Is there a reason why every meson and baryon has an integer electric charge?
What's the deepest reason why QCD bound states have integer charge?
Is it always better to use the whole dataset to train the final model? A common technique after training, validating and testing the Machine Learning model of preference is to use the complete dataset, including the testing subset, to train a final model to deploy it on, e.g. a product.      My question is: Is it always for the best to do so? What if the   performance actually deteriorates?   For example, let us assume a case where the model scores around 65% in classifying the testing subset. This could mean that either the model is trained insufficiently OR that the testing subset consists of outliers. In the latter case, training the final model with them would decrease its performance and you find out only after deploying it.   Re-phrasing my initial question:     If you had a one-time demonstration of a model, such as deploying it   on embedded electronics on-board an expensive rocket experiment, would you trust a   model that has been re-trained with the test subset in the final step without being   re-tested on its new performance? Use Expect in a Bash script to provide a password to an SSH command Plotting bell shaped curve in TikZ-PGF I want to create some bell shaped curves for demonstrating hypothesis testing in statistics. Can anybody help me drawing curves like shown in the following picture? Update: Failed to lock /var/cache/apt/archives/lock Evaluate the integral $$\int_0^{\pi/2} \frac{1}{1+\tan^\alpha{x}}\,\mathrm{d}x$$ I read something that suggests that if $I$ is the $n$-by-$n$ identity matrix, $v$ is an $n$-dimensional real column vector with $\|v\| = 1$ (standard Euclidean norm), and $t &gt; 0$, then $$\det(I + t v v^T) = 1 + t$$ Can anyone prove this or provide a reference? More generally, is there also an (easy) formula for calculating $\det(A + wv^T)$ for $v,w \in \mathbb{K}^{d \times 1}$ and some (invertible) matrix $A \in \Bbb{K}^{d \times d}$? When I visit  I see this:     The "Congratulations! You've earned (badge)" section has dropped down completely beneath the tabs under it. The "You've earned a new privilege!" section has a gap at the bottom:     Shog9 says in comments how to reproduce this:     Gotta have earned a badge, not dismissed the notification, and be looking at your own profile. I'm experimenting with VPNs, so I wanted to install pptpd.  john@desktop:~$ sudo apt-get install pptpd [sudo] password for john:  &lt;snip&gt; The following extra packages will be installed:   bcrelay The following NEW packages will be installed:   bcrelay pptpd 0 upgraded, 2 newly installed, 0 to remove and 11 not upgraded. Need to get 90.5 kB of archives. After this operation, 442 kB of additional disk space will be used. Do you want to continue [Y/n]? y   And everything runs fine until I get down to here:  WARNING: The following packages cannot be authenticated!   bcrelay pptpd Install these packages without verification [y/N]? n E: Some packages could not be authenticated   No authentication?  I'm no security expert, but I'm fairly sure that's a bad thing.  So I .  john@desktop:~$ apt-cache policy pptpd pptpd:   Installed: (none)   Candidate: 1.3.4-3ubuntu1   Version table:      1.3.4-3ubuntu1 0         500 http://us.archive.ubuntu.com/ubuntu/ natty/main i386 Packages john@desktop:~$ apt-cache policy bcrelay bcrelay:   Installed: (none)   Candidate: 1.3.4-3ubuntu1   Version table:      1.3.4-3ubuntu1 0         500 http://us.archive.ubuntu.com/ubuntu/ natty/main i386 Packages   They're coming from Ubuntu.com and not being properly authenticated?  What's going on here? Determinant of a block lower triangular matrix let's say that I have a following file structure:  ~/DIR1 -fileA -DIR2 --fileB   Then, being in ~, I run this script:  cp DIR1 NEWDIR2   And just an error happens:  cp: omitting directory ‘DIR1’   I understand why nothing from DIR1 gets copied (because I didn't use -r switch that would enforce recursion) but I'd expect the DIR1 folder itself copied to NEWDIR2 (so practically a creation of a new "NEWDIR2" in ~ or a creation of ~/NEWDIR2/DIR1).  Could someone explain to me why is my expectation wrong? How to remove some pages from the navigation bullets in Beamer? I'm making a beamer presentation using the miniframes outer theme, with navigation bullets, and the subsection=false option, meaning the bullets are all on a single row. The problem I have is that, if one section has too many pages, the bullets overflow to the next section:  Section 1 Section 2 oooooooooooooooooooo   I tend to have many frames, some of them just asking a question, leading to the next detailed frame. I don't really need those frames to be represented by navigation bullets, and removing them would fix the problem. Any idea how to do that?  I could cheat by using overlays to have the question and the answer on the same frame, but that would be a mess...  Code example to answer percusse's question: this should give you one circle per frame, not one per subsection. If you add many frames in one subsection, the circles will overflow over the next section.  \documentclass[compress]{beamer} \useoutertheme[subsection=false]{miniframes} \begin{document} \section{S1} \subsection{SS1} \frame{1} \frame{2} \frame{3} \subsection{SS2} \frame{1} \frame{2} \frame{3} ... \end{document} How close would you have to be to the merger of two black holes, for the effects of gravitational waves to be detected without instruments? Which version of NFS is my NFS server using? I'm mounting a NFS filesystem on my machine.  How do I figure out what version of the NFS protocol the server uses?  I don't have access to the NFS server machine, but I do have root on my client machine.  Is there anything I can run on my client machine to identify what version of the NFS protocol is being used by the server, or what versions it supports?  I wasn't able to find any useful information in /var/log/messages or kernel debugging output (dmesg).  I have tried running nfsstat, but I'm not sure if it is giving me any useful information.  However, when I run nfsstat -s to request information about the server, I don't see anything useful:  # nfsstat -s Server rpc stats: calls      badcalls   badfmt     badauth    badclnt 0          0          0          0          0          When I run nfsstat -c to request information about the client, I do see some information about Client nfs v3, but I'm not sure how to interpret this.  