I was wondering if given 3 random vars X,Y,Z, assuming we know Cov(X,Z) and Cov(Y,Z), is there a formula to get Cov(X,Y) ? Thank you !
I was asked this question in an interview.  Lets say we have a correlation matrix of the form \begin{bmatrix}1&amp;0.6&amp;0.8\\0.6&amp;1&amp;\gamma\\0.8&amp;\gamma&amp;1\end{bmatrix}  I was asked to find the value of gamma, given this correlation matrix. I thought I could do something with the eigenvalues, since they should be all greater than or equal to 0.(Matrix should be positive semidefinite) - but I don't think this approach will yield the answer. I am missing a trick.  Could you please provide a hint to solve for the same?
A method for the OP to accept comments? Question: What if the author of a question can accept a comment?   The problem:  Time and time again, answers are posted as a comment.  Usually, the person who commented has the best of intention (e.g. it was a trivial answer [to the answerer], and they weren't sure if the OP would immediately edit the question after the comment).  But this leaves the frustrating situation of an unanswered question with a perfectly good answer.  This problem accumulates over the years.     Candidate solution:  The OP can just "accept" the comment, after which a duplicate of the comment is made an accepted CW answer, with a preface indicating that it was originally a comment, e.g. This answer was originally a comment posted by [[blah]] that has been accepted by the author of the question.   Comments on this scheme:   The author of the comment has indicated they're not interested in gaining/losing reputation through their comment (thus CW answer). The original comment is still there. If desirable, the "author" of the accepted question could be a "bot", so the answer is not attributed to a genuine account.  (I suppose it would already be technically possible for Joe Blow to create a script for a bot that creates an answer from a comment in the proposed way.)     A similar question was asked before  (over 3 years ago), and in numerous other questions which have been deemed "duplicates" (,  and ; see also ), so there seems to be demand for such a feature.  The answers to these questions seem (to me, at least) quite weak:   "ask the commenter to add the comment as an answer"   The OP already did this by asking the question in the first place.  And what if the author of the comment doesn't respond (or takes a long time to respond)?  This also adds more chit-chat irrelevant to the question.   "add an answer of your own, referring to the comment, and accept it."   A hacky workaround.  How could this possibly be better than accepting a comment?  (Yet, this is the most up-voted response.)  Moreover, this adds to the user's list of "answers" (thus making the user's page harder to look through).   "When i leave a comment, i do so because i want to leave a comment."   Great; now what about everyone else?  In the immortal words of the FAQ: "Other people can edit my posts?!"  And it's not even an edit, it's copying the post from one box to another (a more hacky version of which is regarded as the "accepted" method). Linux file access monitoring Is there any way in unix to find out who accessed certain file in last 1 week? It may be user or some script ftp it to some other place. Can I get a list of user name who accessed certain file? How can I find out who is accessing particular file?? Fake a webcam using a video loopback device? I want to use a looped video clip as my webcam source in a Google+ Hangout. I installed v4l2loopback and created a . But I haven't figured out how to write the video frames to the device. The given  is:  gst-launch videotestsrc ! v4l2sink device=/dev/video1   So after reading the manual page for gst-launch and trying to understand what was going on, I made my feeble attempt:  sudo gst-launch-0.10 filesrc location=/home/briankb/Videos/darthvaderdancing.mp4 ! v4l2sink device=/dev/video0   Which resulted in:  Setting pipeline to PAUSED ... libv4l2: error getting pixformat: Invalid argument Pipeline is PREROLLING ... Pipeline is PREROLLED ... Setting pipeline to PLAYING ...  (gst-launch-0.10:12622): GStreamer-CRITICAL **: gst_caps_get_structure: assertion `GST_IS_CAPS (caps)' failed New clock: GstSystemClock  (gst-launch-0.10:12622): GStreamer-CRITICAL **: gst_structure_get_name: assertion `structure != NULL' failed Caught SIGSEGV accessing address (nil) #0  0x00007fc1890f0b03 in poll () from /lib/x86_64-linux-gnu/libc.so.6 #1  0x00007fc18962bff6 in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0 #2  0x00007fc18962c45a in g_main_loop_run () #3  0x00007fc189b5c4cb in gst_bus_poll () #4  0x0000000000404587 in ?? () #5  0x0000000000403c34 in ?? () #6  0x00007fc18902b76d in __libc_start_main () #7  0x00000000004043bd in ?? () #8  0x00007fff88253ed8 in ?? () #9  0x000000000000001c in ?? () #10 0x0000000000000006 in ?? () #11 0x00007fff882548c4 in ?? () #12 0x00007fff882548d4 in ?? () #13 0x00007fff882548dc in ?? () #14 0x00007fff88254904 in ?? () #15 0x00007fff88254906 in ?? () #16 0x00007fff8825490f in ?? () #17 0x0000000000000000 in ?? () Spinning.  