Can a system have a negative energy? What is the energy of black hole compared to the normal star?
Is it possible for a system to have negative potential energy?
"This Apple ID has not been yet been used before in the iTunes Store. Tap review to sign in, then review your account information."   I keep getting this message when I try to install free apps from App Store. I don't want to give my account info until I actually purchase something.   Can I not download free apps until I provide my account information? My email verification is complete. I'm using an iPhone 3GS logged in to my iTunes account. There are a variety of characters that are not legally encodeable in XML 1.0, e.g. U+0007 ('bell') and U+001B ('escape'). Most of the interesting ones are non-whitespace 'control' characters.    It's clear from (e.g.)  and others that it's  -- but can anyone illuminate me as to why the XML spec forbids these characters?   It seems like it could have been required that they be encoded in escapes, e.g. as &amp;#x0007; and &amp;#x001B; respectively, but perhaps there's a practical reason that the characters were forbidden rather than required to be escaped?  Answerers have suggested that there is some motivation towards avoiding transmission control characters, but Unicode includes many other control-like characters (consider U+200C "zero width non joiner"). I recognize there may be no good reason for this behavior, but I would still like to understand it better.  It's particularly frustrating because when those character values appear in other encodings data formats, I end up "double-escaping" new XML documents that need to encode this. Continuous function on closed unit ball Take a continuous mapping $f: \bar{B^{n}} \rightarrow \bar{B^{n}}$, where $\bar{B^{n}}$ is a closed unit ball in $\mathbb{R}^{n}$. Assume that $f(x) \neq x$ for every $x \in \bar{B^{n}}$. Define another function $r$ by following the directed line segment from $f(x)$ through $x$ to its intersection with $\partial B^{n}$, and let the intersection point be $r(x)$. Is it immediately evident that $r$ is a continuous function? Is so how does it follow? Thanks. Fast way to copy a large file on a LAN I am having some trouble with NFS, and I'd like to try using just plain old TCP.  I have no idea where to begin, though.  Hardware-wise, I am using an ethernet crossover cable to network two netbooks.  To network them, I type  $ sudo ifconfig eth0 192.168.1.1 up &amp;&amp; ping -c 10 -s 10 192.168.1.2 &amp;&amp; sudo /etc/init.d/nfs-kernel-server start   on the first netbook and  $ sudo ifconfig eth0 192.168.1.2 up $ ping -c 10 -s 10 192.168.1.1 $ mount /mnt/network1   on the second  where /mnt/network1 is specified in /etc/fstab as  192.168.1.1:/home /mnt/network1 nfs noauto,user,exec,soft,nfsvers=2 0 0  as well as in /etc/exports (using the syntax of that file), on the first netbook.  The above works fine, but the files and directories are huge. The files average about half a gigabyte a piece, and the directories are all between 15 and 50 gigabytes.   I'm using rsync to transfer them, and the command (on 192.168.1.2) is  $ rsync -avxS /mnt/network1 ~/somedir   I'm not sure if there's a way to tweak my NFS settings to handle huge files better, but I'd like to see if running an rsync daemon over plain old TCP works better than rsync over NFS.  So, to reiterate, how do I set up a similar network with TCP?  UPDATE:  So, after a good at few hours of attempting to pull myself out of the morass of my own ignorance (or, as I like to think of it, to pull myself up by my own bootstraps) I came up with some useful facts.  But first of all, what led me on this rabbit trail instead of simply accepting the current best answer was this: nc is an unbelievably cool program that resolutely fails to work for me. I've tried the netcat-openbsd and netcat-traditional packages with no luck whatsoever.  The error I get on the receiving machine (192.168.1.2) is:  me@netbook:~$ nc -q 1 -l -p 32934 | tar xv Can't grab 0.0.0.0:32934 with bind tar: This does not look like a tar archive tar: Exiting with failure status due to previous errors   route gives:  me@netbook:~$ route Kernel IP routing table Destination     Gateway         Genmask         Flags Metric Ref    Use Iface default         dir-615         0.0.0.0         UG    0      0        0 wlan0 link-local      *               255.255.0.0     U     1000   0        0 eth0 192.168.0.0     *               255.255.255.0   U     2      0        0 wlan0 192.168.1.0     *               255.255.255.0   U     0      0        0 eth0   But, here's the good news: having the static IP addresses set in /etc/network/interfaces, which I started doing while trying to get nc working, fixed all my NFS problems and rekindled my love for NFS.  The exact configuration I used (with 192.168.1.1 for the first netbook, of course) was:  auto eth0 iface eth0 inet static address 192.168.1.2 netmask 255.255.255.0   With those settings, the two netbooks will be able to ping each other directly after being booted up, without even an ifup.  Anyway, I'd still really like to see nc in action, so I'm hoping someone help me debug this process. Unable to start XAMPP using sudo in Desktop Entry How do you handle multiple instances of setTimeout()? What is the most recommended/best way to stop multiple instances of a setTimeout function from being created (in javascript)?  An example (psuedo code):  function mouseClick() {    moveDiv("div_0001", mouseX, mouseY); }  function moveDiv(objID, destX, destY) {    //some code that moves the div closer to destination    ...    ...    ...     setTimeout("moveDiv(objID, destX, destY)", 1000);    ...    ...    ... }   My issue is that if the user clicks the mouse multiple times, I have multiple instances of moveDiv() getting called.  