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What is data blending? This term appears frequently in the .  Is blending a specific method in data-mining and statistical learning? I cannot get a relevant result from google.  It seems blending is mixing up outcomes from many models and resulting in a better result. Is there any resource that helps me knowing more about it? Proving $f' = - \frac{\partial{F}/\partial{x}}{\partial{F}/\partial{y}}$ My homework has this problem:     Prove that there exist irrational numbers $x$ and $y$ such that $x +  y$ is rational.   There is an easy solution that I found on mathbitsnotebook.com:     \begin{align} \left(2+6\sqrt{7}\right) + \left(-6\sqrt{7}\right) = 2.\end{align}   Can we prove the same without using subtraction? Possible Duplicate:           My name is Greg — this is Greg 's post.  If my name ended with an 's', I am aware of the proper apostrophe usage (James&nbsp;→ James'), but how should this be pronounced?  Phonetically, am I Jameses best friend or James best friend? How can I use quotas in samba? I have a simple set-up with just one public share on my configuration file. How can I restrict its size? What does "Warning: untrusted X11 forwarding setup failed: xauth key data not generated" mean when ssh'ing with -X? Limit of a function with powers (L'Hopital doesnt work) I have a little problem with limit of this function:  $\lim_{x \to \infty} x^2(2017^{\frac{1}{x}} - 2017^{\frac{1}{x+1}})$  I have tried de L'Hopital rule twice, but it doesn't work. Now I have no idea how to do it. I've fitted my model and am trying to understand whether it's any good. I've calculated the recommended metrics to assess it ($R^2$/ AUC / accuracy / prediction error / etc) but do not know how to interpret them. In short, how do I tell if my model is any good based on the metric? Is an $R^2$ of 0.6 (for example) sufficient to let me proceed to draw inferences or base scientific/business decisions?     This question is intentionally broad, to cover a wide variety of situations that members frequently encounter; such questions could be closed as duplicates of this one. Edits to broaden the scope beyond the metrics mentioned here are welcome, as are additional answers - particularly those that offer insight about other classes of metrics. Often (via Google Now, or when searching for a solution to some problem), I'll open a question from a non-favourite site in the app. Sometimes, I'd like to see what else is going on in the site where the question was posted. Currently, I have one of two ways:   Via tag icon:   Click one of the tag icons beneath the title Delete the [tag] text (at which point, thankfully, the sorting changes to Active from Votes).  Via manually searching for the site, which is more cumbersome. I type in hard and Hardware Recommendations is the fourth entry (below Ask Different, Raspberry Pi and Arduino)! This more annoying for meta sites, where I have to pick Meta from the dropdown menu.   Can I pretty please have a way to head to the site directly from the question? Maybe:   the site name in the top bar could become clickable and lead to the site, or an option in the More menu?     This would be especially useful for Meta sites, where questions posted around the same time could be related. Interestingly enough, most questions that Google Now are for metas of sites I visit on occasion, but haven't pinned to the list. maximize volume of triangle in given perimeter OL3 How to select features of a WMS layer Why doesn't my ~/.bash_profile work? I'm using Linux Mint. My login shell (cat /etc/passwd | grep myUserName) is bash.  After I start my graphical desktop environment and run a terminal emulator from it, I can see that .bash_profile is not sourced (environment vars that are exported in it are unset). But if I log in from a text console (ctrl+alt+F1) or manually run bash -l from the terminal emulator, .bash_profile works fine.  Am I wrong when I think that .bash_profile should be sourced when X starts and all export'ed vars should be available in the terminal, running from X?  P.S. Placing everything in .bashrc and sourcing it from .bash_profile is not good idea (): environment stuff should be sourced only once. Windows 10 update broke my MBR / GRUB How deterministic is floating point inaccuracy? Is it worth it to upgrade a Toshiba Satellite C55-A5300 laptop given the latest productivity software requirements? I would like to upgrade a Toshiba Satellite C55-A5300 laptop so it can be as useful as it can be nowadays. I haven’t needed to use much software for a few years now, and I’m not able to gauge whether a maxed out upgrade of this laptop would be up to running the latest versions of MS Office and Acrobat smoothly. Maybe even run some Photoshop and Lightroom? I want to install the best CPU that will work optimally within the parameters of this laptop, including the laptop’s cooling capabilities. I ultimately want improved performance, but I want the performance to be as smooth as possible. From what I gather the Pentium 2030M is the best processor this motherboard will accept. Am I on the right track? I also plan to fully upgrade the memory and the hard disk speed with respective upgrades. What is the max memory per RAM slot (2 total)? What is the fastest speed of RAM I can use?  Copied from the Toshiba spec sheet:  Processor and Graphics Intel® Celeron® Processor 1037U o (2M Cache, 1.80GHz) Mobile Intel® NM70 Express Chipset Mobile Intel® HD graphics with shared graphics memory Memory Memory Configured with 4GB DDR3 1333MHz (max 16GB) 2 main memory slots. One slot is occupied Storage Drive 500GB (5400 RPM) Serial ATA hard disk drive Power 45W (19V 2.37A) 100-240V/50-60Hz AC Adapter What is a glue record? This is a  about DNS glue records.   What exactly (but briefly) is a DNS glue record?  Why are they needed and how do they work? $c\mid a,b\iff c\mid\gcd(a,b)$ [GCD Universal Property] If $c$ is a common divisor of $a$ and $b$ then $c$ divides the greatest common divisor of $a$ and $b$.  What can we use to prove this? The youngest person on Earth in a society of immortals The protagonist is the youngest person on Earth -- the only one to be born in many centuries. Terrestrial society is small but every individual is unique and multitalented, as they are all immortal and have lived for centuries. They live in flying habitats.  No one knows how to raise children anymore; the protagonist wears diapers into adulthood because he was never potty-trained, and his education was haphazard. Consequently, he is quite lost in this advanced society, and only tolerated by the sophisticates around him.   When the Earth is invaded, the immortals put up only a show of a fight, arriving at the scene of battle in a colorful flotilla of unarmed flying habitats. To the horror of the protagonist, they are wiped out by the invaders. He realizes that the immortals were weary of life and welcomed this end.   This is probably a novel, though it might be a novella. It was written in English and I read it probably in the 1970s or 1980s. Fundamentally, what does a friction force do for a rolling object? I am very confused by the way my textbook explains it. In my textbook , it says:  A wheel rolling on a horizontal flat surface at a constant velocity experiences no friction force. Why?   On the same surface, there is an acceleration of the wheel pointing to the right (probably caused by a force), so the ball is angularly accelerating in the clockwise direction. In this case, a friction force appears, and it is also pointing to the right. How come? What does the friction do for this wheel?  On an inclined plane, a ball freely rolls down the surface. The direction of friction is up the ramp, which confuses me because in the previous example the friction force was in the direction of the wheel's acceleration.   And there is a difference if a wheel is freely rolling and if there is a torque acting on the ball's center of mass. WHY???  What does friction do in these cases? How does it cause an object to roll? How to append something to an array?