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| In [[set theory]] and [[computability theory]], '''[[Stephen Cole Kleene|Kleene]]'s <math>\mathcal{O}</math>''' is a canonical subset of the [[natural number]]s when regarded as [[ordinal notations]]. It contains [[ordinal notation]]s for every [[recursive ordinal]], that is, ordinals below [[Church–Kleene ordinal]], <math>\omega_1^{CK}</math>. Since <math>\omega_1^{CK}</math> is the first ordinal not representable in a computable system of ordinal notations the elements of <math>\mathcal{O}</math> can be regarded as the canonical ordinal notations.
| | Today, there are several other types of web development and blogging software available to design and host your website blogs online and that too in minutes, if not hours. You can either install Word - Press yourself or use free services offered on the web today. The effect is to promote older posts by moving them back onto the front page and into the rss feed. s and intelligently including a substantial amount of key words in the title tags, image links, etc. provided by Word - Press Automatic Upgrade, so whenever you need to update the new version does not, it automatically creates no webmaster. <br><br>Creating a website from scratch can be such a pain. If a newbie missed a certain part of the video then they could always rewind. This plugin is a must have for anyone who is serious about using Word - Press. You can up your site's rank with the search engines by simply taking a bit of time with your site. The biggest advantage of using a coupon or deal plugin is that it gives your readers the coupons and deals within minutes of them becoming available. <br><br>It is very easy to install Word - Press blog or website. It was also the very first year that the category of Martial Arts was included in the Parents - Connect nationwide online poll, allowing parents to vote for their favorite San Antonio Martial Arts Academy. Setting Up Your Business Online Using Free Wordpress Websites. If you beloved this article therefore you would like to be given more info pertaining to [http://mmservice.dk/wordpress_backup_plugin_63048 wordpress backup] nicely visit the page. User friendly features and flexibility that Word - Press has to offer is second to none. Socrates: (link to ) Originally developed for affiliate marketers, I've used this theme again and again to develop full-fledged web sites that include static pages, squeeze pages, and a blog. <br><br>Numerous bloggers are utilizing Word - Press and with good reason. Find more information about Design To Wordpress here. To do this, you should link your posts to other relevant posts that you've created. So, we have to add our social media sharing buttons in website. The popularity of Word - Press has increased the demand for Word - Press themes and these themes sells like hot cake on the internet. <br><br>More it extends numerous opportunities where your firm is at comfort and rest assured of no risks & errors. Being a Plugin Developer, it is important for you to know that development of Word - Press driven website should be done only when you enable debugging. As a result, it is really crucial to just take aid of some experience when searching for superior quality totally free Word - Press themes, Word - Press Premium Themes for your web site. Page speed is an important factor in ranking, especially with Google. Definitely when you wake up from the slumber, you can be sure that you will be lagging behind and getting on track would be a tall order. |
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| Kleene (1938) described a system of notation for all recursive ordinals (those less than the [[Church–Kleene ordinal]]). It uses a subset of the [[natural number]]s instead of finite strings of symbols. Unfortunately, there is in general no [[computable function|effective]] way to tell whether some natural number represents an ordinal, or whether two numbers represent the same ordinal. However, one can effectively find notations which represent the ordinal sum, product, and power (see [[ordinal arithmetic]]) of any two given notations in Kleene's <math>\mathcal{O}</math>; and given any notation for an ordinal, there is a [[recursively enumerable set]] of notations which contains one element for each smaller ordinal and is effectively ordered.
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| == Kleene's <math>\mathcal{O}</math> ==
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| The basic idea of Kleene's system of ordinal notations is to build up ordinals in an effective manner. For members <math> p </math> of <math> \mathcal{O} </math>, the ordinal for which <math> p </math> is a notation is <math> | p | </math>. The standard definition proceeds via transfinite induction and the ordering <math> <_{\mathcal{O}} </math> (a [[partial order]]ing of Kleene's <math>\mathcal{O}</math>) is defined simultaneously.
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| * The natural number 0 belongs to Kleene's <math>\mathcal{O}</math> and <math> | 0 | = 0 </math>.
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| * If <math> i </math> belongs to Kleene's <math>\mathcal{O}</math> and <math> |i| = \alpha </math>, then <math> 2^i </math> belongs to Kleene's <math>\mathcal{O}</math> and <math> |2^i|= \alpha + 1 </math> and <math> i <_\mathcal{O} 2^i </math>.
