Why are drop shadows so hard to render, and is there an easier alternative for an aging computer?

Question: I've been using drop shadows on a logo I'm working with and when I apply the effect it takes a good 30-40 seconds for it to finish, and then same amount of time for each edit. I'm using an older macbook here, so it's not super fast but it's no netbook either. Why are drop shadows so intensive to render, and is there something not quite as resource-heavy for improving logo viewability against a mixed-color background?
Comment: Render time is in direct correlation to the object the shadow is being applied to in addition to anything underneath that object. The more complex object stacking is, the more time it takes.
Comment: I know some people won't agree, but frankly you should avoid using drop shadows in vector software. Yes they need to be rendered, although it does help usually to expand the effect or flatten it. If your computer is having issue, an option is to simply create your drop shadows as high resolution raster images in Photoshop and import them under your vector text. It works, it's fast, and it's 100% reliable.
Comment: `Ctrl/Cmd`+`Y`: outline mode?
Comment: Are you using a Mac?
Comment: Also, size will be a factor. Normally, how big (in mm) a vector is makes no difference, but raster (pixel) effects like drop shadow are based on an internal pixels per inch (PPI) setting (probably 300), so if you're designing your logo bigger (in mm) than usual, it'll use more pixels to render than usual, and will be slower than usual. (tip: it's a good idea to avoid pixel effects in logos - they'll be used in so many varied contexts it's best to stick to things which size and render reliably across formats and media)
Answer: If you're happy to edit without the effect until the end, you can temporarily toggle the visibility of the effect in the appearance panel.
That should allow you to focus on completing a different part of the illustration, and then render the effects at the end.
Answer: The least CPU intestive method would be to copy the art, combine all vectors (or select the layer), apply a stroke or contour of a contrasting color. The vector should apply faster than a raster interpolation. Depending on how old your computer is I would expect if it is older than 1998 (it could be this slow) but if it is, you can probably replace it cheaper than you think. If this is less than 10 years old you may have some serious problems with your hard drive. Or you are working with a huge image?


Delphi 2009 where is the 'treat warnings as errors' option?

Question: In Delphi 2009 whereabouts do you turn on the option to treat warnings as errors?
Answer: Just found the answer soon after I posted this!  Might be useful for other people.
Navigate to 'Project -> Options - > Delphi Compiler -> Hints and Warnings' and change the value of 'Output Warnings' to 'as errors'
I was looking for an option similar to what Visual Studio has
Comment: I had looked right at that screen but never did the drop down to see the "as errors" option.  When I saw True as the default I just assumed "False" was the only other option.  Thanks!
Comment: @Paul You probably have to set it on all of the projects individually. It's being set in the project files, so you can write a script to set them all at once. Stick this in a `PropertyGroup` tag with no condition: `error` (for each dproj file).
Comment: This is just for the project you are on at the time. What if I always want all warnings in all projects to be errors? (I really want hints to be the same but I don't think that is possible).
Comment: Thanks but no luck. The program always compiles as success even after reporting a warning. I have added this to every configuration using the GUI and editing the DPROJ file manually. Seems this option is broken in Delphi 10.
Comment: @Paul I noticed the same in my Delphi 10.2 project. When set to 'as errors' the compiler would not convert warnings to errors. However, I did find out that I had some warnings set to False (platform symbols and platform units). When I set them back to True (the default), all warnings are converted to errors. It just does not work anymore when you change any of the defaults in the list of warnings. I am still looking for a solution to disable some warnings globally (for the whole project), but have all other warnings converted to errors. I do not know how to do that...
Answer: On a related note, if you are using the command line compiler (DCC32.exe) the switch is -W^ to have warnings treated as errors.  If you are using this, it's important to note that the default command shell in Windows (cmd.exe) treats the caret (^) as an escape character, so you have to use -W^^ instead if you are executing the compiler directly from the command line, a batch file or even the from the Pre-Build or Post-Build events in the IDE.
It's also worth mentioning that you can have only certain warnings treated as errors.  The switch to do this on the command line would look something like this:  -W^^WARNING-NAME.  You would substitute the string that is associated with the warning you are wanting to have treated as an error.
Answer: The point about -W^ being problematic within BAT files is a good one.  Using -W^^ works if you are modifying the compile line directly.  Otherwise, I found that using surrounding double quotes "-W^" works, for example when building an env var that contains all compiler parameters, that env var being subsequently passed to dcc32.exe. Tested with XE, XE2, XE3, XE4.
SET CompilerParams=-B -M "-W^" -U"..\Source;%dcuoutdir%;%DUnitPath%" -I"..\Source\inc" "-N0%dcuoutdir%" -DDebugMode
"%dcc%bin\dcc32.exe" %FuTFolder%ADDTests.dpr %CompilerParams% -U"%FuTSource%" %ExtraPath32%>%DCCLogFilename%
if errorlevel 1 %Alerter% %DCCLogFilename%


Do I pay twice when transferring domain name to a different registrar?

Question: I'm looking to register a domain for the first time, and I'm trying to weigh the costs. I'm just wondering if you transfer to a different registrar after some time, do you pay both the old and the new registrars? Do publish transfer rates refer to moving out, moving in, or both?
Comment: I think you shouldn't have mentioned that it's for a personal domain ;) Seriously, registrars rarely inflict transfer fees, you mostly pay the minimum which is one extra year of registration tacked on to your domain. Some registrars make renewals less expensive to keep clients, some make transfer-ins less expensive to attract new clients, but ont the registry-registrar backend a transfer imperatively entails a one-year extension of the registration. (You can also vote up comments ;) )
Comment: @Law29 "Some registrars make renewals less expensive" - well, I have never experienced this. In my experience it is more common the other way round... renewals being more experience (or rather, the initial registration fee being "reduced"). Your "average user" will tend not to change their registrar.
Comment: So, does this mean that if I intend to transfer to a different registrar, I should express intention to move out 1 year before (for most registrars)?
Answer: Transferring a domain away from a registrar is free of charge; as long as the domain is unlocked and you provide the correct EPP code to the incoming registrar the transfer can commence.
The incoming registrar will charge you a transfer fee for the incoming domain transfer but will also extend the registration period of the domain by an additional year.

For example: If I purchase example.com from GoDaddy on January 1st 2000 for 1 year the domain will have an expiry date of January 1st 2001. However, If I transfer this domain from GoDaddy to Name.com on May 6th 2000 Name.com charges you a transfer fee for the incoming domain transfer but after the domain has successfully been transferred it's new expiry date would be January 1st 2002. It just adds an additional year.
Comment: @PsychoPunch Yes under certain circumstances a transfer can be denied by the outgoing registrar; however it must be for one of the following reasons: Evidence of fraud, Court order by a court of competent jurisdiction, Reasonable dispute over the identity of the Registered Name Holder, No payment for previous registration period, The transfer was requested within 60 days of the initial registration date or previous transfer date. You can read the policy set forth by ICANN in which registrars have to follow here - https://www.icann.org/resources/pages/policy-2012-03-07-en
Comment: For the record, about three years ago, I transferred a few last domain names from when I was a webhost from Network Solutions to GoDaddy that were ours and got missed. Ooopppssss!!! This was days after a renewal. GoDaddy did not charge a fee at all during the transfer. I only paid the renewal fee a year later. Each registrar is different of course. Most are really really nice because they want your business. Some are not so nice. But they are few and in between and seem to be in countries where there is little to no choice.
Comment: Can a registrar block me from transferring?
Comment: "Transferring a domain away from a registrar is free of charge" - some registrars do charge an _admin fee_. If they do, it should be stated in the T&Cs.
Answer: Some registrars do charge the full fee without refunding what's already been paid with the old one. At least Hetzner explicitly states that this is possible:

If you are transferring a domain to us, we may charge you for the domain again depending on your order. This may happen even if you have already paid for the domain for a year at your old provider/registrar.
Comment: While technically true, I think this is misleading.  You will be charged the renewal fee, but your domain validity will be extended by a year, so if you are keeping the domain name long term it is no more expensive.   Also, some registries (eg .nz) don't charge for moving domains between registrars.


Recording soft body deformations during simulation in bge

Question: I have either simple or complex soft body object, collision plane and run simulation in bge(recording is turned on). What i get is recorded animation containing transformations only. Mesh deforms during simulation- squeeze and stretch, has hull around, but does not record deformations. What would be solution?(tried latest release and older as well, probably have bad setup but can`t figure it out)
Comment: I do not think that is build-in.
Answer: You can write a BGE script that reads the vertex positions from the mesh.
Store this positions in a way that you can match them with the vertices of the according Blender object.
When you run the BGE embedded you can access Blender (bpy) while your game session is running. Be aware changes will apply AFTER the game session ended.
Therefore I suggest:

read the BGE mesh vertex
write to Blender object data (e.g. create shape keys)

I do not have a ready to use script nor do I want to write one here. This answer means is more an idea than a complete solution.
Comment: Thank you, tought it would be, and seems should be straightforward. Will get back to it if i couldnt go with softbody in blender(not bge now.)
Answer: You cannot record a Soft Body deformation with keyframes, since it transforms the vertices of the object itself. There is, however, a Soft Body option in the Physics panel of BI and Cycles. The settings there are more in depth, and you can get an even cleaner simulation than the Game Engine.
Comment: At first i found blender physics too slow(i have complex object as end use) so i tried bullet and bge. I will try again and tweak it maybe. Thank you, you encouraged me.


