What does Tuco throw away in "The Good, The Bad, and The Ugly" when they reach the graveyard?

Question: Right when "The Ecstasy of Gold" starts to play, after he's done being shot with the cannon (really weird, by the way, that they camped out only 100 yards from the graveyard and yet he tried to steal a horse to go to it) he jumps for joy and then throws a bunch of papers away.  What were the papers?
Comment: I don't remember it very well, but can it just be the map or instructions to get there that Tuco has written down? Or maybe a map he used to find the graveyard he only had a name of?
Comment: I don't think he ever wrote down a map or instructions - remember he was trying to hide the secret from Blondie earlier.
Answer: It's a map, either hand drawn or official. The map could be coded in such a way that only Tuco understands it.
Earlier on before the bridge explosion and the drunk captain, Tuco pulls it out looks at it and says to Blondie,

There should be a bridge across that river.

Also the way he folds it when he tells Blondie to trust him indicates that the paper is very large so it's not notes. The only type of document you fold that way is a map.
The scene where he throws it away indicates that he no longer needs the map as he is at Sad Hill Cemetery. Blondie told him the name of the grave was Arch Stanton as well.
Remember Tuco knows the location of the cemetery, Blondie knows the name of the grave.
Comment: It could just have been a standard official map of the area, that Tuco needed since he only knew the name of the cemetery but not where it is. It wouldn't be of much use to Blondie without knowing the name to look for. But on the other hand I don't know how fast the cartographers were at that time, since the cemetery should be rather new, as there lie fallen soldiers from the civil war, which they were still in and which lasted only 5 years at a whole. But well, the cemetery could just have had the name of a nearby town.
Comment: It was a map but Tuco would not have written the destination down.  He remembered the destination and just used the map to find it.
Comment: I'm surprised Blondie didn't just take the map from him if it was so obvious!
Comment: @AndrewLatham Blondie most likely would not have been able to decipher it. It could have been coded in some way that only Tuco understands
Comment: I've watched this again, and get the impression that Tuco was fairly illiterate. Even if it was a map, it probably went unlabeled except in a form only Tuco would be able to understand. Stealing it would probably do no good to anyone.
Comment: @Stefan is right.  Earlier in the movie, when leaving the monastery they show the map.  It is an official printed map of the region.  Twice, Tuco is shown using it for navigation.  No way he marks their destination on it!
Comment: "*They emerged from a stretch of open woods and stopped short. The bridge Tuco had remembered was there—just beyond the bend—but nothing on his map had indicated that now it was guarded by Union pickets.* - In the novelisation he doesn't throw it away, but we know it's what he was consulting


Can't install Ubuntu on a Mac mini 2018

Question: When I boot Ubuntu from a USB key, I do access the Live Ubuntu but Ubuntu can't see the internal Mac mini 2018 disk even though it detects my extra external hard drive. So, it is impossible so far for me to install Ubuntu as a standalone OS. I already remove the T2 chip security and the SIP security (not sure what it is).
Do I have to trash my Mac mini on an Apple store bay (it is quite tough ...) or to burn it ? Or some voodoo ?
Comment: Does the Mac Mini still have MacOS installed on it with the APFS file system?
Comment: Yes, MacOs is still installed on it and with the APFS
Answer: You can't see the internal NVMe because the linux kernel requires custom patch for T2-chip based macs.
This patch is originally for MBP 2016+ but it must work fine with the Macmini 2018.
Patches for arch linux kernel 5.3.5
Original discussion, contains also patches
PS. It has nothing to do with APFS.
Answer: The USB installer doesn't recognize the disk because Ubuntu can't read the APFS filesystem. Your only options are to use recovery mode to erase the filesystem from the disk so that Ubuntu can read it or install Ubuntu on a USB drive and build the apfs-fuse driver from source (you can get it via github) so that Ubuntu can read and format the disk. I recommend the first option as it's far easier.
After that, your Ubuntu installer will see the disk and you can proceed.
Comment: Nope, it has nothing to do with APFS. Custom kernel patching is required to access the internal NVMe drive on new macs

Can't install Ubuntu on a Mac mini 2018


Olympiad calculus problem

Question: This problem is from a qualifying round in a Colombian math Olympiad, I thought some time about it but didn't make any progress. It is as follows.
Given a continuous function $f : [0,1] \to \mathbb{R}$ such that $$\int_0^1{f(x)\, dx} = 0$$ Prove that there exists $c \in (0,1) $ such that $$\int_0^c{xf(x) \, dx} = 0$$
I will appreciate any help with it.
Comment: Where is the $c$ being used?
Comment: @moron: I had a typo, I already corrected it
Comment: @Theo Buehler:  Bad misconception on my part.  Thanks for the heads up.  I realized I was thinking of analytic functions on compact sets, rather than continuous functions.
Answer: [Major edit to make the second part of the proof a lemma]
Define:
$$F(x) = \int_0^x{f(t)\, dt}$$
Then $F(0)=F(1)=0$, and $F'(x)=f(x)$
Integrating by parts, we see that:
$$\int_0^t{xf(x) \, dx} = t F(t) - \int_0^t{F(x)\, dx}$$
To prove the theorem, we must find a $c \in (0,1)$ such that:
$$ F(c) = {\frac{1}{c}}\int_0^c{F(x)\, dx}$$
This is shown with via the following:
Lemma: If $F$ is a continuous function on $[0,1]$ such that $F(0)=F(1)$ then there is a $c\in (0,1)$ such that:
$$ F(c) = {\frac{1}{c}}\int_0^c{F(x)\, dx}$$
Proof:
Define:
$$G(t) = {\frac{1}{t}}\int_0^t{F(x)\, dx}$$
$G(t)$ is continuous and defined on $[0,1]$ (the value at $t=0$ is defined as the limit, and is just $F(0)$ by the fundamental theorem of calculus.)
$G(t)$ is the average of $F(x)$ for $x\in(0,t)$, so $G(t)$ must have as an upper bound the upper bound of $F$.  That is, for all $t\in [0,1]$, $G(t)\leq \operatorname{max}_{x\in[0,1]} F(x).$
Since $F$ is continuous, there must be an $x_M \in [0,1]$ such that $F(x_M)= \operatorname{max}_{x\in[0,1]} F(x)$.  Then, setting $t=x_M$, we see that:
$$G(x_M)\leq \operatorname{max}_{x\in[0,1]} F(x) = F(x_M)$$
Similarly, we have that $G(t)\geq \operatorname{min} F(x)$, and thus, when $F(x_m) = \operatorname{min}_{x\in[0,1]} F(x)$, we have $G(x_m)\geq F(x_m)$.
So the continuous function $H(x)=G(x)-F(x)$ has the property that $H(x_M)\leq 0$ and $H(x_m)\geq 0$.  By the intermediate value theorem, there must be a $c$ between $x_m$ and $x_M$,  inclusive, such that $H(c)=0$, and hence $F(c)=G(c)$.
If we can find $x_m$ and $x_M$ in $(0,1)$ - that is, not on the boundary - then we know that $c\in (0,1)$ and we are done.
If both $x_m$ and $x_M$ are on the boundary, then the maximum and minimum of $F$ are equal, and thus $F$ is constant, and hence we can choose any $c$.
So assume that $F(0)=F(1)$ is the minimum value for $F$, and that $F$ does not take the minimum value anywhere else on $[0,1]$.  Then  $F(x)>F(1)$ for all $x\in(0,1)$, so we know that $G(1)>F(1)$.  So $H(1)>0$ and $H(x_M)\leq0$.  Hence, there must be a $c$ in $[x_M,1]$ with $H(c)=0.$  But $c\neq 1$, so we know that $c \in [x_M,1)\subset(0,1)$.
Comment: Very nice. (You added a little typo, though: $F(1)=0$).
Comment: @user9325 Thanks..  Fixed typo.
Comment: I did not understand the very last sentence. On which two points do you use the intermediate value theorem?
Comment: @user9325 Clarified my answer.
Answer: This is a streamlined version of Thomas Andrews' proof:
Put $F(x):=\int_0^x f(t)dt$ and consider the auxiliary function $\phi(x)={1\over x}\int_0^x F(t)dt$. Then $\phi(0)=0$, $\ \phi(1)=\int_0^1 F(t)dt=:\alpha$, and by partial integration one obtains
$$\phi'(x)=-{1\over x^2}\int_0^xF(t)dt +{1\over x}F(x)={1\over x^2}\int_0^x t f(t)dt\ .$$
The mean value theorem provides a $\xi\in(0,1)$ with $\phi'(\xi)=\alpha$. If $\alpha$ happens to be $0$ we are done. Otherwise we invoke $F(1)=0$ and conclude that $\phi'(1)=-\alpha$. It follows that there is a $\xi'\in(\xi,1)$ with $\phi'(\xi')=0$.
Comment: That's very slick!  I think it might be a case where the slickness obscures what's going on, but slick has its own aesthetic merits.
Comment: @Christian $\phi(0)$ is defined?
Comment: @Lucas: You have $|F(t)|\leq M t$ $(t\geq0)$ for some $M$ and therefore $\left|\int_0^x F(t)\>dt\right|\leq{M\over2} x^2$. This implies $\lim_{x\to0+}\phi(x)=0$.
Answer: Let $G(x) = \int_{0}^{x} t f(t) \, \mathrm{d}t$, and suppose that $G(c) \neq 0$ for all $c \in (0, 1)$. Then by IVT, either

(Case 1) $G > 0$ on $(0, 1)$, or
(Case 2) $G < 0$ on $(0, 1)$.

However,
\begin{align*}
0
= \int_{0}^{1} f(x) \, \mathrm{d}x
= \int_{0}^{1} \frac{G'(x)}{x} \, \mathrm{d}x
= G(1) + \int_{0}^{1} \frac{G(x)}{x^2} \, \mathrm{d}x.
\end{align*}
This is $ > 0 $ in Case 1 and $ < 0$ in Case 2, a contradiction.
Answer: The problem has originally been asked in the 2006 Romanian National Olympiad.

