Paper detail

Universal Graphs at $\aleph_{ω_1+1}$ and Set-theoretic Geology

This thesis consists of two parts: the construction of a jointly universal family of graphs, and then an exploration of set-theoretic geology. Firstly we shall construct a model in which $2^{\aleph_{ω_1}}=2^{\aleph_{ω_1+1}}=\aleph_{ω_1+3}$ but there is a jointly universal family of size $\aleph_{ω_1+2}$ of graphs on $\aleph_{ω_1+1}$. We take a supercompact cardinal $κ$ and will use Radin forcing with interleaved collapses to change $κ$ into $\aleph_{ω_1}$. Prior to the Radin forcing we perform a preparatory iteration to add functions from $κ^+$ into Radin names for what will become members of the jointly universal family on $κ^+$. The same technique can be used with any uncountable cardinal in place of $ω_1$. Secondly we explore various topics in set-theoretic geology. We begin by showing that a class Easton support iteration of $\mathrm{Add}(κ,1)$ at $κ$ regular results in a universe that is its own generic mantle. We then consider set forcings $\mathbb{P}$, $\mathbb{Q}$, $\mathbb{R}$ and $\mathbb{S}$ with respective generics $G$, $H$, $I$ and $J$ such that $V[G][I]=V[H][J]$ and show that $V[G]$ and $V[H]$ must have a shared ground via $(|\mathbb{R}|+|\mathbb{S}|)^+$-cc forcing. This allows a similar analysis of the related situation when $\mathbb{P}$ is replaced by a class iteration and $V[H]$ by a generic ground of $V[G]$. We conclude with a simple characterisation of the mantle of a class forcing extension, and an investigation of the possibilities for a version of the intermediate model theorem that applies to class forcing.