Unless condition_A then condition_b
How should I understand "$A$ unless $B$"?
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Probably I need some help in number theory Is there a function $f\colon\mathbb{R}\to\mathbb{R}$ such that every non-empty open interval is mapped onto $\mathbb{R}$? I wonder  whether there is a function $f\colon\Bbb R\to\Bbb R$ with the folowing characteristic?   for every two real numbers $\alpha,\beta,\alpha\lt\beta$,  $$\{f(x):x\in(\alpha,\beta)\}=\Bbb R$$   I can't  say such a function does not exist, neither can I construct a example  Thanks a lot! What should I do when I see a question that is a duplicate of another one?   Should I answer it? Should I downvote it? Should I comment? Should I edit the question to indicate it's a duplicate? If I can, should I (vote to) close the question? What happens when I vote to close as duplicate? Or: Why did a comment with my name on it just appear? Should I flag it for moderator attention? What about similar or related questions? What tense should I use after the conjunction 'lest'? Must it always be an infinitive? Apache error "Could not reliably determine the server's fully qualified domain name" When I restart my Apache server using the command  sudo /etc/init.d/apache2 restart   I get the following error:  Restarting web server apache2 apache2: Could not reliably determine the server's fully qualified domain name, using 127.0.1.1 for ServerName ... waiting apache2: Could not reliably determine the server's fully qualified domain name, using 127.0.1.1 for ServerName   Is the server using 127.0.1.1 instead of 127.0.0.1? What causes this error? How to check for an active Internet connection on iOS or macOS? Named blocks to limit variable scope: good idea? How should I handle questions which are answered in the comments? Sometimes I have asked a question which got no answers, but is answered in the comments to the question.  How should I handle these questions? I can't accept an answer, so it is never an accepted question.  For example this one:   (ok here the answer in the comments is from me, but nevertheless I also have others of them) How can an 18 month old be taught that bullying is not ok? He seems to like causing the trouble it brings upon him, and bullying, mean acts also seem to please him. He likes to sit on his younger cousin (my son), push him down, and steal his toys. His parents seem to be at a loss about it, and I don't really have any good suggestions to give. Thanks. SUMIF Horizontal I am searching for a column header and return a value under it.  Example:  _|A|B|C  1|X|Y|Z  2|1|2|3   If the value is Y, it must return 2. I tried =SUMIF(A1:C1,"B", A2:C2), but realized that IFSUM does not support searching horizontal.  I would appreciate an alternative solution! How to avoid the 'not properly formatted as code' alert Deriving the Coulomb force equation from the idea of photon exchange? Since Newton's law of gravitation can be gotten out of Einstein's field equatons as an approximation, I was wondering whether the same applies for the electromagnetic force being the exchange of photons. Is there an equation governing the force from the exchange of photons? Are there any links which would show how the Coulomb force comes out of the equations for photon exchange? I know that my question is somewhat similar to the one posted here , but it doesn't really have an answer to my question specifically.