Define a function $\mathcal{G}(y)=y\int_0^{y}f(x)dx-\int_0^{y}xf(x)dx$
Using chain rule and Leibnitz rule on differentiation under integral sign we get $\mathcal{G}\prime(y)=\int_0^yf(x)dx$
On inspection we get that $\mathcal{G}\prime(1)=\mathcal{G}\prime(0)=0.$
Now we will use Flett's variant of LMVT that for some $c\in(0,1)$ $\quad\mathcal{G}\prime(c)=\frac{\mathcal{G}(c)-\mathcal{G}(0)}{c}$
$\implies c\int_0^cf(x)dx=c\int_0^{c}f(x)dx-\int_0^{c}xf(x)dx\implies\int_{0}^{c}xf(x)dx=0.$

Olympiad calculus problem


Reference request: Measure Theory and Probability Theory

Question: Yes, I know that this question has been asked before, but let me explain why I am asking again.
I have had the experience of taking a measure theory course using the book by Donald Cohn on the subject, and the exercises were tough.
Let me explain some features of a measure theory book I would be interested in:

exercises of various levels of difficulty (if possible, in sorted
order or marked to indicate difficulty level)
a solutions manual to (at least some of) these exercises
would like the book to explain some of the tricks used in harder analysis problems
would like the book to have good exposition that helps to build intuition
would like the book to have most of the standard material on measure theory required for graduate mathematics

I don't claim that these criteria are anything more than vague, but I hope that they give a sense of what I'm looking for. For example, I liked Aluffi's Algebra book, because it has many of these feature with out sacrificing too much rigor/material needed in a graduate textbook. If possible, it'd be great for this book to cover the needed material on measure theory required for graduate mathematics. I mention Probability Theory in the title as well, on the off chance that you happen to know of a book that does these things for Probability Theory.
Answer: I recommend for
Measure Theory:
Real analysis, Gerald Folland.
An Introduction to measure theory, Terence Tao.
Probability Theory (first course):
Probability theory and examples, Rick Durrett.
Probability with martingales, David Williams.


How to undo autocorrect in libreoffice calc

Question: I would like to prevent libreoffice calc to make autocorrecion in some cases in some cell. For example, sometimes libreoffice decides that some text I enter is a hyperlink and it formats the cell as a hyperlink. How I can undo this autoformatting case? (When I press Ctrl+Z, calc removes my entered text). I do not need completely turn off autocorrection in libreoffice calc.
Comment: Hi Loom, it appears that you have two very similar questions which have slightly different scopes and information. Perhaps you could merge the two and delete one or the other? (cf http://superuser.com/questions/936904/switch-off-autocorrect-in-libreoffice-calc-for-specific-document-column-cell)
Comment: Hi @bertieb. Yes, answers for both questions are for solving one problem. However, answers for both questions are differ, so I couldn't select right  answer for the suggested merged question
Comment: Perhaps I'm missing something, neither of these questions have answers at time of writing- is there another set of questions you are referring to?
Comment: It's looks my English obstructs my attempts to explain) I meant, that if each of my questions would have an answer, then these answers would differ. I would like to know answers for both questions. If I merged my questions I could lose one of the answers.
Comment: I understand now. In my opinion, the questions are similar enough that you could ask both in one post (as two bullet points), but you are entitled to your opinion as well.
Answer: I think there's currently no way of doing this cleanly, here's a workaround:
Assuming you have entered e.g. http://test.com into a cell, and it got auto-formatted as a hyperlink.

Click the cell so that it becomes active (make sure it has indeed become active, because clicking the hyperlink itself will do nothing)
Select the text in the formula bar
Cut it
Press Enter (thus getting empty cell)
Press Up (or click the cell)
Press Ctrl+Shift+V to call Paste Special
In the Paste Special dialog which appears click OK, making sure "Unformatted text" is selected in the list

Now you should have the link pasted without any special formatting.


A word that means you need to be better or do better but not necessarily better than others

Question: I am looking for a word or phrase that means that you have a feeling or need to improve or enhance yourself just for the sake of it.
Answer: I think "self-improvement" conveys that pretty well. "Self-enhancement", as well. To express the "need for" element you could add "drive" to the end of either one of those hyphenated alteratives.
E.g., "Joe has an incredible self-improvement drive; he's always challenging himself and doing something new. Unfortunately, he's really pedantic and goes around saying, 'I'm not in competition with anybody but myself.' All the time; it never stops".
And, by the by, "social comparison" is what we say in psychology for process of gauging how well you're doing using others as a measuring stick.
Answer: Your question is vague, but the phrase "for my own edification" comes to mind.  The gist is, "tell me for no other reason than to allow me, and only me, to improve myself."
edification -

If something is for your edification, watch out! It means it's designed to improve you in some way, whether morally, educationally, or spiritually.
Comment: Sorry about that. Could not get a clear thought of what I meant but thanks for the help.
Answer: Self-growth as an inner experience.
Comment: Synonims : autodidacticism (self-learning).
Answer: Excel, e.g. She likes to excel in everything she does.
Get ahead in the world, e.g. As a young man, So-and-So had a real drive to get ahead in the world.  (Note, this is a pretty vague expression, but I think it relates more to business than to academics.)
Do your best, e.g. I'm not overly perfectionistic or anything, but I do like to do my best.


Excel Auto-populate

Question: I have 40 employees in an Excel spreadsheet 1 in Column A, Rows 2-41.
Column Heading is Employee's Name (in A1).
In Column B, I have a place for his Wage/Hour, but this Column is blank.
I need a formula to take from spreadsheet 2 (having all the employee's name and wages in Columns A and B), their actual wages which are in Column B to auto-populate their wages into spreadsheet 1 column B when I type their name in Column A of spreadsheet 1.
If this can work, I will delete all of the names in Column A, spreadsheet 1. Once it is determined which employees worked on a special project, I will type their name in spreadsheet 1, Column A, and expect that his wages will populate from spreadsheet 2 Column B, into Spreadsheet 1, Column B.
Comment: Please add some relevant linebreaks to improve the readability of your question ...
Answer: vlookup("cell with name", "lookup array name and wage", 2, false)
Not sure if your lookup array is a master table containing just latest wage rate or a transaction table containing wage data for every pay period.
Also consider using data validation to create a pulldown list of names so you don't misspell.

Excel Auto-populate


"This form cannot be opened in a web browser. To open this form, use MS Infopath"...why am I getting this message?

Question: I'm creating a simple out of the box workflow in SP2010 for a calendar.  Content Type is ALL, Approval - SharePoint 2010.  Then when I click continue.  I receive the "This form cannot be opened in a web browser.  To open this form, use MS InfoPath".  I didn't use InfoPath.  Has anyone else experienced this?  I've searched all the solutions are individuals using InfoPath.  Is there something else that's causing this?
Comment: did you used Nintex workflow or form?
Comment: @MSallal, I'm using an out of the box workflow.  However, further investigation from the customer, the customer accidently deleted the existing workflow.  Now I do see Nintex on SiteActions option so it might have been a Nintex form?
Comment: the easiest way is to install info path and open it using info, so that you can override the form
Answer: This is the known issue in SharePoint out of the box workflow because it sends the item URL directly which only opens in the browser and there is no configuration for out of the box workflow form template.  As an alternate solution you may develop your own workflow to do the same where you can configure the template form as we need.
Here is details answers for the same issue in MSDN forum :
"
That's the problem with using the OOTB workflows. They simply send a link directly to the XML file, but that link isn't what opens the form in the browser.  The browser URL is way longer and much different, so you need to send that instead.  Since you can't modify OOTB workflows, then it's not something you can configure.  If you make your own workflow with SharePoint Designer, then you can dynamically generate the browser URL for the form so that it opens in the browser.  If you want to do that, I can show you quickly how to do it.  If you must stick with the OOTB workflow, then the forms will only open in the browser if the user does not have InfoPath 2007"
Reference URL :
Approval Workflows with Browser-Based InfoPath Forms Not Opening In Browse


Radio button group or Checkbox?

Question: Should I use a radio button or checkbox or something else to indicate which row is the primary selection?
Answer: Radio buttons are correct in this case, unless you can have more than one primary supervisor, in which case, checkboxes are correct. However, if you have more than a few rows, and especially if you're using pagination, it might become tricky to quickly see which supervisor is the primary.
Another way to set this is to add all of the supervisors into a dropdown menu below the table, and let the user select the primary there.
Comment: I suggested that but the dev team said this is easier rather than writing code logic for dropdown lists and disabling items in the dropdown list if one primary is already selected.
Comment: Why would anything in the drop down list need to be disabled? It could just default to whomever was selected as primary if that was previously set. But radio buttons work as well.
Comment: If there's only one primary supervisor, the logic for setting the 'checked' attribute on the radio input element is going to require pretty much the same effort as setting the 'selected' attribute on an option element. If there's multiple supervisors, the added backend logic scales pretty much equally to the frontend code. If you can afford to, try and understand a little better what their actual concerns are. Maybe it's more involved than I can tell from the sideline, but it does have a whiff of a fallacy.
Answer: This blog article has been mentioned on Twitter with regard to when to use a checkbox and when to use a radio button.
In short: If you ask a question use radio buttons, if you want confirmation / selection of a statement use a checkbox.
Answer: This is a very weird edge case actually - good find!
In my opinion, checkbox would be the right choice here. Let me explain:
Technically, this is what is called a radio button group, as @izquierdo is saying. Meaning a bunch of options belonging together, user can only pick one, best practice radio buttons, bob is your uncle. Like:
What is your favorite animal?
 Cat
⭕ Dog
⭕ Horse

The thing is that (at least for me) it is very hard  to perceive the three buttons as a group, because every person seems to be a group in itself, if you know what I mean?
It is almost a multidimensional form you built there: Each person is a unit, but then the is-supervisor question is also a group stretching over all the people. Meaning that if you only perceive the people as their own groups, the three inputs for Primary Supervisor are "pulled apart" and don't feel like a group at all.
And I think asking your user to understand this double-grouping is indeed a big ask.
Instead, what I perceive is a list of people where for each person you are asking whether that guy or gal is the/a "Primary Supervisor". And as @yahalnaut is hinting at, that is a yes/no question, meaning check box. To illustrate my mental model:
Person A, some data, ☐ is this person the Primary Supervisor?
---
Person B, some data, ☒ is this person the Primary Supervisor?
---
Person C, some data, ☐ is this person the Primary Supervisor?

Now you have the extra programming requirement to disable the other two checkboxes whenever a new one is selected (which might feel silly because that is what radio groups do anyways, but is easily implemented), but if my mental model is anything like that of your users, it is better and more quickly understood.
Usertest this!
Answer: You should use checkbox here. The key being radio buttons present set of mutually exclusive options. Checkbox presents - "yes", "no" and optionally "maybe". From Apple design guidelines good read- please visit:

If you need to show that a setting or item has a mixed state, consider using a checkbox instead.

Prefer a set of radio buttons to present mutually exclusive options. If you need to let people choose multiple options in a set, use checkboxes instead.

Avoid listing too many radio buttons in a set. A long list of radio buttons takes up a lot of space in the interface and can be overwhelming. If you need to present more than about five options, consider using a component like a pop-up button instead.

I do not develop for iOS or MacOS but these guidelines are really well written and consistent. I recommend having a read.
Answer: Alright. We did away with check boxes, radio buttons. Instead i'm going to create a table and maybe use a dropdown list/ checkbox to indicate primary supervisor.
Not sure if it's the best approach, but that's what the product said he wanted.

Radio button group or Checkbox?


Does an electron move in a conductor?

