# Unsolvable problems asked seriously as questions

Are there problems posed with any frequency on MSE that are known to be unsolvable in a strong formal sense, such as being independent, undecidable, or blocked by Goedel's theorem? Requests for solution of NP-hard problems or graph isomorphism recur, but has there been any stream of questions like:

prove mathematics is consistent
solve the Continuum Hypothesis
test if two groups are isomorphic from their presentations
algorithm to solve arbitrary Diophantine equations

?

Clarification (edit). This is not intended as a question about policy, allowed questions, or what to close, but about the content of the traffic on MSE. I am requesting links or memories of examples, and briefly had added the [big-list] tag to the original posting, but dropped it when a list of questions in that tag looked like it is being used for different things. Adding the tag again.

• Rolling back the edit : this is not asking about "allowed-questions" or suggesting these questions not be allowed. Of course they are on-topic and the askers probably do not know that they are requesting the impossible. – zyx Jun 7 '13 at 15:57
• Also the problem arises with problems that are conjectured intractable, e.g. a recent example is this question on a problem related to the $\,3x+1\,$ Collatz conjecture. It may be that some classes of this problem are tractable, but probably only experts would be able to decide in the half-hour it took to quickly close it (which probably was not the optimal way to welcome a relatively new user who spend much time composing the question). It's not clear if it was closed for being conjectured intractable, or for other reasons (e.g. not focused). – Key Ideas Jun 7 '13 at 16:06
• In any case, if we wish to attract further experts, it certainly helps to leave difficult questions open, so that they will find some things of interest when they stumble upon MSE. – Key Ideas Jun 7 '13 at 16:07
• @KeyIdeas : amazing, not in a good way, that the Collatz variant was closed. The OP raised an interesting matter of whether the rule he proposes for assigning a multiplicative constant is a valid way to gauge difficulty. I cannot see how any objection to random posting of unsolved problems should block a discussion of that. – zyx Jun 9 '13 at 21:23
• So this question is not about the recent rise in Beal conjecture related questions? – Hagen von Eitzen Jun 15 '13 at 22:22