Please run 'gdb gst-launch 12622' to continue debugging, Ctrl-C to quit, or Ctrl-\ to dump core.   The clip I'm trying to use an h264 video at 29.97 fps, 320x240, and in an mp4 container (it's from Youtube). "going to do" vs. "going to be doing" Note: this post is different to , which just cites part from somewhere else, without any further analysis and thinking. Furthermore, none of the answers to that post answers my question - the situations where &quot;will (going to) be doing&quot; is more appropriate.   I will be participating in IELTS test next Tuesday afternoon.  In an online class, the teacher says the following at the beginning of the lesson.  I will be using NotePod (pointed out by red rectangle) to write relevant points we talk about today.   I guess &quot;will be using&quot; is more appropriate than &quot;will use&quot; there, right?  A  explains  This &quot;tense&quot; is used for reporting a scheduled activity with special focus on its timing— as if one is standing in the future and experiencing the flow of the time. The focus is on the temporary, ongoing, or repetitive nature of the activity.   I understand the grammar rules of those verb tenses and the difference between this group  I am going to drive home tomorrow.   I am going to be driving home tomorrow.  The former is just convey my activity in the near future while the latter probably makes an implication that &quot;don't disturb me as I am driving in that time&quot; However, in the following situations, I don't see the difference between 2 verb tenses.  Welcome back to this class. Today's lesson is for the IELTS students who are going to be taking the IELTS test. it's for everybody to be honest with you. this lesson is useful for anybody who needs to do some writing.   who are going to take the IELTS test ... How do Horcruxes work? A few questions regarding the Horcruxes.   Had Voldemort actually succeeded in using a Horcrux, would he come back the same age (biologically and/or aesthetically) he was when he made the Horcrux, or the age he would biologically be if he had never died (half died)? If Harry was a Horcrux, then in the graveyard, when Wormtail added Harry's blood to the potion to bring Voldemort back, didn't that technically use up the Horcrux inside of Harry? So in Deathly Hallows when Voldemort kills Harry, shouldn't Harry have actually died rather than just have the Horcrux inside him destroyed, seeing as it was technically used in the Goblet of Fire? libsensors5 or libsensors-config upgrade problem MacBook is displaying the message "no hardware installed" when I click on Wi-Fi I have a MacBook Pro 2009 running MacOS El Capitan. Recently the Wi-Fi has stopped working.   When I click on the Wi-Fi icon it displays the message "No hardware installed", but Ethernet still works.   Is there any way of fixing this by using a Wi-Fi dongle or something of the sort, if so do I need a specific dongle and how do I install it? velocity of an object after leaving a ramp Given the length ot the sides $a , b$ and $c$  of $ \triangle ABC$.  What is the length of the radius of the circumcribed circle?  After some formula substitution I came to the monster formula:  $$ \frac {a b c}{\sqrt{2a^2b^2+2a^2c^2+2b^2c^2-a^4-b^4-c^4}} $$  Can this formula be simplified? What is the origin of "a cut above/below the rest"? I'm looking to make a "Drop Folder" in a windows shared drive that is accessible to everyone. I'd like files to be deleted automagically if they sit in the folder for more than X days.  However, it seems like all methods I've found to do this, use the last modified date, last access time, or creation date of a file.  I'm trying to make this a folder that a user can drop files in to share with somebody. If someone copies or moves files into here, I'd like the clock to start ticking at this point. However, the last modified date and creation date of a file will not be updated unless someone actually modifies the file. The last access time is updated too frequently... it seems that just opening a directory in windows explorer will update the last access time.  Anyone know of a solution to this? I'm thinking that cataloging the hash of files on a daily basis and then expiring files based on hashes older than a certain date might be a solution.... but taking hashes of files can be time consuming.  Any ideas would be greatly appreciated!  Note: I've already looked at quite a lot of answers on here... looked into File Server Resource Monitor, powershell scripts, batch scripts, etc.  They still use the last access time, last modified time or creation time... which, as described, do not fit the above needs. How do I calculate spoke length? I am on a Mac with MacTex 2020 and ran into a strange problem with Biber. Working on a document all was fine, I did not change the bibliography (says my git) and only fixing typos (not even adding cites) and suddenly latexmk stopped working after a Biber run, while that looked like  INFO - This is Biber 2.14 INFO - Logfile is 'doc.blg' INFO - Reading 'doc.bcf' INFO - Found 31 citekeys in bib section 0 INFO - Processing section 0 INFO - Globbing data source 'literature.bib' INFO - Globbed data source 'literature.bib' to literature.bib INFO - Looking for bibtex format file 'literature.bib' for section 0 INFO - LaTeX decoding ... INFO - Found BibTeX data source 'literature.bib'   so harmless in my view. Then with --debug I additionally get in the end.  