Question: The definition of current is flow of electric charge. But recently I have heard that the electrons cannot move, that they just transmit energy to the other electrons and so on.
Answer: Classically, electrons do move in a conductor that is passing direct current – but much more slowly than you might think. Let's break this down:
Current in a wire is defined as the amount of charge that passes through a cross-section of that wire in a single second. By this definition alone, it is clear that a current relies on the motion of some charged particle. It is possible that there could be a system where electrons transfer energy to each other, but in classical terms this would not be considered a "current." However, as I mentioned before, electrons actually move pretty slowly, even in very high-power currents. This might be what you're thinking of – how even very slow-moving electrons transfer a lot of power.
As a matter of interest, let's look at exactly how quickly electrons move. We need a common identity, $I = qnA\overline{v},$ where $q$ is the charge of the charge carrier, $n$ is the number of those particles per unit volume, $A$ is the cross-sectional area of the wire, and $\overline{v}$ is the average speed of these particles. This identity is fairly simple to derive – $q n$ is the charge density per unit volume, and $A \overline{v}$ is the average volume of particles that passes through a cross-section of the wire in a given second, so $q n A \overline{v}$ is the total charge that passes through a cross-section of a wire in a given second, or equivalently the current $I$.
Now, suppose we have a copper wire. Let's make it pretty thick – say, 1cm in diameter. Classically, a current in a copper wire is transmitted by electrons. So the charge of the charge carrier is $q = e = 1.6 \cdot 10^{-19}$ C.
To find $n$, we note that, from Wikipedia, "Copper has a density of $8.94$ g/cm$^3$, and an atomic weight of $63.546$ g/mol, so there are $140685.5$ mol/m$^3$. In 1 mole of any element there are $6.02\cdot 10^{23}$ atoms (Avogadro's constant). Therefore in $1$ m$^3$ of copper there are about $8.5 \cdot 10^{28}$ atoms ($6.02 \cdot 10^{23} \cdot 140685.5$ mol/m$^3$). Copper has one free electron per atom, so $n$ is equal to $8.5 \cdot 10^{28}$ electrons per m$^3$."
For $A$, our wire is circular and has diameter $1 cm$, so its cross-sectional area in square meters is $A = \pi r^2 = \pi (0.5 \cdot 10^{-2})^2 = 7.85 \cdot 10^{-5}$ m$^2$
Now let's suppose we have a current of $1$ ampere – a fairly strong current by most standards. The velocity of the moving electrons in the wire is
$$
\overline{v} = \frac{I}{q n A} = \frac{1 \text{ C/s}}{1.6 \cdot 10^{-19} \text{ C} \cdot 8.5 \cdot 10^{28} \text{ m}^{-3} \cdot 7.85 \cdot 10^{-5} \text{ m}^2} \approx 9.37 \cdot 10^{-7} \text{m/s}.
$$
So a centimeter-thick copper wire carrying an ampere of current only requires its electrons to move $9.37 \cdot 10^{-7}$ m/s on average. That's very slow! You'll note from the relation $\overline{v} = I / q n A$ that the thicker the wire becomes, the smaller the velocity of the electrons is – that is, as $A$ grows larger, $\overline{v}$ grows smaller. Obviously, the wider a wire is, the more electrons can move through it. The more electrons moving through a wire, the slower they have to go to move the same total net charge.
The point is, power transferred in a wire is a result of massive numbers of electrons moving very, very slowly. They are moving, though.
Of course, this all only holds for direct current – as some other users have mentioned, alternating current is another common current in which electrons move back and forth with some predetermined frequency. In this case, the current is constantly switching directions, hence the name "alternating." This also could be what you're thinking of – as the electrons are moving back and forth but staying in essentially the same place. It is not the individual electrons that are transferring energy to each other, however, but the electric field pervading the wire which is constantly switching direction and forcing the electrons in the wire to change their direction of motion.
Answer: When an electric field is applied on a conductor there exists a drift velocity for the electrons in the medium . Electrons in conductors exist in what is called the Fermi level, a  band  level common to the whole solid. Depending on the field they can accumulate on one side or the other leaving a positive charge from the combined field of the molecules on the opposite side.
When an alternating currentis applied they move back and forth with the applied frequency.

Does an electron move in a conductor?


Rigging default alternatives

Question: I was studying rigs done by pros the other week and ran into a unique style of rigging I have never seen before. After trying it out I learned that it is so much better than the OG style everyone is taught. I am curious as to the downfalls of this style? I mean it may be really painful for higher bone count is all I can really think of. I am sharing this to show the style, but also because I am intrigued by it. It has so many questions to be tossed at it, and to be honest I suck at rigging and animation but this style is really comfy if that makes sense. If you haven't tried it out give it a try and then for the pros who do maybe you can help me out with this intriguing puzzle. Is this an old style, or this some weird new hybrid monstrosity? My model is a bit separated but that is okay for what I am doing.
Comment: Looks an interesting alternative.  Not sure I see an advantage.  The bones still have to be seated where they belong, they're just not laid along the limbs?  Wouldn't that be confusing?  But - if it's 'comfy' - why not use it!  :)  What about when you're setting bone targets?  Do they still work as in Tracking to the target?
Comment: Haven't tried targets. I am doing basic rigging for a game. Just where the joint is. Makes it less confusing no need to align the bone it is already aligned well by whatever axis. You just click the bone, you can look at the name at the top left if need be, or have names on if you want to know what you clicking, then do whatever. Makes finding bones easier honestly. I just look at where the pivot is, paint what I want to be pivoted and done used to be away more annoying. I dunno my brain likes this better dunno why lol.
Comment: @Edgel3D. I made the above rig then went through like 2 or 3 rerigs of preset skeletons and they actually were worse to use for me. The hand for instance would not bend at all worth a crap no matter what weight paint I used. With the above everything just goes smoothly. Felt like slipping on an old pair of slippers. I am unique in the head so maybe that is why lol. I visualize it better, and feels like I have more control plus the bones as stated are easier to assess. I wish I could explain how it works but I cannot find words. It is a weird unique thing that I know is weird.
Comment: Keep doing what works for you!  I for one will be experimenting with this...
Answer: It's not really a different style of armature.  It's just an armature without meaningfully placed bone tails.
The essence of a bone, for skeletal animation purposes, lies in it having two characteristics:

An origin about which it can rotate (and/or scale; translation doesn't really need an origin);
A hierarchical structure from which it can inherit a transformation from its parents.

To do those two things, you don't need bone tails.  And in fact, many skeletal deformation systems have no conception of bone tails-- their bones are points, not lines.  The only thing you need for a bone is the head of the bone.  This is common in game engines, where any unnecessary data is scrubbed out in the name of efficiency, and that's why you're going to see game rip models without meaningful bone tails.  The game never stored their tails, because it didn't need them.
That doesn't mean that meaningfully placed bone tails aren't useful in Blender.  In Blender, bone tails do three things:

They establish bone length data for autoweights and bone envelopes;
They, along with bone roll, establish per-bone local axes;
They act as an extra pocket of information that you can use with bone constraints.

All three of those things are very useful.  Autoweights are a great start on weights that save a ton of time.  It is a pain to fix an A-pose arm twist bone when its local axes are oriented with the world axes at rest.  And good luck creating even a simple leg IK without a meaningfully placed shin bone tail.
However, if you want, you can work with an armature like this.  You can weight paint manually.  You can create non-deforming control bones to deal with constraints, or just not use any constraints.  You can pose it entirely visually, never limiting rotations to any local bone axes.
It's just, you lose out on some easier ways of doing things by ignoring your bone tails.
Comment: guess this is why it is good for me. I most of the time have LP model, and end up having to paint everything myself. I usually do the vertex assign painting style then use weight paint. Auto weight never works for me and sometimes even manual painting does not work the same everytime. So what you saying is good for what I need it for but not everyone else. Good for those who do not require a Pixar animation as well to a degree. Works for me and makes sense lol. I have always seen the world in an a skewed way.

Rigging default alternatives


Is an image with 5 labels equivalent to 5 images with 1 label?

Question: I am collecting data to train an object detection model using and was wondering if 5 labels in the same image and 5 images with 1 label each provided the same quality of input training data. Example: an image with 5 labeled apples vs. 5 images with 1 apple each.
Comment: If I may ask, why '5 images with 1 label each'? Technically speaking, having setup the dataset like this may lead to intentional duplicates in your data (somewhat, since your labels are changing, you may have less problem, but it is exactly same image), and in general duplicates introduces bias to your data! You may want to try it out to compare on a set of unseen dataset.
Answer: No. These are two separate problems. Multi-label classification and multi-class classification. In general, when we talk about classification we mean multi-class classification i.e. there are a certain number of categories and the input training samples fall into only one of these which is your case of 5 images with 1 label. In the case of 1 image with 5 labels, it is a multi-label classification, which is a generalization of multi-class classification. Both of these are different tasks and the use case depends on the problem that you are trying to solve!


How to Manipulate a Dataset

Question: I have a raw dataset of Images I got from Kaggle, It has been classified already, but I want to randomly split the information in a 80:20 ratio between train and test, problem is since all information is classified, I'd love to keep that folder classification in my new train set while all is mixed in the test folder. I cant do it myself since I want it to be randomly split, how do I do this?
Answer: From this dataset, you should prepare new variables namely, X and y; where X = feature matrix and y = class variable. Now, use the following code to split the data into train and test dataset of desired proportion -
from sklearn.model_selection import train_test_split

X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.2, random_state=42)

The test_size helps you prepare a train-test split of 80:20; and
the random_state is the seed value for randomisation.
Each different seed shall yield a different randomised split.

How to Manipulate a Dataset


control digital signing

Question: What is the secure way to support dev mode and production modes on embedded systems that support digital signing. Dev mode will skip signature verification.
Answer: The same way that Windows 10 does:

Add a setting to it that lets its owner/administrator enable "Developer Mode". This is secure as it gets unless your security model is something evil like "keep the device safe from its owner so I can do DRM".

control digital signing


does anybody know of a GNU/Linux project which has support for both gcc as well as clang?

Question: Does anybody know of a project (apart from the Linux monolithic kernel) where a/the program can be compiled by both the compilers and can be used to observe their performances, any recommendations ?  Preferably the program/package is in debian, is not too big (no 200 MB plus downloads please) and yet good enough to say who is king of the hill for now.
Comment: I'm not sure what you are asking, "shootout"? If I had to guess, I'd say you are asking about software in a Linux distribution that can be compiled with both gcc and Clang. But if that is what you mean, you should say so.
Answer: Not sure what you are asking, but Slackware supports and comes with both Gcc and Clang with its default install.
Also, there is no such thing as the GNU/Linux kernel. There is a GNU kernel (Hurd) and a Linux kernel (Linux).
Answer: If you want to compare performance of the compilers, there are many benchmarks floating around.
Be careful, it is certainly advantageous for a compiler to give "good" benchmark results, so it has been known that compilers added "optimizations" that hurt real uses in order to get better benchmarks (perhaps in good faith; benchmarking is easy, relevant real use data is very hard to come by), or even detect the benchmark and cut corners. Not accusing GCC or LVM here, just take comparisons with a shovelful of salt. Besides, a benchmark is a limited set of programs to be compiled/run, if your use differs substantially from the benchmark the results will be meaningless for you.
In the end, unless your application is to be run extensively (i.e., very long runs on your machine, running frequently on hundreds of machines and performance is critical), getting the "fastest program" is completely irrelevant (just look at how much of your Linux sysadmin commands are written e.g. in Python, which is a nice language but certainly holds no speed record; or how much "real use" software is being developed in byte-compiled Java or .Net). Your time as developer/troubleshooter is much, much more valuable than the computer's time, except under extreme circumstances. I'd evaluate a compiler by (a) reliability (if it crashes, or miscompiles my code, I'll be royally pissed off), (b) standards compliance (might be "bug- and extension-compatible to GCC du jour", not strictly "standard", if you want to compile or link against software developed by others), (c) user-friendly error messages, even (d) compilation speed. Performance of the generated code, if it even comes into consideration, much later.


