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5
votes
1answer
131 views

What language $L \in NCM$ has $\overline{L} \not \in NCM$?

$NCM$, the class of non-deterministic reversal-bounded counter machines, has a lot of interesting dependability and closure properties. It's known that, unlike the deterministic version, NCM is not ...
10
votes
1answer
192 views

What is the simplest computational model for which the emptiness problem is undecidable?

What is the simplest computational model for which the emptiness problem is undecidable? Emptiness problem for a computational model (e.g. finite state automaton, alternating pushdown automaton, ...
18
votes
2answers
417 views

Problems with efficient solution except for a small fraction of inputs

The halting problem for Turing machines is perhaps the canonical undecidable set. Nevertheless, we prove that there is an algorithm deciding almost all instances of it. The halting problem is ...
1
vote
0answers
118 views

research on systematically attacking multiple instances of undecidable problems

this question is inspired by a recent popular question [1] on a boundary relating to decidable and undecidable problems (ie open problems in this area), a sort of counterpoint. there are at least ...
2
votes
0answers
63 views

Regaining decidability by adding axioms that model real world situation

It is known that first order logic is too general to be decidable. Adding axioms with special meaning (e.g. expressing notions such as necessity/obligation, provability, etc.) leads us to modal logics ...
13
votes
1answer
220 views

Good reference about approximate methods for solving logic problems

It is known that many logic problems (e.g. satisfiability problems of several modal logics) are not decidable. There are also many undecidable problems in algorithm theory, e.g. in combinatorial ...
8
votes
2answers
297 views

Is meta-undecidability possible?

There are problems that are decidable, there are some that are undecidable, there is semidecidability, etc. In this case I wonder whether a problem can be meta-undecidable. This means (at least in my ...
3
votes
1answer
113 views

Decidability in Extensional Type Theory

What are the ways in which one can add a decidable equivalence relation in a type system with undecidable type checking/extensional equality?
5
votes
0answers
632 views

Eliminate ambiguity from CFG

CFG here stands for context-free grammar. I understand that: Deciding whether a CFG $G$ is ambiguous is undecidable. Deciding whether a CFL $L$ is inherently ambiguous is undecidable. My question ...
10
votes
3answers
2k views

Does P contain languages whose existence is independent of PA or ZFC? (TCS community wiki)

Answer: not known. The questions asked are natural, open, and apparently difficult; the question now is a community wiki. Overview The question seeks to divide languages belonging to the ...