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So all wizards get an invitation to Hogwarts at 11... How do they know whom to invite?  If a wizard is Muggle-born, then nobody would know about their existence.  Or do they register some weird magical stuff being done by them before 11? How to only perform LTS upgrades? My wife and I didn't know we're pregnant and in a weekend fun with friends we smoked a bit of weed - we don't usually smoke weed or cigarettes.  This is not something we're regulary doing as I said, but how bad to the fetus can it be ? Idempotents in a ring without unity (rng) and no zero divisors. Question: Given a ring without unity and with no zero-divisors, is it possible that there are idempotents other than zero?   Def: $a$ is idempotent if $a^2 = a$.   Originally the problem was to show that $1$ and $0$ are the only idempotents in a ring with unity and no zero-divisors, but I wonder what happens if we remove the unity condition.   I am trying to find a ring with idempotents not equal to $0$ or $1$. So far my biggest struggle has been coming up with examples of rings with the given properties.   Does anyone have any hints? How should I attack this problem? I have set up my site with sample pages and data (lorem ipsum, etc..) and Google has crawled these pages. I deleted all these pages and actually added real content but in webmaster tools, i still get a lot of 404 errors Google trying to crawl these pages. I have set them to "mark as resolved" but some pages still come back as 404.   Furthermore, I have a lot of these sample pages still listed when i do a search of my site on Google. How to remove them. I think these irrelevant pages are hurting my rating.   I actually wanted to erase all these pages and start getting my site being being indexed as a new one but I read it's not possible? (I have submitted a sitemap and used "Fetch as Google.") Berry naming scheme list of LaTeX font families How to convert SSH public key from PEM to DER format? Is the double dash still the posix standard for long switches? I recentely tried to log-in / sign-up for  using my SE account (which works on ,  and sign-up here) but it showed me an OpenID related error. (See below)  I could reproduce this error across multiple days and browser refreshes and using my other OpenID provider.  Error message (shown in red, obfuscated my e-mail address):  Unable to log in with your OpenID provider:  The openid.return_to parameter (http://area51.stackexchange.com/users/authenticate/?s=bd394805-cf55-4e8e-ad73-21485a09a989&amp;dnoa.userSuppliedIdentifier=https%3A%2F%2Fopenid.stackexchange.com%2Fuser%2F642b1038-5c91-40d4-83ab-eb92db46c3d1) does not match the actual URL (https://area51.stackexchange.com/users/authenticate/?s=bd394805-cf55-4e8e-ad73-21485a09a989&amp;dnoa.userSuppliedIdentifier=https%3A%2F%2Fopenid.stackexchange.com%2Fuser%2F642b1038-5c91-40d4-83ab-eb92db46c3d1&amp;openid.claimed_id=https%3A%2F%2Fopenid.stackexchange.com%2Fuser%2F642b1038-5c91-40d4-83ab-eb92db46c3d1&amp;openid.identity=https%3A%2F%2Fopenid.stackexchange.com%2Fuser%2F642b1038-5c91-40d4-83ab-eb92db46c3d1&amp;openid.sig=4emyzuA31ChHU0LabPGJXldbN8GeKN%2FFBJXJy69yD10%3D&amp;openid.signed=claimed_id%2Cidentity%2Cassoc_handle%2Cop_endpoint%2Creturn_to%2Cresponse_nonce%2Cns.alias3%2Calias3.mode%2Calias3.type.alias1%2Calias3.value.alias1%2Calias3.type.alias2%2Calias3.value.alias2&amp;openid.assoc_handle=hrQL%21IAAAAGzBtFeNGBLU2o04XSWr7ISRAbO-B2tZCK3L61IzAsuQQQAAAAEdoTN6rf9fqaP1oekJGXGXJeB8fNSVuByptp5yx1VpQbAzQwF6HJzh6a0DwedydNJ-wwpJQoN5dOLEFmndZt-L&amp;openid.op_endpoint=https%3A%2F%2Fopenid.stackexchange.com%2Fopenid%2Fprovider&amp;openid.return_to=http%3A%2F%2Farea51.stackexchange.com%2Fusers%2Fauthenticate%2F%3Fs%3Dbd394805-cf55-4e8e-ad73-21485a09a989%26dnoa.userSuppliedIdentifier%3Dhttps%253A%252F%252Fopenid.stackexchange.com%252Fuser%252F642b1038-5c91-40d4-83ab-eb92db46c3d1&amp;openid.response_nonce=2015-06-12T12%3A30%3A42Z7tADDeYZ&amp;openid.mode=id_res&amp;openid.ns=http%3A%2F%2Fspecs.openid.net%2Fauth%2F2.0&amp;openid.ns.alias3=http%3A%2F%2Fopenid.net%2Fsrv%2Fax%2F1.0&amp;openid.alias3.mode=fetch_response&amp;openid.alias3.type.alias1=http%3A%2F%2Faxschema.org%2Fcontact%2Femail&amp;openid.alias3.value.alias1=mycorrectaddress%40web.de&amp;openid.alias3.type.alias2=http%3A%2F%2Faxschema.org%2FnamePerson&amp;openid.alias3.value.alias2=SOJPM) the request was made with.    As noted in the comments I'm not the only one, so I'd guess it may be not something I can fix. So will this be fixed?  Repro steps (using SE OpenID):   Get an Stackexchange OpenID account (like I have) (optional?) Add other OpenID providers and check they're working (StartSSL in my case) Please note: "You" don't have an "account" for the Area 51 subpage yet and still need to do the initial sign-in. Go to the Area 51 SE page Hit "Log-in" in the top-bar Hit the "log-in using stackexchange" button Enter your e-mail and password and submit Get the above error   Repro steps (using other OpenID)   Perform as above (until step 4), but this time step 2 is mandatory Hit "more OpenID options" Provide your OpenID link XXX.provider.com (works on crypto, server fault, ...) Authenticate against your provider Receive a variation of the above error (can be included if needed) Where did Magister Illyrio Mopatis get the dragon eggs from? Why is $\pi$ irrational if it is represented as $C/D$? How do I apply differential cryptanalysis to a block cipher? I have read a lot of summaries of block ciphers particularly with regards to the NIST competitions stating that reduced-round block ciphers are – for example – vulnerable to differential cryptanalysis.  