superscript greater than 0, as in the analytical hierarchy. The superscript 0 indicates quantifiers over numbers, the superscript 1 would indicate quantification over functions from numbers to numbers (type 1 objects), the superscript 2 would correspond to quantification over functions that take a type 1 object and return a number, and so on.

Examples

  • The set of natural numbers that are indices for Turing machines that compute total functions is Π20. Intuitively, an index e falls into this set if and only if for every m "there is an s such that the Turing machine with index e halts on input m after s steps”. A complete proof would show that the property displayed in quotes in the previous sentence is definable in the language of Peano arithmetic by a Σ10 formula.
  • Every Σ10 subset of Baire space or Cantor space is an open set in the usual topology on the space. Moreover, for any such set there is a computable enumeration of Gödel numbers of basic open sets whose union is the original set. For this reason, Σ10 sets are sometimes called effectively open. Similarly, every Π10 set is closed and the Π10 sets are sometimes called effectively closed.
  • Every arithmetical subset of Cantor space of(or?) Baire space is a Borel set. The lightface Borel hierarchy extends the arithmetical hierarchy to include additional Borel sets. For example, every Π20 subset of Cantor or Baire space is a Gδ set (that is, a set which equals the intersection of countably many open sets). Moreover, each of these open sets is Σ10 and the list of Gödel numbers of these open sets has a computable enumeration. If ϕ(X,n,m) is a Σ00 formula with a free set variable X and free number variables n,m then the Π20 set {Xnmϕ(X,n,m)} is the intersection of the Σ10 sets of the form {Xmϕ(X,n,m)} as n ranges over the set of natural numbers.

Properties

The following properties hold for the arithmetical hierarchy of sets of natural numbers and the arithmetical hierarchy of subsets of Cantor or Baire space.

Relation to Turing machines

The Turing computable sets of natural numbers are exactly the sets at level Δ10 of the arithmetical hierarchy. The recursively enumerable sets are exactly the sets at level Σ10.

No oracle machine is capable of solving its own halting problem (a variation of Turing's proof applies). The halting problem for a Δn0,Y oracle in fact sits in Σn+10,Y.

Post's theorem establishes a close connection between the arithmetical hierarchy of sets of natural numbers and the Turing degrees. In particular, it establishes the following facts for all n ≥ 1:

The polynomial hierarchy is a "feasible resource-bounded" version of the arithmetical hierarchy in which polynomial length bounds are placed on the numbers involved (or, equivalently, polynomial time bounds are placed on the Turing machines involved). It gives a finer classification of some sets of natural numbers that are at level Δ10 of the arithmetical hierarchy.

See also

References

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    Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.

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  • Many property agents need to declare for the PIC grant in Singapore. However, not all of them know find out how to do the correct process for getting this PIC scheme from the IRAS. There are a number of steps that you need to do before your software can be approved.

    Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.

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    Once you place down the one per cent choice price to carry down a non-public property, it's important to accept its situation as it is whenever you move in – faulty air-con, choked rest room and all. Get round this by asking your agent to incorporate a ultimate inspection clause within the possibility-to-buy letter. HDB flat patrons routinely take pleasure in this security net. "There's a ultimate inspection of the property two days before the completion of all HDB transactions. If the air-con is defective, you can request the seller to repair it," says Kelvin.

    15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.

    To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010.
  • Many property agents need to declare for the PIC grant in Singapore. However, not all of them know find out how to do the correct process for getting this PIC scheme from the IRAS. There are a number of steps that you need to do before your software can be approved.

    Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.

    In case you are in search of an actual estate or Singapore property agent on-line, you simply should belief your intuition. It's because you do not know which agent is nice and which agent will not be. Carry out research on several brokers by looking out the internet. As soon as if you end up positive that a selected agent is dependable and reliable, you can choose to utilize his partnerise in finding you a home in Singapore. Most of the time, a property agent is taken into account to be good if he or she locations the contact data on his website. This may mean that the agent does not mind you calling them and asking them any questions relating to new properties in singapore in Singapore. After chatting with them you too can see them in their office after taking an appointment.

    Have handed an trade examination i.e Widespread Examination for House Brokers (CEHA) or Actual Property Agency (REA) examination, or equal; Exclusive brokers are extra keen to share listing information thus making certain the widest doable coverage inside the real estate community via Multiple Listings and Networking. Accepting a severe provide is simpler since your agent is totally conscious of all advertising activity related with your property. This reduces your having to check with a number of agents for some other offers. Price control is easily achieved. Paint work in good restore-discuss with your Property Marketing consultant if main works are still to be done. Softening in residential property prices proceed, led by 2.8 per cent decline within the index for Remainder of Central Region

    Once you place down the one per cent choice price to carry down a non-public property, it's important to accept its situation as it is whenever you move in – faulty air-con, choked rest room and all. Get round this by asking your agent to incorporate a ultimate inspection clause within the possibility-to-buy letter. HDB flat patrons routinely take pleasure in this security net. "There's a ultimate inspection of the property two days before the completion of all HDB transactions. If the air-con is defective, you can request the seller to repair it," says Kelvin.

    15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.

    To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010.

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