Past exam of the mathematics course of the University of Cambridge / 2002 / iii / Paper 21 / 4

For the effective counterexample, let $K$ be the diagonal halting set and let $K_s$ be the finite approximation obtained by running the first $s$ programs for $s$ steps. For $x<y<z$, define the recursive triple colouring with no computable infinite homogeneous set byThis is a total recursive two-coloring: only bounded computations are involved. An infinite homogeneous set $H$ cannot have color one. Fix $x\in H$ and take $y,z\in H$ sufficiently large that every halting computation with index at most $x$ that ever halts has already halted by stage $y$; then the color is zero. If $H$ has color zero, it computes $K$: given $e$, find $x<y$ in $H$ with $x\ge e$ and inspect $K_y(e)$. Agreement with every later stage $z\in H$, and the unboundedness of $H$, give $K_y\cap \{0,\dots ,x\}=K\cap \{0,\dots ,x\}$. Hence this is the correct answer. The set $K$ is not recursive: a purported halting decider would produce a program that halts exactly when the decider says it does not halt on its own index. Thus there is no recursive infinite homogeneous set. Here infinite is essential: every finite subset is recursive, and sets with fewer than three elements are vacuously homogeneous.

A precise finite-arity Erdős-Rado theorem for finite arities iswhere ${\beth }_0(\lambda )=\lambda$ and ${\beth }_{r+1}(\lambda )=2^{{\beth }_r(\lambda )}$. We prove the closed elementary-submodel construction of an end-homogeneous sequence and then induct. We work with AC, as in the usual cardinal partition calculus.

Put $\theta =(2^{\mu }{)}^{+}$, where $\mu$ is infinite, and let $c:[\theta {]}^{m+1}\to \lambda$ with $\lambda \le \mu$ and $m\ge 1$. Choose a regular cardinal $\chi$ large enough that this coloring and its domain belong to $H_{\chi }$, the sets of hereditary size below $\chi$. There is $M\prec H_{\chi }$ of size $2^{\mu }$ containing $c,\theta ,\mu$, every ordinal below ${\mu }^{+}$, and closed under externally given sequences of length at most $\mu$. Here is the closure construction: start with a Skolem hull of the named parameters and the ordinals below ${\mu }^{+}$; iterate hulls after adjoining all at-most-$\mu$-sequences of the preceding hull for ${\mu }^{+}$ stages. Every stage and the union have size $2^{\mu }$, since $(2^{\mu }{)}^{\mu }=2^{\mu }$ and ${\mu }^{+}\le 2^{\mu }$. The elementary chain theorem gives elementarity. Any sequence of at most $\mu$ members of the final union lies in a bounded stage, because ${\mu }^{+}$ is regular, and is added at the next stage. This proves the required closure.

Let $\beta =\sup (M\cap \theta )$. Regularity of $\theta$ gives $\beta <\theta$, and $\beta \notin M$: otherwise $\beta +1$ would also belong to $M\cap \theta$. Recursively choose increasing $x_{\alpha }\in M\cap \theta$ for $\alpha <{\mu }^{+}$ so that, for every $m$-subset $u$ of the earlier points,To justify the recursion, the earlier sequence has length at most $\mu$, hence belongs to $M$. There are at most $\mu$ constraints. All their points and color values are in $M$, so their coded table belongs to $M$ by closure, even though $\beta$ itself is external to $M$. The supremum of the earlier points, plus one, also belongs to $M\cap \theta$ and is below $\beta$. Thus $\beta$ witnesses in $H_{\chi }$ that a point above that bound satisfying the table exists. Elementarity supplies such a point in $M$. This completes the recursion.

The sequence is end-homogeneous: on any increasing $(m+1)$-tuple from it, the color depends only on its first $m$ entries. Define the induced $m$-tuple coloring by $g(u)=c(u\cup \{\beta \})$. For the induction, the arity-one assertion ${\lambda }^{+}\to ({\lambda }^{+}{)}_{\lambda }^1$ is the infinite pigeonhole principle: a union of $\lambda$ sets each of size at most $\lambda$ has size at most $\lambda$. At step $r\ge 1$, take $\mu ={\beth }_{r-1}(\lambda )$, so $\theta ={\beth }_r(\lambda {)}^{+}$. The constructed sequence has size ${\mu }^{+}={\beth }_{r-1}(\lambda {)}^{+}$. The induction hypothesis makes $g$ constant on a subset of size ${\lambda }^{+}$; end-homogeneity makes $c$ constant on its $(r+1)$-subsets. This proves the theorem. In particular $\lambda ={\aleph }_0$ gives an uncountable homogeneous set for every finite-arity coloring with countably many colors, on a sufficiently large cardinal.

There is no unrestricted infinite-exponent analogue under AC. For any infinite cardinal $\kappa$, choose one representative $R$ from each equivalence class of $[\kappa {]}^{\omega }$ modulo finite symmetric difference. Color $X$ by the parity of $|X△R|$. Removing one element of $X$ leaves it in the same class and reverses the parity. Every infinite proposed homogeneous set contains both a countably infinite $X$ and $X\setminus \{x\}$, so cannot be homogeneous. Thus the finite-symmetric-difference colouring of infinite subsets provesThe contrast is between arbitrary colorings at countably infinite arity and the finite arities of the theorem; restrictions on the definability of an infinite-arity coloring can change the problem.