The option I have seen is to create a flag, that only allows the timeout to be called if no other instance is available...is that the best way to go?  I hope that makes it clear.... Is there a way to use the covariance matrix to find coefficients for multiple regression? For simple linear regression, the regression coefficient is calculable directly from the variance-covariance matrix $C$, by  $$ C_{d, e}\over C_{e,e} $$ where $d$ is the dependent variable's index, and $e$ is the explanatory variable's index.  If one only has the covariance matrix, is it possible to calculate the coefficients for a model with multiple explanatory variables?   ETA: For two explanatory variables, it appears that $$\beta_1 = \frac{Cov(y,x_1)var(x_2) - Cov(y,x_2)Cov(x_1,x_2)}{var(x_1)var(x_2) - Cov(x_1,x_2)^2} $$ and analogously for $\beta_2$. I'm not immediately seeing how to extend this to three or more variables. When a user is suspended, their account is reduced to 1 rep and all of his privileges are stripped. However, they can still join chat rooms.   This leads to various forms of possible abuse (all of which I've seen by suspended users):   Join/leave spams Expletives in profile messages Hold one sided conversations with users of the room via    Suspended users cannot be kicked, and even moderators can do very little (aside from completely removing the account, which is often overkill).  My proposal is to extend  for suspended users to one of two levels:   Prevent them from being shown in the user list at all. Strip their profile profile picture and description from the profile view visible from chat rooms (not from profile pages). How do I reboot a Ubuntu Server from the command line? sudo reboot now never seems to work for me. Because its a server, I don't have a window manager installed, so I can't do it from a GUI (which always seems to work).    I've had the problem for years on different versions of the operating system. It happens in virtual machines and on real metal. The same problem exists with sudo shutdown now, too.  I've lived with it in the past. The difference now is I don't want to just hit the power button because I might lose data. Below is a visual proof (!) that $32.5 = 31.5$. How could that be?    (As noted in a comment and answer, this is known as .) Prove every element of $G$ has finite order. Let $G$ be a group such that the intersection of all its subgroups which are different from $\{e\}$, is a subgroup different from $\{e\}$. Prove that every element of  $G$  has finite order.  Assume that $a$ has infinite order in $G$. Then let $y\neq e$ belong to the intersection of all subgroups of $G$ different from $\{e\}$. Then $y=a^n$ and $y$ will also belong to subgroup generted by $a^2$ hence  $y=a^{2m}$ for some integers $m$. This contradicts that $a$ has infinte order.  Is this correct? Find all integral solutions of $y^2=x^3+7$ Find all integral solutions of $y^2 = x^3+7$.  I tried to use many different moduli, but it never works. With modulo $9$, you can get $x$ is divisible by $3$. the (surely simplistic) model of a bipolar junction transistor one is taught in foundational physics course appears to be symmetric. - So, what is the difference between the collector and the emitter of a real BJT? If the transistor were symmetric one would not make this distinction...  Also:   Do BJTs have a 0.6V voltage drop like diodes? Are BJTs conductive in both directions, i.e. E-C and C-E?   Many thanks. To prove Cayley-Hamilton theorem, why can't we substitute $A$ for $\lambda$ in $p(\lambda) = \det(\lambda I - A)$? Cayley Hamilton Theorem states that if $A$ is an $n \times n$ matrix over the field $F$ then $p(A) = 0$.  We note that $p(\lambda) = \det(\lambda I - A)$. Hence, why can't we just substitute $\lambda$ with $A$ and directly prove Cayley-Hamilton Theorem by saying that $p(A) = p(\lambda) = \det(\lambda I - A) = \det(AI - A) = 0$? I am trying to prove this inequality  $$|e^z-1|\leq e^{|z|}-1\leq |z|e^{|z|}$$  I've tried calculating the difference in their power series   $$\sum_{k=0}^\infty\frac{|z|^k}{k!}-1-\left|\,\sum_{k=0}^\infty\frac{z^k}{k!}-1\,\right|$$   but did not get anywhere. Making dual power supply from single for opamp? For the circuit of opamp 741. we have to supply the dual power for it. How can I make a dual power supply from single. For example, make a (+9V)-GND-(-9V) from single 9V battery ? Why does light in a room not form constructive and destructive interference patterns? I have a simple question.     over centuries / for centuries   what is the difference?  Thank you. Parameter Sniffing (or Spoofing) in SQL Server A while ago I had a query that I ran quite a lot for one of my users. It was still being evolved and tweaked but eventually it stablised and ran quite quickly, so we created a stored procedure from it.   So far, so normal.   The stored procedure, though, was dog slow. No material difference between the query and the proc, but the speed change was massive.   [Background, we're running SQL Server 2005.]  A friendly local DBA (who no longer works here) took one look at the stored procedure and said "parameter spoofing!" (Edit: although it seems that it is possibly also known as 'parameter sniffing', which might explain the paucity of Google hits when I tried to search it out.)   We abstracted some of the stored procedure to a second one, wrapped the call to this new inner proc into the pre-existing outer one, called the outer one and, hey presto, it was as quick as the original query.   So, what gives? Can someone explain parameter spoofing?   Bonus credit for    highlighting how to avoid it  suggesting how to recognise possible cause discuss alternative strategies, e.g. stats, indices, keys, for mitigating the situation The calling thread cannot access this object because a different thread owns it