Is there current flowing through this capacitor?

Question:
Let's say \$ V_s \$ is some time-varying voltage source. Is it necessarily true that the current going through \$ C_1 \$ is zero? I would think that the answer is yes, because the current flowing through R_1 must be equal to the current flowing through \$ R_2 \$ (as these are the current out and in of the voltage source). I think the op-amp should be in negative feedback and shouldn't contribute any current through its terminals.
Comment: **If** no current was flowing through \$C_1\$ then it would not be influencing the circuit. But \$C_1\$ does influence the circuit, look up the transfer function of a Sallen-Key filter. The opamp **is** in negative feedback mode, - input and output are shorted so it behaves as a **voltage buffer**. Note how one side of \$C_1\$ is connected directly to the output. The output of the opamp does carry a current, the current through \$C_1\$ + any current the load on the output needs.
Comment: Thanks for pointing me towards the Sallen-Key filter! I am still a bit confused though -- let's say there isn't any additional load on the output. Isn't it true that the current flowing out of the voltage source must be equal to the current flowing into the voltage source, which implies that the current flowing through $R_1$ is equal to the current flowing through $R_2$?
Comment: *the current flowing out of the voltage source must be equal to the current flowing into the voltage source* That is true but that says nothing about the currents through R1 and R2. If the I(R1) = I(R2) was true then I(C1) would need to be zero. Also I could then replace R1 and R2 by a single resistor. Again, that would change the behavior of the circuit. So I(R1) ≠ I(R2).
Comment: According to Kirchoff \$ I_{R1} + I_{R2) + I_{C1} = 0 \$.  That's the current into node 'A'. Now assume there is no current in either op-amp input and both inputs are at the same voltage you should be able to calculate its response from here.  As pointed out by others this is a Sallen-Key filter.
Comment: Your assumption about current flow is wrong. There's a path for the current to return to the source ground via the output of the op-amp (i.e. through its low output impedance).
Answer: Consider this picture: -

The voltage at the non-inverting input is \$A\times\dfrac{XC_2}{R_2+XC_2}\$ and, given that the op-amp is configured as a unity gain buffer (gain = 1) the output voltage is the same.
This means that the voltage across C1 cannot be zero and hence current will flow through C1 if that voltage has an AC signal content produced by Vs. If the voltage produced by Vs is purely DC then no current will flow once the circuit has stabilized.
Comment: @ScottSeidman I shall amend to mention that this applies to AC voltages!
Comment: The *derivative* of the voltage difference needs to be nonzero for current to flow
Comment: ... or billions of bypass caps just got very very hot
Answer: This is a low pass filter. When the input frequency is high the capacitors impedance will be low and of course there will be some current flowing through C1 and then goes to the op amp and returns to it's source. The output voltage will be damped due to C2 low impedance which somehow grands the output. In low frequencies however there is a very low current flowing through all the circuit and of course C1.
You can use simulation tools to see for yourself.
Comment: It's not a high pass filter, as DC bias passes on from input to output.


Noise on the ancilla for syndrome measurement

Question: To detect noise through stabiliser measurement, in the circuit formalism, there is usually an ancilla which stores a parity check result. However, what happens if also the ancilla suffers from the same noise. Does this nullify all the theory? Isn't the assumption of a perfect ancilla too impractical?
Comment: this is the difference between error correction (in particular as described upon first studying it) and fault-tolerant error correction, which adjusts for all the ancillas etc being noisy as well.
Answer: The ancilla being noisy has two effects: making the measurement output noisier, and creating "hook errors" which look like multiple data qubit errors. An $n$-qubit stabilizer can have hook errors equivalent to up to $\lfloor n/2 \rfloor$ data errors (which occurs when the error on the ancilla occurs exactly halfway through the stabilizer measurement operations), with the specific subset of data qubits depending on the order of operations. Hook errors can be mitigated by sticking to small stabilizers, or by using techniques like flag qubits to distinguish the hook errors from other errors.

Does this nullify all the theory?

What? Of course not. Of course people have thought about the ancilla being noisy!
Whenever a paper says it is doing simulations with "circuit noise", they are considering noise on the ancilla interspersed with the operations the stabilizer measurement has been decomposed into. There are tons of examples of this... an extremely recent one is "A circuit-level protocol and analysis for twist-based lattice surgery".

Isn't the assumption of a perfect ancilla too impractical?

Yes, it would be a ridiculous thing to assume would hold in practice.
Often research will ignore noise on the ancilla as a starting point, to make a problem easier to get a hold on. But that's different from assuming it doesn't have to be dealt with or that it doesn't exist in reality.
Answer: Syndromes are analyzed in space and time.
An error on ancilla (also equivalent in effect to measurement error) seems as a sign flip of the syndrome in time, in only 1 ancilla.
While error on data qubit will affect the 2 ancillas next to him:
In the general case, data qubits will show even number of errors, while ancilla error, will create an odd number of errors - just a flip over time.
You can see here X/Y/Z error effect on the syndrome, and M is an effect of the error on ancilla/ measurement error


iTunes Store vs App Store (iPhone 4)

Question: Why are some apps installed via iTunes Store while others via App Store? is there any significance in that? Many thanks.
Comment: Which ones are in iTunes Store?
Answer: You're confused because on Mac, you access iOS App Store from iTunes, but it is actually iOS App Store. So all your apps are from iOS App Store, not the Mac App Store which is a separate App like iOS App Store is separate on iDevices than iTunes.
Answer: No apps are installed via the iTunes Store. The iTunes Store is for music, movies, TV shows, etc. All apps are in the App Store.

iTunes Store vs App Store (iPhone 4)


equivalent norms in Banach spaces of infinite dimension

Question: Suppose $ X $ is a Banach space with respect to two different norms, $ \|\cdot\|_1 \mathrm{ e } \|\cdot\|_2 $. Suppose there is a constant $ K > 0 $  such that
$$ \forall  x \in X, \|x\|_1 \leq K\|x\|_2 .$$
show then that these two norms are equivalent
Answer: This is a variant of the open mapping theorem. If we consider the identity map $i$ on $X$ as a linear mapping from $(X, \|\cdot\|_2)$ to $(X, \|\cdot\|_1)$, then your condition says that $i$ is continuous. Then by the theorem $i$ is open and hence a homeomorphism, so its inverse is also continuous and the norms are equivalent.
Comment: Beautiful!! Thanks!
Answer: Use the open mapping principle: the identity map $F: (X, \Vert \cdot \Vert_2) \to (X, \Vert \cdot \Vert_1)$ is linear and continuous (thanks to hypothesis). Its rank is of the second category, since $X$ is Banach (in particular, a complete metric space under the metric induced by $\Vert \cdot \Vert_1$).
By open mapping, it follows the this is an open map, i.e. an homeomorphism. In particular, the inverse map $F^{-1}: (X, \Vert \cdot \Vert_1) \to (X, \Vert \cdot \Vert_2)$ is still continuous; from this you easily get the other inequality you need in order to prove that the norms are equivalent.


How can I find the original bag listing for 42083 Bugatti Chiron?

Question: I'm buying a used set (42083 Bugatti Chiron to be specific) and I would like to sort the parts in their original module bags. However, I can only find the complete part list, not the parts per module. Does anyone have a pointer as to how to retrieve the original sorting?
Answer: As far as I know there is no such resource on the internet.
It's asked from time to time on other fora and groups I frequent, mainly from people who have a collection of bags and want to gather entire sets.
The best that can be done to get the parts per module is disassembling the set in reverse order of the instructions...


Are the crown given to the first horseman in Revelation 6:2 and the crown in Isaiah 28:1 the same kinds of crowns?

Question: Are the crown given to the first horseman in Revelation 6:2 and the crown of the drunkards of Ephraim given to the strong one in Isaiah 28:1 the same kinds of crowns?
Answer: The word used in Rev 6:2 is "stephanos", a "chaplet" (see David Bentley Hart's translation and W E Vine Expository dictionary) of (usually) laurel leaves made into a wreath given to victors at Olympic games.  In the LXX version of Isa 28, the same word is used.  The NIV correctly translates this as "wreath".
Note that in Rev 6 the "stephanos" signifies victory (Gr: "nike") as the rider goes "conquering and to conquer".  In Isa 28 the wreath of the drunkards symbolises the fertile valley (v1, v4, etc) about to be despoiled, and is contrasted with the LORD Almighty who will become the victory wreath of the remnant (v5).
Answer: Revelation 14:14 (the same person as it Rev. 6:2)
Then I saw, and look! a white cloud, and seated on the cloud was someone like a son of man [Jesus], with a golden crown on his head and a sharp sickle in his hand.
Isa. 28:1 talks of the 10 tribe kingdom, also called Ephraim, which was acting as if drunk with power and pride when making friends with Syria and other nations, eneimes of God and judah the 2 tribe kingdom, thus a false crown of power!!
ethos


Holomorphic function constant on the boundary

Question: Let $D$ be a domain and $f$ holomorphic on $D$ and continuous on its closure $\overline D$. Now, if $f$ doesn't have any zeros in $D$ one can show that if $f$ is constant on $\partial D$ it must be constant on $D$. Does this hold as well if $f$ has any zeros inside $D$? If not, can someone provide an example?
Answer: You need some assumptions about the boundary of $D$, namely that the boundary is not a discrete subset of the complex plane. Otherwise, your claim is clearly false (think of $D={\mathbb C}- \{a_1,...,a_n\}$ and a certain degree $n$ polynomial on ${\mathbb C}$).
The relevant theorem than is due to Rado and Cartan:
Theorem. Suppose that $D$ is a domain in ${\mathbb C}$ and $p\in \partial D$ is a non-isolated boundary point. Suppose that $U\subset \partial D$ is a neighborhood of $p$ (in the subspace topology). Assume that $f: D\to {\mathbb C}$ is holomorphic and has continuous extension to $D\cup U$ which is constant on $U$. Then $f$ is constant.
See Ch. 11, section 8, Theorem 2 in
R.Narasimhan, Complex Analysis in One Variable, Springer Verlag.


Does software based file recovey solutions have the same results on real physical device vs dd image file?

