Complexity and Avoidance

Abstract

In this dissertation we examine the relationships between the several hierarchies, including the complexity, $\mathrm{LUA}$ (Linearly Universal Avoidance), and shift complexity hierarchies, with an eye towards quantitative bounds on growth rates therein. We show that for suitable $f$ and $p$, there are $q$ and $g$ such that $\mathrm{LUA}(q) \leq_\mathrm{s} \mathrm{COMPLEX}(f)$ and $\mathrm{COMPLEX}(g) \leq_\mathrm{s} \mathrm{LUA}(p)$, as well as quantify the growth rates of $q$ and $g$. In the opposite direction, we show that for certain sub-identical $f$ satisfying $\lim_{n \to \infty}{f(n)/n}=1$ there is a $q$ such that $\mathrm{COMPLEX}(f) \leq_\mathrm{w} \mathrm{LUA}(q)$, and for certain fast-growing $p$ there is a $g$ such that $\mathrm{LUA}(p) \leq_\mathrm{s} \mathrm{COMPLEX}(g)$, as well as quantify the growth rates of $q$ and $g$. Concerning shift complexity, explicit bounds are given on how slow-growing $q$ must be for any member of $\rm{LUA}(q)$ to compute $\delta$-shift complex sequences. Motivated by the complexity hierarchy, we generalize the notion of shift complexity to consider sequences $X$ satisfying $\operatorname{KP}(\tau) \geq f(|\tau|) - O(1)$ for all substrings $\tau$ of $X$ where $f$ is any order function. We show that for sufficiently slow-growing $f$, $f$-shift complex sequences can be uniformly computed by $g$-complex sequences, where $g$ grows slightly faster than $f$. The structure of the $\mathrm{LUA}$ hierarchy is examined using bushy tree forcing, with the main result being that for any order function $p$, there is a slow-growing order function $q$ such that $\mathrm{LUA}(p)$ and $\mathrm{LUA}(q)$ are weakly incomparable. Using this, we prove new results about the filter of the weak degrees of deep nonempty $\Pi^0_1$ classes and the connection between the shift complexity and $\mathrm{LUA}$ hierarchies.