July | 2007 | A kind of library

Goodstein sequences

July 27, 2007

Will Sladek, a student at Caltech, wrote an excellent introductory paper on incompleteness in $\mathsf{PA}$ , The termite and the tower. While Will was working on his paper, I wrote a short note, Goodstein’s function, on how to compute Goodstein’s function. Please let me know of any comments of corrections to either article.

Update: My paper has been published.

Goodstein’s function. Revista Colombiana de Matemáticas 41 (2)(2007), 381–391. MR2585906 (2011c:03139). Zbl 1156.03053.

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2 Comments | 116b: Computability and incompleteness | Tagged: Goodstein's function, Undecidability and incompleteness | Permalink
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Mathoverflow
- A translation of the Cantor set contained in the irrationals February 22, 2014
  Let $C$ be the standard Cantor middle-third set. As a consequence of the Baire category theorem, there are numbers $r$ such that $C+r$ consists solely of irrational numbers, see here. What would be an explicit example of a number $r$ with this property? Short of an explicit example, are there any references addressing this question? A natural approach would […]
  Andrés E. Caicedo
- Answer by Andrés E. Caicedo for Does determinacy in $L(\mathbb{R})$ implies projective determinacy (in $V$)? October 1, 2017
  Suppose $M$ is an inner model (of $\mathsf{ZF}$) with the same reals as $V$, and let $A\subseteq \mathbb R$ be a set of reals in $M$. Suppose further that $A$ is determined in $M$. Under these assumptions, $A$ is also determined in $V$. The point is that since winning strategies are coded by reals, and any possible run of the game for $A$ is coded by a real, […]
  Andrés E. Caicedo
- Answer by Andrés E. Caicedo for Is ZFC+(negation of a large cardinal axiom) arithmetically sound? September 29, 2017
  Yes. This is obvious if there are no such cardinals. (I assume that the natural numbers of the universe of sets are the true natural numbers. Otherwise, the answer is no, and there is not much else to do.) Assume now that there are such cardinals, and that "large cardinal axiom" is something reasonable (so, provably in $\mathsf{ZFC}$, the relevant […]
  Andrés E. Caicedo
- Answer by Andrés E. Caicedo for MathSciNet Reference Fixes September 26, 2017
  Please send an email to mathrev@ams.org, explaining the issue. (This is our all-purpose email address; any mistakes you discover, not just regarding references, you can let us know there.) Give us some time, I promise we'll get to it. However, if it seems as if the request somehow fell through the cracks, you can always contact one of your friendly edit […]
  Andrés E. Caicedo
- Answer by Andrés E. Caicedo for Reals which must, can't or might be added by forcing September 25, 2017
  The characterization mentioned by Mohammad in his answer really dates back to Lev Bukovský in the early 70s, and, as Ralf and Fabiana recognize in their note, has nothing to do with $L$ or with reals (in their note, they indicate that after proving their result, they realized they had essentially rediscovered Bukovský's theorem). See MR0332477 (48 #1080 […]
  Andrés E. Caicedo
Math.StackExchange
- Answer by Andrés E. Caicedo for What statements are equivalent to the Axiom of Determinacy? October 5, 2017
  I give an example (perhaps the best-known example) below, but let me first discuss equiconsistency rather than straight equivalence. Usually an equiconsistency is really the sort of result you are after anyway: You want to establish that certain statements in the universe where choice holds correspond to determinacy, which implies the failure of choice. The […]
  Andrés E. Caicedo
- Answer by Andrés E. Caicedo for Is the axiom of choice necessary here? October 4, 2017
  The other answers have correctly identified the issue. Let me highlight the difficulty: it is relatively consistent with the axioms of set theory except for the axiom of choice that there are infinite sets which do not contain a copy of the natural numbers (that is, there are infinite sets $X$ such that there is no injection $f\!:\mathbb N\to X$). This means […]
  Andrés E. Caicedo
- Answer by Andrés E. Caicedo for How to evaluate $\prod_{i\in \omega}\aleph_i$ October 4, 2017
  This is $\aleph_\omega^{\aleph_0}$. First of all, this cardinal is an obvious upper bound. Second, if $A\subseteq\omega$ is infinite, $\prod_{i\in A}\aleph_i$ is clearly at least $\aleph_\omega$. The result follows, by splitting $\omega$ into countably many infinite sets. In general, the rules governing infinite products and exponentials are far from being w […]
  Andrés E. Caicedo
- Answer by Andrés E. Caicedo for How is raising 2 to the power of a cardinality equivalent to taking the power set? September 25, 2017
  If $\lambda$ and $\kappa$ are cardinals, $\lambda^\kappa$ represents the cardinality of the set of functions $f\!:A\to B$ where $A,B$ are fixed sets of cardinality $\kappa,\lambda$ respectively. (One needs to check this is independent of which specific sets $A,B$ we pick, of course.) At least for finite numbers, this is something you may have encountered in […]
  Andrés E. Caicedo
- Answer by Andrés E. Caicedo for Which sets are Lebesgue measurable in $ZFC$? August 20, 2017
  R. Solovay proved that the provably $\mathbf\Delta^1_2$ sets are Lebesgue measurable (and have the property of Baire). A set $A$ is provably $\mathbf\Delta^1_2$ iff there is a real $a$, a $\Sigma^1_2$ formula $\phi(x,y)$ and a $\Pi^1_2$ formula $\psi(x,y)$ such that $$A=\{t\mid \phi(t,a)\}=\{t\mid\psi(t,a)\},$$ and $\mathsf{ZFC}$ proves that $\phi$ and $\psi […]
  Andrés E. Caicedo
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