Question: I have a flash drive and I want to recover some files from it. Can I take a dd image of the device and perform the recovery operation on dd image instead? Does it have the same results when performing recovery using software on physical flash drive?
Comment: It depends on how the data was damaged and how deeply you are trying to recover. if the recovery is required due to partial magnetic erasure for instance, you can only see the data that DD could understand. Tools like Spinrite which reassess the magnetic properties of the bit and rewrite it at full signal strength could only work on the original media. likewise blocks too corrupt for DD/DDRescue could not be contained in an image. That said, It is standard practice to take an image and work from it and for most forensics tasks that is a really good idea!
Answer: Yes!
It's standard forensic procedure to work from a "Physical Copy", which is what a dd bit-for-bit image is. It will have the same results as working from the drive. In fact the hash checks verify that they are the same.
This assumes of course that the dd image was done appropriately. An example of an inappropriate dd image would be one done against a multisession DVD as DVDs are not actually block devices and only the last session would be extracted.
A flash drive is a block device and would image as expected via a dd.
Wear leveling is a bugaboo that's often thrown around but it's not a factor here.
The one extremely unlikely difference with an SSD (Solid State Drive) is a massively delayed TRIM function. If the TRIM runs delayed by many minutes, a dd image may actually extract data before the TRIM runs quicker that performing the analysis directly against the SDD, in which case the image would be better but the hashes would not match causing problems. I've never seen this occur!
Answer: Working on a dd image is the same as working on the disk, if the disk is healthy.
On a working flash device, it does not matter. A dd image is basically the disk. On a damaged drive it differs a little. If dd cannot read the start of a block, for example, it will write all zeroes for that block, even if the end of the block can be read.
If the device is damaged, it's better to use ddrescue. It will copy full-block where possible, and decrease the block size and retry reading until all possible data is copied back.
And if the device is damaged, it's better to work with a image, because further device activity may damage it even further.


A minimum reputation of 50 to comment - why?

Question: If someone can please clarify, I don't get the minimum reputation of 50 required to comment on someone else's answer or question. This is simply too high for new users. I have lost track of about 10-20 questions/answers which I wanted to comment (with a comment as simple as 'please cite sources') - which I can no way remember and go back to do after my reputation has gone up.
Comment: Its more of SE policy then hindusim.se's , and its discussed again and again over main meta [example](http://meta.stackexchange.com/questions/51408/remove-reputation-minimum-to-comment)
Answer: It's not just Hinduism.SE is having such a rule, its the same across StackExchange network. Coming to your question that why such a rule is because new users tend to chat is because we want don't want spam from new users.
It might happen that few people can make fake accounts and start spamming across the site which is not only bad for the website but it will be tough for the users and moderators to control and keep a watch for that all the time.
Indeed we may lose some valuable information a new user might want to contribute in a comment but chances of this is too low. It's like 1 in 10 comments might be useful.

Coming to the second part of your question which says I have lost track of about 10-20 questions/answers which I wanted to comment so for this we do provide a feature which is called "Mark as Favorite" by clicking on the star besides the question. This will not only remind you for commenting but you will also get notifications for whatever activity is carried on that question.
I hope this solves your doubt. Also you have 51 reputation now so happy commenting.
Comment: That is, in my opinion, not a good system because those who cannot comment will post an answer. A bad comment is much better than a bad answer.
Comment: Thanks for the explanation. Not every question/answer I read or comment is my favorite, so "Mark as Favorite" is only a workaround not a _feature_ designed for _come back later to comment when your reputation is up_. We have another _feature_ for the same purpose it's called _browser history_ ;-)
Comment: _It's like 1 in 10 comments might be useful_ I wouldn't make any such assumptions having read others questions/clarifications/suggestions to improve the answers in comments including some of the mods' on Hinduism.SE.
Comment: @sv. Again, these stats are from StackExchange where they collect the data of how many general comments are deleted by moderators, I will find a link for you
Comment: http://meta.stackexchange.com/questions/214173/why-do-i-need-50-reputation-to-comment-what-can-i-do-instead Here you go


Are the eigenvalues of Laplacian times Diagonal times Laplacian matrix non-negative?

Question: Consider two (not identical) symmetric Laplacian matrices $L_1$ and $L_2$ and a diagonal matrix $D > 0$. My question is if the eigenvalues of the product $A =L_1 D L_2$ have non-negative real-part. It is well known that the product of two positive semi-definite matrices have eigenvalues with non-negative real part. (see e.g. the book by Bernstein: Matrix Mathematics, Theory, Facts, and Formulas). Extensive simulations show that $A$ has indeed eigenvalues with non-negative real part and that this not true in general if $L_1$ and $L_2$ were `only' positive semi-definite matrices. However, I have been, trying similarity transformations and Sylvester's law of inertia, unable to find a proof. Of course, a counterexample would be very useful as well.
Answer: The answer is no. A random counterexample: the eigenvalues of
$$
A=\pmatrix{1&0&-1\\ 0&1&-1\\ -1&-1&2}
\pmatrix{1\\ &3\\ &&1}
\pmatrix{2&-1&-1\\ -1&1&0\\ -1&0&1}
=\pmatrix{3&-1&-2\\ -2&3&-1\\ -1&-2&3}
$$
are $0$ and $\frac12(9\pm i\sqrt{3})$, so $A$ has non-real eigenvalues. When the matrix sizes are larger (such as $5\times5$), we can even obtain easily a counterexample in which $A$ has a negative real eigenvalue.
Edit. Another counterexample: the matrix product
\begin{align*}
A&=\pmatrix{
 1& 0& 0&-1& 0\\
 0& 2&-1& 0&-1\\
 0&-1& 2& 0&-1\\
-1& 0& 0& 1& 0\\
 0&-1&-1& 0& 2}
\pmatrix{1\\ &3\\ &&3\\ &&&3\\ &&&&3}
\pmatrix{
 2&-1& 0& 0&-1\\
-1& 2& 0& 0&-1\\
 0& 0& 1&-1& 0\\
 0& 0&-1& 1& 0\\
-1&-1& 0& 0& 2}\\
&=\pmatrix{
 2&-1& 3&-3&-1\\
-3&15&-3& 3&-12\\
 6&-3& 6&-6&-3\\
-2& 1&-3& 3& 1\\
-3&-12&-3& 3&15},
\end{align*}
has an eigenpair $(\lambda,v)$ where $\lambda\approx-0.2111<0$ and $v\approx(0.46902,\,0.30552,\,-0.61103,\,-0.46902,\,0.30552)^T$.
Comment: Thank you. I clarified my question that I meant with non-negative, non-negative real part. Can you provide an example where $A$ has a negative real eigenvalue? You mention that it is easy, but unfortunately I do not find one.
Comment: @galirt Not meant to be rude, but for 5-by-5 matrices, it is indeed easy to obtain a counterexample with a real negative eigenvalue. There may be some limitations in the simulations you've done.
Comment: Thanks. I have rerun my simulations, and I find indeed examples when the Laplacians (at least one) do not correspond to connected graphs (like your example). Initially, I excluded this possibility in my simulations as my associated graphs are always connected. Do you think this absence of connectivity is essential to find a counterexample?
Comment: @galirt It's quite plausible. I cannot obtain a counterexample (for matrices with sizes $\le8$) with a connected graph too.


getrawtransaction - find destination and "change" addresses

Question: How to get destination address and amount from result of getrawtransaction and how to distinguish it from "change" address using rpc api.
Comment: try decoding the raw transaction `bitcoind decoderawtransaction ` or you can just work with the original transaction in the first place `bitcoind gettransaction  `
Answer: The getrawtransaction RPC will provide you with details about a particular transaction.  However, answering your question further is complicated by several issues:

Transactions are not required to have change addresses.  Good wallets will attempt to avoid using change addresses when change isn't necessary.
Transactions may have more than two outputs, so there's not necessarily a single payment address.
Transactions may pay a bare (non-P2SH) multisig output which comprises several public keys.  GetRawTransaction will display these as multiple addresses.
In a well-implemented wallet, change should be indistinguishable from a payment within a single transaction.  You would need to analyze multiple transactions or have information from the party that received payment in order to be able to determine which output is a change address.

In short, I don't think GetRawTransaction or any other RPC will provide you with the information you want for arbitrary block chain transactions.  However, if you're only interested in information about transactions in the local wallet, the gettransaction RPC is what you're looking for.


Prehung Door Frame's Latch Hole

Question: I purchased some prehung interior doors. Is there a standard way to fill/cover the back of the frame's latch hole? I'd prefer if there wasn't a hole exposing the cavity between the frame and wall.
Comment: Have you compared the length of the latch bolt to the thickness of the casing? You might need some space beyond the casing.
Comment: The latch bolt does not extend past the frame. But do most people just leave the latch cut-out open in the frame as-is? Seems strange.
Answer: There are strike plates which include a dust box behind them; you could get one of those rather than using the one that comes with the lockset.
Even better, as long as you're paying a few dollars to upgrade, consider a high-security strike. These come with hardware that secures them to the doorway's framing studs, not just the relatively thin wood enclosing the doorway itself, and significantly increase the door's strength. And they usually include a dust box of the sort I mentioned above.

Prehung Door Frame's Latch Hole


When has the phrase "dirty sex" started to be used?

Question: We say 'dirty sex' nowadays,quite often. When we're referring to hardcore sex that includes role play, bondage, sex toys and whatever else goes beyond the simple act of copulation, we say 'Dirty sex'. But when has this phrase first started to be used? Is it something people would say in North America at the beginning of the 20th century?
Comment: I'd say sometime around the [1920's](https://books.google.com/ngrams/graph?content=dirty+sex&year_start=1885&year_end=2005&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Cdirty%20sex%3B%2Cc0), although some of the variants you mention might be more accurately called _kinky sex_, which become a rather common expression about [50 years later](https://books.google.com/ngrams/graph?content=dirty+sex%2C+kinky+sex&year_start=1885&year_end=2005&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Cdirty%20sex%3B%2Cc0%3B.t1%3B%2Ckinky%20sex%3B%2Cc0).
Comment: Do we? I've never said it.
Comment: Google's Ngram agrees with J.R.'s response.  If the questioner is not familiar with the information Ngrams provide, I invite him to simply google  "Google Ngram."  It's not perfect data, of course, but usually interesting.
Answer: The earliest use in the New York Times is from 1972:

New York Times Movie Mailbag BRENDA H. HAWLEYDEC. 10, 1972  To THE
  EDITOR: In his selection of these film roles and what he selects to
  admire about them, Hirsch clearly reveals one of the bases of the
  Tuesday Weld cult. Tuesday has long been admired by fellers like him
  who see in her the embodiment of the exploitative child‐woman. The sex
  kitten who consumes and destroys. Tuesday of "the angelic face" who
  plays the dirty sex game. But now there is "Play It As It Lays" and
  that image is gone. . .

Another, in an article about farm communities (NYT Magazine), 1973:
Out in the Country
By MARK KRAMER FEB. 17, 1973

The trouble is more manifest in the numbness of the thousands of
  victimized young people who grew up on farms and, having been turned
  away from that life by the economic and social conditions of our
  times, live Instead with TV, discount stores, and an ethos crackling
  with the mystique of dirty sex and violent punishment, daringly
  expensive playthings, and the universal veneer of middle‐class grace,
  unwittingly learned from the goodnatured and hollow folk who populate
  the world of TV.

