Electric lamp and City electricity frequency
What invisible flicker do different types of light bulbs have?
Does 'rm .*' ever delete the parent directory? The expression .* is expanded by bash to include the current and parent directories:  $ ls -la total 2600 drwxrwxrwx   2 terdon terdon 2162688 Sep 10 16:22 . drwxr-xr-x 142 terdon terdon  491520 Sep 10 15:34 .. -rw-r--r--   1 terdon terdon       0 Sep 10 16:22 foo $ echo .* . ..   If I run rm -rf .* on my Debian using GNU bash, version 4.2.36(1)-release and rm from rm (GNU coreutils) 8.13, I get this message:  $ rm -rf .* rm: cannot remove directory: `.' rm: cannot remove directory: `..'   Is this a GNU thing or is it POSIX? Are there any *nix systems where the command above will silently delete . and ..?  Also, is this a safety feature of the shell or of the rm command itself? Closure of identity element in a topological group is the intersection of its neighborhood system Consider a topological group $G$. I want to prove that      if $\cal{U}$ is the set of all neighborhoods of $e$ (the identity element), then $\overline{\{e\}}=\cap_{U\in \cal{U}}U$.   What I thought was:  i) If $x\in \overline{\{e\}}$, then every neighborhood $V$ of $x$ intersects $\{e\}$, hence every $V$ is a neighborhood of $e$ as well, i.e. $V\in \cal{U}$.  ii) On the other hand, if $x\in \cap_{U\in \cal{U}}U$, then $x\in U$ for all $U \in \cal{U}$, meaning every $U$ is a neighborhood of $x$ intersecting $\{e\}$.  Now, I think that the missing part is an argument justifying that there is no neighborhood of $e$ that is not a neighborhood of $x$ (part i)), and that there is no neighborhood of $x$ that is not a neighborhood of $e$ (part ii)). My idea for this is to argue that there is a homeomorphism taking $e$ to $x$, therefore the neighborhood system of one cannot have more elements that the neighborhood system of the other.  Is this correct? If not, is there a way I can fix it? My furnace pilot light keeps going out so I changed the thermocouple. Now the furnace will come on and heat up to the degree set on my thermostat, then the pilot light will go out and the furnace will not come back on.    Could the thermostat be bad and cause the pilot light to go out? What's the difference between denizen, resident, inhabitant? What's the difference between , , and ?  Even if their ODO definitions suggest they are interchangeable most of the time, are there fine differences in meaning between them? How can I tell which cards or sets are currently legal in various formats? I haven't played Magic: the Gathering since the days of Onslaught and Mirrodin, and I would like to try to catch back up. Since all my cards are quite dated now, I may be more interested in formats other than the most up-to-date one.  Rather than ask what is currently legal in various formats (since that is likely too localized), I would rather pose the question of how can I tell which sets/cards are legal in each format. Why do employers require you to spread your 401(k) contributions throughout the year to get the maximum match? I think the title says it all. I can't see any reason employers have to incentivize employees to contribute to their 401(k) evenly throughout the year. Instead couldn't they use a relatively simple formula that is based on how much of their income the employee has contributed to their 401(k) thus far for the year? If that or their current contribution is over the threshold, they get the match this pay period, even if they didn't contribute anything.  EDIT: To clarify, my employer matches 100% of the first 3% of my contribution and 50% of the next 2%. The match is added every pay period along with my contribution. I am pretty sure there is no "true-up" feature, so if I were to hit the $17.5K limit early, and not be able to contribute at least 5% of my salary, I would not get the full match. So I have to be careful to make my contributions evenly throughout the year to get the maximum match. Just trying to figure out my employer's motivation for requiring this. A regular expression to exclude a word/string Why do close votes expire? I nominated  to be closed as a dup. I see that two other people upvoted my comment, so they might have voted to close it as well. Now, we didn't gather enough votes, and all close votes expired ... and I can't re-vote to close. What is the motivation behind expiring close votes ... especially for close-as-duplicate? Update: Please take into consideration:  Beta Stack Exchange sites have a smaller community, and fewer people with voting privileges Closing as a duplicate is &quot;usually the right thing&quot; IMHO - much less controversial than closing as off topic. After a vote is reset, the original users can't vote again. On a small site, this makes it even harder to get a consensus of five votes. what question should be asked to make a persona I've been going through  and have a question on the inductive proof for the . Recurrent relation:  $J(1) = 1$ $J(2n) = 2J(n) - 1$ $J(2n+1) = 2J(n) + 1$  Closed form hypothesis:  $J(2^m + l) = 2l + 1$, for $m \ge 0$ and $0\le l&lt;2^m$  Inductive Proof (for the even case):  $J(1)=1$ $J(2^m+l)= 2J(2^{m-1}+l/2)-1=2(2l/2+1)-1=2l+1$  How do the authors get from $2J(2^{m-1}+l/2)-1$ to $2(2l/2+1)-1$? What is this colored fiber in my chicken? I smoked a whole chicken. When I cut into the breast I found this:    What is it and is my chicken ruined? In C, are arrays pointers or used as pointers? Why are there directories called Local, LocalLow, and Roaming under \Users\\AppData? I have a workstation running Windows Server 2008 that's logged into an  domain, and I have a dual boot with Ubuntu Linux. When running Linux, I'd like to be able to use the same Thunderbird profile I use under Windows, so I pointed Thunderbird to use a profile I found under:  \Users\(myname)\AppData\Local\Thunderbird\Profiles   It turns out it was not the correct profile -- it had the right configuration, but the Inbox was an old version. I eventually found out the correct path was:  \Users\(myname)\AppData\Roaming\Thunderbird\Profiles   What's the rationale behind those different places to store application data? How can slightly blurred photos be improved in post processing? Where are static variables stored in C and C++? Is it possible to "hack" a MacBook so as to be able to install macOS Mojave? Short story: rocket fuel which oxidizes iron destroys life on earth I'm looking for the title of a short science fiction story, probably published in a Hugo winner collection 30 years ago. In the story, scientists have developed an extremely efficient rocket fuel that has one dangerous side-effect: it oxidizes iron. A man takes off in a rocket powered by this fuel but returns to find the earth completely devoid of life. His subsequent investigation reveals that a single drop of the fuel began a domino effect, as the substance oxidized all iron it came into contact with. In the end, the man is dying.  He drags himself to the ocean, believing that if he can get his living cells into the water they will restart the primordial soup. Two monitors when only having one in 13.10 I just did a fresh install of Ubuntu 13.10 64bit, and it says that I have 2 monitors, when I actually only have one    I have a Dell XPS 15 with Nvidia graphics card if that helps in anyway I try to press the F6 key and in the YouTube tutorials those who make the video pressing them all go well, but not me :( Bibliography contents (Journal names) not abbreviated even with IEEEabrv I am using the following syntax, with no errors or warnings. However, the journal names are still not abbreviated. I compiled the file numerous times, as well as the bibliography. Everything compiles with no errors.  The syntax:   \bibliographystyle{IEEEtran} \bibliography{IEEEabrv,references}   Additional info: I am using BibDesk and TeXStudio on a macbook.  Do you have any idea what might be the issue? and how to solve it? Thank you.  Edit:  Example entry from the IEEEabrv file:  @STRING{IEEE\_M\_COM        = "{IEEE} Commun. Mag."}   Example entry from my references file:  @article{dai:2015,     Author = {Test, T.},     Journal = {IEEE\_M\_COM },     Month = {September}     }