Which app uses my GPS

Question: I have discovered battery drain and it is caused by GPS but I don't understand how is that possible when my location is turned off and location permission is denied for every app installed and also any app I use don't need gps. I don't have any unfamous or pirated apps installed, and didn't get any new apps since 5 days, and problem started today. So is there any way to see which app is using gps since here is just blank name..?
Answer: Go to setting--> Location
Then you will see list of recent apps who requested location.
If you are still seeing apps there who is using location service then you can find which app is using your location service .
Comment: Nothing special here only google play services "low battery use"
Answer: May be Google Play Services is the culprit here. It drains a battery a lot, infact it uses wifi scan in the background too!. So turning permissions off for it might help with the battery drain.
Comment: I turned off all location permissions, however you can't turn off permission for play services since it's location provider, you can turn off location what I did, and it didn't helped. Also I turned off wifi and bluetooth scanning.
Answer: Whatever have happened here it is solved by rebooting phone (d'oh). I hate this way of solving bugs on R2D2's.
Answer: In the settings you can go to apps and there should be a menu where you can see which app has which permission, there you can click on location and you will see.
Sincerely
Rikvola
Comment: There is 14 apps that have permission but as I said none of them have it granted..
Comment: Now i have a good solution:
root your phone and don't install any google stuff. It will use your battery as well but not that much. This is always on for the system it isn't used by any apps.

And probably some apps have a "hack" to overrule the permissions.

Which app uses my GPS


How to move files from one VM to other VM in a linux basedserver using SSH?

Question: Specifically I am looking for any CLI commands that can do the job.Two VM's exist on a server both are red hat linux based.Thank you
Answer: While the best way would be to use the network, like SSH, SCP, rysnc or NFS, there are ways to share files at the virtual host level between 2 VMs.  However, this is typically harder to configure and more prone to errors.  I would suggest a network approach for all but special use cases.
In VMware for example, you can setup a virtual disk and share it to the two VMs.  However, you need to be very careful with how you do this - because most OSes will expect to have their own disk and have full control over it, and might behave unexpectedly when another VM modifies that disk.
Protocols like NFS take care of this for you, and NFS is a lot cleaner solution for this problem.
Answer: <code>scp /path/to/file <othervm's ip>:/tmp/
</code>
Comment: Do you wanna really use ssh without ip???
Comment: ssh uses tcp protocol, tcp is an ip based protocol. you can't make ssh connection without ip.
Comment: Buddy actually my knowledge in this area is minimal.So plz feed me some info about it
Comment: k thank u but is  there any way to transfer files other than using SSH??
Comment: yes, nfs. https://en.wikipedia.org/wiki/Network_File_System It's also ip based.
Comment: Sorry buddy that link did not answer my question
Answer: Well the whole point of VMs is to be able to isolate your running code.  For example, you want to run two Apache instances but you don't want anything to be shared by them so you build two VMs and now you can run them in isolation on the same server (where the VMs run).  So the only way to transfer files between VMs is by normal server file sharing mechanisms like scp or rsync or nfs mounted file systems or samba mounted file systems.  Otherwise you are hacking the virtual machine environment.  Maybe instead of running VMs you want to run something a little more hackable like containers or even a chrooted environment to "isolate" your running code?


Alcohol level still the same after being soaked with dried fruit

Question: Prunes soaked in gin, does the gin still have same alcohol content after 2 months?
Answer: Assuming that the container you have them in is tightly sealed then the total amount of alcohol in the container will be the same. However, the concentration in the liquid will have changed somewhat, how much depends on how many prunes you put in relative to how much gin.
Basically what happens is that the fruit absorb some of the alcohol and water from the liquid meaning that the gin/liquid has a bit less alcohol in it overall (essentially it is diluted out by the volume of the fruit too). However, I don't know how much of the alcohol would be absorbed by the fruit per volume.
Answer: I wouldn't expect the alcohol level to change much, but it will change some. Gin and other liquors are actually mostly water, or at least a significant percentage will be water. Typically gin is somewhere around 60% water and 40% alcohol, any flavorings are trace elements.
Prunes are dried so they aren't going to contribute moisture to the gin, they will absorb both water and alcohol from the liquor. The question is whether the prunes will absorb more alcohol or more water, or both equally. I have no scientific basis to answer that, so I'll apply some rough numbers to show what the possibilities are.
Say alcohol is absorbed 30% more than water in the fruit, which is a guess but about the maximum difference I'd expect either way. If you have 500ml of gin which is 40% alcohol (80 proof) and the fruit soaks up 100ml of gin then it will absorb about 10ml more alcohol than if it was absorbed evenly. This would reduce the alcohol percentage by around 2.5% of the total volume of the liquor. The opposite works, if the fruit absorbs 30% more water than alcohol the alcohol level would go up 2.5%. These are rough figures, the true figure is dependent on the fruit itself, ratio of fruit to alcohol, alcohol percentage in the liquor, and other factors.
Comment: Prunes are one of the wonders of the world @rumtscho, you don't give them enough credit!
Comment: @rumtscho I see a potential route involving osmosis, as the prunes are made up of cells  full of sugar (soluble in water and not alcohol).  Whether it would have any significant effect is another matter.  Of course if you eat the prunes and drink the gin, you get all the alcohol anyway. A friend who made flavoured gins used to drain the fruit  (usually sloes) and coat it in dark chocolate; they were certainly boozy
Comment: I may be wrong, but separation of alcohol from water by filtration sounds like it needs something much more high-tech than a prune!


Are Agra main attractions closed on 31st of December?

Question: I'm planing a trip to India for the end of this year, and I want to know if Agra main attractions are closed on the 31st of December (like the Taj Mahal and Agra fort). I didn't find anything in the internet with respect to this. I only see that the Taj is closed on all Fridays.
Comment: Isn't the Red Fort in Delhi?
Comment: @Richard yes it is. But they may be referring to Agra Fort, which is red.
Comment: @thanks for the correction im refering to Agra Fort.
Comment: India does not usually observe end of the year holidays
Comment: @RedBaron so what you are saying is that all tourist attractions are open at end of year in India?
Comment: Yup. Only christmas (25 dec) is a holiday. Some places may have one on new year (1 jan) but 31 dec is mostly open.
Answer: The Indian government's Taj Mahal page FAQ says the Taj Mahal is open sunrise to sunset, all days except Friday. (The page also has information on the Agra Fort.) There is no mention of any holiday or new year's closures.
If this is insufficiently specific, the linked page also contains a "Contact Us" link at the top right, with telephone and email contact information.
Comment: Of course I had a look at the page before asking. And it doesn't answer my question
Comment: @AlejandroAndrade Good, many who come to Stack Exchange don't do any research first, even 'though it's expected here. Did you go on to contact the tourism authorities, either by email, snail mail, or telephone?


Cookies in the new browser when using HyperlinkButton in Silverlight

Question: When you use the HyperlinkButton control in silverlight in IE to open a link in a new window using TargetName="_blank", does the new window have the same cookies as the original IE window.  If not, how do you make it have the same cookies?
Answer: Well windows don't have cookies, the HTTP Request will have cookies.  The cookies included in the request will be determined by the cookies in the browsers cookie cache and the URL being requested.
Hence in a typical case where you have application cookies set by responses from an ASP.NET application and they you open a new window to display another url in the same application the request for that url will carry those cookies previously set.
Answer: When I say the windows 'have' cookies, I'm referring to what the browser has in it's cookie cache.  It appears that what the browser launched from clicking on a SL HyperlinkButton control has cached is stale, ie; the JSESSIONID is from a previous session.  Right now, we've hacked it so that we call System.Windows.Browser.HtmlPage.Window.Invoke("openUri", someURL), where openUri is a JS function which calls window.open(someUrl, "_blank").  It appears the difference here is that with the SL implementation, a new instance of IE is launched with stale cache.  With the JS implementation, the new window is launched under the same process.  Is there a way to mimick the js implementation with the hyperlinkbutton?


Can I map half my midi controller keyboard to one octave and the other half to a different octave?

Question: I want to play Canon which requires more octaves than the 25 keys. I'm using an Alesis Q25 and Ableton 9 LE.
Answer: Certainly. You want to define a keyboard "split point", which is a certain MIDI note number. Above this number you want the keys to transmit on one of the 16 MIDI channels, and below this number you want the keys to use a different MIDI channel. Then you can have two instrument patches on those two different MIDI channels, set up to respond to notes in transposed octaves. You can achieve these settings on your MIDI controller keyboard and on your DAW, or in some combination of the two. Read your owners manuals and look for the term "define split point".
Answer: You can also do this by setting each instrument to the same MIDI channel and disabling the unwanted keyboard regions within each instrument.
This method allows you to define different settings for each MIDI channel. This also allows you to quickly switch between different instrument settings. This is useful especially if you are doing live performances. You can just progress to the next MIDI channel output of the keyboard to pass to the next instrument setting required in the song.


How many numbers divide $n$?

Question: My questions started with a random homework dump from Stack Exchange (sadly closed):
Someone posted code that performs $n$ operations for each $i \in \{1 \dots n\}$ that divides $n$, asking about its overall complexity.
In itself, that question was not very interesting. But I thought it does hold some interesting problem: given $n \in  \mathbb{N} $, how many numbers divide $n$?
Consider the following:
$\bullet$ $D(n) = \{$number of divisors of $n$ $ \}$
$\bullet$  for $n$ that is prime, $D(n) = 2$ ($n$ and $ 1$)
Therefore, there is probably little hope finding a closed expression for $D(n)$. How about an asymptotic bound?