These appear to be the more modern use.
Answer: I looked for the source of Daniels extraordinary find and found the novel
by Josephine Daskam Bacon (1909) Margarita's Soul: The Romantic Recollections of a Man of Fifty (full text eg here).
The relevant context is " When you shall have learned, dear ladies, to keep your domains clean without so furiously getting them clean, you will have earned, in our eyes, your somewhat dubious title of housekeepers. Meanwhile, continue, in heaven's name, to think us the contentedly dirty sex!!" –
First I thought this to be an early mention of "dirty sex" so I checked for all other occurrences of sex in this text and realized all mentions of sex in this book imply the female sex, women. including the above quote.
to think us to be the contentedly dirty sex
So this is not really an example of the expression "dirty six" but an assertion about cleanliness!
UPDATE
Looking a bit further there are a few combinations of dirty + sex from 1914
One of them is this:
"The man who tells dirty sex stories should be suppressed.
Let him tell them to his own son and daughter." (source, eg this text)
Comment: I saw it wasn't the phrase as used now, we would probably need to go through NGram and look when the meaning may have changed.
Comment: Actually, in the quote you mention the "dirty sex" refers to the *male* sex. But either way, it's irrelevant to the original question; the phrase is only coincidentally the same.
Answer: A good way to discover when phrases were first used is to use Google Ngram, almost certainly they date from before then.
The first use of the phrase dirty sex, that I could find, was in 1908 or 1909 in the The American Magazine.

EDIT: I looked up dirty sex vs kinky sex too, the later dates to 1904 afaics.


Automating Interpolation in QGIS

Question: I have a project in QGIS which involves interpolating several different variables on several different maps, then using the raster calculator to multiply some of the resulting interpolations together.  I'm now trying to automate it so I can create this map on a daily basis.  What specific libraries must be installed so I can run interpolation automatically (via a python script, or any other method)?  Can anyone share an example of how this can be done?
Answer: You could automate some tasks using the QGIS modeler which allows you to build and run models on selected layers like the following:

There's a nice tutorial in the following link which goes into quite great detail on how to create a complex model:
Automating Complex Workflows using Processing Modeler.

You could also use the Python Console to call tools from the Processing Toolbox. For example, to see a list of interpolation tools, you could run the following to find all tools with the keyword "interpolat" in their description (some tools contain "interpolation" and "interpolate", hence why "interpolat" is used):
<code>import processing
processing.alglist("interpolat")

r.fillnulls - Fills no-data areas in a raster layer using v.surf.rst splines interpolation or v.surf.bspline interpolation--->grass7:r.fillnulls
r.resamp.interp - Resamples raster map to a finer grid using interpolation.--->grass7:r.resamp.interp
r.resamp.rst - Reinterpolates using regularized spline with tension and smoothing.--->grass7:r.resamp.rst
r.surf.idw - Surface interpolation utility for raster layers.--->grass7:r.surf.idw
v.surf.bspline - Bicubic or bilinear spline interpolation with Tykhonov regularization.--->grass7:v.surf.bspline
...
</code>
You can find more information on how to run these commands from the documentation:
Using processing algorithms from the console.

Automating Interpolation in QGIS


iPhone Human Interface Guidelines question

Question: iPhone Human Interface Guidelines -- Do they apply for OpenGL games?  GL games typically have custom buttons, custom lists, etc.  Take the Quake player list for example, if ported, it wouldnt be a typical UI list object.  Does this mean the reviewer in question would reject anything that wasn't using standard UI?  Or, is there an exception for games.
Comment: I'm voting to close this question as off-topic because [we are not customer support for your favorite company](https://meta.stackoverflow.com/questions/255745/).
Answer: You might want to refer to the section in the iPhone Human Interface Guidelines on "Immersive Applications".
Comment: Excellent link. I think a sentence in that section says a lot: "Users expect seeking and discovery to be part of the experience of an immersive application, so the use of nonstandard controls is often appropriate."
Answer: Yes and no. It's certainly true that you're not required to use the standard UIKit elements for games - buttons, labels, and so forth - but the core principles of the guidelines still hold. You should still be designing a UI that is responsive, consistent, and intuitive.
Keep in mind that these are guidelines. However strict they may be, if you have a good reason for disregarding them in creating your game, do so - just make sure that your UI is not obviously or fundamentally broken.
Answer: The reviewers dont reject apps because they dont meet the guidelines… that is, they reject the apps because you are using something that contradicts a direct and purposeful guideline.
For instance, I have had many apps sent back because I used an icon of a man for showing your people.  It is intended to show your contacts.  Small things like this might get you into trouble, but if the context is clear and there is no confusion with elements that look like they came from cupertino, then you are ok.


Generalising the Penrose Twistor Fibration

Question: As is well known, there exists a fibration $\mathbb{CP}^3 \to S^4$, of the four sphere by complex projective $3$-space, called the Penrose twistor fibration. Does this fibration admit a "canonical" generalisation to a fibration of $S^6$ by some projective $n$-space, or possibly flag manifold. What about general $2n$-spheres?
Comment: The Whitney sum formula for Pontryagin classes imposes strong conditions on the fiber tangent bundle of such a fibration.  Similarly, the long exact homotopy sequence and the Leray spectral sequence impose conditions.  It looks like the fibers must be $\mathbb{CP}^{n-3}$.  Then the cohomology is determined by $c_3$ of the associated $SU(n-2)$-bundle.  On $BSU(n-2)$, does $c_3$ pair as $\pm 1$ with a generator of $\pi_6(BSU(n-2)) \cong \mathbb{Z}$?  If not, there is no fibration of $\mathbb{CP}^n$ by $\mathbb{CP}^{n-3}$ over $S^6$ (analogous to the Penrose fibration).
Answer: Yes, there is such a twistor fibration over each $S^{2n}$, and the resulting manifold is a complex manifold endowed with a holomorphic $n$-plane field transverse to the fibers of the mapping. Namely, one writes $S^{2n} = \mathrm{SO}(2n{+}1)/\mathrm{SO}(2n)$ and then, using the inclusion $\mathrm{U}(n)\subset\mathrm{SO}(2n)$, one has the coset fibration
$$
Z_n = \mathrm{SO}(2n{+}1)/\mathrm{U}(n)\longrightarrow\mathrm{SO}(2n{+}1)/\mathrm{SO}(2n).
$$
The manifold $\mathrm{SO}(2n{+}1)/\mathrm{U}(n)$ canonically has the structure of a complex manifold and is now known as the twistor space of $S^{2n}$.  Interestingly, this space was already used in 1967 by Calabi in his study of minimal $2$-spheres in $S^{2n}$ (Minimal immersions of surfaces in Euclidean spheres, J. Differential Geom. 1 (1967), 111–125), where, using a quite different language, he showed that such immersions all have lifts to the twistor space as holomorphic curves that are tangent to a certain holomorphic plane field on $Z_n$.
Now, there are generalizations of this picture for each of the so-called 'inner' symmetric spaces $G/K$ where $K$ is the fixed subgroup of an involution that is an inner automorphism of $G$.  The twistor fibration is of the form $G/U\to G/K$ where $U\subset K$ is a subgroup such that $K/U$ (the typical fiber of the fibration) is an Hermitian symmetric space.  (There are also other kinds of twistor spaces over $G/K$ that are flag manifolds of the form $G/T$ where $T\subset K$ is a maximal torus.)
For more details on these topics, especially the generalization to the inner symmetric spaces and flag manifolds, one can, for example, have a look at my paper Lie groups and twistor spaces,  Duke Mathematical Journal 52 (1985), pp. 223–261.
Comment: Beautiful answer as always ! Sorry for not citing you in my answer. I digged a little hoping to add better references and discovered Burstall & Rawnsley's book (MR1059054), citing in particular your work.
Comment: A long time since I've thought about this but maybe worth saying the $(4n+2)$-dimensional case differs from the $4n$-dimensional: you get a pair of "twistor spaces" with an anti-holomorphic isomorphism between them instead of a real structure. The case of the six sphere mentioned in the OP is especially interesting because you end up with a complex 6-dimensional quadric with a real structure (the original $S^6$) and another pair of 6-dimensional quadrics with the anti-holomorphic map between them. I seem to remember this paper: http://www.sciencedirect.com/science/article/pii/0393044095000364
Answer: One possible generalization of Penrose twistor fibration is the fibration $\pi:\mathcal{J}_M\to M$ over any oriented $2n$-dimensional conformal manifold $(M,[g])$, whose fiber $\pi^{-1}(x)$ is the space of orthogonal almost complex structures on $(T_xM,[g_x])$, namely the orthogonal $J_x:T_xM\to T_xM$ squaring to $-\mathrm{id}$ such that the complex orientation $(e_1,Je_1,e_2,Je_2,\dots)$ is the given one.
For $n=2,3$, the fibers are projective spaces $\mathcal{J}_4=\mathbb{CP}^1$, $\mathcal{J}_6=\mathbb{CP}^3$, and for general $n$ complex projective varieties of dimension $n(n-1)/2$. This is because $\mathcal{J}_{2n}$ identifies with the grassmannian of $n$-dimensional totally isotropic subspaces of $\mathbb{C}^{2n}$ for any non degenerate quadratic form, e.g. $z_1 w_1+\dots+ z_n w_n$. For $n=4$, one gets $\mathcal{J}_{8}=Q_6$, the six dimensional smooth complex quadric hypersurface in $\mathbb{CP}^7$. As a real manifold, it is also the homogeneous space $SO(2n)/U(n)$, with the central $U(1)$ giving the complex structure on tangent spaces.
For $M=S^{2n}$ the round sphere, the total space $\mathcal{J}_{S^{2n}}$ identifies with $\mathcal{J}_{2n+2}$, by identifying $T_xS^{2n}\oplus\mathbb{R}^2$ with $T_xS^{2n}\oplus\mathbb{R}x\oplus\mathbb{R}e=T_x\mathbb{R}^{2n+1}\oplus\mathbb{R}e=\mathbb{R}^{2n+2}$ in the natural way, and any $J_x \in \mathcal{J}_{S^{2n}}$ to $\hat J_x\in \mathcal{J}_{2n+2}$ which coincides with $J_x$ on $T_xS^{2n}$ and sends [edit] $x$ to $e$ (and $e$ to $-x$) [/edit].
So the fibration over $S^6$ is $\mathbb{CP}^3\to Q_6\to S^6$, and in general $\mathcal{J}_{2n}\to\mathcal{J}_{2n+2}\to S^{2n}$. Note that, contrary to the case $n=2$, the $n=3$ fibration has a section, meaning that $S^6$ has an orthogonal almost complex structure (noted by Ereshman, but maybe ealier).
I don't have a reference at hand, but a first guess is that it should be found in Lawson & Michelsohn "Spin Geometry" and maybe also in Harvey "Spinors and calibrations".
The link to spinors is convenient to identify $\mathcal{J}_{2n}$ for low $n$, because it identifies with the projectivization of the cone of "pure (complex) spinors" of $\mathbb{R}^{2n}$, which are all of them for $n\leq 3$, and a quadric cone for $n=4$.
Comment: For the $\mathbb{CP}^3$-bundle over $S^6$ whose total space is $Q_6$, for the associated $SU(4)$-bundle over $S^6$, $c_3$ equals $2$ times the Poincare dual of a point class.
Comment: @Jason Starr : it's a long time since you posted this, but I missed it somehow. May I ask what was your point ?


