Why do shadows seems to "unite" before the object does
Why do shadows from the sun join each other when near enough?
Why is "nodev" in /etc/fstab so important? How can character devices be used for hacking? I am learning about linux security and struggling to understand why a USB stick with a character device on it is potentially dangerous.  If I have a USB stick with a bash executable that has setuid root on it, the danger is obvious: Anybody with such a USB stick can gain root privileges on my computer if I have an entry like   /dev/sdb1 /media/usbstick auto defaults 0 0   in my /etc/fstab because defaults includes suid.  But what about character devices? How can I use a character device to gain root privileges or break stuff if a USB stick with a character device on it gets mounted with dev or defaults? Bought a new battery, do I need to calibrate it? My original battery is bloated and is dying quickly after a full charge so I now bought a replacement. Do I need to calibrate this new battery using a calibration app? In my class design, I use abstract classes and virtual functions extensively. I had a  feeling that virtual functions affects the performance. Is this true? But I think this performance difference is not noticeable and looks like I am doing premature optimization. Right? How to add a directory to the PATH? How do I add a directory to the $PATH in Ubuntu and make the changes permanent? I need to clip a vector layer to the extent of a raster. It has to be very precise that's why I would not want to draw a polygon myself (and clip the two polygons). There seem to be some ways to clip a raster to a polygon but I have difficulties to find a solution to clip a polygon to a raster. I already tried to create a polygon out of my raster several times in ArcGIS and R, so I could clip the result. But every time it crashes because the raster is to big. There must be a simple way to solve this, and somehow I can just not see it at the moment.  How can I do this using GDAL? A correct preposition - account closed due/by/at 12th of May I asked my bank to close my account on 12th of May. Also     I expect the account to be closed [?] 12th of May   . What preposition should be there? I think about by or due. Also if I want to ask someone a question: What date was your account closed [missing preposition?] How do I mount shared folders in Ubuntu using VMware tools? Experts. I've successfully installed VMware tools for Ubuntu. Everything seems to work fine, but shared folders were not mounted automatically. How do I get them to work?  If I run vmware-hgfsclient in terminal, I get the list of shared folders, but ls -l /mnt/hgfs is empty. Actually there's no hgfs dir in /mnt. I know I should probably use the vmware-hgfsclient tool, but I realy don't know how.   P.S. I wouldn't ask if I could understand the vmware-hgfsclient help I've read.   Any suggestions? Clustering on the output of t-SNE I've got an application where it'd be handy to cluster a noisy dataset before looking for subgroup effects within the clusters.  I first looked at PCA, but it takes ~30 components to get to 90% of the variability, so clustering on just a couple of PC's will throw away a lot of information.    I then tried t-SNE (for the first time), which gives me an odd shape in two dimensions that is very amenable to clustering via k-means.  What's more, running a random forest on the data with the cluster assignment as the outcome shows that the clusters have a fairly sensible interpretation given the context of the problem, in terms of the variables that make up the raw data.  But if I'm going to report on these clusters, how do I describe them?  K-means clusters on principal components reveal individuals who are nearby to one another in terms of the derived variables that comprise X% of the variance in the dataset.  What equivalent statement can be made about t-SNE clusters?  Perhaps something to the effect of:     t-SNE reveals approximate contiguity in an underlying high-dimensional manifold, so clusters on the low-dimensional representation of the high-dimensional space maximize the "likelihood" that contiguous individuals will not be in the same cluster   Can anyone propose a better blurb than that? Why do certain sites require more reputation for privileges? So, let's take Skeptics.SE for an example. You can become a trusted user when you acquire 4000 rep., not 20000  I was like, wow, so is it like this because of the lack of users on Skeptics.Se, as in, not so much as Stack Overflow or something?  Why do some sites need less reputation to be a trusted user? Meaning of "stackexchange" What does stackexchange mean? I have checked out this  to read about the Stack Exchange Network. Can somebody explain "leveraged debt investment positions" and "exposures" in this context for me, please? "The strong bond performance has led to a surge in leveraged debt investment positions that are ultimately funded by the banks' interbank exposures and WMPs. But nobody really knows how levered these positions are, who has financed them, and by how much," S&amp;P said. "The contagion risks for the financial sector could be high."  source: Is it possible to trade Pokemon from the 1st generation games to the most recent ones? I know that you can trade Pokemon between games of the same generation (Gold/Silver, Ruby/Sapphire) and even trade between 1st gen and 2nd gen. However, I haven't been able to find a way to trade Pokemon so that I can play with my 1st generation Pokemon in HeartGold, for instance. If it is possible, how many steps would it take and what equipment would I need? The set of functions which map convergent series to convergent series Suppose $f$ is some real function with the above property, i.e.  if $\sum\limits_{n = 0}^\infty {x_n}$ converges, then $\sum\limits_{n = 0}^\infty {f(x_n)}$ also converges.   My question is: can anything interesting be said regarding the behavior of such a function close to $0$, other than the fact that $f(0)=0$? How to properly secure a wireless home network I'm soon going to be setting up a wireless network for me and some buddies and (hopefully) making it secure. I have some questions and concerns regarding this and figured someone here could give me a hand and steer me in the right direction.   I have a WRT55AG (version 2) loaded with the factory firmware. I've heard that it's possible to load your own firmware onto routers like these. Is it worth it? What can be achieved with it? I was wondering what kind of security I should use on my wireless. Ideally I would want all devices to be 'accepted', but I'm not sure how to accomplish this. I'm guessing it has something to do with MAC addresses, but I don't know the first thing about them. It would be great that ALONG WITH encryption (I'm told WPA2 is the best), every device would need to be added to a list of some sort before even being able to access the AP. Another feature I was thinking of would be completely hiding my wireless from showing up on other people's computer. Is this possible (like an SSID broadcast thing). Is it worth the trouble? One more option I was looking to implement would be bandwidth calculators. Ideally something in the router to measure the amount of bandwidth being used and how much has been used for that month (this has to do with bills and limits, obviously). Is it possible to throttle down speed when a certain limit is reached?   These are alot of points but I was hoping someone who has more knowledge than me in this subject could give me a hand and a few pointers. Hopefully all this is possible and I'm not dreaming :) Generate two random numbers with constraints Killing form on $\mathfrak{sp}(2n)$ I have the same question as  from a long time ago. Is there an easy way to see that the Killing form on $\mathfrak{sp}(2n)$ is $\kappa(x,y) = (4n+2) \mathrm{tr}(xy)$?  For example, the Killing form for $\mathfrak{sl}(n)$ can be found from the easier to calculate case of $\mathfrak{gl}(n)$ using the decomposition $$\mathfrak{gl}(n) = \mathfrak{sl}(n) \oplus \mathbb{C}.$$ I don't know a similar argument for $\mathfrak{sp}(2n)$ though. I covered hyperbolic trigonometric functions in a recent maths course. However I was never presented with any reasons as to why (or even if) they are useful.  Is there any good examples of their uses outside academia? If I want my app to behave differently on a jailbroken iPhone, how would I go about determining this? SQL to find the number of distinct values in a column I can select all the distinct values in a column in the following ways:   SELECT DISTINCT column_name FROM table_name; SELECT column_name FROM table_name GROUP BY column_name;   But how do I get the row count from that query? Is a subquery required? How to set the same font size for all tables?