I am a UK passport holder currently living in Canada. I recently visited Alaska and travelled via air and was issued an electronic I94. The I94 website shows that my I94 has an admit until date that's valid for another 2 months.  Now I'm looking to enter the US via a land border (rail). Last time I did this trip I was issued a paper I94 which I returned to the CBSA officer upon my re-entry to Canada.  I have a couple of questions:   Is my electronic I94 still valid for entry via a land border? If I use my electronic I94 for entry, how do I prove that I have exited the USA if there is no record of my departure?
I need to end my visa waiver period when leaving States to Canada so that I don't exceed the period of 90 days. But form I-94 is no longer a physical card, it's only online.  How do I know or ensure that it will be terminated and will not continue counting the days once I leave to Canada?
It is given that $X,Y \overset{\text{i.i.d.}}{\sim} N(0,1)$  Show that $\frac{XY}{\sqrt{X^2+Y^2}},\frac{X^2-Y^2}{2\sqrt{X^2+Y^2}} \overset{\text{i.i.d.}}{\sim} N(0,\frac{1}{4})$  I was thinking of making polar transformations $X=r \cos \theta, Y=r \sin \theta$  Then I am getting stuck at ranges of $\theta$ Lazy evaluation in C++ C++ does not have native support for lazy evaluation (as Haskell does).  I'm wondering if it is possible to implement lazy evaluation in C++ in a reasonable manner. If yes, how would you do it?  EDIT: I like Konrad Rudolph's answer.  I'm wondering if it's possible to implement it in a more generic fashion, for example by using a parametrized class lazy that essentially works for T the way matrix_add works for matrix.  Any operation on T would return lazy instead. The only problem is to store the arguments and operation code inside lazy itself. Can anyone see how to improve this? As of patch 1.5, I am completely unable to see underwater in Skyrim.  It's not even that it's murky or dark - it's just a solid grey or green screen.  I can still skim across the surface and see into deep waters, but if I go any deeper than that, nothing is visible.  I can still access items like chests underwater if I figure out their location first (by skimming across the surface) and then point my character there and hope really hard that it gets me to the right place.  I was never able to see much underwater, but I at least used to be able to see small things like wood floating near a shipwreck.  Now, there's just nothing. I am running the Steam version of Skyrim with graphics on Ultra High, with the high res textures DLC.  I've tried disabling the high res textures (along with all my other mods) and it still happens.  In case it's relevant, my GPU is a GTX 560 Ti.  Does anyone have any solutions to this? What is Stack Overflow's business model? I once , but that was . Now, , and they have , but where's the business model? The  its own Q&amp;A about its site (since it wasn't included in the ), and just about the only dig in there that halfway has merit is the business model dig:  We switched to a premium model to keep out of the Venture Capital business (been there, done that, got the t-shirt). Companies like StackExchange couldn't do it without the VC cookie jar. Where's the model? Huh, Spolsky? (I’m sorry. Did that slip out?)  It hurt, but I can't help but wonder if they're on to something.  How would Stack Overflow Internet Services pay back the VCs? Ad revenue alone?  There is only one 'paid' product, Stack Overflow careers, but it doesn't seem like that could sustain a business as large as Stack Overflow (Joel once mentioned on his blog that his jobs board brought in an extra million a year in revenue). So what does Stack Overflow do when the VC cash runs out? Is this a get-big-and-take-the-company-public maneuver? Is that the only viable business model for Stack Exchange? Edit: With the size of Stack Overflow's team, it isn't a stretch to say that payroll is at least $2 million  a year, probably as much as 4 when you think about benefits (average 120K for a developer in New York City; x2 for benefits/taxes/all-that-other-good-stuff). Office space can't be cheap, so how long does $6 million last before you need another VC infusion? How to install LXDM instead of GDM3? I'm running Debian Wheezy and my default Display Manager is GDM3, and I would like to replace it with LXDM. The problem is that I'm trying to run # apt-get install lxde and it's not working, what am I doing wrong?  