Turing Machines, Variants and REC vs RE
The Turing machine as a tuple with configurations, accept, reject and loop outcomes; a full decider for a^n b^n with its move table and step counts; equivalent variants (multitape, multitrack, two-way infinite tape, stay option, nondeterministic, two-stack PDA, enumerator) with the cost of simulation; recursive versus recursively enumerable languages, deciders versus recognisers and enumerators; linear-bounded automata and context-sensitive languages in brief; and countability. The decider was simulated on every string up to length 10.
