I read a book a while ago in which a device is invented which opens up "tunnels" back through time. These tunnels cannot be traveled through, but can be used to view what happened in the past. I believe it starts out narrating the invention of the device, then skips ahead to after it is widely in use. There is another device created, a cloak of sorts,  which hides the user from the view of someone using the tunnel device. Does this ring a bell with anyone?
Read it in the 90's. I remember one specific part where the inventor of the peeking mechanic has his Eureka moment, and has the presence of mind to give a thumbs-up/wave to the future, when they look back on his Eureka moment.  I also remember that they had computer screens that were like plastic cloth that they would just unfold on whatever surface was handy.  Sorry I don't have much more to go on.  Please and thank you!
Android popup window dismissal What is time, does it flow, and if so what defines its direction? Creating System-Wide Keyboard Shortcut To Launch Applications I recently switched from Ubuntu, and I was feeling very comfortable with using Ctrl + Alt + T to launch terminal. Can I assign Shift + Cmd + T to do the same thing in Yosemite? Specifically, I want a new terminal window to be opened. This might be a stupid question, but I just don't get it. In Hamiltonian mechanics when examining conditions for a $(\boldsymbol{q},\boldsymbol{p})\rightarrow(\boldsymbol{Q},\boldsymbol{P})$ transformation to be canonical one starts with $$ \dot{q}_ip^i-H(\boldsymbol{q},\boldsymbol{p},t)= \dot{Q}_iP^i-\bar{H}(\boldsymbol{Q},\boldsymbol{P},t)+\frac{d}{dt}W(\boldsymbol{q},\boldsymbol{Q},t)$$ where $\bar{H}$ is the transformed Hamiltonian, and $W$ is the generating function (now a function of $\boldsymbol{q}$ and $\boldsymbol{Q}$). This term shouldn't break Hamilton's principle, since $$ \delta\int_{t_1}^{t_2} dt\frac{d}{dt}W(\boldsymbol{q},\boldsymbol{Q},t)=\delta W(\boldsymbol{q},\boldsymbol{Q},t)|_{t_2}-\delta W(\boldsymbol{q},\boldsymbol{Q},t)|_{t_1}=0-0=0 .$$ But I don't see why the variation of $W$ should disappear at the endpoints (say at $t_1$). Expanding leads to: $$ \delta W(\boldsymbol{q},\boldsymbol{Q},t)|_{t_1}=\left(\frac{\partial W}{\partial q_i}\right)_{t_1}\underbrace{\delta q_i(t_1)}_{=0}+ \left(\frac{\partial W}{\partial Q_i}\right)_{t_1}\delta Q_i(t_1)=\left(\frac{\partial W}{\partial Q_i}\right)_{t_1}\delta Q_i(t_1).$$ $\boldsymbol{Q}$ is itself a function of $\boldsymbol{q}$ and $\boldsymbol{p}$, so $$ \delta Q_i(t_1)=\left(\frac{\partial Q_i}{\partial q_k}\right)_{t_1}\underbrace{\delta q_k(t_1)}_{=0}+\left(\frac{\partial Q_i}{\partial p_k}\right)_{t_1}\delta p_k(t_1)=\left(\frac{\partial Q_i}{\partial p_k}\right)_{t_1}\delta p_k(t_1). $$ It seems as if we also needed the variation of $\boldsymbol{p}$ to vanish at the endpoints, and I don't get this because (at least in cartesian coordinates) $\boldsymbol{p}=m\dot{\boldsymbol{q}}$ and the velocity can be different along the original and the varied orbitals even at the endpoints (they can point in totally different directions), so in general $\delta \dot{\boldsymbol{q}}(t_1)\neq 0$. What am I doing wrong? Can someone help me with this, please? I'm a first time Linux user. My laptop wasn't connecting to the wifi so I followed the instructions from .  On typing sudo apt-get update after that, I got this error:  E: Type 'sudo' is not known on line 2 in source list /etc/apt/sources.list E: The list of sources could not be read.   What can I do? What is the proper way of creating installation media from Ubuntu iso? This is the third time I've tried to install Ubuntu Server 14.04.03 64-bit on the same laptop, and each time it caused different problems.  Each time I wrote the same iso image to a USB flash drive by using a different application and OS.   Universal USB Installer (pendrivelinux.com) from Windows WinImage writer from Windows built-in image writer from Linux Mint 17.2    The iso image is the same in all three cases, the USB flash drive is the same, and the laptop is the same. However each time the installation of Ubuntu gave me different installation options. For example, the last time I installed Ubuntu, it didn't show all the packages to choose while installing (Basic Ubuntu server, etc.). Best compiler warning level for C/C++ compilers? Laurent series expansion I had created the xml document with xml version="1.0".  In that document I need to use the greater than symbol &gt; and less than symbol &lt;.  How should I include those symbols? It's not working.  &amp;gt; and &amp;lt; are not working for me.  Is there any special encoder for this? I tried to choose a answer for a question on SA and someone immediately answered with the correct response. I try to click the check box and it says wait 2 min. I keep clicking until it changes from 2 min to 1 min. I wait 60 seconds on my watch and click again and it says please wait 59 seconds.  The 2 minute mark should have said 3 and the 1 minute mark was really 2.  It seems that this has , which is a lot more inconvenient than 59 seconds. How do I deal with a bored or distracted player bringing down the group? My group have been playing various D&amp;D style games for a few years,  I have one player however that often will drift off and start playing on his phone  and several times has actually fallen asleep (with his character having to be "nudged" along by his fellow players).  He insists he's "just tired" and "isn't bored" — and besides, he keeps insisting we all get together for D&amp;D and wants to play.  Unfortunately we're all pretty busy, so if I'm to get the group together at all it's usually late on a Sunday. After talking to the player he insists it's not the game, or the story, he just happens to be tired. But, it's the same excuse every week, and the odd times we've played at different times or situations, the falling asleep has happened.  