How can I halt the spread of corruption after winning hell?

Question: After defeating the Wall of Flesh, corruption is spreading much faster and bypasses sunflowers. Are there any other methods of halting it than hallowing?
Comment: possible duplicate of [How can I save the world from both good and evil corruption?](http://gaming.stackexchange.com/questions/41839/how-can-i-save-the-world-from-both-good-and-evil-corruption)
Answer: You just gotta make sure the Corruption is contained (i.e., has nothing to spread to). Any Corrupted block can spread via these blocks:

Exposed Dirt and Grass
Stone
Sand
Jungle Grass
Mud

Additionally, it can jump up to three blocks (that is, clear 2-wide gaps). Therefore, digging a trench lined with incorruptible materials should stop it. However, preserving the natural biome isn't very practical in the long run, for several reasons:

The Corruption can spread underground via the abundant Stone, and possibly tunnel under your trenches and corrupt everything in a heartbeat.
Corrupted grass can develop thorns, which could reach across your trenches and start corrupting.

Creating a Hallowed biome really is your best bet against the Corruption. It spreads just like the Corruption, but they cancel each other out. Corrupt blocks will not infect hallowed ones and vice versa.
It's still rather dangerous to be in, but I believe it is the lesser of two evils.
Answer: I believe a Hallowed Biome will prevent them from spreading. These can be created by planting Hallowed Seeds, which are gained by smashing a Demon Altar with a PwnHammer.
Edit: I've discovered I was mistaken about how to get Hallowed Seeds. You don't directly get them from the Demon Altar, but instead buy them from the Dryad after smashing the altar.
Comment: This is what I ended up doing; entirely hallowed surface FTW!
Comment: That's what I'm working on right now, but I've planted corrupted seeds on a couple of my flying islands, in case I want something from corruption.
Answer: Depending on what you need, you can stop Corruption with the following methods:

If you need to keep corruption away from your housing, you could 'dome' the house with a non-corruptible material such as Clay, Glass, any ores, bricks, etc. Note, though, that your dome must be 4 blocks across or corruption can go to the other side.
If you need a quick temporary fix, you Can buy a lot of Purification Powder from the Dryad and purify your corruption, which will slow it.
If you want to somewhat cheat, I think Tshock has the ability to convert corruption to normal blocks.
If you have access to the Steampunker you can buy the Clentaminator and Green Solution. However, this is very expensive and requires you to defeat a Hardmode Boss to spawn the Steampunker.
Answer: Neither Corruption or Crimson can get past a mushroom biome. If you have a jungle nearby that you want to preserve, purchase some dark blue solution from the shroomite, dig a hole down to the underworld, and line both sides with the dark blue solution using the clentaminator. mushroom biomes don't spread, so it acts as a static wall between your jungle and the corruption/crimson.
I've found that simply keeping a clentaminator with me at all times is the best way to go about getting items; if I need a specific biome, I simply create one right around me. I most commonly use this when I need a pure (non hallow non corrupt) underground area.
Answer: If you play on an Xbox or PlayStation, you can make a wall of dirt separating the corruption/crimson from the forest and plant Hallowed seeds. The hallow will eventually take over the corruption/crimson. Although the hallow is very dangerous as well as the corruption and crimson, it's a way to make your world less evil.


Is it possible to exchange seeds in uTorrent?

Question: Lets say a torrent release has 100 seeds, and I am connected to 10 of them right now. If  those seeds have very bad upload speed (I can see it from peer stats), can I add those seeds to a blacklist or something and connect to 10 different ones instead?
Answer: All clients based upon BitTorrent (including uTorrent) have a setting in the Preferences, under the Advanced tab, below the "gui_" section, that says ipfilter_enable.
If you enable this, then create/download an ipfilter.dat file & add the IP of the machine you wish to block, & restart uTorrent, any IP you choose can be blocked.
As for adding peers, just select the torrent in your list, open the Peers tab, right click inside it & choose Add Peer, then paste the IP of your buddy.
Answer: I have used torrents for a long time and have never came across a blacklisting feature in any torrent clients. I know this doesn't answer your question but its the best i can do.


SVN Mac - Stripping files of SVN meta data?

Question: I downloaded some source files on a Mac that were previously part of some working copy on the authors computer, I need to use these files in another repository but the SVN client "Versions" for Mac is picking up on the data from this old repository. I can't find the ".SVN" folder anywhere... any idea on how to "cleanse" these files so I can commit them to my repository?
Comment: the directories are .svn in lowercase which are hidden by default in Finder, and ls in terminal without doing ls -la
Answer: The .svn folder is just a hidden directory which will appear in every (or at least most) of the directories in your source tree.
To remove all of them, you can open up Terminal, navigate to your source directory, and then use the following line (found here):
find . -name ".svn" -type d -exec rm -rf {} \;

