Editing List of undecidable problems (section)

==Problems about abstract machines==
* The [[halting problem]] (determining whether a [[Turing machine]] halts on a given input) and the [[mortality (computability theory)|mortality problem]] (determining whether it halts for every starting configuration).
* Determining whether a Turing machine is a [[Busy beaver#Non-computability|busy beaver champion]] (i.e., is the longest-running among halting Turing machines with the same number of states and symbols).
* [[Rice's theorem]] states that for all nontrivial properties of partial functions, it is undecidable whether a given machine computes a partial function with that property.
* The halting problem for a [[register machine]]: a finite-state automaton with no inputs and two counters that can be incremented, decremented, and tested for zero.
* Universality of a nondeterministic [[pushdown automaton]]: determining whether all words are accepted.
* The problem whether a [[tag system]] halts.