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| * Suppose <math> \{ e \} </math> is the <math> e </math>-th partial recursive function. If <math> e </math> is total, with range contained in <math> \mathcal{O} </math>, and for every natural number <math> n </math>, we have <math>\{ e \} (n) <_{\mathcal{O}} \{ e \} (n + 1) </math>, then <math> 3\cdot 5^e </math> belongs to Kleene's <math>\mathcal{O}</math>, <math> \{ e \} (n) <_\mathcal{O} 3\cdot 5^e </math> for each <math> n </math> and <math> | 3 \cdot 5^e | = \lim_k |\{e \} (k) | </math>, i.e. <math> 3 \cdot 5^e </math> is a notation for the limit of the ordinals <math> \gamma_k </math> where <math>| \{ e \}(k)| = \gamma_k </math> for every natural number <math> k </math>.
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| * <math> p <_\mathcal{O} q </math> and <math> q <_\mathcal{O} r </math> imply <math> p <_\mathcal{O} r </math> (this guarantees that <math> <_\mathcal{O}</math> is transitive.)
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| This definition has the advantages that one can recursively enumerate the predecessors of a given ordinal (though not in the <math> <_\mathcal{O} </math> ordering) and that the notations are downward closed, i.e., if there is a notation for <math> \gamma </math> and <math> \alpha < \gamma </math> then there is a notation for <math> \alpha </math>.
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| == Basic properties of <math> <_\mathcal{O} </math> ==
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| * If <math> |i| = \alpha </math> and <math> |j| = \beta </math> and <math> i <_{\mathcal{O}} j \,,</math> then <math> \alpha < \beta </math>; but the converse may fail to hold.
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| * <math> <_\mathcal{O} </math> induces a tree structure on <math>\mathcal{O} </math>, so <math>\mathcal{O} </math> is well-founded.
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| * <math>\mathcal{O} </math> only branches at limit ordinals; and at each notation of a limit ordinal, <math>\mathcal{O} </math> is infinitely branching.
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| * Since every recursive function has countably many indices, each infinite ordinal receives countably many notations; the finite ordinals have unique notations, <math> n </math> usually denoted <math> n_\mathcal{O} </math>.
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| * The first ordinal that doesn't receive a notation is called the [[Church–Kleene ordinal]] and is denoted by <math>\omega^{CK}_1</math>. Since there are only countably many recursive functions, the ordinal <math>\omega^{CK}_1</math> is evidently countable.
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| * The ordinals with a notation in Kleene's <math>\mathcal{O}</math> are exactly the [[recursive ordinal]]s. (The fact that every recursive ordinal has a notation follows from the closure of this system of ordinal notations under successor and effective limits.)
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| * <math> <_\mathcal{O} </math> is not [[Recursively enumerable set|recursively enumerable]], but there is a recursively enumerable relation which agrees with <math> <_\mathcal{O} </math> precisely on members of <math>\mathcal{O} </math>.
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| * For any notation <math> p </math>, the set <math>\lbrace q \mid q <_{\mathcal{O}} p \rbrace </math> of notations below <math> p </math> is recursively enumerable. However, Kleene's <math>\mathcal{O}</math>, when taken as a whole, is <math>\Pi^1_1</math> (see [[analytical hierarchy]]).
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| * In fact, <math>\mathcal{O} </math> is <math>\Pi^1_1</math>-complete and every <math>\Sigma^1_1</math> subset of <math>\mathcal{O} </math> is effectively bounded in <math>\mathcal{O} </math> (a result of Spector).
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| * <math>\mathcal{O} </math> is the universal system of ordinal notations in the sense that any specific set of ordinal notations can be mapped into it in a straightforward way. More precisely, there is a recursive function <math> f </math> such that if <math> e </math> is an index for a recursive well-ordering, then <math> f(e) </math> is a member of <math>\mathcal{O} </math> and <math> <_e </math> is order-isomorphic to an initial segment of the set <math> \{ p \mid p <_\mathcal{O} f(e) \} </math>.
