Editing Zero sharp

{{short description|Concept in set theory}}
In the mathematical discipline of [[set theory]], '''0<sup>#</sup>''' ('''zero sharp''', also '''0#''') is the set of true [[formula (mathematical logic)|formulae]] about [[indiscernibles]] and order-indiscernibles in the [[Gödel constructible universe]]. It is often encoded as a subset of the [[natural number]]s (using [[Gödel numbering]]), or as a subset of the [[hereditarily finite set]]s, or as a [[Baire space (set theory)|real number]].  Its existence is unprovable in [[ZFC]], the standard form of [[axiomatic set theory]], but follows from a suitable [[large cardinal]] axiom. It was first introduced as a set of formulae in [[Jack Silver|Silver's]] 1966 thesis, later published as {{harvtxt|Silver|1971}}, where it was denoted by Σ, and rediscovered by  {{harvtxt|Solovay|1967|loc=p.52}}, who considered it as a subset of the natural numbers and introduced the notation O<sup>#</sup> (with a capital letter O; this later changed to the numeral '0').

Roughly speaking, if 0<sup>#</sup> exists then the universe ''V'' of sets is much larger than the universe ''L'' of constructible sets, while if it does not exist then the universe of all sets is closely approximated by the constructible sets.

== Definition ==

Zero sharp was defined by Silver and [[Robert M. Solovay|Solovay]] as follows. Consider the language of set theory with extra constant symbols <math>c_1</math>, <math>c_2</math>, ... for each nonzero natural number. Then <math>0^\sharp</math> is defined to be the set of [[Gödel number]]s of the true sentences about the constructible universe, with <math>c_i</math> interpreted as the uncountable cardinal <math>\aleph_i</math>.
(Here <math>\aleph_i</math> means <math>\aleph_i</math> in the full universe, not the constructible universe.)

There is a subtlety about this definition: by [[Tarski's undefinability theorem]] it is not, in general, possible to define the truth of a formula of set theory in the language of set theory. To solve this, Silver and Solovay assumed the existence of a suitable large cardinal, such as a [[Ramsey cardinal]], and showed that with this extra assumption it is possible to define the truth of statements about the constructible universe. More generally, the definition of <math>0^\sharp</math> works provided that there is an uncountable set of indiscernibles for some <math>L_\alpha</math>, and the phrase "<math>0^\sharp</math> exists" is used as a shorthand way of saying this.

A closed set <math>I</math> of [[indiscernibles|order-indiscernibles]] for <math>L_\alpha</math> (where <math>\alpha</math> is a limit ordinal) is a set of ''Silver indiscernibles'' if:
*<math>I</math> is unbounded in <math>\alpha</math>, and
*if <math>I\cap\beta</math> is unbounded in an ordinal <math>\beta</math>, then the [[Skolem hull]] of <math>I\cap\beta</math> in <math>L_\beta</math> is <math>L_\beta</math>. In other words, every <math>x\in L_\beta</math> is definable in <math>L_\beta</math> from parameters in <math>I\cap\beta</math>.

If there is a set of Silver indiscernibles for <math>L_{\omega_1}</math>, then it is unique. Additionally, for any uncountable cardinal <math>\kappa</math> there will be a unique set of Silver indiscernibles for <math>L_\kappa</math>. The union of all these sets will be a proper class <math>I</math> of Silver indiscernibles for the structure <math>L</math> itself. Then, <math>0^\sharp</math> is defined as the set of all Gödel numbers of formulae <math>\theta</math> such that

<math>L_\alpha\models\theta(\alpha_1,\alpha_2\ldots\alpha_n)</math>

where <math>\alpha_1 < \alpha_2 < \ldots < \alpha_n < \alpha</math> is any strictly increasing sequence of members of <math>I</math>. Because they are indiscernibles, the definition does not depend on the choice of sequence.

Any <math>\alpha\in I</math> has the property that <math>L_\alpha\prec L</math>. This allows for a definition of truth for the constructible universe:

<math>L\models\varphi[x_1...x_n]</math> only if <math>L_\alpha\models\varphi[x_1...x_n]</math> for some <math>\alpha\in I</math>.

There are several minor variations of the definition of <math>0^\sharp</math>, which make no significant difference to its properties. There are many different choices of Gödel numbering, and <math>0^\sharp</math> depends on this choice. Instead of being considered as a subset of the natural numbers, it is also possible to encode <math>0^\sharp</math> as a subset of formulae of a language, or as a subset of the hereditarily finite sets, or as a real number.

==Statements implying existence==

The condition about the existence of a Ramsey cardinal implying that <math>0^\sharp</math> exists can be weakened.  The existence of <math>\omega_1</math>-[[Erdős cardinal]]s implies the existence of <math>0^\sharp</math>. This is close to being best possible, because the existence of <math>0^\sharp</math> implies that in the constructible universe there is an <math>\alpha</math>-Erdős cardinal for all countable <math>\alpha</math>, so such cardinals cannot be used to prove the existence of <math>0^\sharp</math>.

[[Chang's conjecture]] implies the existence of <math>0^\sharp</math>.

==Statements equivalent to existence==
Kunen showed that <math>0^\sharp</math> exists if and only if there exists a non-trivial elementary embedding for the [[Gödel constructible universe]] <math>L</math> into itself.

[[Donald A. Martin]] and [[Leo Harrington]] have shown that the existence of <math>0^\sharp</math> is equivalent to the determinacy of [[lightface analytic game]]s.  In fact, the strategy for a universal lightface analytic game has the same [[Turing degree]] as <math>0^\sharp</math>.