INFO - LaTeX decoding ... warning: overriding existing definition of macro "jun" warning: overriding existing definition of macro "jul" warning: overriding existing definition of macro "sep" warning: overriding existing definition of macro "jan" [...] warning: overriding existing definition of macro "nov" INFO - Found BibTeX data source 'literature.bib' Unicode::UCD: failed to find unicore/version in /var/folders/hm/gt63cn0j2r7bwbzw7c1hww1r0000gn/T/par-626572676d616e6e/cache-2f8a664c3c933a44d096bcc8d018a5e92bced897/inc/lib /var/folders/hm/gt63cn0j2r7bwbzw7c1hww1r0000gn/T/par-626572676d616e6e/cache-2f8a664c3c933a44d096bcc8d018a5e92bced897/inc CODE(0x7fd026aeb5a8) CODE(0x7fd026aebb90) at /var/folders/hm/gt63cn0j2r7bwbzw7c1hww1r0000gn/T/par-626572676d616e6e/cache-2f8a664c3c933a44d096bcc8d018a5e92bced897/inc/lib/Biber/Input/file/bibtex.pm line 1011.   It seems unrelated to the library itself since deleting a few entries (going through deleting one half, then the other), the problem persists. It does not happen for an empty literature, but as soon as a first entry is added (just author/journal/title/year/volume/pages - no utf8-only chars) this error message appears.  what does this last error message mean?  How can I solve this? I tried getting a good measurement with my caliper, but the flexibility of the tire doesn't help much. Let's say I have a 26&quot;x2.125&quot; tire. I know this fits on an ISO 559mm bead seat diameter rim, and 26&quot; = 660.4mm. 26&quot;:  The OD of the tire including the tread? The OD of the tire excluding the tread? When aired up or deflated? Means it fits a 559mm ISO BSD None of the above?  2.125&quot;:  Width of tread? Width of sidewall? Inflated or deflated?  Essentially, I'm converting the tire sizes to &quot;height off rim in mm&quot; and drawing them with a wheel in a web application, so I need to know how &quot;tire size&quot; translates to this measurement. I'm somewhat assuming that the fractional sizes can be converted to the same measurement, but I'm aware that they have certain differences - so any help there would be of value as well.  has been helpful, but still leaves me with the questions above. Upload files upto 2GB from Apex How to prevent updating of a specific package? Because of  I'd like to prevent apt-get upgrade and Update Manager from updating the "libgtk2.0-0" package.   How can this be achieved? Remove all files/directories except for one file I have a directory containing a large number of files. I want to delete all files except for file.txt . How do I do this?  There are too many files to remove the unwanted ones individually and their names are too diverse to use * to remove them all except this one file.  Someone suggested using   rm !(file.txt)   But it doesn't work. It returns:  Badly placed ()'s    My OS is Scientific Linux 6.  Any ideas? Are the rules regarding absolute modifiers too absolute? A common grammar lesson that was taught to me in the US and that I've had to teach abroad in EFL classrooms is that we're not to use adverbs of emphasis with absolute modifiers, just as we're not supposed to use them as comparatives or superlatives.  Classic egregious examples of this mistake include very unique and more perfect, which seem obviously flawed to me. Other instances of further modified absolute modifiers are similarly meaningless or contradictory in nature.  I wonder, though, if the rule that I learned isn't overly broad, and whether it's taught similarly in the UK and elsewhere. I find myself modifying absolute modifiers quite regularly, and in some circumstances I think it's not only logical and correct, but quite meaningful. Phrases like almost exactly or virtually all or practically infinite possess a specific meaning that can't really be replicated using nonabsolutes like very nearly or most or quite long.  In my own speech and writing, I use such phrases exactly as I see fit, so this isn't a request for permission, per se. I'm more curious about whether there's a more appropriate rule regarding absolutes that you rely on (and might be used in a classroom). Is there a way to buy SO reputation? A problem about an $R$-module that is both injective and projective. Let $R$ be a domain that is not a field, and let $M$ be an $R$-module that is both injective and projective. Prove that $M= \left \{ 0 \right \}$.   This is exercise 7.52 of Rotman's Advanced Modern Algebra. Using theorems before exercises, because $M$ is injective and $R$ is a domain, I conclude that $$\forall m\in M ,\forall r\in R\ (r\neq 0) ,\exists {m}'\in M \Rightarrow m=r{m}'$$ and also because $M$ is projective there is a surjective $\psi$ from free $R$-module $F$ with basis $\left \{ e_{i} \right \}_{i\in I}$ to $M$ and thus we can conclude that for every $m\in M$ we have $$m=\sum r_{i}\Psi (e_{i})$$ now I don't know how should I use these together.  The idea of what is happening or a suggestion or a hint will be great.