Towsner’s stable forcing

It is well known that a model $\newcommand{\MN}{\mathfrak{N}}\MN$ of $\newcommand{\RCA}[1]{\mathsf{RCA}_{#1}}\RCA0$ satisfies $\Sigma^0_n$-induction ($\newcommand{\Ind}[1]{\mathsf{I}{#1}}\Ind{\Sigma^0_n}$) if and only if it satisfies bounded $\Sigma^0_n$-comprehension: if $\phi(x)$ is a $\Sigma^0_n$-formula (with parameters) then the set $\set{x \lt b: \phi(x)}$ exists for every number $b$ in $\MN$. Thus, it follows that $\Pi^0_n$-induction also holds and indeed induction holds for all boolean combinations of $\Sigma^0_n$ formulas. However, $\Ind{\Sigma^0_n}$ offers no control over sets of higher complexity than that. In particular, $\Delta^0_{n+1}$-induction may fail very badly in a model of $\Ind{\Sigma^0_n}$. Indeed, by a result of Slaman [1] $\Delta^0_{n+1}$-induction is equivalent to $\Sigma^0_{n+1}$-bounding ($\newcommand{\Bnd}[1]{\mathsf{B}{#1}}\Bnd{\Sigma^0_{n+1}}$) and it is known that we have a strict hierarchy: $$\Ind{\Sigma^0_1} \quad\WHEN\quad \Bnd{\Sigma^0_2} \quad\WHEN\quad \Ind{\Sigma^0_2} \quad\WHEN\quad \Bnd{\Sigma^0_3} \quad\WHEN \cdots$$ The actual size of the gaps between $\Ind{\Sigma^0_n}$ and $\Bnd{\Sigma^0_{n+1}}$ has recently been quantified by Henry Towsner, who included a beautiful forcing argument in his preprint [2] that shows that $\Ind{\Sigma^0_n}$ gives absolutely no control whatsoever on $\Delta^0_{n+1}$-definable sets.

Theorem (Towsner). If $\MN$ is a model of $\RCA0$ and $X$ is any external set of numbers in $\MN$ then there is an $\omega$-extension $\MN’$ of $\MN$ that satisfies $\RCA0$ and such that $X$ is $\Delta^0_2$-definable in $\MN’$.

Theorem (Towsner). If $\MN$ is a model of $\RCA0+\Ind{\Sigma^0_n}$ and $X$ is any external set of numbers in $\MN$ then there is an $\omega$-extension $\MN’$ of $\MN$ that satisfies $\RCA0+\Ind{\Sigma^0_n}$ and such that $X$ is $\Delta^0_{n+1}$-definable in $\MN’$.

I will only outline the proof of the first result. The Limit Lemma gives a very combinatorial way to understand $\Delta^0_2$-definable sets over $\MN$. Namely, the external set $X$ is $\Delta^0_2$-definable in $\MN$ exactly when there is a function $c:\N^2\to\set{0,1}$ in $\MN$ such that $\lim_{y\to\infty} c(x,y)$ exists for every $x$ and $x \in X \IFF \lim_{y\to\infty} c(x,y) = 1$. In general, a function $f:\N^2\to\N$ such that $\lim_{y\to\infty} f(x,y)$ exists for every $x$ is called stable. Towsner’s strategy is to generically add a stable coloring $c:\N^2\to\set{0,1}$ such that $x \in X \IFF \lim_{y\to\infty} c(x,y) = 1$ and show that the forcing to add such a coloring preserves $\RCA0$.

Towsner doesn’t name this forcing, so I will take the liberty of coining a name for it — Towsner’s Stable Forcing and I will denote it $\newcommand{\ST}{\mathbb{ST}}\ST_X$ — at least for the purpose of this post. The forcing can be described as follows. Conditions of $\ST_X$ are pairs $p = (c_p,V_p)$ where $c_p$ is (a code for) an $\MN$-finite partial function $\N^2\to\set{0,1}$ and $V_p$ is a truly finite set of stability promises (discussed shortly); and the ordering of conditions is $q \leq p$ iff $c_q$ extends $c_p$ and $V_p \subseteq V_q$. The stability promises consist of pairs $(x,y) \in \N^2$ that promise that the values of the generic coloring with first coordinate $x$ stabilize from $y$ on to the correct value predicted by $X$. More precisely, if $(x,y) \in V_p$ then $(x,y) \in \dom(c_p)$, $c_p(x,y) = 1 \IFF x \in X$, and $c_p(x,y’) = c_p(x,y)$ for all $y’ \gt y$ such that $(x,y’) \in \dom(c_p)$. Since $X$ is not in $\MN$ and $\MN$ usually has no way of distinguishing truly finite sets from others, the forcing $\ST_X$ is not definable in $\MN$ though all conditions are in $\MN$. It is therefore hopeless try to internalize this forcing but we will see that if $G$ is a (sufficiently) generic filter over $\ST_X$, then $c(x,y) = i \IFF (\exists p \in G)(c_p(x,y) = i)$ defines a total coloring $c:\N^2\to\set{0,1}$ and that $\MN’ = \MN[c]$ is as required for the theorem.

The main difficulty is in showing that $\MN[c]$ still satisfies $\Ind{\Sigma^0_1}$. The trick is that if an extension $q \leq p$ forces a bounded statement, then only an $\MN$-finite amount of the information of the information that $q$ knows about $c$ is actually used to witness that fact. Though $V_q$ contains an infinite amount of promised information, we can trim down $V_q$ and throw all the information actually used into $c_q$ to get a new condition $q’ \leq p$ that still forces the bounded statement and has no more stability promises than $p$ had. Thus, if $p \Vdash \exists v\phi(v)$ where $\phi(v)$ is bounded, then there are an extension $q \leq p$ with $V_q = V_p$ and $x \in \N$ such that $q \Vdash \phi(x)$. Using this, given a condition $p$ and a $\Sigma^0_1$-formula $\phi(v)$ we can work in $\MN$ to find an extension $q \leq p$ with $V_q = V_p$ such that $$p \Vdash \forall v\lnot\phi(v) \quad\text{or}\quad p \Vdash \phi(x) \land (\forall v \lt x)\lnot\phi(v) \text{ for some $x$.}$$ Since $V_q = V_p$, we know that $q$ actual condition in $\ST_X$, provided that $p$ is one in the first place, without worrying about $X$, true finiteness, or anything that is not within reach of $\MN$.

Of course, this forcing construction is only a small part of Towsner’s excellent preprint, I highly recommend reading it!


  1. T. A. Slaman, $\Sigma_n$-bounding and $\Delta_n$-induction, Proceedings of the American Mathematical Society 132 (2004), 2449–2456. [doi:10.1090/S0002-9939-04-07294-6]
  2. H. Towsner, On maximal conservative extensions, preprint. [arχiv:1302.1488]

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