It follows from [[Jensen's covering theorem]] that the existence of <math>0^\sharp</math> is equivalent to <math>\omega_\omega</math> of ''V'' being a [[regular cardinal]] in the constructible universe <math>L</math>.

Silver showed that the existence of an uncountable set of indiscernibles in the constructible universe is equivalent to the existence of <math>0^\sharp</math>.

== Consequences of existence and non-existence ==
The existence of <math>0^\sharp</math> implies that every [[Uncountable set|uncountable]] [[Cardinal number|cardinal]] in the set-theoretic universe <math>V</math> is an indiscernible in <math>L</math> and satisfies all [[large cardinal]] axioms that are realized in <math>L</math> (such as being [[Ineffable cardinal|totally ineffable]]).  It follows that the existence of <math>0^\sharp</math> contradicts the ''[[axiom of constructibility]]'': <math>V=L</math>.

If <math>0^\sharp</math> exists, then it is an example of a non-constructible <math>\Delta^1_3</math> set of natural numbers. This is in some sense the simplest possibility for a non-constructible set, since all <math>\Sigma^1_2</math> and <math>\Pi^1_2</math> sets of natural numbers are constructible.

On the other hand, if <math>0^\sharp</math> does not exist, then the constructible universe <math>L</math> is the core model—that is, the canonical [[inner model]] that approximates the large cardinal structure of the universe considered. In that case, [[Jensen's covering lemma]] holds:

:For every uncountable set <math>x</math> of ordinals there is a constructible <math>y</math> such that <math>x\subset y</math> and <math>y</math> has the same [[cardinality]] as <math>x</math>.

This deep result is due to [[Ronald Jensen]]. Using [[forcing (mathematics)|forcing]] it is easy to see that the condition that <math>x</math> is uncountable cannot be removed. For example, consider [[List of forcing notions#Namba forcing|Namba forcing]], that preserves <math>\omega_1</math> and collapses <math>\omega_2</math> to an ordinal of [[cofinality]] <math>\omega</math>. Let <math>G</math> be an <math>\omega</math>-sequence [[cofinal (mathematics)|cofinal]] on <math>\omega_2^L</math> and [[generic filter|generic]] over <math>L</math>. Then no set in <math>L</math> of <math>L</math>-size smaller than <math>\omega_2^L</math> (which is uncountable in <math>V</math>, since <math>\omega_1</math> is preserved) can cover <math>G</math>, since <math>\omega_2</math> is a [[regular cardinal]].

If <math>0^\sharp</math> does not exist, it also follows that the [[singular cardinals hypothesis]] holds.<ref>P. Holy, "[https://www.dmg.tuwien.ac.at/holy/dip.pdf Absoluteness Results in Set Theory]" (2017). Accessed 24 July 2024.</ref><sup>p. 20</sup>

== Other sharps ==
If <math>x</math> is any set, then <math>x^\sharp</math> is defined analogously to <math>0^\sharp</math> except that one uses <math>L[x]</math> instead of <math>L</math>, also with a predicate symbol for <math>x</math>.  See [[Constructible universe#Relative constructibility]].

== See also ==
* [[Zero dagger|0<sup>†</sup>]], a set similar to 0<sup>#</sup> where the constructible universe is replaced by a larger inner model with a [[measurable cardinal]].

== References ==
* {{cite book|author=Drake, F. R.|title=Set Theory: An Introduction to Large Cardinals (Studies in Logic and the Foundations of Mathematics; V. 76)|publisher=Elsevier Science Ltd|year=1974|isbn=0-444-10535-2}}
* {{cite journal | last=Harrington | first=Leo | author-link=Leo Harrington| title=Analytic determinacy and 0 # | journal=Journal of Symbolic Logic | volume=43 | issue=4 | date=1978 | issn=0022-4812 | doi=10.2307/2273508 | mr=518675 | pages=685–693| jstor=2273508 }}
* {{cite book | last1=Jech | first1=Thomas | author1-link=Thomas Jech | title=Set Theory | edition=Third Millennium | publisher=[[Springer-Verlag]] | location=Berlin, New York | series=Springer Monographs in Mathematics | isbn=978-3-540-44085-7 | year=2003 | zbl=1007.03002 }}
* {{cite book|last=Kanamori|first=Akihiro|year=2003|publisher=Springer|author-link=Akihiro Kanamori|title=The Higher Infinite : Large Cardinals in Set Theory from Their Beginnings|title-link=The Higher Infinite|edition=2nd|isbn=3-540-00384-3}}
* {{cite journal | last=Martin | first=Donald A. | title=Measurable cardinals and analytic games | journal=Fundamenta Mathematicae | volume=66 | issue=3 | date=1970 | mr=0258637 | issn=0016-2736 | doi=10.4064/fm-66-3-287-291 | doi-access=free | pages=287–291}}
* {{cite journal | last=Silver | first=Jack H. | title=Some applications of model theory in set theory | journal=Annals of Mathematical Logic | volume=3 | issue=1 | date=1971  | mr=0409188 | doi=10.1016/0003-4843(71)90010-6 | doi-access=free | pages=45–110}}

* {{cite journal | last=Solovay | first=Robert M. | title=A nonconstructible Δ{{su|p=1|b=3}} set of integers | journal=Transactions of the American Mathematical Society | volume=127 | issue=1 | date=1967 | doi=10.2307/1994631 | mr=0211873 | issn=0002-9947 | pages=50–75 | jstor=1994631 }}

===Citations===
{{reflist}}

[[Category:Real numbers]]
[[Category:Determinacy]]
[[Category:Large cardinals]]
[[Category:Constructible universe]]