Complexity and Avoidance

TL;DR

This dissertation explores the relationships and hierarchies of complexity, LUA, and shift complexity, providing bounds, comparisons, and structural insights into their growth rates and computational properties.

Contribution

It establishes new bounds and equivalences between complexity hierarchies, introduces generalized shift complexity notions, and analyzes the structure of LUA hierarchies using bushy tree forcing.

Findings

Quantified growth rates of functions in hierarchy relations

Established weak incomparability of LUA hierarchies for different order functions

Connected shift complexity with properties of deep nonempty Pi^0_1 classes

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.