Complexity and Avoidance

Abstract

While close connections between recursively bounded $\mathrm{DNR}$ sequences and complex sequences have been well-documented, explicit bounds on growth rates in these results has not been possible due to the nature of $\mathrm{DNR}$'s definition. Recent observations show that this problem can be circumvented, opening the door to quantification of those relationships. We follow the approach of Simpson, replacing $\mathrm{DNR}$ with a class we term $\mathrm{LUA}$ (\textbf{L}inearly \textbf{U}niversal \textbf{A}voidance).

In this dissertation we examine the relationships between the several hierarchies, including the complexity, $\mathrm{LUA}$, 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 $\mathrm{LUA}(q)$ 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 $\ldnr$ hierarchies.