$\mathbf{Lower}$ $\mathbf{Bound:}$
Let $n$ be some non-prime natural number.
$\bullet $ $p_i$ denotes the $i$-th prime number
$\bullet $ $\alpha_i$ denotes the number of times $p_i$ occurs in the factorization of $n$. For such primes $p_j$ that don't occur at all, $\alpha_j = 0$
$\bullet $ $p_t$ denotes the largest prime number that divides $n$
The factorization of $n$ is given by:
$$n=p_1^{\alpha_1}*p_2^{\alpha_2} *p_3^{\alpha_3} \dots *p_t^{\alpha_t}$$
Example:
the factorization of $18$ is given by:
$$18=2^{1}*3^{2} $$
It is clear then, that since for any $p_i$, $p_i \geq 2$:
$$ p_1^{\alpha_1}*p_2^{\alpha_2} \dots p_t^{\alpha_t}\geq 2^{\alpha_1 + \alpha_2 \dots + \alpha_t } $$
Which gives us the lower bound: $$D(n) = \Omega (\log(n))$$

And here is my question: can we find an upper bound for $D(n)$? Is there a similar way to prove $D(n) = O(\log(n))$, and that the bound is tight?
Comment: See https://en.wikipedia.org/wiki/Divisor_function#Growth_rate
Answer: A classical result is that for all $n$,
$$\sum_{k \leq n} D(k)= n \ln(n)
+ (2\gamma-1)n + O(\sqrt{n})$$

How many numbers divide $n$?


Two-way communication between kernel-mode driver and user-mode application?

Question: I need a two-way communication between a kernel-mode WFP driver and a user-mode application. The driver initiates the communication by passing a URL to the application which then does a categorization of that URL (Entertainment, News, Adult, etc.) and passes that category back to the driver. The driver needs to know the category in the filter function because it may block certain web pages based on that information. I had a thread in the application that was making an I/O request that the driver would complete with the URL and a GUID, and then the application would write the category into the registry under that GUID where the driver would pick it up. Unfortunately, as the driver verifier pointed out, this is unstable because the Zw registry functions have to run at PASSIVE_LEVEL. I was thinking about trying the same thing with mapped memory buffers, but I'm not sure what the interrupt requirements are for that. Also, I thought about lowering the interrupt level before the registry function calls, but I don't know what the side effects of that are.
Comment: You're already using conventional I/O from the driver to the application to pass the URLs; why not use conventional I/O to return the result as well?
Comment: Because the request initiates from the driver, I couldn't figure out how to make it work. The app makes an I/O request, the driver completes it with the URL in the output buffer, then how does the app get information back to the driver since the I/O request is complete? Also, the operation is time-sensitive because I'm making a decision about whether to block a web page.
Answer: You just need to have two different kinds of I/O request.
If you're using DeviceIoControl to retrieve the URLs (I think this would be the most suitable method) this is as simple as adding a second I/O control code.
If you're using ReadFile or equivalent, things would normally get a bit messier, but as it happens in this specific case you only have two kinds of operations, one of which is a read (driver->application) and the other of which is a write (application->driver).  So you could just use WriteFile to send the reply, including of course the GUID so that the driver can match up your reply to the right query.
Another approach (more similar to your original one) would be to use a shared memory buffer.  See this answer for more details.  The problem with that idea is that you would either need to use a spinlock (at the cost of system performance and power consumption, and of course not being able to work on a single-core system) or to poll (which is both inefficient and not really suitable for time-sensitive operations).
Answer: There is nothing unstable about PASSIVE_LEVEL. Access to registry must be at PASSIVE_LEVEL so it's not possible directly if driver is running at higher IRQL. You can do it by offloading to work item, though. Lowering the IRQL is usually not recommended as it contradicts the OS intentions.
Your protocol indeed sounds somewhat cumbersome and doing a direct app-driver communication is probably preferable. You can find useful information about this here: http://msdn.microsoft.com/en-us/library/windows/hardware/ff554436(v=vs.85).aspx
Comment: Sorry, I could have phrased that better. It's "unstable" in the sense that it doesn't crash every time, but the WFP filter functions run at DISPATCH_LEVEL so the registry functions are inappropriate.
Answer: Since the callouts are at DISPATCH, your processing has to be done either in a worker thread or a DPC, which will allow you to use ZwXXX. You should into inverted callbacks for communication purposes, there's a good document on OSR.
I've just started poking around WFP but it looks like even in the samples that they provide, Microsoft reinject the packets. I haven't looked into it that closely but it seems that they drop the packet and re-inject whenever processed. That would be enough for your use mode engine to make the decision. You should also limit the packet capture to a specific port (80 in your case) so that you don't do extra processing that you don't need.


Microlocal proof of Wigner semicircle theorem?

Question: Something I really enjoy about Tao's writing is that he proves the same theorem over and over.  While I complain a bit sometimes about clarity, this is a heuristic that I very much believe in.
This question is about the Wigner Semi-Circle Law from his 254A random matrix theory.  He does it two different ways:

for a broad class of matrix ensembles Notes 4
for a Gaussian Ensembles  Notes 6

They are Hermitan matrices: $M = \overline{M}^\dagger$ (equal to it's own adjoint) with elements $\xi_{ij}$ chosen under special conditions:

$\xi_{ij} = \overline{\xi_{ji}}$ is a complex variable, zero mean $\mathbb{E}[\xi_{ij}] = 0$ and unit variance $\mathbb{E}[\xi_{ij}^2] = 1$
Diagonal matrics  $\xi_{ii} \in \mathbb{R}$ with bounded mean and variance.

So that semi-circle theorem works under very broad conditions, but let's set $\xi_{ij}$ and $\xi_{ii}$ be appropriate $\mathcal{N}(0,1)$ random variables.  Before getting to the "main" proof in Notes 6, there are some rather unique and technical estimates that I'm going to skip, basically 90% of the proof.
He shows that GUE is a determinantal process with kernel basic on Hermite polynomials:
$$ K_n(x,x) = \sum_{j=0}^{n-1} \phi_j(x)^2 = \phi'_n(x)^2 + (n - x^2/4)\phi_n(x)^2  $$
with a related formula for $\mathbb{E}(\mu_{M_n/\sqrt{n}})$ .  He explains to us the various notions of convergence for probability measures and how these interplay with matrix norms in general.  And works a few of these cases out.
I recognize the Hermite polynomials (weighted with Gaussians)
$$ L_h \phi = - h^2 \phi'' + \frac{x^2}{4} \phi = \left(- h^2 \frac{d^2}{dx^2} + \frac{x^2}{4} \right)\phi $$
Intuitively, I can picture what this "phase space" should look like.  There should be a coordinate axis marked "$x$" and a momentum axis marked "$p$" and the Hamiltonian should read:
$$  L_h = p^2 + x^2/4 \leq 1 \text{ with }p = - i\hbar \frac{d}{dx} $$
which is indeed a circle (rather... an ellipse).  There's some sort of projection operator:
$$ \pi_{V_n} = L_{1/\sqrt{n}} \Big|_{[0,1]} =_{symbol} \dots (4-x^2)_+^{1/2}$$
Unfortunately, Tao warns us this requires semi-classical analysis or even microlocal analysis.
What does he mean by that?  He does some type of "WKB approximation" except that's also not rigorous.  So he settles for a mix of

variation of parameters
Gronwall's inequality
steepest descent

Google searching for "microlocal" analysis leads me astray.  I found work of Kashiwara which I shall not even mention here...
How does WKB fail to be rigorous?  What kind of machinery gets introduced with microlocal analysis?
Answer: You need the theory of $h$-pseudodifferential operators.
Standard references are:

"Spectral Asymptotics in the Semi-Classical Limit" by Dimassi and Sjöstrand.
"An Introduction to Semiclassical and Microlocal Analysis" by Martinez.
"Semiclassical analysis" by Zworski.


Forward Squid Traffic To Tor Network

Question: I have installed squid on my Ubuntu machine and I would like to forward requests from the squid proxy to the Tor network. Is this possible without iptable changes? I know that Tor proxies are socks proxies on the client end.
Comment: I fail to see how setting up something to connect to Tor network losses "Professional" title. Or maybe its just me.
Comment: "Server Fault is for system administrators and desktop support professionals, people who manage or maintain computers in a professional capacity." I fail to see how this falls into "Professional".
Answer: I figured that it wont be possible using Squid alone. I installed privoxy and set it up in between Squid and the Tor network. Privoxy has  built-in capability to connect to a socks proxy.