Does this tell me anything about the protocol being used between my client machine and the NFS server?  Does it mean I am currently using v3 of the NFS protocol?  Does it tell me anything about what versions of the NFS protocol the server supports, e.g., NFS v4? How do you join on the same table, twice, in mysql? I have 2 tables. One (domains) has domain ids, and domain names (dom_id, dom_url).  the other contains actual data, 2 of which columns require a TO and FROM domain names. So I have 2 columns rev_dom_from and rev_dom_for, both of which store the domain name id, from the domains table.  Simple.  Now I need to actually display both domain names on the webpage. I know how to display one or the other, via the LEFT JOIN domains ON reviews.rev_dom_for = domains.dom_url query, and then you echo out the dom_url, which would echo out the domain name in the rev_dom_for column.  But how would I make it echo out the 2nd domain name, in the dom_rev_from column? What should the standard spelling be - British or US? I just saw someone edit the title of  question to  the spelling from favourite (The British spelling) to favorite (The US-English spelling).  Does SOFU have an accepted standard on language and spelling? Which is it? Origin of the term "fun fact" Where does the term "fun fact" originate?-- namely, not with the compositional meaning but rather with the idiomatic usage to introduce some sort of unusual, esoteric, absurd or otherwise "controversial" fact? How can I diagnose and repair missing drive space? I have a small ext3 / partition on a 2T drive which appears to have a discrepancy in its free space. How can I determine what is causing it and how it can be fixed.   After trying everything I could think of, I forced a fsck on boot which didn't have any affect.  I have other systems configured identically which don't exhibit this condition.  # df -h / Filesystem            Size  Used Avail Use% Mounted on /dev/sda1             9.7G  8.9G  318M  97% /  # du -shx --max-depth 1 / 5.2G    /  # lsof | awk 'BEGIN {t=0} /(deleted)/ {t+=$7} END {print t}' 0  # tune4fs -l /dev/sda1 | grep -E 'state|Free|Reserve|size|Inode' Filesystem features:      has_journal ext_attr resize_inode dir_index filetype needs_recovery sparse_super large_file Filesystem state:         clean Inode count:              2621440 Reserved block count:     131029 Free blocks:              212317 Free inodes:              2487281 Block size:               4096 Fragment size:            4096 Reserved GDT blocks:      639 Inodes per group:         32768 Inode blocks per group:   1024 Reserved blocks uid:      0 (user root) Reserved blocks gid:      0 (group root) Inode size:               128   Edit:  fsck says the partition is 2% non-contiguous.  For comparison to du above (sparseness):  # du -hxs --apparent-size / 4.9G    / Is it safe to change the lens on my new DSLR? I recently bought myself a shiny new DSLR camera. Before I can use it, I had to do something kind of terrifying: mount the lens.  Now of course, half the advantage of SLR cameras is that the lenses are interchangeable. But sitting there holding an extremely expensive brand new camera and staring into its bare innards, I'm just terrified that if I make one wrong move, a hair or a piece of dust could fall in there, and then the camera will be irreparably damaged and I'll never be able to use it again.  This might sound like a stupid question, but&hellip; is it actually &ldquo;safe&rdquo; to change the lens on the camera? Is this likely to destroy it?  (Obviously it goes without saying that you should limit the time spent with no lens attached to the minimum number of seconds possible. But even if you have lightning fingers, the camera will still be lenseless for some number of seconds. And it's not like I'm standing in a class 100 cleanroom here! Besides, you don't want to be too rough with the delicate lens mounting either&hellip;) Find $\lim_{x \to 0} (\frac{\tan(x)}{x})^{\frac{1}{x}}$ Find $$\lim_{x \to 0} (\frac{\tan(x)}{x})^{\frac{1}{x}}$$   My first idea to solve this was to try to evaluate it and then apply the L'Hospital's rule. This is what I managed to achieve: $$\left(\frac{\tan(x)}{x}\right)^\frac{1}{x}=e^{\frac{1}{x}\ln\left(\frac {\tan(x)}x \right)} $$ However, this is problematic, because the L'Hospital's rule is not applicable in the exponent, therefore my transformation was not a good shot. What transformation should I apply to get the desired form? I have a 24/7 always-on Debian Jessie based headless home server that has a large 1TB SSD for the OS and all of my frequently accessed files. This same system has 4 larger hard disk drives in a SnapRAID array. These are mainly for archiving infrequently accessed Blu-rays and want those drives to remain spun down in standby unless I actually read or write to them. They are all formatted as ext4 and mounted with noatime and nodiratime enabled.  So even though no process or program should be regularly accessing those drives in any direct way, the hard drives constantly get spun up from standby. It seems to be related to graphical programs that provide a gui file browser, even something like Chromium. If I don't even browse into those drives, I'm thinking that these processes by simply getting a list of available drives spins up the hard disks. Much like blkid does. The problem is, it's hard to determine the root cause of this since none of these processes are actually reading or writing the filesystem on those drives, so no files are actually changing or being touched. Is there some sort of cache that I can populate or a buffer to prevent these programs from spinning up the hard drive simply by getting a list of available disks? This is honestly driving me insane, since I can't find a reliably way to keep these disks spun-down even though there is no direct access of the filesystem.  UPDATE: Thanks to Stephen's answer, I was able to trace the disk activity to gvfs and udisks. It's a real shame that these processes insist on waking up disks in standby when they aren't actually being accessed to do any real I/O with the filesystem. So far I just uninstalled them, knowing that it will remove some functionality from PCManFM and the like.