My /etc/apt/sources.list is:  deb cdrom:[Debian GNU/Linux testing _Wheezy_ - Official Snapshot i386 DVD Binar$  deb http://ftp.br.debian.org/debian wheezy main deb-src http://ftp.br.debian.org/debian/ wheezy main  deb http://security.debian.org/ wheezy/updates main contrib deb-src http://security.debian.org/ wheezy/updates main contrib Is the empty set a group? US / Mexican border data I'm interested in locating free GIS shapefile data representing the US / Mexican border. I have found a point layer depicting the border crossing points but I'm interested in a polyline feature representing the actual border. Optimizing logical sector size for physical sector size 4096 HDD With many new hard drive disks the physical sector size is 4096. Would it be possible to make the system use a logical sector size of the same size, rather than the default logical sector size of 512?  Will it speed up bulk reads and writes? Where can it be configured? What is momentum really? How to avoid the wrinkle at the poles of the UV sphere? The prepositions “with”, “at” and “for” are also used to associate a business title with a company's name. It seems that they are interchangeable, with no (significant) difference in meaning. The following examples taken from The New York Times website seem to confirm that:      Until now, said Justin Nielson, a senior analyst for the media   research firm SNL Kagan, “there really hasn't been any direct   competitor . . .      “The consumer is still deleveraging,” said Jason Goldberg, a senior   analyst with Barclays. “It's not a lack of supply; it's a lack of   demand.”      “There’s the party of fear and the party of despair,” Nikos Xydakis, a   political analyst and an editor at Kathimerini daily, said.   Are they really the same as for meaning? If it isn't so, what is the difference between them? Massive 404 attack with non existent URLs. How to prevent this? The problem is a whole load of 404 errors, as reported by Google Webmaster Tools, with pages and queries that have never been there. One of them is viewtopic.php, and I've also noticed a scary number of attempts to check if the site is a WordPress site (wp_admin) and for the cPanel login. I block TRACE already, and the server is equipped with some defense against scanning/hacking. However, this doesn't seem to stop. The referrer is, according to Google Webmaster, totally.me.  I have looked for a solution to stop this, because it isn't certainly good for the poor real actual users, let alone the SEO concerns.  I am using the Perishable Press mini black list (), a standard referrer blocker (for porn, herbal, casino sites), and even some software to protect the site (XSS blocking, SQL injection, etc). The server is using other measures as well, so one would assume that the site is safe (hopefully), but it isn't ending.  Does anybody else have the same problem, or am I the only one seeing this? Is it what I think, i.e., some sort of attack? Is there a way to fix it, or better, prevent this useless resource waste?  EDIT I've never used the question to thank for the answers, and hope this can be done. Thank you all for your insightful replies, which helped me to find my way out of this. I have followed everyone's suggestions and implemented the following:   a honeypot a script that listens to suspect urls in the 404 page and sends me an email with user agent/ip, while returning a standard 404 header a script that rewards legitimate users, in the same 404 custom page,  in case they end up clicking on one of those urls. In less than 24 hours I have been able to isolate some suspect IPs, all listed in Spamhaus. All the IPs logged so far belong to spam VPS hosting companies.   Thank you all again, I would have accepted all answers if I could. "Hot" tab in is badly skewed and misleading - time to replace it with actual hot questions? Each site got a &quot;hot&quot; tab, where its contents are calculated based on . However take a look on the screenshots below and see for yourself that &quot;hot&quot; got nothing to do with many of the questions listed there. :    :    As you can see, there are questions with negative score, no answers, and even closed. I won't be surprised to see spam in there as well. This can't be any worse. What I suggest, instead of fixing the formula or patching it, simply have the hot tab show the same questions as the Hot Network Questions sidebar. (from the specific site only, of course.) Since the hot questions &quot;pool&quot; (as seen ) is limited to 100 for the whole network, either expand it or have the per-site list show questions with less hotness points - it would still be better than what we have now. I have recently a purchased Lenovo Thinkpad E480 laptop, I have installed Ubuntu 18.04 on it, everything is working fine except am not able to connect to network via WiFi. Please help.  