Forward Squid Traffic To Tor Network


A question about electrons in a positively charged sphere

Question: I understand why negative charge spreads out evenly across the outside of a sphere (as electrons repel each other), but I am having difficulty understanding why that would be the case with a positively charged sphere:
That is to say, for the positive charge in a positively charged sphere to be spread out evenly across the surface, the few electrons around presumably need to be somewhere on the inside. But why would they flock to the inside if they repel each other, while positive charges are around both at the surface and further inside?
Answer: Imagine an electron exactly on the surface of the sphere.  We assume the region outside the sphere is neutrally charged, so there are no electrostatic forces.
The sphere itself is positively charged, so there is an unbalanced attraction on the electron.  It must experience a force that accelerates it into the sphere.
Yes, the other electrons are repelling each other, but there are not as many of them as there are protons.  The electrons have to move (slightly) away from the surface for them to balance.
Answer: Net positive charge on the surface of the sphere would have its field projected outside of the sphere and have no effect on the electrons on the inside.
On the other hand, net positive charge inside the sphere would attract electrons surrounding it and make them move away from the surface, leaving the surface with the net positive charge.


different resistors connected parallel, finding the maximum power dissipation for specified values of resistances and wattages

Question: what will be the resultant wattage when 4 resistors having individual resistances and wattage given as (2.4ohm,1W) , (5.1ohm,2W) , (10ohm,5W) , (22ohm,10W) connected in parallel across an DC voltage source? which resistor would dissipate more power in the resultant circuit?
please help me on this question. i had spent lot of time trying to understand it in detail.
Comment: This smells like a homework question. Just use basic math to calculate the flowing circuits and then compare...
Comment: Can you determine the power dissipation if its only one resistor connected to the power supply ? What is the consequence of all resistors being "in parallel with the power supply" ?
Comment: it's a question asked in an examination last year, i was revising that paper again and struck with a doubt regarding wattage of resistor combinations in particular, not with the calculation of power dissipation rather the role of wattage in series and parallel connection of resistors. Thank u.
Answer: When you know the voltage across a resistor, the power is given by:
$$P = \frac{V^2}{R}$$
Given that the voltage across all resistors in parallel is the same, what does this tell you about the relationship between resistance and power?
Comment: yes, i can see the inverse proportionality between resistance and power dissipation. but, i am more confused regarding the role of wattage, in particular about what would be the final total wattage of the combination.
Comment: @Rama Krishna (3:49) The way to approach that is to find out which resistor will hit its power limit first as the voltage increases. This would be the highest voltage you could safely apply to the set of them. Then calculate the total power all of them dissipate.
Answer: The fact that the resistors are connected in parallel is irrelevant except to establish that the same voltage is impressed across each resistor.
Likewise, a resistor's power rating has nothing to do with the actual power dissipated by a resistor, it's merely a rating based on how hot the resistor can get (how much work it can do / how much power it can dissipate) before its temperature rises to the point where it'll be damaged in some way.
What decides the actual dissipation is the product of the voltage across the resistor and the current through it.
For example, if we have a one ohm resistor and we place one volt across it, the current through it will be
$$ I =\frac{E}{R} = \frac{1V}{1 \Omega} = \text {1 ampere}$$
and it'll dissipate
$$ P = IE = 1V \times 1A = 1 \text{ watt}$$
If the resistor is rated to dissipate one watt, then that means that its temperature will rise to, say, 100C when it's dissipating one watt which is ,say, the highest temperature it can safely tolerate.
If the resistor had a power rating of two watts, however, it would still dissipate one watt, but it would be running cooler than if it were rated at one watt.
In that vein, then, since:
$$ P = \frac{E^2}{R}, $$
if we arbitrarily choose 10 volts, DC, as the excitation source, we can write for the first resistor:
$$ P = \frac{E^2}{R}=\frac{10V^2}{2.4\Omega} \approx 42\text{watts.} $$
Considering that the resistor is rated to dissipate one watt indicates that it will be destroyed in short order.
The calculation for the other three resistors is identical except for the values of resistance, so determining which one will dissipate the most power is easily determined by comparing the quotients, and the total power dissipated by the circuit will be the sum of the power dissipated by each resistor.
Finally, since there's no voltage specified in the problem, it might be an interesting exercise to work out, academically, the voltage which will cause the 2.4 ohm resistor to dissipate its rated power and use that voltage to work out the rest of the problem. :)
Like this:
Since $$ P =\frac{E^2}{R} $$
we can rearrange and solve for E like this:
$$ E = \sqrt {PR} = \sqrt {1W\times 2.4\Omega} \approx 1.55 \text{volts,} $$
We now have the voltage which will appear across all of the paralleled resistors, so it should be a simple matter of plugging numbers into formulas and watching answers fall  out.
Comment: firstly,thanks for the reply.
i do know the formula for power P=V^2/R or I^2 *R
my doubt was regarding the role of wattage in deciding the power dissipation of any resistor.
Comment: Also how can we calculate the total wattage of the above equivalent circuit finally?
Comment: @RamaKrishna: Please see my edited answer.
Comment: @RamaKrishna: There are two ways to calculate the total power dissipated, one being to calculate the power dissipated in each resistor and then to add up the watts, and the other is to calculate the equivalent resistance of all four resistances in parallel and then calculate the power dissipated as if they were a single resistor.
Comment: yes,but what would be the total wattage,i think it should be different from the total power dissipation, because wattage is the maximum possible power dissipation without burning out.isn't it?
Comment: I think you're confusing power rating with power.  Power rating is how much power, in watts, a resistor (or anything, for that matter) is designed to dissipate, and power is, well, [here ya go](https://en.wikipedia.org/wiki/Electric_power)


Will apt-get dist-upgrade upgrade my system to a newer version?

Question: I am using Ubuntu 12.04 right now. If I use the command sudo apt-get dist-upgrade will that upgrade my Ubuntu to 12.10? Does that mean I won't be using the LTS version after that? I don't want to use Ubuntu 12.10 because I've already tried it and it's very heavy for my netbook.
Answer: You can read the man-page of apt-get to see what each command do.
Open the manual
man apt-get

Find the section dist-upgrade and read

dist-upgrade
       in addition to performing the function of upgrade,
       also intelligently handles changing dependencies with new versions
       of packages; apt-get has a "smart" conflict resolution system, and
       it will attempt to upgrade the most important packages at the
       expense of less important ones if necessary. So, dist-upgrade
       command may remove some packages. The /etc/apt/sources.list file
       contains a list of locations from which to retrieve desired package
       files. See also apt_preferences(5) for a mechanism for overriding
       the general settings for individual packages

See also: What is "dist-upgrade" and why does it upgrade more than "upgrade"?
In order to upgrade Ubuntu to a newer release you have to run in terminal
sudo do-release-upgrade

We read from the man-page
man do-release-upgrade

DESCRIPTION
Upgrade  the  operating  system  to  the  latest  release from the
  command-line.  This is the preferred command if the machine has
  no graphic    environment  or  if the machine is to be upgraded over a
  remote connection
Comment: I'd love a TL;DR; too, though
Comment: If you prefer a nice GUI, use:
`sudo do-release-upgrade -f GTK`
Comment: TL;DR; No it won't
Answer: No, sudo apt-get dist-upgrade will not upgrade to a new Ubuntu release. It will just install available updates for the Ubuntu release you already have installed.
Unlike sudo apt-get upgrade it may install new packages or remove installed packages if that is necessary to satisfy dependencies. So be careful when using it.
Comment: It may remove a package if there's a conflict between packages or dependencies that can't be satisfied. Usually you don't need to use `dist-upgrade`. It may be useful if `upgrade` tells you that some packages are hold back, but you should carefully review what it wants to do, especially if it want to remove packages.
Comment: @PeterV.Mørch In fact, the do-release-upgrade tool does exactly that: update the contents of /etc/apt/sources.list, disables or modifies the extras in /etc/apt/sources.list.d/ and then calls apt-get dist-upgrade.
Comment: what do you mean by remove installed packages? why it remove installed packages? So how suppose I use dist-upgrade?
Comment: I'll claim this answer is false. If the `/etc/apt/sources.list*` has been altered to point to a new release, then `apt-get dist-ugprade` indeed _will_ (attempt to) upgrade to a new release. Not something that is likely to happen to a casual user, though.
Comment: I think the concepts are inverted.
Answer: If you do not want to upgrade to non-LTS versions, follow these steps:

Open Ubuntu Software Center.
Go to Edit -> Software Sources.
Go to the Updates tab and make sure you have selected For long-term support versions in the Notify of a new Ubuntu version.

Open a terminal and run sudo apt-get update (just to be sure :) ).
Now, when you run sudo apt-get dist-upgrade it'd be upgraded to the next LTS version and not non-LTS versions.
Comment: How do you do this operation from the command line?
Comment: In Ubuntu 16 this program is called `Software & Updates`
Comment: From command line: `software-properties-gtk`
Answer: you cannot jump from 12.04 to 13.04 directly you would have to have update manger set to normal then do
sudo apt-get dist-upgrade

or
sudo do-release-upgrade

the difference between the commands is that dist-upgrade wants you to change your config files during the upgrade
they both will upgrade your dist when the time comes
sudo apt-get upgrade
only upgrades the software on your system it does not install or remove packages like dist-upgrade.
Comment: If you are running an alternative desktop like cinnamon or mate and you run do-release upgrade it will remove cinnamon and keep the config files and your system will break cause it's looking for cinnamon and it's not there if you ran dist-upgrade it will change the config files system will remove cinnamon and system will use gnome 3 instead you would have reinstall the desktop after doing the upgrade though


Rails non-table drop down list

Question: I need to have a drop down list in which the user selects the day of the week they want to come in every week. The values will never change right.  It's just Sunday, Monday, ..., Saturday right?  It seems like more work than needed to make a table and put the days of the week in. I certainly don't need the ability to create, update or delete them.  Is there a simple way to handle something like this? Or maybe instead of days of the week it could be status like off, park, reverse, neutral, drive.  The main thing is that the values are never going to change.  There are just a few of them.  So why make a table? I am thinking that there is a way to create a model that already has data in it but I could be wrong.
Answer: Why create a model? Just use select.
DAYS = ['Monday', 'Tuesday', 'Wednesday', ...]
select(:event, :day, DAYS)

It's usually better practice to place the constant in the relevant model and use it from there.
In your model:
class Event < ActiveRecord::Base
  DAYS = ['Monday', 'Tuesday', 'Wednesday', ...]
end

and then in your view:
select(:event, :day, Event::DAYS)

and here's another trick I use a lot:
select(:event, :day, Event::DAYS.collect {|d| [d, Event::DAYS.index(d)]})

Comment: I don't want to be rude or something, but as a newbie rails developer as I am, I cannot understand how to apply your suggestion? What will my view contain and how would I access what you suggest?
Comment: the select() goes in your view, wherever you want the dropdown list to appear. the DAYS constant goes in your relevant model.
Comment: if there's no relevant model (you're not storing this anywhere), then DAYS could go in your view too (or better yet, in an initializer).
Answer: Note that Ruby has the English names for the days of the week already built into its date class.  You should try to leverage that if you can.  Here's the rdoc.
Then as Can has suggested just do the following:
select(:event, :day, Date::DAYNAMES)