I have a general idea that the application of differential cryptanalysis is to look at the difference between inputs;  makes that fairly clear. However, I could take any two inputs for any given block cipher and I am pretty certain I'd be staring at random differences.    I am aware this is the idea of a well written block cipher; however, assuming a broken or vulnerable cipher (feel free to provide simple examples) how do I go about choosing differences to try?  Are there any clues in algorithm design that would inform a decision on which values to choose?  How does being vulnerable to differential cryptanalysis impact a cipher in the wild?  If all I have are differences between known plain-texts and known keys as my analysis and a captured ciphertext as my data to exploit, what can I actually deduce? If $f:\mathbb R \to \mathbb R$ satisfies $f'(x) <L$ for some $L<1$, then $f$ has a unique fixed point I have noticed that the confidence interval for predicted values in an linear regression tends to be narrow around the mean of the predictor and fat around the minimum and maximum values of the predictor. This can be seen in plots of these 4 linear regressions:    I initially thought this was because most values of the predictors were concentrated around the mean of the predictor. However, I then noticed that the narrow middle of the confidence interval would occur even if many values of were concentrated around the extremes of the predictor, as in the bottom left linear regression, which lots of values of the predictor are concentrated around the minimum of the predictor.  is anyone able to explain why confidence intervals for the predicted values in an linear regression tend to be narrow in the middle and fat at the extremes? Natbib -- change author conjunction in citation call-out from 'and' to '&' I'm using natbib, german babel and natbib creates output of the form   Foo and Bar (1337)   I would like to get   Foo &amp; Bar (1337) for citet and (Foo &amp; Bar 1337) respectively for citep. So I recently saw add comment (requires 50 reputation) on a deleted answer. After looking around a bit, it seems to be on all deleted posts now (only deleted posts, normal posts still say just add comment).  I'm sure it wasn't there before (I think it just said add comment (unclickable) or possible nothing).  It is also on , presumably the root cause is the same.  And yes, I do have more than 50 reputation (in case anyone was wondering).  Just to clarify, requires 50 reputation obviously shouldn't be there for users with 50 or more reputation. Choices:   Display nothing Unclickable add comment - probably not ideal Unclickable add comment (disabled for deleted posts) or something along those lines   FYI - You may be somewhat confused right about now if you can't  yet. What are WPF Preview Events? How can I remap Caps lock to both Escape and Control? I generally run Linux. I use Vim a lot, so the Escape key is extremely important. I also use other keyboard shortcuts a lot, so Control is "key" as well. On Linux, I've solved this by using  to map the Caps lock key to Escape if pressed by itself, or Control if pressed in combination with another key. This puts the best of both worlds within quick reach of my pinky finger. Is there any way I can do this on a Mac? Do votes get canceled if voting privilege is lost? If I'm below 15 reputation, I can't alter the public score of an answer/question.  Suppose I gain the privilege by getting the required rep and vote some posts up. Then, for any reason, I fall below 15 and lose the ability to vote. What about the votes I already did cast?  Do post scores get altered by ignoring my vote, or will they stay the same? In other words, is losing a privilege retroactive?  If yes, what about when I gain my privilege back? Are my votes automatically restored, or do I have to cast them again from scratch?  I made the easiest example, but this applies to any higher privilege How to make a through hole using inward extrusion in Blender 2.8? I am trying to make a through hole using inward extrusion. I enterd into edit mode, then I drawed an outline of my hole using knife tool. then I turn into wireframe shading, rotate my view an select snap to faces and then extrude the hole outline inward  so the hole is created successfully, but I still have a face at the hole bottom. I can select this face separately  But If I try to delete it, I siply delete this face (I can't select it anymore after deleteting), but I do not see through hole, so the basic cube face is still here. So how to remove the hole bottom?