During my childhood, it was common to use magnifying glasses to burn paper and dead leaves.    But would it be a good idea to place a lens above a solar panel, making sure that all the magnified light hits every solar cell on the solar panel? I know that  to harvest more energy, but I was wondering if this is reliable and durable.   Wouldn't the magnified light burn the solar panel or would it be converted to electrical energy right away?
I was just wondering why Fresnel Lenses are not widely used in the production of solar electricity. Their use there would mean that you could produce heat within a fraction of a second, up to a few minutes and run a turbine to produce electricity.  Though it is used in welding, I am not sure what are the problems in producing electricity, as stated by :     New applications have appeared in solar energy, where Fresnel lenses can concentrate sunlight (with a ratio of almost 500:1) onto solar cells.
So I get a square matrix $A$ is idempotent when $A^2 = A$.  How can you prove that if $A$ is an $n \times n$ matrix that is idempotent and invertible, then  $A = I$??  I'm quite confused on this.  Any help much appreciated, Thanks. I remember my English teacher saying that there are only two valid ways to make a one-word sentence:   A question:      Why? Where?  A command:     Go! Stop!    Is this correct? Multivariable limit - perhaps a trickier problem I am stuck on. I am trying to solve the following limit:  $\lim_{(x,y) \to (0,0)} \frac{x^4y^4}{(x^2 + y^4)^3}$  (This is a more challenging problem from Folland Calculus, it seems).  I am pretty sure this limit does not exist (however, this is just a guess, and I am not 100% sure.)  I am trying to approach via the paths y = mx and x = my but... they don't seem to work. I have tried simpler cases such as x = 0 and y = 0.   Any help appreciate. Follow up: instance control in Java without enum Can I skip my train leg booked with a KLM flight? I'm using an old iBook G4 (PowerPC) running Tiger to serve iTunes to the house. For the past week or so, the mac boots fine, and then sometime during the day starts generating   An unexpected error occurred (error code -50).   errors when iTunes attempts to save its library file.  Checking reveals that the boot drive (main drive, in the laptop) behaves as if it is write-protected - I get the same error code when I attempt to create a folder on the desktop, for instance. If I attempt to create files via the terminal, e.g. via     ls > foo.txt I get an "invalid argument" error.   I've set the iBook to run in Target Disk Mode and run Disk Utility to repair the disk (from a Macbook running Snow Leopard) and get "The volume appears to be ok".   I don't have the option in Disk Utility to Repair Permissions in Target Disk Mode. When I ran Repair Permissions on the iBook itself, it came back with no errors, no permissions corrected, but did not in fact fix the problem.  Rebooting always solve the problem - for the next 5-10 hours or so.   Would Disk Warrior help, or some other software fix, or is it time to buy a new drive? (And, in any case, what's going wrong?) The new GNU Octave 4.0.1 was released March 23, 2016.  I would like to find some standard way to install it in El Capitan (mine OS X 10.11.3) because it has significantly better support in some functions than Matlab, discussed in the thread .  I do brew search octave and get  homebrew/science/octave    Caskroom/cask/octave       Caskroom/cask/xoctave       where one of the first two should be a correct one. The project's homepage  for OS X is down  about installing Octave on OS X systems.  There is also ruby's way of installing Octave described in the thread  but for Yosemite.  I would like to find some standard way of getting Octave for OS X.   I do successfully brew update and brew upgrade before the commands. My Homebrew is 0.9.5 (git revision 80d1; last commit 2016-03-24).  . I run brew info octave but I get  Error: No available formula with the name "octave"     How can you install Octave in some standard way for El Capitan? How can I make extra money with photography? I have photos in Getty Images, in Shutterstock, Imagekind and I recently added some to RedBubble, but I'm not having the expected results.  Does anyone have some history of success selling photographs or working as a photographer?  Examples of my work are available in .    Also asked by   I am an amateur who sometimes takes pictures of nature or random things. I've been told by several people that my pictures are worth money.  How can I monetize my best shots?  I don't really want to do a lot of work, editing, SEO or whatever. I'm looking for an easy way where I would just upload my pictures somewhere and let it go.  Maybe stock photos? People buying prints? I have no idea how that works...  EDIT: maybe a link to see which type of photography I do can help Integral of $\frac{1}{(1+x^2)^2}$ I am in the middle of a problem and having trouble integrating the following integral:  $$\int_{-1}^1\frac1{(1+x^2)^2}\mathrm dx$$  I tried doing partial fractions and got:  $$1=A(1+x^2)+B(1+x^2)$$  I have no clue how to solve this since it is obvious there is no way to cancel out either $A$ or $B$ to get the other variable. Please guide me.  Thank you. What are good basic statistics to use for ordinal data? subsurf splits the mirrored model into two objects I tried to subsurf this model that I mirrored but when I use subsurf it splits the model into two objects.  Before Subsurf:   After Subsurf:   Any help would be appreciated. Why "themselves" and "himself" Possible Duplicate:             In the itemize environment, how do I use the ampersand symbol &amp; without LaTeX trying to use it as a command? Is there any canon evidence that FN-2187 actually worked in sanitation? Finn tells Han that he worked in "Sanitation" on Starkiller.  