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| * There is a recursive function <math> +_\mathcal{O} </math>, which, for members of <math>\mathcal{O} </math>, mimics ordinal addition and has the property that <math> \max \{ p,q \} \leq p +_\mathcal{O} q </math>. (Jockusch)
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| == Properties of paths in <math>\mathcal{O} </math> ==
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| A path in <math> \mathcal{O} </math> is a subset <math> \mathcal{P} </math> of <math> \mathcal{O} </math> which is totally ordered by <math> <_\mathcal{O} </math> and is closed under predecessors, i.e. if <math> p </math> is a member of a path <math> \mathcal{P} </math> and <math> q <_\mathcal{O} p </math> then <math> q </math> is also a member of <math> \mathcal{P} </math>. A path <math> \mathcal{P} </math> is maximal if there is no element of <math> \mathcal{O} </math> which is above (in the sense of <math> <_\mathcal{O} </math>) every member of <math> \mathcal{P} </math>, otherwise <math> \mathcal{P} </math> is non-maximal.
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| * A path <math> \mathcal{P} </math> is non-maximal if and only if <math> \mathcal{P} </math> is recursively enumerable (r.e.). It follows by remarks above that every element <math> p </math> of <math> \mathcal{O} </math> determines a non-maximal path <math> \mathcal{P} </math>; and every non-maximal path is so determined.
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| * There are <math> 2^{\aleph_0} </math> maximal paths through <math> \mathcal{O} </math>; since they are maximal, they are non-r.e.
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| * In fact, there are <math> 2^{\aleph_0} </math> maximal paths ''within'' <math> \mathcal{O} </math> of length <math> \omega^2 </math>. (Crossley, Schütte)
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| * For every non-zero ordinal <math> \lambda < \omega_1^{CK} </math>, there are <math> 2^{\aleph_0} </math> maximal paths within <math> \mathcal{O} </math> of length <math> \omega^2 \cdot \lambda </math>. (Aczel)
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| * Further, if <math> \mathcal{P} </math> is a path whose length is ''not'' a multiple of <math> \omega^2 </math> then <math> \mathcal{P} </math> is not maximal. (Aczel)
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| * For each r.e. degree <math> d </math>, there is a member <math> e_d </math> of <math> \mathcal{O} </math> such that the path <math> \mathcal{P} = \lbrace p \mid p <_{\mathcal{O}} e_d \rbrace </math> has many-one degree <math> d </math>. In fact, for each recursive ordinal <math> \alpha \geq \omega^2 </math>, a notation <math> e_d </math> exists with <math> |e_d| = \alpha </math>. (Jockusch)
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| * There exist <math> \aleph_0 </math> paths through <math> \mathcal{O} </math> which are <math>\Pi_1^1 </math>. Given a progression of recursively enumerable theories based on iterating Uniform Reflection, each such path is incomplete with respect to the set of true <math> \Pi_1^0 </math> sentences. (Feferman & Spector)
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| * There exist <math> \Pi^1_1 </math> paths through <math>\mathcal{O} </math> each initial segment of which is not merely r.e., but recursive. (Jockusch)
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| * Various other paths in <math> \mathcal{O} </math> have been shown to exist, each with specific kinds of reducibility properties. (See references below)
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| == See also ==
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| * [[Recursive ordinal]]
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| * [[Large countable ordinal]]
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| * [[Ordinal notation]]
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| ==References==
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| *{{citation|last=Church|url=http://www.ams.org/bull/1938-44-04/S0002-9904-1938-06720-1/ |title=The constructive second number class
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| |journal= Bull. Amer. Math. Soc.|volume= 44|issue=4 |year=1938|pages= 224–232|doi=10.1090/S0002-9904-1938-06720-1|first1=Alonzo}}
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| *{{citation|title= On Notation for Ordinal Numbers
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| |first= S. C.|last= Kleene
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| |journal= The Journal of Symbolic Logic|volume= 3|issue= 4|year= 1938|pages= 150–155|doi= 10.2307/2267778|jstor= 2267778|publisher= Association for Symbolic Logic }}
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| *{{Citation | last1=Rogers | first1=Hartley | title=The Theory of Recursive Functions and Effective Computability | origyear=1967 | publisher=First MIT press paperback edition | isbn=978-0-262-68052-3 | year=1987}}
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| *{{citation|last1=Feferman|first1=Solomon|last2=Spector|first2=Clifford|title=Incompleteness along paths in progressions of theories|journal=Journal of Symbolic Logic|volume=27|issue=4|year=1962|pages=383–390|doi=10.2307/2964544}}
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| [[Category:Ordinal numbers]]
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