This document summarizes key concepts from Unit 5, including types of Turing machines, undecidability, recursively enumerable languages, Post's correspondence problem, and counter machines. It defines undecidable problems as those with no algorithm to solve them. Examples of undecidable problems include the halting problem and determining if a Turing machine accepts a language that is not recursively enumerable. Post's correspondence problem and its modified version are presented with examples. Recursively enumerable languages are defined and properties like concatenation, Kleene closure, union, and intersection are described. Counter machines are defined as having states, input alphabet, start/final states, and transitions that allow incrementing, decrementing, and checking if a