Help! 2005 Mazda 3 brakes

Question: I noticed recently that my brake goes soft and almost to the floor. Several months ago I had new brakes put on and I just took it in again to bleed the brakes.
I just picked up the car and the brakes feel the same.  Any ideas?
The mechanic noticed that there was hardly any oil in the car. It was puzzling because in October I just had the oil changed.  Thank you!
Comment: You have two unrelated issues. You should create two questions to cover. Also, give us some more details, like what does "brakes done" mean to you, and have you looked/seen oil where the car sits. Any other pertinent details. As your question is, there's a plethora of things which could be going on. Help us help you diagnose your issues.
Answer: A vehicle fitted with a modern brake servo, as the Mazda 3 would have 'servo travel'. To determine how much travel you have: Engine off, pump the brake several times until it is hard and will not increase its pedal height anymore, press the brake pedal and hold it pressed, car in neutral start the engine still holding the brake pedal down with your foot. You will find the brake pedal travels down an amount. This is your servo working as it should. Is there any great differance in brake travel when operating the brake whilst driving, or is it about the same? If it is about the same then you now know it is your servo's operation giving you its travel. But if the travel is greater in use you have a problem and the brake system needs to be pressure tested, and a careful examination of its hoses undertaken by a competent mechanic. As for the low oil level, was it fully filled up on the oil change? Do you have blue smoke out of the exhaust? Sump plug washer and oil filter seal good? The longer it is before the low oil level is noticed to be low the harder it would be to say why the oil level was found to be low.

Help! 2005 Mazda 3 brakes


Can a toilet on concrete slab be moved about 4 1/2 inches?

Question: My toilet is situated such that it doesn't allow much room to get in and out of the shower. I'd like to move it back to the wall as close as possible, to allow more space. The toilet is on concrete slab. Is this possible? I saw this similar question (http://diy.stackexchange.com/questions/6123/can-a-toilet-be-moved-6-inch-to-the-right) was asked but that is for a toilet over subfloor. Also, I know there are different sized toilets but is the distance from the hole on bottom of toilet to the back of the toilet a standard size?
Answer: In the US, the typical toilet rough in size is 12", which I believe goes from the toilet bolts (center of the drain) to the studs (so subtract 1/2" for drywall). You can also find toilets made for 10" and 14" rough ins. But, to get more clearance, the easiest thing to do is swap out the style of toilet from an elongated bowl to a round bowl. If you're a man visiting the library, it means you have less space to hang free, but it gives you a lot more space to get around the bathroom.
To answer your first question, you can move the drain, but it involves taking a jackhammer to the concrete. Not a fun project if you don't like messes.
Comment: We ended up going the jackhammer route. To make matters worse the plumber discovered a y-split where he was going to attach the new pipe, so he had to do extra work there. Overall it was the right decision. Cost $550. When the plumber filled the concrete it was about 1/8" higher than the rest of the slab. I thought it'd be better to be lower than higher but that's what he did.
Answer: Build a platform for the toilet to set on.  It must be tall enough so that the drain can be redirected towards the existing drain hole and maintain the correct slope.
Comment: Interesting idea but I'm not sure how it would be possible to keep the platform hidden
Comment: @Andrew - If you really need to 'hide' such a platform, you'd have to basically platform the whole room, so that's not much of a win. If you think about it, you can probably integrate such a platform into your decor without it being a problem, but it clearly is going to be visible.


Can I run Windows programs on Ubuntu + Virtual Box?

Question: I have Ubuntu on my computer. I was asked to do programming and spreadsheet by Visual Studio and Excel. Can I do this by installing Virtual Box from https://www.virtualbox.org/ , Windows 10, Visual Studio and Excel?
Comment: Yes - this is a common usage of Virtualbox on Linux.
Answer: Yes virtual box allows you to run an operating-system, and to then run programs on it.
Some use-cases such as 3D-cad or 3D-games or 3D-virtual reality, may be to much for the virtual machine. However if you have modern hardware (almost anything in the last 10 years), then any other use-case will be fine. It is even possible to have Microsoft's Windows run faster in Virtual-box than on real hardware.
Also consider other alternatives: Mono-develop it a .net development system. It is both better and worse than Visual Studio Professional (the most expensive one). However if your (or your customers) goal is to make a product that is incompatible with non-MS-Windows, then this may not be your best option.
Answer: There's no logical reason why you can't install a Windows 10 guest OS in VirtualBox depending on the configuration of the guest's OS virtual hardware and what applications you install in it. Basically if the system requirements of the software that you plan to install on the guest OS are compatible with the resources allocated to the guest OS in the VirtualBox guest OS settings, the applications that are installed in the guest OS should run OK whatever they are.
In particular you need to allocate whatever virtual hardware resources to Visual Studio that it needs.


setKeepAliveTimeout is deprecated in iOS9

Question: The current API changes for iOS9 state that -setKeepAliveTimeout:handler: is deprecated.
Up to now, this was the only way that a VoIP SIP app on iOS could maintain its registration with the SIP-server.
This technique is used by various apps like LinPhone and others.
Does anybody have a view on the proposed alternatives by Apple ?
Or will SIP be crippled starting from (post-)iOS9 ?
See: https://developer.apple.com/library/prerelease/ios/documentation/UIKit/Reference/UIApplication_Class/index.html#//apple_ref/occ/instm/UIApplication/setKeepAliveTimeout:handler:
http://www.linphone.org/docs/liblinphone/group__IOS.html
Comment: I have same issue. My VoIP app does registration refresh up on KeepAlive timeout. It is supposed to work in lan environment even w/o internet connectivity which means no remote notifications. My app is working right now with this deprecated API but not sure how long it will.
Answer: Apple document the alternative in the page you linked:

Discussion
In iOS 8 and later, voice-over-IP (VoIP) apps register for UIRemoteNotificationTypeVoIP remote notifications instead of
  using this method.
Comment: UIRemoteNotificationTypeVoIP allows an external trigger to wake-up the voip app. This does not cover the case where the SIP client needs to re-register actively to the SIP server.
Answer: It seems that you have to leak the signaling event to the apple push notification server by registering for https://developer.apple.com/library/prerelease/ios/documentation/UIKit/Reference/UIApplication_Class/index.html#//apple_ref/occ/instm/UIApplication/registerForRemoteNotifications .
This is quite unfortunate, but seems unavoidable. The idea behind this is probably to save battery by having fewer active network connections on the phone.
So this means that you have to leak the information that an event has reached your application to apple services. You can probably just send a 'wake' notification to your app and then handle loading the event type yourself - but this might incur too much latency, so you might need to leak more of the signaling information to apple services. :-/
Answer: From Apple's Docs:

In iOS 8 and later, voice-over-IP (VoIP) apps register for registerForRemoteNotifications remote notifications instead of using this method. Using remote notifications eliminates the need for a timeout handler to check in with the VoIP service. Instead, when a calls arrives for the user, the VoIP service sends a VoIP remote notification to the user's device. Upon receiving this notification, the device launches or wakes the app as needed so that it can handle the incoming call.

In the past, the setKeepAliveTimeout call used to allow a handler to be called a the end-time and would have a max of 10 seconds to exit or be forced to terminate, also calls to the handler are not guaranteed to be within the timeout value.
The new (registerForRemoteNotifications) would be ok to work with since the handler is internal (to IOS) and would in-turn call your app when a remote-event occurs (this would even wake your app if it's in-sleep).
Either way, both should do the same thing, the older version you would handle the code, and the new one, you would receive a notification (also handle it somewhere), but you will no longer control the timeout.
From Apple docs:

Call this method to initiate the registration process with Apple Push Notification service. If registration succeeds, the app calls your app delegate object's application:didRegisterForRemoteNotificationsWithDeviceToken: method and passes it a device token. You should pass this token along to the server you use to generate remote notifications for the device. If registration fails, the app calls its app delegate's application:didFailToRegisterForRemoteNotificationsWithError: method instead.

If you want your app's remote notifications to display alerts, play sounds, or perform other user-facing actions, you must call the registerUserNotificationSettings: method to request the types of notifications you want to use. If you do not call that method, the system delivers all remote notifications to your app silently. Because the registration process takes into account the user's preferred notification settings, requesting access to user-facing notification types also does not guarantee that they will be granted. To find out what notification settings are available, use the currentUserNotificationSettings method.
And Finally (for Un-Registration):

You should call this method in rare circumstances only, such as when a new version of the app removes support for all types of remote notifications. Users can temporarily prevent apps from receiving remote notifications through the Notifications section of the Settings app. Apps unregistered through this method can always re-register.

Not sure, but I hope this helps.
Regards,
Heider Sati
Answer: If we integrate PUSHKIT, it will take care of everything regarding wakeup the application. If you send the push notification while getting VoIP call with Push notification via PUSHKIT, it will work if its in backgroundstate. I have done same thing.
I hope this will help you.


