Extensions Theorems, Orbits, and Automorphisms of the Computably Enumerable Sets

Abstract

We prove an algebraic extension theorem for the computably enumerable sets, $\mathcal{E}$. Using this extension theorem and other work we then show if $A$ and $\hat{A}$ are automorphic via $\Psi$ then they are automorphic via $\Lambda$ where $\Lambda \restriction \L^*(A) = \Psi$ and $\Lambda \restriction \E^*(A)$ is $\Delta^0_3$. We give an algebraic description of when an arbitrary set $\Ahat$ is in the orbit of a \ce set $A$. We construct the first example of a definable orbit which is not a $\Delta^0_3$ orbit. We conclude with some results which restrict the ways one can increase the complexity of orbits. For example, we show that if $A$ is simple and $\hat{A}$ is in the same orbit as $A$ then they are in the same $\Delta^0_6$-orbit and furthermore we provide a classification of when two simple sets are in the same orbit.