Fixed-point lemma for normal functions: Difference between revisions

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{{Short description|Mathematical result on ordinals}}

The '''fixed-point lemma for normal functions''' is a basic result in [[axiomatic set theory]]; it states that any [[normal function]] has arbitrarily large [[fixed point (mathematics)|fixed point]]s and can often be used to construct [[ordinal number]]s with interesting properties. A formal version and proof (using the [[Zermelo-Fraenkel axioms]]) follows.

The '''fixed-point lemma for normal functions''' is a basic result in [[axiomatic set theory]]; it statesstating that any [[normal function]] has arbitrarily large [[fixed point (mathematics)|fixed point]]s and(Levy can1979: often be used to construct [[ordinal number]]s with interesting propertiesp. 117). AIt formalwas versionfirst andproved proof (using theby [[Zermelo-FraenkelOswald axiomsVeblen]]) followsin 1908.

== Formal version ==

Let ''f'' : [[ordinal number|Ord]] → Ord be a [[normal function]]. Then for every α ∈ Ord, there exists a β ∈ Ord such that β ≥ α and ''f''(β) = β.

== Background and formal statement ==

A [[normal function]] is a [[proper class|class]] function <math>f</math> from the class Ord of [[ordinal numbers]] to itself such that:

* <math>f</math> is '''strictly increasing''': <math>f(\alpha)<f(\beta)</math> whenever <math>\alpha<\beta</math>.

* <math>f</math> is '''continuous''': for every limit ordinal <math>\lambda</math> (i.e. <math>\lambda</math> is neither zero nor a successor), <math>f(\lambda)=\sup\{f(\alpha):\alpha<\lambda\}</math>.

It can be shown that if <math>f</math> is normal then <math>f</math> commutes with [[supremum|suprema]]; for any nonempty set <math>A</math> of ordinals,

:<math>f(\sup A)=\sup f(A) = \sup\{f(\alpha):\alpha \in A\}</math>.

Indeed, if <math>\sup A</math> is a successor ordinal then <math>\sup A</math> is an element of <math>A</math> and the equality follows from the increasing property of <math>f</math>. If <math>\sup A</math> is a limit ordinal then the equality follows from the continuous property of <math>f</math>.

A '''fixed point''' of a normal function is an ordinal <math>\beta</math> such that <math>f(\beta)=\beta</math>.

The fixed point lemma states that the class of fixed points of any normal function is nonempty and in fact is unbounded: given any ordinal <math>\alpha</math>, there exists an ordinal <math>\beta</math> such that <math>\beta\geq\alpha</math> and <math>f(\beta)=\beta</math>.

The continuity of the normal function implies the class of fixed points is closed (the supremum of any subset of the class of fixed points is again a fixed point). Thus the fixed point lemma is equivalent to the statement that the fixed points of a normal function form a [[club set|closed and unbounded]] class.

== Proof ==

WeThe knowfirst step of the proof is to verify that ''<math>f''(γ\gamma) ≥ γ\ge\gamma</math> for all ordinals γ<math>\gamma</math> and that <math>f</math> commutes with suprema. WeGiven nowthese declareresults, inductively define an increasing sequence &lt;α<submath>''\langle\alpha_n\rangle_{n''<\omega}</submath>&gt; (''n'' &lt; ω) by setting α_{0</submath>\alpha_0 = α\alpha</math>, and α<submath>''\alpha_{n''+1}} = ''f''(α<sub>''n''\alpha_n)</submath>) for ''<math>n''\in\omega</math>. &lt;Let ω,<math>\beta and= define\sup_{n<\omega} β\alpha_n</math>, =so sup &lt;α<submath>''n''\beta\ge\alpha</submath>&gt;. ClearlyMoreover, βbecause ≥ α. Since ''<math>f''</math> commutes with [[supremum|suprema]], we have

:<math>f(\beta) = f(\sup_{n<\omega} \alpha_n)</math>

:''f''(β) = ''f''(sup {α_''n'' : ''n'' &lt; ω})

:<math>\qquad = \sup_{n<\omega} f(\alpha_n)</math>

:&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; = sup {''f''(α_''n'') : ''n'' &lt; ω}

:<math>\qquad = \sup_{n<\omega} \alpha_{n+1}</math>

:&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; = sup {α_''n''+1 : ''n'' &lt; ω}

:<math>\qquad = \beta</math>

:&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; = β

(The last stepequality usesfollows from the fact that the sequence &lt;α<submath>''n''\langle\alpha_n\rangle_n</submath>&gt; increases). <math> \square </math>

As an aside, it can be demonstrated that the <math>\beta</math> found in this way is the smallest fixed point greater than or equal to <math>\alpha</math>.

== Example application ==

It is easily checked that theThe function ''f'' : Ord → Ord, ''f''(''α'') = אω_''α'' is normal (see [[alephinitial numberordinal]]);. thusThus, there exists an ordinal Θ''θ'' such that Θ''θ'' = אω_Θ''θ''. In fact, the above lemma shows that there areis infinitelya manyclosed, unbounded class of such Θ''θ''.

==References==

{{unreferencedrefbegin}}

* {{cite book

| author = Levy, A.

| title = Basic Set Theory

| year = 1979

| publisher = Springer

| id = Republished, Dover, 2002.

| isbn = 978-0-387-08417-6

| url-access = registration

| url = https://archive.org/details/basicsettheory00levy_0

}}

*{{cite journal

| author= Veblen, O.

| authorlink = Oswald Veblen

| title = Continuous increasing functions of finite and transfinite ordinals

| journal = Trans. Amer. Math. Soc.

| volume = 9

| year = 1908

| pages = 280&ndash;292

| doi= 10.2307/1988605

| issue = 3

| jstor= 1988605

| issn= 0002-9947| doi-access = free

}}

{{refend}}

[[Category:SetOrdinal theorynumbers]]

[[Category:Fixed-point pointstheorems|Normal Functions]]

[[Category:Lemmas in set theory]]

[[Category:Articles containing proofs]]