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If it turns out that a problem is equivalent to a known open problem, then the open-problem tag is added. After that, the question essentially becomes, "What is known about this problem? What are some possible ways to approach this problem? What are some ways that people have tried to attack it before, and with what results?"

13 votes
Accepted

Can any finite lattice be realized as an intermediate subgroups lattice?

This is an open problem. See Wikipedia article: http://en.m.wikipedia.org/wiki/Finite_lattice_representation_problem Palfy and Pudlak's result (see open-source description in Palfy's article Interva …
Bjørn Kjos-Hanssen's user avatar
33 votes
Accepted

Is there a known Turing machine which halts if and only if the Collatz conjecture has a coun...

$\newcommand\PA{\mathit{PA}}$Let's note that this is not a question of whether Collatz is undecidable. The statement $\neg\mathrm{Con}(\PA)$ is undecidable (by $\PA$, assuming $PA$ is consistent) but …
Bjørn Kjos-Hanssen's user avatar