Activity for Derek Elkinsâ€
| Type | On... | Excerpt | Status | Date |
|---|---|---|---|---|
| Comment | Post #278382 |
This is useless. People don't need detailed guidance about topics that are obviously on-topic anymore than they need guidance about topics that are obviously off-topic. This fails to accomplish even that as it's incomplete, as you say, and, more importantly, topicality isn't just a matter of subject ... (more) |
— | over 4 years ago |
| Edit | Post #278269 | Post edited | — | over 4 years ago |
| Comment | Post #278270 |
A problem that is almost certainly related is that sometimes the Markdown processor will convert substrings delimited by underscores to italics when those underscores are part of the MathJax, thereby breaking the MathJax. Peter Taylor's issue and this one are almost certainly due to the Markdown proc... (more) |
— | over 4 years ago |
| Edit | Post #278280 | Initial revision | — | over 4 years ago |
| Question | — |
Can we constructively find a third element of a set $X$ satisfying $X \cong X \times X$ given two distinct elements? Consider a set $X$ such that $X \cong X\times X$. It's quite easy to prove in a classical set theory, e.g. ZF, that $X$ must be the empty set or a singleton set or an infinite set. In other words, if we additionally assume that there exists $a, b \in X$ and $a \neq b$, then we know that $X$ is an inf... (more) |
— | over 4 years ago |
| Comment | Post #278270 |
I've suggested an edit that "fixes" the post (so if the edit is accepted people should look in the history to see the original form). That said, the intuition of the change I made is that the backslashes needed to be escaped. However, they probably shouldn't need to be escaped and so some MathJax or ... (more) |
— | over 4 years ago |
| Suggested Edit | Post #278269 |
Suggested edit: There seems to be some issue escaping backslashes(?). This edit fixes the issue now but may cause problems (extra vertical space) if the underlying issue is resolved. (more) |
helpful | over 4 years ago |
| Edit | Post #278145 | Initial revision | — | over 4 years ago |
| Answer | — |
A: Computational hardness of the uniform halting problem According to Wikipedia, the set of (indices of) Turing machines that compute total functions, i.e. which halt on all inputs, is a $\Pi2$ set. If we use the variation of the definition of arithmetical hierarchy which includes primitive recursive functions, then it is fairly straightforward^[If you are... (more) |
— | over 4 years ago |
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