Keep in mind that this solution is NOT particularly i18n friendly.  If i18n is an issue, I would also check out the localized dates plugin and the changes that were made in Rails 2.2 to support i18n.
Answer: Try this:
<%= select_tag(:week_day, options_for_select([['Sunday', 'Sun'], ['Monday', 'Mon'], ...])) %>

See http://guides.rubyonrails.org/form_helpers.html#theselectandoptionstag

Rails non-table drop down list


Lion Apache Subversion (SVN) ignore problem

Question: I hope this is the right place to ask this kind of problem.
So, before the Lion, when I tried to ignore something on Apache Subversion (svn), I just typed the following command: svn propedit svn:ignore .
So this opened a temporary file for the current directory with the selected editor and I could write there my patterns, which where ignored by the svn.
After the I installed Lion, when I type this command the following error appears: The document "svn-prop.tmp" could not be opened. The file doesn't exist.
Did anybody else met this error before? (I tried googling, but I didn't find any solution).
Comment: What editor is configured for the shell/SVN?
Comment: It is the default, it is what I got with the system. I didn't configure any new.
Comment: Which editor does the command launch then?
Comment: TextEdit, I've used this since I am using svn.
Comment: That is definitely not the default and it surprises me it worked before.
Answer: The problem with GUI editors is that when launched from the command-line, they generally do not halt the process in the shell. That's why when editing a property, svn will create the temporary file, launch the editor, and wait for the exit signal to read and delete the temporary file contents. Because the GUI editor returns the exit signal immediately upon launching, the file will no longer be there.
Some GUI editors like TextMate and Sublime Text support a -w (wait) option from the command-line, forcing them to halt the SVN process until the file is closed. I am not aware of TextEdit supporting such an option.
You probably want to change your SVN editor to something more appropriate, like vim or nano. You can change the editor command in ~/.subversion/config
Comment: Yes, you are true, but I also found a solution to start TextEdit. I will post it soon.
Answer: As Gerry pointed out calling TextEdit directly will not work but you can use open:
open -e -W -n

in your case
export SVN_EDITOR='open -e -W -n'

-e opens with TextEdit (you can use -a to specify another application)
-W waits that the application exits
-n opens a new instance (so that you will not have to quit an eventually open TextEdit)
Comment: Yes, this is the answer. Thanks to post it here too. :)


Semiconductor theory

Question: Why is there no resistor in the circuit when analyzing hole and electron currents according to semiconductor theory?
Comment: Hoàng Lê - Hi, Please note the [site rule](/help/referencing) which requires that if a post copies or adapts content (e.g. text, image, photo etc.) from elsewhere, that content must be correctly referenced. As a minimum, for online material, the *original* source webpage / PDF / video etc. should be named & linked (see that rule regarding references for books / articles etc.). Therefore please [edit] your question to add the original image source name & link (and please remember it's your responsibility to do that in future). Thanks.
Comment: Question (with regard to Graham Nyes answer): Does your problem desription involves the INTERNAL resistor of the device or EXTERNAL parts (resistors) in some application circuits?
Comment: it is a reistor in circuit in some application
Answer: Whilst the semiconductor block will have some resistance these diagrams are not intended to represent realistic circuits.
The battery exists to show a bias of a specific polarity across the junction to allow discussion of how the semiconductor works. Real world requirements like external resistors have been omitted to focus on the semiconductor action. The diagram shows you that forward bias is defined as having an external voltage with the positive side connected to the p layer and the negative side connected to the n layer (and that the depletion region narrows under forward bias). The diagram isn't there to tell you anything else.
A real circuit will have some external load so that the semiconductor action can be put to some practical use. In a practical circuit forward biasing a semiconductor junction with no external resistance to limit the current flow may result in such a high current flowing that the junction will be damaged.
Once a textbook has covered the fundamental operation of a p-n junction individually and in combination to form more complex semiconductor devices (e.g. transistors, thyristors) it will hopefully go on to explain how semiconductor devices are used in practical circuits.
Answer: The resistance is there. I remind you that the current density J in the semiconductor subjected to a potential difference at two ends is given by the conductivity σ, reciprocal of the resistivity, of the semiconductor for the electric field intensity E, i.e.: J=σE. The conductivity σ depends on the concentration n of electrons and p of holes but also on their mobility μ1 and μ2 constants at T constant , i.e. σ=nμ1+pμ2.
Answer: The shown figure represents the classical pn-junction (diode).
The corresponding V-I relationship is given by the well-known exponenetial Shockley-equation.
Such a non-linear function current I versus voltage V always allows to specify two resistances:

The static resistance is defined as the ratio of the two DC quantities: Rstat=V/I. This resistance is identical to the inverse slope of the line between the origin and the actual working point on the curve I=f(V).

The dynamic (differential) resistance rd=d(V)/d(I) is identical to the inverse slope of the function I=f(V) at the actual working point.

Semiconductor theory


UDLD in VSS configurations

Question: Since a VSS is a single system image, does it make sense to enable UDLD for links between the switch pair? We've seen some recovery problems that we feel might be due to UDLD mistakenly identifying an ISL link as unidirectional, especially in a configuration with ASA-SMs in an HA configuration.
Comment: Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can post and accept your own answer.
Answer: If you are referring the the VSL between the to VSS switches, I don't think it makes sense to enable UDLD because LMP will detect a unidirectional link.

LMP is designed to run on each VSL member link to maintain multiple state machines with the same peer over different ports. In a case in which a unidirectional condition on a port is detected, LMP marks it as down and attempts to restart VSLP negotiation—instead of err-disabling the port.

For this and more LMP and VSS info: http://www.cisco.com/en/US/docs/solutions/Enterprise/Campus/VSS30dg/VSS-dg_ch2.html#wp1055977

UDLD in VSS configurations


What muscles would get sore if humans had wings and could fly?

Question: I'm intrigued by the idea of humans with feathered wings as a 6th appendage, but with prolonged flight, if such a thing were actually possible for humans naturally as it is in my book, I'm not sure which muscles would become sore over long periods with little rest between, especially for newer fliers, or people unaccustomed to those long distances. As it would be with any other type of muscle fatigue there is the expected amount of soreness and weakness, shaky legs or burning and cramping, but which muscle group would this affect? Feel free to elaborate, anything and everything will help.
Comment: Without a clear picture of what these humans would look like, this question isn't really answerable. If you want bat-like wings, it would be the arm and chest muscles, as well as the heart. However, bat-like wings (or bird-like, for that matter) entail _replacing_ normal human arms with wings, which I suspect isn't what you had in mind. If you want a separate set of wings from the back, like angels, the answer is "whatever muscles you imagine", because our existing muscles wouldn't be able to move such wings: you'd have to invent new ones, and make a lot of other changes to the body as well.
Comment: Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking.
Comment: Your back muscles: The lats
Answer: The muscles would have to be just as imaginary as human wings themselves are, you would need to develop a new set of muscle groups, analogous to those we use for our arms, but with more focus on performance in a single plane (the direction of beating).
In addition to these imaginary muscle groups, your core muscles (Thoracic and abdominal) would likely fatigue as well, not to mention your heart and lungs from the level of activity that would need to be sustained.
You'd also likely have to lighten the skeleton, and that would be very risk from an evolutionary standpoint as humans rely heavily on their strong bones for survival.


