These exercises (due September 28) are mostly meant to test your understanding of compactness.

Let be a nonstandard model of Show:

(Overspill) Suppose that is definable (with parameters) and that Show that is finite.

(Underspill) Suppose that is definable and that Show that there is some infinite such that all the elements of are larger than

Let be a nonstandard model of Here, is treated as a relation, and in we may have placed whatever functions and relations we may have need to reference in what follows; moreover, we assume that in our language we have a constant symbol for each real number. (Of course, this means that we are lifting the restriction that languages are countable.) To ease notation, let’s write for The convention is that we identify actual reals in with their copies in so we write rather than etc.

Show that is a nonstandard model of the theory of problem 1. (In particular, check that the indicated restrictions of and have range contained in )

A (nonstandard) real is finite iff there is some (finite) natural number such that Otherwise, it is infinite. A (nonstandard) real is infinitesimal iff but for all positive (finite) natural numbers one has that We write to mean that either is infinitesimal, or else it is Show that infinite and infinitesimal numbers exist. The monad of a real is the set of all such that which we may also write as and say that and are infinitesimally close. Show that the relation is an equivalence relation. Show that if a monad contains an actual real number, then this number is unique. Show that this is the case precisely if it is the monad of a finite number. In this case, write to indicate that the (actual) real is in the monad of We also say that is the standard part of

Show that a function is continuous at a real iff for all infinitesimal numbers

Suppose that is continuous on the closed interval Argue as follows to show that attains its maximum: For each positive integer there is some integer with such that Conclude that the same holds if is some infinite natural number, i.e., there is some (perhaps infinite) “natural number” with such that Let and argue that the maximum of is attained at

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Given a language and an -structure a set is definable iff there is a formula with (distinct) free variables and there are elements such that, letting be the set of assignments such that for then for all with

In human: is definable if it is the set of elements of that satisfy some formula. We allow said formula to use parameters, i.e., to refer to some fixed elements of

Now my concern is: if 0 is not an infinitesimal, then is reflexive. Namely, if then . That is, for all positive . But . So, cannot be infinitesimal. What am I missing here?

Type ‘latex’ immediately following the dollar sign, leave a space, and then the math text as you’d do in latex usually. See this announcement for more info.

The wordpress people tweak with the way latex is compiled every now and then, so sometimes strange errors that were not there before appear; but it works pretty decently, and it is getting better. (There seem to be a few silly things still: you want to write {} right before a [ if this is the first symbol in a math display, for example.)

Luca Trevisan devised a nice program, LaTeX2WP, to make the use of in WordPress pleasant rather than traumatic; I use it whenever I have a long post.

(1) Patrick Dehornoy gave a nice talk at the Séminaire Bourbaki explaining Hugh Woodin's approach. It omits many technical details, so you may want to look at it before looking again at the Notices papers. I think looking at those slides and then at the Notices articles gives a reasonable picture of what the approach is and what kind of problems remain […]

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 (Intern […]

I learned of this problem through Su Gao, who heard of it years ago while a post-doc at Caltech. David Gale introduced this game in the 70s, I believe. I am only aware of two references in print: Richard K. Guy. Unsolved problems in combinatorial games. In Games of No Chance, (R. J. Nowakowski ed.) MSRI Publications 29, Cambridge University Press, 1996, pp. […]

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 […]

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, […]

The two concepts are different. For example, $\omega$, the first infinite ordinal, is the standard example of an inductive set according to the first definition, but is not inductive in the second sense. In fact, no set can be inductive in both senses (any such putative set would contain all ordinals). In the context of set theory, the usual use of the term […]

I will show that for any positive integers $n,\ell,k$ there is an $M$ so large that for all positive integers $i$, if $i/M\le \ell$, then the difference $$ \left(\frac iM\right)^n-\left(\frac{i-1}M\right)^n $$ is less than $1/k$. Let's prove this first, and then argue that the result follows from it. Note that $$ (i+1)^n-i^n=\sum_{k=0}^{n-1}\binom nk i^ […]

I think it is cleaner to argue without induction. If $n$ is a positive integer and $n\ge 8$, then $7n$ is both less than $n^2$ and a multiple of $n$, so at most $n^2-n$ and therefore $7n+1$ is at most $n^2-n+1

Let PRA be the theory of Primitive recursive arithmetic. This is a subtheory of PA, and it suffices to prove the incompleteness theorem. It is perhaps not the easiest theory to work with, but the point is that a proof of incompleteness can be carried out in a significantly weaker system than the theories to which incompleteness actually applies. It is someti […]

What was the precise definition of “definable” again. I can’t find it in the book anywhere.

Given a language and an -structure a set is

definableiff there is a formula with (distinct) free variables and there are elements such that, letting be the set of assignments such that for then for all withIn human: is definable if it is the set of elements of that satisfy some formula. We allow said formula to use parameters, i.e., to refer to some fixed elements of

Thanks.

Is 0 considered an infinitesimal? By the definition above, 0 would be, but I always thought it was otherwise.

Ah, you are right! I’ve modified the text accordingly.

Making infinitesimals different from 0 now forces us to change slightly the definition of so I’ve done that as well.

Now my concern is: if 0 is not an infinitesimal, then is reflexive. Namely, if then . That is, for all positive . But . So, cannot be infinitesimal. What am I missing here?

[Addressed by the revised definition. -A.]Thank you. It’s clear now.

Also, how do you get LaTex to work on your blog? I noticed that you got the approximation symbol to show, but when I tried \approx it didn’t work

Type ‘latex’ immediately following the dollar sign, leave a space, and then the math text as you’d do in latex usually. See this announcement for more info.

The wordpress people tweak with the way latex is compiled every now and then, so sometimes strange errors that were not there before appear; but it works pretty decently, and it is getting better. (There seem to be a few silly things still: you want to write {} right before a [ if this is the first symbol in a math display, for example.)

Luca Trevisan devised a nice program, LaTeX2WP, to make the use of in WordPress pleasant rather than traumatic; I use it whenever I have a long post.