FINN The flooding tunnels are over that ridge. We'll get in that way. HAN What was your job when you were based here? FINN Sanitation. HAN Sanitation? Then how do you know how to disable the shields? FINN I don't. I'm just here to get Rey   It just occurs to me - the "I was in sanitation" line given to Han seems to be the only actual canon evidence of "Sanitation" job.   But was it the truth"?!?!  Finn is always trying to create a positive impression of himself with the Resistance, from the get-go:   He tells Poe that he's rescuing him because that's the "Right thing to do" (with Poe quickly seeing through the fib: "You need a pilot!") He tells Rey that he's Resistance (she buys that because she's a romantic 19 year old still playing with X-Wing pilot helmet) He tells Han that he's Resistance (that one lasts about a minute - Han was not a romantic 19 year old even at 19 :)   In reality, we know that he was 1% of his class, model stormtrooper cadet, personally groomed for promotion by Captain Phasma (; ).  However, I am pretty sure telling Han "Hey, I was 1% of my class, model stormtrooper cadet, groomed for promotion by Captain Phasma"  isn't high on his list of brand management plans.  Far better to pretend to be a lowly "sanitation" worker than top-flight cadet that he really was.  So, my question is - do we have any direct canon evidence proving that the "sanitation" job was real and not a fib to make himself look better to Han/Resistance?  Knowing where trash compactor was isn't direct evidence, though it is circumstantial one. A "non-believer" of C# was asking me what the purpose to extension methods was. I explained that you could then add new methods to objects that were already defined, especially when you don't own/control the source to the original object.  He brought up "Why not just add a method to your own class?" We've been going round and round (in a good way). My general response is that it is another tool in the toolbelt, and his response is it is a useless waste of a tool... but I thought I'd get a more "enlightened" answer.  What are some scenarios that you've used extension methods that you couldn't have (or shouldn't have) used a method added on to your own class? Trying to open a http website on any web browser (event telnet) gives a ‘ERR_CONNECTION_REFUSED’ or ‘connection timed out’.  However, the same sites can be opened using https. I tried doing a traceroute and it works fine.  I am new to OS X but have experience with Linux. How do I go about troubleshooting this issue?  Update  I think I've tracked down the culprit to a rule in pf (firewall).  The result of sudo pfctl -s nat is:  No ALTQ support in kernel ALTQ related functions disabled rdr pass inet proto tcp from 192.168.0.101 to any port = 80 -&gt; 127.0.0.1 port 9882   192.168.0.101 is the current ip address of the computer.  I tried to remove the rule with sudo pfctl -F all, but it is back after a few seconds. I didn't find the rule in the default configurations (/etc/pf.conf). And disabling pf with sudo pfctl -d has no effect on it.  Is this a malware issue? What books, journals, and electronic resources are most valuable for expanding knowledge of GIS? What books, journals, and/or electronic resources have you found most valuable for expanding your knowledge in the GIS field, and why? Should the top navigation be frozen (optional)? In Excel, the window can be frozen in place in such a way that you can always see the name of the column even though you go way down below the 'fold' of the screen.  On SE, when you scroll, the whole page goes up (or down). This means the top navigation bar is scrolled off screen. I'm not sure if it would be a good idea or not to fix the top navigation to the top of the screen and let the rest of the content scroll.  What do you think about this, in terms of pros and cons?  [Update]  With the new Top navigation bar, kudos to the design team, I think this question becomes relevant again. I'm aware of the busload of downvotes, but still.  The top navigation bar doesn't occupy much space and has two notification areas:   Inbox Achievements   Having these two notifications visible at all times would be nice, IMO.  [Update]  I've just noticed that I haven't used the word "optional" in this discussion... My bad. In Excel, the Freeze function is controlled by the user. You can optionally Freeze part of the interface, and you can eliminate the Freeze at will. Its very easy to prove $p\mid n^p-n$ for p=3,5,7, it fails for p=9 because $$     (n+1)^9-(n+1)= n^9+9n^8+36n^7+84n^6+126n^5+126n^4+84      n^3+36n^2+8n $$ and $84= 2²\times 3\times7$. Is it true for any $p$ prime? I have checked many primes, but have no idea for the general case. Clothing store access in Pokémon X & Y Once the player obtains access to Lumiose City for the first time, they'll be able to visit Boutique Couture from the city's Vernal Avenue (South fork, connecting from South Boulevard). Upon entering the store without performing any other actions, the player will be turned away and informed they "aren't stylish enough" and that they should visit more places in Lumiose City before visiting again.  This description is dubious, and doesn't inform the player of what they need to accomplish to gain access. Specifically:  Even beating the game (completing the Pokémon League) fails to allow access, as does modifying the player's look, visiting most shops in all areas of Lumiose City, and performing all hair stylist, PR studio, and associated actions. Completing a set of triple battles in Restaurant Le Wow did set the event flag that allowed access in my copy of Y, but it is not clear if this is the only requirement for access.  So other players may benefit, and so I know how to repeat this process if I obtain a copy of X or the future release of Z, what exact requirements yield access to this store for the first time?