Real Skew Symmetric $3\times 3$ matrix has one eigen value $2i$

Question: Real Skew Symmetric $3\times 3$ matrix has one eigen value $2i$ so another eigen value is $-2i$ another eigen value must be $0$ right? as we know these kind of matrix has eigen values $0$ or purely imaginary.
Comment: Yes. $\quad\mu^{*} = \mu\quad$ and $\quad\mu^{*} = -\mu\quad$
$\Longrightarrow\quad\mu = 0$.
Answer: In general, a real skew-symmetric $3 \times 3$ matrix $K$ looks like
$K = \begin{bmatrix} 0 & a & b \\ -a & 0 & c \\ -b & -c & 0 \end{bmatrix}, \tag{1}$
so that the characteristic polynomial is
$\det(\lambda - K) = \lambda^3 + (a^2 + b^2 + c^2) \lambda, \tag{2}$
which is easily seen to have roots
$\lambda = 0, \pm i\sqrt{a^2 + b^2 + c^2}, \tag{3}$
which follow the general pattern the OP suggested; apparently the matrix she/he was thinking of satisfies $\sqrt{a^2 + b^2 + c^2} = 2$; but $0$ will always be an eigenvalue for such $K$ in any case.
Hope this helps.  Cheers.
Answer: Since a real matrix $A$ is skew if, and only if, $A=-A^T$, since $\text{spec} (A)=\text{spec}(A^T)=-\text{spec}(-A^T)$ and since the characteristic polynomial has degree $3$ and non-real roots come in pairs, it follows that $0\in\text{spec}(A)\cap \text{spec}(A^T)$.
Answer: Yes, the characteristic polynomial of a real $3\times 3$ matrix will be a real cubic polynomial.  If not identically zero, the real skew matrix will have one pair of conjugate imaginary eigenvalues and a zero eigenvalue, each of multiplicity one.
Comment: Does this eliminate the possibility of eigenvalues $2i,2i,-2i$?
Comment: @Rebecca: Yes, it does (for a 3x3 real skew matrix).
Comment: In case someone has doubts, the characteristic polynomial being a real cubic "eliminates the possibility" of three purely imaginary nonzero roots.  Since these must occur in conjugate pairs, divding out a monic quadratic with one such pair as roots gives as quotient a real degree 1 polynomial having the third purely imaginary nonzero root (contradiction).
Answer: We know that a skew symmetric matrix $A=(a_{ij})_{3 \times 3}$ satisfies $A^T=-A$.  This means that $a_{ii}=-a_{ii}$ or equivalently $a_{ii}=0$ for all $i \in \{1,2,3\}$.
We know the trace of $A$ is the sum of its eigenvalues, which is $0$ since the main diagonal of $A$ comprises of zeroes.
Since a $3 \times 3$ matrix has $3$ (not necessarily distinct) eigenvalues (given by the roots of the characteristic polynomial [a degree $3$ polynomial with leading coefficient either $1$ or $(-1)^3$, depending on how it's defined]), there is precisely one other eigenvalue $\lambda$ and we must have $2i-2i+\lambda=0$, which implies $\lambda=0$.
Answer: Here's another way of looking at it, which I thought of after posting my previous answer.
The approach here seemed sufficiently different that I felt a separate answer was merited.  Hope I'm not overdoing it.  Anyway . . .
First, note that for any skew-symmetric, real matrix, the only possible real eigenvalue is zero.  For, if $K$ is real skew-symmetric, so that $K^T = -K$, and if
$v \ne 0$ is an eigenvector with real eigenvalue $\lambda$, so that
$Kv = \lambda v, \tag{1}$
we have
$\langle Kv, v \rangle = \lambda \langle v, v \rangle, \tag{2}$
but
$\langle Kv, v \rangle = \langle v, K^Tv \rangle = \langle v, -Kv \rangle = -\langle Kv, v \rangle, \tag{3}$
or
$\lambda \langle v, v \rangle = -\lambda \langle v, v \rangle, \tag{4}$
and since $\langle v, v \rangle \ne 0$ this shows that
$\lambda = - \lambda, \tag{5}$
whence
$\lambda = 0. \tag{6}$
Here I take $\langle \cdot, \cdot \rangle$ to be the standard inner product on the real vector space on which $K$ operates.
Now argue as follows:  since $2i$ is an eigenvalue of $K$, and the characteristic polynomial of $K$ is real, $-2i$ is an eigenvalue as well.  This means
$\lambda^2 + 4 \mid p_K(\lambda), \tag{7}$
where
$p_K(\lambda) = det(\lambda - K) \tag{8}$
is the characteristic polynomial of $K$.  But $\deg p_K(\lambda) = 3$ in the present case, so the quotient polynomial $q(\lambda)$ is of degree $1$, i.e. we must have
$p_K(\lambda) = (\lambda^2 + 4)q(\lambda) \tag{9}$
with
$q(\lambda) = \lambda - a, \tag{10}$
$a$ real.  But this of course implies $a$ is a real eigenvalue of $K$, whence
we must have
$a = 0. \tag {11}$
Well, I hope this answer is more than mere mathematical logorrhea, and it sheds a little more light on the subject at hand.
Cheers!


ipe latex text not able to run

Question: I was trying to add latex text/symbols in ipe tools, but it's popping out error shown in the image.
I tried to add the path as an environment variable as C:\Users\Pramod\AppData\Local\ipe where "runlatex" file located , still got the same error. Please could you please help?
Comment: Did you install LaTeX, e. g. MiKTeX?
Comment: Just add the LaTeX bin directory to your path. How you do that depends on your Windows version.
Comment: Maybe read the last sentence of the error message more closely. This sounds like a convenient solution.
Comment: @Manuel ,yes I have installed MikTeX  2.9,
Comment: The last-mentioned solution is not working even after enabling online compile.
Comment: a few days back I have modified the registry for some software and forgot to take backup, Is this error anything to do with the registry ?
Answer: Since I don't have enough reputation to comment, replying here, the issue is resolved - The problem was is in the registry, solved using this .hence command prompt was also not opening.
Comment: Is it possible that you are the same person as the original poster, but have two accounts?  If so, you can get them merged: https://meta.stackexchange.com/q/18232

ipe latex text not able to run


Make a property visible in DataGridView but NOT in PropertyGrid?

Question: Let's say I have a property which I want shown in a DataGridView, but not when the same object is shown in a PropertyGrid.  I know I can use [Browsable(false)], but that hides it in both views.  I can also do a gridView.Columns["blah"].Visible = false;, but this is the opposite of what I want, as it hides in the DataGridView but not in PropertyGrid.  Is there some way to do the reverse?  (Short of creating a whole new DataTable just to hold the same data minus one field, and rebinding everything to that instead - that's really a kludge way to do things.)  Alternatively, I could live with a solution which adds a column to the DataGridView that is not present on the actual class.
Comment: I'll accept them when they actually answer my questions.  Half my questions have barely had any views, let alone good responses.
Comment: I'm calling this a bug. `Browsable` specifically states that it determines the visibility of a member in a properties window. This has nothing to do with a DGV. You could open a connect issue, but the odds that they will fix it are exactly zero. A fix would undoubtedly break existing code. I think you are out of luck here.
Comment: Damn.  Of course, they could add a new decorator rather than try and fix Browsable, but still sucks that there's no way to do this.  You'd think it wouldn't be that uncommon a request...
Comment: Yea, there is another reason they won't fix it, even if it were a non-breaking fix: WinForms has not been touched since 2.0; It is considered to have been superceeded by WPF.
Answer: it is possible to solve this issue by using the BrowsableAttributes property of a PropertyGrid.
First, create a new attribute like this:
public class PropertyGridBrowsableAttribute : Attribute
{
    private bool browsable;
    public PropertyGridBrowsableAttribute(bool browsable){
        this.browsable = browsable;
    }
}

Then add this attribute to all those properties which you want to be shown in your PropertyGrid:
[DisplayName("First Name"), Category("Names"), PropertyGridBrowsable(true)]
public string FirstName {
    get { return ... }
    set { ... }
}

Then set the BrowsableAttributes property like this:
myPropertyGrid.BrowsableAttributes = new AttributeCollection(
    new Attribute[] { new PropertyGridBrowsableAttribute(true) });

This will only show the attributed properties in your property grid and the DataGridView can still access all properties with only a little bit more coding effort.
I would still go with Tergiver and call this behaviour a bug, since the documentation of the Browsable attribute clearly states its use for property windows only.
(Credit goes to user "maro" at http://www.mycsharp.de/wbb2/thread.php?postid=234565)
Comment: Well, it's 3 months since I last touched this project, and parts of it related to this were completely refactored, so I can't really test this anymore, but I'm going to go ahead and accept this answer anyhow, because it's the only one I've got, and it _looks_ valid...
Comment: I stumbled upon this a few days ago, having the same problem. It definitely works like a charm!
But it is very interesting to see that this seems to have been touched only very few times and only ONE person has come up with a solution... Well, now it is available in English as well.


How do I learn how to write QA test scripts for software products?

Question: Writing Quality Assurance course test scripts for software products is a critical part of ensuring the quality and reliability of the software. Test scripts are sets of instructions that QA testers follow to verify that the software functions correctly and meets its requirements.
Comment: This is rather broad and I'm not really sure what you're asking about here. Can you narrow the question down to specific technologies or domains? Thanks. QA test scripts for web apps in Playwright is totally different than desktop or mobile QA test scripts.
Comment: You can't prove a negative: you can not prove the absence of bugs. You can reduce bugs, but doing it at a late stage is too late. The software devs need to adopt practices that reduce defects. They need to test their own code. Also, scripts should be automated, humans should be un-scripted using intuition.
Comment: Normally in software we just jump in and flail around until we get better at it.
Answer: Think like the user from hell.

Check that it dose what it should. (knowledge of the spec is needed for this one)
Check that you can achieve the goals of a user, easily. (user insight is needed for this one)
Check that it does not break, including giving wrong results (some sort of insight of what can go wrong in the software/hardware is useful for this one).

Depending on which of the roles that you will be doing. You may benefit from:

Legal knowledge: it does not do what you want, but it meets the spec, so hand over the moneys.
Business/user experience, that is having a deep knowledge of the users of the system. What task are they trying to get done. What is the easiest way to get it done.
Programming experience. I would leave these tests to the programmers.


Sufficient number of sample to learn Bayesian network?

