arXiv subjects
Publications and source records attributed to Robert Rust.
The Whitehead Model of free groups can be used to measure the complexity, or degree, of automorphisms of free groups. The bound for the degree of the $f \circ g$ for deg$(f) =$ deg($g) = 0$ had previously been discovered. We extend this result to the case where at least one of our automorphisms has degree 0.
Ramsey's theorem states that for all finite colorings of an infinite set, there exists an infinite homogeneous subset. What if we seek a homogeneous subset that is also order-equivalent to the original set? Let $S$ be a linearly ordered set and $a \in N$. The big Ramsey degree of $a$ in $S$, denoted $T(a,S)$, is the least integer $t$ such that, for any finite coloring of the $a$-subsets of $S$, there exists $S'\subseteq S$ such that (i) $S'$ is order-equivalent to $S$, and (ii) if the coloring is restricted to the $a$-subsets of $S'$ then at most $t$ colors are used. Ma\v{s}ulovi\'{c} \& \v{S}obot (2019) showed that $T(a,\omega+\omega)=2^a$. From this one can obtain $T(a,\zeta)=2^a$. We give a direct proof that $T(a,\zeta)=2^a$. Ma\v{s}ulovi\'{c} and \v{S}obot (2019) also showed that for all countable ordinals $\alpha < \omega^\omega$, and for all $a \in N$, $T(a,\alpha)$ is finite. We find exact value of $T(a,\alpha)$ for all ordinals less than $\omega^\omega$ and all $a\in N$.