I followed this , but am unable to solve the issue.  Below are the outputs of some commands.  lsb_release -a  No LSB modules are available. Distributor ID: Ubuntu Description:    Ubuntu 18.04.1 LTS Release:    18.04 Codename:   bionic   uname -r  4.15.0-33-generic   rfkill list all  0: tpacpi_bluetooth_sw: Bluetooth     Soft blocked: no     Hard blocked: no 1: hci0: Bluetooth     Soft blocked: no     Hard blocked: no   lspci -nnk | grep 0280 -A3  05:00.0 Network controller [0280]: Realtek Semiconductor Co., Ltd. RTL8821CE 802.11ac PCIe Wireless Network Adapter [10ec:c821]     Subsystem: Lenovo RTL8821CE 802.11ac PCIe Wireless Network Adapter [17aa:c024] 06:00.0 SD Host controller [0805]: O2 Micro, Inc. SD/MMC Card Reader Controller [1217:8621] (rev 01)     Subsystem: Lenovo SD/MMC Card Reader Controller [17aa:5068]   sudo lshw -C network   *-network                         description: Ethernet interface        product: RTL8111/8168/8411 PCI Express Gigabit Ethernet Controller        vendor: Realtek Semiconductor Co., Ltd.        physical id: 0        bus info: pci@0000:03:00.0        logical name: enp3s0        version: 10        serial: 8c:16:45:a3:d9:f1        size: 100Mbit/s        capacity: 1Gbit/s        width: 64 bits        clock: 33MHz        capabilities: pm msi pciexpress msix vpd bus_master cap_list ethernet physical tp mii 10bt 10bt-fd 100bt 100bt-fd 1000bt 1000bt-fd autonegotiation        configuration: autonegotiation=on broadcast=yes driver=r8169 driverversion=2.3LK-NAPI duplex=full firmware=rtl8168g-3_0.0.1 04/23/13 ip=192.168.2.2 latency=0 link=yes multicast=yes port=MII speed=100Mbit/s        resources: irq:16 ioport:d000(size=256) memory:f1204000-f1204fff memory:f1200000-f1203fff   *-network UNCLAIMED        description: Network controller        product: Realtek Semiconductor Co., Ltd.        vendor: Realtek Semiconductor Co., Ltd.        physical id: 0        bus info: pci@0000:05:00.0        version: 00        width: 64 bits        clock: 33MHz        capabilities: pm msi pciexpress cap_list        configuration: latency=0        resources: ioport:c000(size=256) memory:f1100000-f110ffff   It seems I have to clone, compile and install the required WiFi driver, please guide me which WiFi driver to use and how to install (with steps if possible).  P.S.: Let me know if any further information is needed to solve this issue, I will add the required info (if any) in the question as per the guidance given. I know that Windows XP 32-bit can be configured, through , to support more than 3.5&nbsp;GB of RAM.  Is there a good tutorial to do this with Windows 7 32-bit?  As to why I don't simply use 64-bit Windows 7: The software for my Internet connection (cell phone-as-modem) will only work in 32-bit environments. How can a file size be zero? Just something I ran into and couldn't think of a proper explanation. If I create an empty *.txt file on my PC and then look at its size, it shows 0. But how is that possible? I mean even if the file itself is empty, it still must have some size, just to store its own name. How can this be explained? (Non OS specific) This is similar to , except it's a bug report, not a feature request.  Currently, the nominee's comments on his/her own post are not highlights, unless you click "add / show more comments"  When the page is loaded it looks like:    AfterI click "add / show more comments", it becomes:    Could we have the blue highlighting on the post owner's comments there all the time? It becomes difficult to see the nominees responses to questions, especially on nominations where there aren't enough comments to enable the "add / show more comments" features, like Matt's. Show that $A$ is an invertible matrix if $\left(A+I\right)^3=0$ and find $A^{-1}$ How do I programmatically log in a user with a POST request? Comma after introductory words, phrases, clauses: unacceptable, obligatory or optional?