Question: I want to construct Bayesian network for a 800  genes(genes are my node/variables). I have only 30 cancer samples and 30 normal sample. So I want to create network for cancer samples and for the normal samples
whether my data is reasonable to learn Bayesian network?
Comment: You could learn a network with 60 samples, but the quality may not be that good. You could use the PC algorithm by Spirtes et al. (or something more advanced like the MMHC algorithm by Tsamardinos et al.) with the Pearson partial correlation test. Consider setting the maximum conditioning size to 1 or 2, as you do not have many samples.
Comment: @George: it turns out that the PC algorithm is unstable in high dimensions - you get very different results just by permuting the order of variables in your data table. Colombo & Maathuis have a modification called the [PC-Stable algorithm](http://arxiv.org/pdf/1211.3295.pdf), which almost completely eliminates this order-dependence. For an 800-gene network I would definitely use PC-stable rather than PC.
Comment: @LizzieSilver You are of course correct. I didn't think about it when I wrote the comment. I think that MMHC does not have this problem (i.e. it is invariant under permutation of the variables), but I'm not 100% sure. Another way to avoid the stability problem is to use a score-based method. However, for those specific data, I wouldn't try to learn a Bayesian network at all, as, apart from the small sample size, latent confounders and violations of faithfulness (cycles) probably exist.
Answer: I strongly recommend you read up on k-dependent bayesian networks.  You need O(2^K) examples where K is the maximum number of nodes on the bayesian network that can be connected to another node.  So in other words, the more dependencies you consider between your features, very very rapidly you need more examples.  So a full bayesian network for 800 genes means you need 2^800 examples - astronomical.
Nevertheless you could consider only connecting considerably less genes.  The way you would do this is by using information theoretic clustering algorithms.
If you only have 30 examples, then I'd suggest only considering pairs, or triples of genes being dependent.
Furthermore if you have such few examples, and (as most people do) your going to use Maximum Likelihood + some kind of Laplacian Smoothing for probability estimation, then your probabilities are going to be way out and quite meaningless.  Maximum likelihood only starts making sense when you have 100s of examples.
Comment: Can you provide some references on k-dependent bayesian networks and/or provide justification for your explanation of the number of samples required?


what speed was the "negative 4G inverted dive" in Top Gun

Question: I realise that the "negative 4G inverted dive" described in Top Gun is a hypothetical manoeuvre, but I was wondering what sort of speed they would be flying at. I'm guessing low subsonic Mach 0.3, but I have no idea really. I guess the 4G force, depends on radius of the vertical loop and the speed, so different radii would give different speeds. The aircraft were theoretically an F-14 Tomcat (top) and F-5 Tiger (below). I am interested in doing some CFD analysis of the 2 aircraft in close proximity, so it would be useful to have a rough estimated of the flight speed. Thanks.
Comment: No, this is clearly about the original Top Gun movie, not Maverick.
Comment: I think you're likely referring to "Top Gun: Maverick"; if so, you may want to edit your question to specify that movie.
Answer: The maneuver described is so non-sensical that I don't think knowing an airspeed range would help.  In fact I don't think you could properly model it at all...
Start by asking yourself what the words "negative 4G inverted dive" actually mean, compare them to the movie footage of the alleged event, and then ask yourself some questions about what the aircraft was actually doing.
For example:

If you were inverted and pushing negative 4Gs, would you be in a dive?
If you were inverted, canopy to canopy with another aircraft that was upright, (as depicted in the scene) and the other plane was pushing negative 4Gs in a dive, would your G loading be positive or negative?
Starting from level, upright or inverted, if you entered a dive at 4Gs, how long might you be able to sustain 4Gs before it becomes an "Split-S"?
How much might your airspeed change during such a maneuver, and how would that affect your calculations?

And then look at the (phony) footage - what are they actually doing?  Really just flying along in close formation, canopy to canopy.  Maverick is inverted, but it doesn't look like they are pulling any Gs at all really, certainly the maneuver is gentle enough to leisurely snap a photo.
Even if the dive wasn't intended to be shown in the movie and happened just after Goose took the photo, Maverick's description of the event doesn't even make sense to me.  My advice would be to not waste any time trying to reconcile these Hollywood anomalies with reality.
But, if the airspeed parameter is the only thing holding you back from doing something you find interesting, I would suggest you start the maneuver at around 300KIAS, and presume that by the time the plane reaches 90 degrees nose down, they would be going at least 500.
P.S.  If you need any help answering the bulleted questions above please mention it in the comments.


Does installing JSS package on sitecore modify database schema

Question: I am planning to install JSS package on my sitecore instance to use GraphQL APIs. I want to know wether installing JSS package change/alter my database schema or solr indexes. In case I want to uninstall the JSS package later, will it be a clean uninstall or it will impact my db also?
Answer: JSS does not alter the SQL database schema. Sitecore modules rarely, if ever, do. It does install a number of items and alters permissions on <code>/sitecore/content</code>. It does not directly alter the Solr indexes, but the managed schema may be updated based on the additional templates and items included in the JSS module.


Almost sure convergence for measurability

Question: So it is pretty obvious that the limit of measurable random variables is also measurable if they converge...but I'm not sure it's true of almost sure convergence.  Suppose there is a probability space over three states $\omega_1, \omega_2, \omega_3$, but with $\mathbb{P}[\omega_1] = 1$ and $\mathbb{P}[\omega_2]=\mathbb{P}[\omega_3] = 0$.  Then define $X_n$ to be the random variable where $X_n(\omega_1) = 1$, $X_n(\omega_2) = 1$, and $X_n(\omega_3) = 1$, which converges almost surely to $X$ where $X(\omega_1) = 1$, $X(\omega_2) = X(\omega_3) = 2$.   Let $\mathcal{F}$ be the trivial $\sigma$-algebra.  Then $X_n$ is $\mathcal{F}$-measurable but $X$ is not...so how come so many textbooks say that $X$ must also be $\mathcal{F}$-measurable, even from just almost sure convergence?
Comment: it's measurable wrt to the sigma field of the measurable sets.
Comment: It's not though because $\{\omega \in \Omega | X(\omega) \le 1\} = \{\omega_1\}$ which is not in $\{\Omega, \emptyset\}$.
Comment: the trivial sigma field is not the field of measurable sets. all sets of measure $0$ are always measurable. observe there's no hope in obtaining measurability for an arbitrary sigma field since the notion of convergence you have in mind (almost sure) is directly related to the particular measure you're using, hence also to the sigma field associated to it
Answer: Random variables are not really
functions on the probability space, but rather equivalence classes of such functions, where two functions are deemed equivalent if they are equal almost surely.  So while the function $X$ is not $\mathcal F$-measurable, it is equal
almost surely to one that is.


How to find serial number of Solaris server?

Question: Is there any command which will be available on Sun Solaris Netra T5220 server, to get the machine's serial number?
Comment: Specify Which series of server are you looking for.
Comment: Which series `T` series, `M` Series  or any other.
Comment: our solaris server is  5.10
Comment: our server is T series
Comment: Please use following command `prtdiag -v`
Answer: As Yascan stated, prtdiag works well, if you need to obtain this info from the OS and cant get or dont have sneep then I would try the following
I use this is in my script
    # prtdiag -v | awk '/Chassis Serial/{getline; getline; print}'

get a different look
    # prtdiag -v | awk '/Chassis Serial/{print; getline; print; getline; print}'

Answer: Login to ALOM and do the following and run the below command.
sc> showplatform
SUNW,SPARC-Enterprise-T5120
Chassis Serial Number: BEL00011GA

Comment: i have also checked... but it doesnt match with serial number of back door of server
Comment: There is no command which can give you the serial number. you have to check it physically on the server
Answer: There is not -- unless you manually put it into the EEPROM using sneep:

Oracle Serial Number in EEPROM (SNEEP) provides a software-accessible
Chassis Serial Number (CSN) for all Oracle Solaris hardware platforms.
SNEEP uses the system EEPROM for persistent storage of the Chassis
Serial Number and other important user-defined data such as asset
information, contract ID, or the serial numbers of attached storage
devices.
The presence of the software-accessible serial number and other
service-related information can significantly simplify activities
related to system service and asset management.
Without SNEEP, only a subset of the Solaris-based hardware platforms
have a mechanism to maintain a software-accessible serial number.
Among those platforms, there is a wide variety of mechanisms for this,
making consistent access to this information difficult.
SNEEP provides one simple and consistent interface to the management
of this information on all Solaris hosts, domains, and zones. SNEEP
can also reference and maintain the serial number in the configuration
files for the Oracle Explorer Data Collector.

...

If your hardware includes the serial number in its EEPROM, then you can view it with the following command:

sneep -t hostname,serial,model

If your hardware does not already store the serial number in EEPROM, then you can use SNEEP to include it.
You will need to first obtain the serial number and manually enter it into EEPROM.

(my emphasis)

On most Sun hardware, the serial number is always located on a physical tag somewhere on the machine. The Sun System Handbook tells exactly where to find it for all Sun machines. The handbook can be accessed at My Oracle Support (click the Knowledge tab and then select Sun System Handbook in the left-hand navigation): https://support.oracle.com

...

WARNING:
The decimal number reported in the SPARC boot banner as the serial number when the machine is powered on is NOT the true serial number - it is the decimal form of the "host id". The host id is ordinarily seen in hexadecimal as the output of the Solaris hostid command.
Answer: As noted by Jeff, sneep is a useful tool to track the serial # on older hardware as well as from the OS.  Most of the newer hardware, you can successfully get the serial number via the ILOM or XSCF.
And when in doubt, not only have your property team track it, but also the team in charge of the server maintenance as you'll need it for renewals and opening cases with Oracle.
Answer: cfgadm -alv | grep SN works on my Solaris installation:
# cfgadm -alv | grep SN
sata1/0::dsk/c3t0d0            connected    configured   ok         Mod: WDC WD100EFAX-68LHPN0 FRev: 83.H0A83 SN: 1SH5Y67Z
sata1/1::dsk/c3t1d0            connected    configured   ok         Mod: WDC WD100EFAX-68LHPN0 FRev: 83.H0A83 SN: 1SH6LBAZ
sata1/2::dsk/c3t2d0            connected    configured   ok         Mod: WDC WD100EFAX-68LHPN0 FRev: 83.H0A83 SN: 1SHJKKXZ
sata1/3::dsk/c3t3d0            connected    configured   ok         Mod: WDC WD100EFAX-68LHPN0 FRev: 83.H0A83 SN: 1DGP5ETZ


Wingdings font in Firefox 3.5?

Question: How can I display Wingdings characters in Firefox 3.5?
<code>http://www.purplehell.com/riddletools/wingdingschart.htm
</code>
This page will render the special characters in IE6 but does not show the correct symbols in Firefox 3.5.
Comment: In what OS are you having the problems?
Comment: Tried both Mac OS 10.5 and OpenSolaris.
Answer: Firefox deliberately does not support webdings and wingdings.  See this "bug".  "Specifying Wingdings font is contrary to the published specifications, has never been a documented feature of HTML, is not reliable, and should not be done", according to this, which explains that you should be using unicode characters instead.
Answer: Though the question is about Dingbats, not Wingdings: maybe the note on using "Arial Unicode MS" at Get Dingbats to appear in Firefox 3? can help.