How do I cope with:   The morale destroying (for me) idea of your players falling asleep through a campaign and story you've put lots of work into. The general tone of the room when this player loses interest. (Occasionally a couple of others zone out into Facebook when they're not "active" because the players have split up / gone unconscious).   Given that it's an "all or nothing" situation with the group (there are only 3 players and they won't play unless all 3 are around), how do I best deal with it? How to invite friends to Stack Overflow, Server Fault and Super User? Since we're all into this OpenID thing, I think it would be really useful (and quite easy) to implement a friend invitation feature that grabs a list of friends from one's webmail contact list with an option to choose who to invite to SO, SF or SU. Something in the line of Facebook's friend invitation and recommendation feature. Computing the $n^{\textrm{th}}$ permutation of bits. I've seen  post about the $n^{\textrm{th}}$ permutation of a set but that is not what I need. If you have a bit string (ones and zeros only) there are  to quickly permute the NEXT lexicographically ordered bit permutation. For example take $$000111 \rightarrow 001011 \rightarrow 001101\rightarrow\cdots$$ etc. If the string is long then there are going to be approximately one Bajillion of these things. And I want to know how to compute the $n^{\textrm{th}}$ guy without doing the exhaust.  Backgound: This is for a parallel computing job where I need to farm out the search space of elements of a set.  Foreground: I own a copy of Knuth's 4th volume of "The Art of Computer Programming" and I think the answer is in there but I can't seem to find it. (It's like 900 pages).  I'm posting here in the hopes that someone has knowledge of this (obviously). Even if you can point me to a source, say the part in Knuth's book where he describes this problem I would be most grateful. When can a sum and integral be interchanged? Let's say I have $\int_{0}^{\infty}\sum_{n = 0}^{\infty} f_{n}(x)\, dx$ with $f_{n}(x)$ being continuous functions. When can we interchange the integral and summation? Is $f_{n}(x) \geq 0$ for all $x$ and for all $n$ sufficient? How about when $\sum f_{n}(x)$ converges absolutely? If so why? I'm trying to install Windows 8 on my pc via Microsoft Virtual PC.  However I get this annoying, non-descriptive error:    I'm looking to find out if Microsoft Virtual PC supports Windows 8, and if so how do I solve this issue?  NOTE: I'm NOT trying to ask for alternatives to Virtual PC, but rather trying to figure out if it's possible, and if so how, to use Virtual PC. Many of the various items have a list of enchantments that would be ideal. For instance, I might want Infinity, Power, Flame, and Punch all on a bow... and at the highest levels. Barring incredible luck, I will never get those with an enchantment table.  I can enchant books until I find those spells, and add them to the bow with an anvil. However, at some point (I haven't yet figured out how many times) it will no longer allow you to add enchantments to an item, saying that it's "too expensive". There is a hidden counter that is incremented every time, and once this goes past a threshold you can never add more magic (though, I've read in places you can continue to repair the item at great cost indefinitely... not clear if that's true or not).  This counter is cumulative, such that if you combine two items' magic into a single item, that won't sidestep the limit.  However, I've only just caught on that one can also combine two books. I can take two Power I books, and make a single Power II book. And I can take two of those, and get it up to Power III, and so on, but also have two spells combined in a book.  If I do this until I get all the magic I need onto a single book (or even just two or three books, supposing the books max out), will I get to the point where the books will no longer combine with the item because it's too expensive? Or are books exempt from the cumulative counter thing? It would suck to spend so much time and effort to get a powerful book and discover that it can't actually be used on anything.  Also, is it true that you can indefinitely repair, as long as you have the repair material and are at level 39? Or is that a misunderstanding of the rules (or maybe just an earlier version of the game and it no longer works like that... I'm on 1.8.9)? Does that only work for those items that can be repaired with diamond/iron, or will it work for fishing rods and bows as well? What is the difference between the two "worlds" when going thru the day/night wormhole at the altar? Reference request: Standard textbook for first-order predicate logic in english I am writing my bachelor thesis in english, but I have studied logic in my different native language. Since a rely on elementary results from first-order predicate logic (especially satisfaction in structures), I should reference the reader to a textbook for details.   Is there a standard textbook that starts with elementary propositional logic and goes to Incompleteness theorems? basic geometry angle problem Two straight lines, AB and AD, lie in the same vertical plane. AB makes an angle of 25 degrees with the horizontal, and AD makes 32 degrees with the vertical. What is the angle between AB and AD?  The answer is 33 degrees or 93 degrees.   I cannot visualize what this question is asking. It says both AD and AD lie on the same vertical plane, then immediately says AD makes 32 degrees with the vertical. Isn't this a contradiction? I've been using gksudo nautilus and sudo nautilus through Alt+F2.   What's the difference? They look very similar!