SVN Error E175002 while checking out code from repository

Question: While trying to checkout code from a repository online I got the following error:

E175002: REPORT of '/repos/xxx/!svn/vcc/default'

I am trying to checkout the code from a remote computer.
Answer: After long research, I finally found a solution the solution was to put allow from all in the svn configuration on httpd.conf:
<Directory /repos>
...
allow from all
...
</Directory>

The svn checkout is working fine now
The above are for apache 2.2
if you use apache 2.4
<Directory /repos>
    ...
    require all granted
    ...
    </Directory>

Comment: In 1.7.2 the support for Directory directives is lost as mod_dav_svn will no longer map URLs to the filesystem.  Instead be sure to use the Location directive. http://subversion.apache.org/docs/release-notes/1.7.html#mod_dav_svn-fsmap
Comment: If you use apache 2.4 use require all granted instead
Answer: In my case I had E175002 because of missing DNS records of Subversion server.
Answer: I had 2 issues:
a) The svn folder was not readable by apache user
b) Possibly the Directory was not setup correctly
In my example, SVN repository is installed under /home/svn.
1) chown apache.svn -R /home/svn
2) Modify subversion.conf:
<VirtualHost svn.xxx.com:443>
    ServerName svn.xxx.com
    DocumentRoot /home/svn/

  <Directory "/home/svn/">
        AllowOverride None
        Order allow,deny
        Allow from all
        Options None
    </Directory>

SSLEngine on
SSLCipherSuite ALL:!ADH:!EXPORT56:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv2:+EXP:+eNULL

SSLCertificateFile /etc/pki/tls/certs/ca.crt
SSLCertificateKeyFile /etc/pki/tls/private/ca.key

<FilesMatch "\.(cgi|shtml|jsp|html|htm|php?)$">
    SSLOptions +StdEnvVars
</FilesMatch>

SetEnvIf User-Agent ".*MSIE.*" \
         nokeepalive ssl-unclean-shutdown \
         downgrade-1.0 force-response-1.0

CustomLog /var/log/httpd/ssl_request_log \
          "%t %h %{SSL_PROTOCOL}x %{SSL_CIPHER}x \"%r\" %b"

alias /svn /home/svn

<Location /svn>
 DAV svn
 SVNPath /home/svn
 SVNListParentPath On
 AuthType Basic
 AuthName "Repositorio Subversion"
 AuthUserFile /etc/httpd/dav_svn.passwd
 Require valid-user
 allow from all
</Location>

</VirtualHost>

Answer: I have changed the windows password and then I have tried checking out the code and I end up with the above unknown host error.
I have restarted eclipse and restarted virtual machine where my eclipse has been set up.
Now I am able to checkout. Hope this helps.
Answer: I had the same error, but the cause for the error was, that I am behind a proxy server. I could resolve this issue by configuring SVN to use this proxy server.
Details how to do this can be found in the following discussion:
How to configure a HTTP proxy for svn
Answer: in OSX environment, can check ~/.subversion/server
if you are behind proxy, can edit http-proxy-xxxxx attribute
Answer: This same problem happened for me and it was possible to finish checking the project out by running the UPDATE SVN command on the incomplete checked out folder.
Answer: I had this issue in Code Composer Studio today where only one of the four files I was trying to commit produced the error. Using the command line solved it for me.
More detail:
No commands from the plugin would clear the error, but commit from the command line worked fine. (That likely means either a bug or misconfiguration in the plugin. But as long as the CLI works, I can live with it. The plugin--subclipse--seems to be no longer supported, so I use it when it seems to work, and fall back to the command line when it doesn't. So far, every issue that's appeared in the plugin has not occurred with the command line.)


How are energy conserved and momentum both conserved in this system?

Question: In space, a photon with momentum $P$ is reflected off a previously at rest mirror, imparting momentum to the mirror. Even if the wavelength of the reflected photon is much longer than the that of the incident, the reflected photon still has some momentum vector pointing the opposite direction, meaning that the momentum of the mirror must be greater than $P$ in order to follow the conservation of momentum of the entire system. But this would all mean that the sum of the magnitudes of the momentum vectors has increased, so somehow the energy of the system has increased? Where did I go wrong?
Comment: Let me know if I should clarify
Answer: The wavelength of the reflected photon will be slightly larger, so that its energy and momentum have decreased. So not all of the momentum vector magnitudes increase. The photon loses energy to provide it to the mirror. This effect is tiny, which is why usually we don't notice mirrors moving or wavelengths changing after reflecting off mirrors.
Related:
Energy conservation in reflection of light from a perfect mirror
Question about transfer of momentum in photons
Comment: Sorry I meant to say longer instead of shorter. But this helps anyway thx
Answer: Let's look at this one by one:
I'm assuming here that the mirror does not have infinite mass, and that it is perfectly reflecting (only absorbs energy by the momentum change of the photon, doesn't absorb any energy from the photon directly).
Now,
1. Momentum Conservation: We know that the momentum of a photon is p=h/λ. When the photon is reflected, it changes direction. If the mirror was large enough, we could assume that the photon didn't give any energy to the mirror, but this is not the case here.
So, some of the photon's energy (and hence, momentum) goes to the mirror.
Let P(1) and P(2) be the initial and final momenta of the photon, P(m) be the final momentum of the mirror, λ(1) and λ(2) be the initial and final wavelength of the photon.
By momentum conservation: P(1)=P(m)+P(2)
Taking direction of P(1) as positive, h/λ(1)=P(m) - h/λ(2)  (Since some momentum is lost, P(1)>P(2), and so λ(1)<λ(2). So yes, the momentum of mirror is indeed greater than the initial momentum of photon.
2. Energy Conservation: We know that the energy of a photon is hc/λ. Since we've established that the mirror absorbs some energy (only due to the momentum change of photon), we can say that the photon loses energy, and is redshifted. Again, E(1) and E(2) are the initial and final energies of photon, and E(m) is the final energy of mirror.
By energy conservation: E(1)=E(m)+E(2), and so hc/λ(1)=hcλ(2)+E(m).
So, the mirror does absorb energy(and momentum), but no conservation laws are violated.
Comment: No, that's why I added for the sake of simplicity. In reality, the final speed of the ball will be less than the initial speed, based on the masses of the ball and the box. So, net kinetic energy will be constant. However, since the direction is reversed, the momentum of the box will be greater than the initial momentum of the ball.
Comment: So, the momentum of the mirror is greater than than the initial photon's, but it's energy is less than the initial photon's?
Comment: Yeah. It's seeming a bit counterintuitive to you because you may have developed a false intuition of momentum. Let me present it like this if I throw a ball at a stationary box, and (for the sake of simplicity), the ball rebounds off the box with the same velocity, the momentum the ball will impart on the box will be twice the initial momentum of the ball. While, the energy will obviously not be more than the ball. The reason for this is energy can't be negative, while momentum can be. So, a body can impart more momentum on an object than it has, if the body reverses direction.
Comment: Ok, sure, but now the ball is moving at the same speed, but the box is now also moving. So the total kinetic energy has increased somehow.


What kind of name is "Underground Railroad"?

Question: The "Underground Railroad" wasn't a real railroad but it was given that name. Is there a word for what kind of name "Underground Railroad" is? It's a made up name for a phenomenon that's also sort of a metaphor (as in if it were not a recognized proper name, someone might still write "they traveled along the underground railroad" as a metaphor). Is there such thing as a proper name metaphor?
Comment: For the most part it wasn't  "underground" either.
Comment: Historical names are not like other names. What about the Velvet Revolution?
Answer: Sounds like a kenning to me.  A kenning is a compound, usually consisting of 2 nouns, which metaphorically describe something or someone.
Examples:
"Missile toes"---a marathon winner.  "I'd never participate in a marathon; I'm no missile toes."
"Gutsquatters"---intestinal parasites.  "Those darn gutsquatters I picked up in South America have been plaguing me."
"Word pruner"---editor.  "After a word pruner finishes checking over a manuscript, as much as half of it might be gone."
(http://examples.yourdictionary.com/examples-of-kenning.html)
Comment: Hey Wordster, this is really interesting. I had never heard this word before and I think it applies. On this site, though, we try to give complete answers and to not make people follow links. (Give the links, but also give the relevant content.) Go check out some well scored answers on the site. When you've edited let me know and I'll come back to upvote.
Comment: Thanx, @Unrelated.  I did my best editing the answer, but am thinking that it's a bit redundant in the face of Kris having provided a thorough answer.  I appreciate the upvote!
Comment: Although it's related to OE/Old Norse, some linguists seem to use this term when they see Bahuvrihi samasa. In that sense this answer is also relevant and useful. I had up voted this answer early on.
Answer: Figurative might be what you are looking for.
The figuratively named "underground railroad" helped to smuggle slaves to free land.
Answer: It's a linguistic device originating in Sanskrit and also found in Old English and Old Norse poetry.
It's a compound expression known as Bahuvrihi, similar to an idiomatic expression but here the literal meanings of the component words are also significant.
Bahuvrihi (WP)

A bahuvrihi compound (from Sanskrit: बहुव्रीहि, literally meaning "much rice" but denoting a rich man) is a type of compound in Sanskrit grammar, that denotes a referent by specifying a certain characteristic or quality the referent possesses.
Examples

"Houndstooth", a woven fabric with a patterns resembling dog's teeth: "She's wearing houndstooth."
"Old money", members from established upper-class who have usually inherited their wealth: "He's definitely old money."
"Bluestocking", an educated, intellectual or artistically accomplished woman: "Auntie Maud will never marry; she's a bluestocking."
Comment: As much as I love Sanskrit in general and bahuvrīhi compounds in particular, this one is not an example IMO. In a bahuvrīhi compound (also called "exocentric" in English) not only is the "head" not present in the compound, but also it is related to the components of the compound. For example, "[cloth with] hounds' tooth", "[one with] old money", "[one wearing] blue stocking" etc. (See a good set of examples [here](https://www.victoria.ac.nz/lals/about/staff/publications/Bauer-EnglishExocentricCompounds.pdf), though the author is wrong about Sanskrit and how narrowly "bahuvrīhi" is used there.)


facebook cocos2d-x integration invalid key hash

Question: I followed How to create Android Facebook Key Hash? keytool -exportcert -alias androiddebugkey -keystore debug.keystore | openssl sha1 -binary | openssl base64 and the result matches what is on my facebook app settings page I still got the same error
Answer: I was trying it recently. And I used android platform.
I found there are two key hashes I have to find out.

The key hash at my build environment saved at /.android/ like you did.
The key hash from the device, which mentioned in this link( the reply of "you can use code below to get the Hash key" ).
Key hash for Facebook Android SDK

I filled them both at facebook app setting. I hope this helps you.


Slow light and gravitational lensing

Question: It has been proposed that the Sun's gravitational lens be used to observe distant objects, but according to relativity a spacecraft would need to be 550 AU away in order to take advantage of the effect.
Would it be possible to instead slow down the light while it is being deflected using, for example, a Bose-Einstein condensate such as a large quantity of very cold sodium placed into orbit around an asteroid?
An answer to this question mentioned that it is useless to do the same thing using an ordinary material with a high refractive index because of dispersion, but as I understand it the mechanism for producing slow light changes the group velocity, which is the speed at which energy is carried, instead of the phase velocity.
Answer: The limiting factor in optical observations isn't usually the magnification but rather how much light can be collected. The advantage of using the Sun as a lens isn't that it's especially powerful, but that it's exceptionally big!
So you could sit on an asteroid to make your observations, but there would be no benefit as you'd be getting little more of the light from the distant object than if the Sun wasn't there. The point of going out to 550AU is that all the light bent by the Sun's gravitational field converges at this point so you get a huge increase in the brightness of the image.
Comment: Thank you!  I'm sorry, I think my question was a little confusing.  What I meant to propose was that we use the asteroid as a lens.  Does your answer mean that this still won't work because not enough light could be collected?  If so, why do we care about reducing the focal length in an ordinary lens?
Comment: @Tom: the point is that without the Sun the asteroid sized lens can capture some amount of light, call this $L_0$, and with the Sun present the asteroid can capture a bit more light, $L_1$, because of the Sun's focussing effect. However when you're near to the Sun its focussing effect is tiny, and $L_0 \approx L_1$ - i.e. the presence of the Sun makes no significant difference. If you go out to 550 AU the Sun will have strongly focussed the light and your asteroid sized lens will capture orders of magnitude more light i.e. $L_1 \gg L_0$.

Slow light and gravitational lensing