Wingdings font in Firefox 3.5?


Wifi problem. Internet stops working after few minutes of connection

Question: I am using ubuntu 16. Whenever I am connected to a WiFi,  Internet stops working after few minutes, even when the WiFi shows connection.
I have to disconnect and then connect again to that WiFi.(Internet starts working)
This process keeps on coming after every 10-15 minutes.
After running network diagnostic test my result is,  paste.ubuntu.com/24373253
Comment: Welcome to AskUbuntu! Could you please [run the network diagnostics](//askubuntu.com/a/425205/175814) and [edit] your question to include a link to the result? I know it may seem a bit overwhelming for a novice but your info so far is a bit vague and the diagnostics will likely cover all options that may cause your issue. Thanks.
Comment: Hi, did you check if anything changed from when it was working to when it stopped? For example, check your IP, Gateway and DNS when it's working and when it stop working and see if they are different. Also, please do some tests likes trying to ping the gateway, google dns etc.
Does it happens only on wifi or wired connection as well? All wifi or a particular one? Thanks
Comment: I have added link to the result of running network diagnostic test. Please check.
Answer: For me this ugly workaround does the job, until I find a better solution:
Try the following command when the connection drops next time:
sudo ip link set <your device> down && sudo ip link set <your device> up

If the connection comes back add this command to crontab:
sudo crontab -e

Than add this line:
  * *  *   *   *     ip link set <your device> down && ip link set <your device> up


Can i use the name of a real UK Prime Minister in a fictional Novel?

Question: I am writing a fictional novel that rewrites history surrounding the Gulf war. It involves at one point the actions of the english Prime Minister at the time. In the novel he takes us to war with Iraq under false pretences (some would say that was the case anyway) however my explanation as to why he invades Iraq is different from the accepted reasons, and some might consider it to be unjustified. Am I on dodgy ground here? Could I be done for libelling the PM? Can i mention his name, and if not, will changing his name take away from the realism of the novel? Or should I just not mention his name even though everyone will know who I am talking about.
Comment: Not a lawyer so can't answer the question, but I can make a suggestion: You could go for a slight skew of perspective so that the story is never written from his perspective, but instead from the perspective of an advisor who always refers to him as "the Prime Minister" or "Mister Prime Minister" or "Sir".
Comment: I think you should put your question in [this](http://area51.stackexchange.com/proposals/48901/legal) Area51 Stackexchange proposal. It is about legal matters.
Comment: Seems like a duplicat of this: http://writers.stackexchange.com/q/9215/1993
Comment: There are no lawyers that I know of on the site...we should look into retaining a publishing lawyer.  That said, I have never heard of issues resulting from the use of a public official's persona in a work of revisionist fiction...at least not in the US/UK/Western Europe...
Answer: If you just mention that e.g. Tony Blair was prime minister of the UK at the time of your fictional events, then that is similar to stating that there is a city called London. It will be no problem at all.
If you describe Tony Blair doing what all prime ministers do (e.g. be friendly to a journalist) or what is a historical, documented, published and proven fact (and you mention this documentation and give your sources in an appendix), there will be no problem at all. Just make sure you quote the source correctly: if the source says "it might have been", you must describe the events with the same possibility of doubt.
But as soon as you have any real and existing person do anything in any kind of fictional work that either you have made up or that may be interpreted as reflecting negatively on that person, from picking his nose to starting a war, you will most certainly recieve a letter from that person's lawyer.
I would never do it. The newspapers are full of lawsuits for journalists mentioning the names of the children of celebrities, stating that a celebrity dyed their hair (German chancellor Gerhard Schröder sued a newspaper over this) and similar apparent banalities. I can't imagine the backlash you will face when you imply a politician might have lied to the public.
If the person is dead, go ahead, but as long as he is alive, make up an imaginary prime minster, just like American movies always have fake presidents in them.
If your whole idea hangs on working with something that everybody knows to be true but has never been legally proven, talk to a lawyer experienced in the relevant field (journalism, personal rights, rights over one's own image, slander, etc.) and pay for legal advice that you can rely on (and never trust anything written by anonymous persons on the web).
Answer:
I am writing a fictional novel that rewrites history surrounding the Gulf war ... Could I be done for libelling the PM?

Possibly.  The laws about slander and libel vary widely from nation to nation, and a fig leaf of "it's fictional" would probably not be enough in and of itself to survive a challenge.
You're better off making a thinly veiled analog with a different name and significantly different characteristics, especially if your novel's PM is at all villainous in any way.  If omitting all personally identifiable details can be done, that may also suffice.
Also, in addition to a possible claim of libel, be aware that you might be hit with trademark or privacy laws as well.  Make a novelization about a news story of some living person's heroics, and you may be presented with a bill for using their likeness.

In general, you shouldn't worry too much about legal affairs (aside from copyright) when writing.  Once you have a completed work and want to move into publishing is the time you'll want to worry about what changes may be necessary to comply with libel, trademark, or privacy laws.
Comment: I like this answer.  Write what you want.  If you get to the point of publishing then you can swap out fake names/personas etc if necessary.
Answer: I agree with what others have said, both here and in the above-referenced question (which I reproduce here: writers.stackexchange.com/q/9215/1993).  To wit (always wanted to say that), change the person.  In your specific case, start your novel with Tony Blair losing the election.  Instead, the PM is Snidely Beckinsworth.  You can then make Snidely whatever you want -- egotistic, money-grubbing, cowardly, sadistic, stupid, war-mongering, etc. -- and nobody can complain.  Snidely is your stand-in for Tony (with cosmetic details changed), but you are covered because your novel already makes clear that Snidely is not Tony.  I suppose Mr. Blair could still try to sue for libel, but then he'd have to prove that, despite the internal evidence of your novel, your unflattering PM is obviously him.  (But that's self-defeating.  If it's obviously him, then it isn't libel.)  The free publicity from such a lawsuit would easily pay for a five-quid barrister.  Just don't make Snidely exactly like Tony, except that he's a lying jerk, because that WOULD get you into trouble.


What is the significance of return 0 in C and C++?

Question: I need the simplest possible answer to this. I have looked up in various sites for the answer. If the inputs to a programme is such that the output is 10, will command return 0 force the programme to return the value 0 instead of 10? I have written simple C programmes on Borland IDE without return 0 and they work fine.
Comment: [Here you'll find a good explanation](http://stackoverflow.com/questions/204476/what-should-main-return-in-c-and-c).
Answer: Nothing to add to Maroun's link. That's the way if you want to know more about return values.
Just a few words for a novice:
int main()
{
  return 0;
}

and
int main()
{
  return 22;
}

and
void main()
{

}

will all behave the same way if you're not checking the return value for a program and compile on most default-set compilers.
When you execute a program on the command line (windows) or shell (linux), you can check whether that program failed or completed its tasks successfully by checking the return value, that is, exactly that value you're returning.
Following the standard is a good habit by the way, and I therefore strongly recommend it to you.
Answer: I assume you're talking about the main() function: If you return anything else than 0 (which is the same as omitting return altogether) this is assumed to be an error code, and can be used to notify other programs that the current program failed to do what it is supposed to do.
Answer: The main function is generally supposed to return a value and after it returns something it finishes execution.The return 0 means success and returning a non-zero number means failure. Thus we "return 0" at the end of main function. But you can run the main function without the return 0.It works the same . Sources :
What is the proper declaration of main?
Answer: The return code of the main function is passed back to the operating system. What it does with this depends. If you execute a program on the commandline you can simply ignore it. In scripts this returncode can be used to determine how to proceed.
If you use void main() it will return 0.
Here is an example for DOS:
rem test.bat
@echo off
MainReturnValTest
@if "%ERRORLEVEL%" == "0" goto good
:fail
    echo Execution Failed
    echo return value = %ERRORLEVEL%
    goto end

:good
    echo Execution succeeded
    echo Return value = %ERRORLEVEL%
    goto end

:end

The same can of course be done on other OSes using different syntax. For example in Linux you could use:
myprogram && echo success

which would only print success if the returncode was 0.
The operation of your program is not affected of course.
Answer:
The return value has nothing to do with your Output value . That is, even if your output is 10, you return 0.
Every command returns an exit status (sometimes referred to as a return status or exit code). A successful command returns a 0, while an unsuccessful one returns a non-zero value that usually can be interpreted as an error code. Well-behaved UNIX commands, programs, and utilities return a 0 exit code upon successful completion, though there are some exceptions.


Why does my van shudder vibrate only after 20mph and only in gear?

Question: 1979 austin/leyland sherpa petrol o series engine.
At about 20mph my van starts to shudder vibrate quite badly and gets progressively worse if I dare go at a higher speed.  The thing is it only happens when the van is in drive. If I free wheel it disappears completely.  The problem appeared all of a sudden like something had broken rather than progressively worn.  It was so sudden I thought it was the quality of the road. It seems like a suspension problem like I'm driving over a continuous series of ribs in the road except the fact it goes when free wheeling rules out a suspension problem, no?
Comment: Loosing a balance weight off the propshaft could cause this.
Answer: A failed engine mount could possibly cause this symptom.
You could get someone to watch the engine as you select drive and reverse while stationary with the brakes on.  If the engine moves excessively then it is likely an engine mount.  Make sure the spectator stands to the side of the vehicle, not in front of it.