Will Sladek, a student at Caltech, wrote an excellent introductory paper on incompleteness in 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.

[…] a nice introduction to incompleteness and Goodstein’s theorem, see Will Sladek’s paper here. Possibly related posts: (automatically generated)Partitioning numbersUS concerns of decline in […]

[…] (an undergraduate student of mine at Caltech wrote a nice paper on this a few years ago, “The termite and the tower.“). There are others. A nice one is about a game, Hercules and the […]

Yes. This is a consequence of the Davis-Matiyasevich-Putnam-Robinson work on Hilbert's 10th problem, and some standard number theory. A number of papers have details of the $\Pi^0_1$ sentence. To begin with, take a look at the relevant paper in Mathematical developments arising from Hilbert's problems (Proc. Sympos. Pure Math., Northern Illinois Un […]

I am looking for references discussing two inequalities that come up in the study of the dynamics of Newton's method on real-valued polynomials (in one variable). The inequalities are fairly different, but it seems to make sense to ask about both of them in the same post. Most of the details below are fairly elementary, they are mostly included for comp […]

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 […]

Not necessarily. That $\mathfrak m$ is consistently singular is proved in MR0947850 (89m:03045) Kunen, Kenneth. Where $\mathsf{MA}$ first fails. J. Symbolic Logic 53(2), (1988), 429–433. There, Ken shows that $\mathfrak{m}$ can be singular of cofinality $\omega_1$. (Both links above are behind paywalls.)

Ignas: It is not possible to provide an explicit expression for a non-linear solution. The reason is that (it is a folklore result that) an additive $f:{\mathbb R}\to{\mathbb R}$ is linear iff it is measurable. (This result can be found in a variety of places, it is a standard exercise in measure theory books. As of this writing, there is a short proof here. […]

What is the context? In the setting of analysis, $\sum_{i\in\mathbb N}x_i$ is defined as usual; other than that, the infinite sum $\sum_{i\in I}x_i$ is defined only when $\sum_i|x_i|$ is defined (so, we do not have a proper theory of conditionally convergent series). Assume then that the $x_i$ are non-negative, in which case $\sum_i x_i$ is defined as the un […]

There are several nice proofs of the result. The most intuitive I'm aware of is the following: We may as well assume that $A$ and $B$ are disjoint. Consider the directed graph whose set of vertices is $A\cup B$, in which you add an edge from $a$ to $b$ precisely if $a\in A$, $b\in B$, and $f(a)=b$, or $a\in B$, $b\in A$, and $g(a)=b$. Now consider the c […]

A forcing collapses cardinals iff (by definition) some cardinal of the ground model is no longer a cardinal in the forcing extension. Naturally, this means that there is some $\kappa$ in the ground model whose cardinality in the extension is strictly smaller than $\kappa$ (e.g., let $\kappa$ be the first cardinal that witnesses the definition above). Note th […]

$\mathrm{HOD}$ always contains $L$ because any inner model contains $L$, by absoluteness. How easy it is to exhibit a difference really depends on your background. For instance, $0^\sharp$, if it exists, is a real that always belongs to $\mathrm{HOD}$ but is not in $L$. If you are not too comfortable with large cardinals, but know forcing, you may enjoy prov […]

The classical definition of $0^\sharp$ is as (the set of Gödel numbers of) a theory, namely, the unique Ehrenfeucht-Mostowski blueprint satisfying certain properties (coding indiscernibility). This is a perfectly good definition formalizable in $\mathsf{ZFC}$, but $\mathsf{ZFC}$ or even mild extensions of $\mathsf{ZFC}$ are not enough to prove that there are […]

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[…] a nice introduction to incompleteness and Goodstein’s theorem, see Will Sladek’s paper here. Possibly related posts: (automatically generated)Partitioning numbersUS concerns of decline in […]

[…] (an undergraduate student of mine at Caltech wrote a nice paper on this a few years ago, “The termite and the tower.“). There are others. A nice one is about a game, Hercules and the […]