Comment: As usual, please be careful when messing about in the Terminal :)
Answer: The .svn folder can sometimes be named _svn. You can try looking for those folders and removing them.
Also, if your Subversion client (Versions) sees the folder as a working copy, you can try to perform SVN export from the folder to itself. It should clean the SVN meta-data.
Answer: As an indirect answer, I would highly recommend the Finder replacement shell PathFinder to any developers on Macs.  It's useful for many reasons, the relevant one to this post being the ability to show hidden files/folders by default (which I always keep on).  I've done this exact thing before -- simply find all ".svn" folders in your root folder using PathFinder and nuke them -- much safer than mass deleting in terminal (at least for some of us :)
[No, I have no relation to the company, I'm merely a happy customer.]
Answer: Remove Subversion .svn Folders on Mac OS X
If you need to clean up a folder on Max OS X that was previously under Subversion, you need to remove the hidden .svn/ folders.
Here is a quick terminal command that can do the job:
Open Terminal (Applications -> Utilities)
cd to the folder you want to clean up
Run
find ./ -name ".svn" | xargs rm -Rf

You are now free of Subversion's source control.


I can't find the Einstein relation for spin diffusivity

Question: I'm studying the article "Thermodynamic analysis of interfacial transport and of the thermomagnetoelectric system" (PhysRevB.35.4959, Mark Johnson and R. H. Silsbee); they use this relation
$$
D=\frac{\sigma \beta^2}{\chi e^2}
$$
calling it "Einstein relation", but I can't find it anywhere on textbooks nor online, where the actual Einstein relation is written in terms of the mobility and thermal energy
$$
D=\mu k_B T
$$
What is the origin of that relation? Where I can find it?
Answer: I am not sure about the $\beta^2$ part. Coleman derives $\sigma = e^2\chi D$ in his book Introduction to many-body physics Sec 10.4. The short version of it is that the density-density response function receives a correction when there are impurities. In perturbation theory in momentum $q$, one can identify the coefficient of $q^2$ as the diffusion constant.
Edit: The field theory is very nice. But I came across another intuitive picture in the book Electronic Transport in Mesoscopic Systems by Datta. I will be paraphrasing Sec 1.7 of that book in the following.
Consider applying a small field $E_x$ onto a 2DEG. Denote the changed Fermi energy at the left edge $\mu_1$, and $\mu_2$ at the right edge.  To make the connection with diffusion, remember that in a Fermi liquid, currents are carried by electrons near the Fermi surface. In other words, the electrons that actually make contributions are the ones in the energy interval $\mu_2<E<\mu_1$. The net effect of the electric field is to produce a density gradient across the 2DEG. Using the diffusion equation $J = -eD\nabla n = eD\chi(\mu_1-\mu_2)/L = e^2D\chi E_x$, we get $\sigma = e^2\chi D$. $\chi$ here is the density of states.
One can also think the current as simply $J = env_d$, where $n$ is the electron density and $v_d$ is the drift velocity. Since mobility $\mu = e\tau/m$ (and $v_d = eE_x\tau/m$), conductivity $\sigma = en\mu$. Comparing with the expression above, we get the usual $D=E_f\mu/e=v_f^2\tau/2$.
Notice this picture only works for a Fermi liquid or a degenerate (semi)conductor. For a non-degenerate (semi)conductor, the important energy scale is $k_BT$ rather than $E_f$. Then we get the second relation you have: $D = \mu k_BT/e$.


What will "rm -rf /" actually do?

Question: I know that this is a very dangerous command, but what does it actually do. In other words, to what extend does it delete the hard drive. Does it delete only the user data? Or does it delete root OS files as well. If someone were to activate this command, would it break the OS?
Running as sudo on Yosemite
Comment: possible duplicate of [Legendary sudo rm -rf /](http://apple.stackexchange.com/questions/125156/legendary-sudo-rm-rf)
Comment: It's going to depend quite a bit on whether you run the command as root (i.e. with `sudo`), as an administrator, or as a standard user. It'll also depend on which version of OS X you're using -- IIRC there was at least one version where `rm` failed as soon as it'd taken out /bin for some reason. On the other hand, when run as root under OS X Yosemite, the destruction is impressively complete.
Answer: The command itself is "rm", which is a program used to remove a file.
The "-rf" is the first argument to the program. Arguments can be thought of as "settings" or other information that you want the program to use while performing its task.
"-rf" is really a short hand for two seperate arguments, "-r" and "-f":
-r means that you want the program to not only remove files, but also directories, and that you want the program to remove not only the files/folders that you request, but also any subfolders and their contents.
-f means that you do not want to be prompted for confirmation on each delete, and that attempts should be made to delete every file/folder listed, no matter if you have permissions to do so orr not.
The last part of the command "/" is actually the list of what you want the program to delete. In this case "/" means the "root" (the top) of the file hierarchy. In practice this means to do delete everything on your system - including files on external drives that may be mounted.
The command does not discriminate between user data or operating system files. As long as you have permissions to delete a file/folder, it will be deleted.
When run as sudo (i.e. with root permissions) under Yosemite, it will break the system so that it doesn't function afterwards.
For newer versions of macOS, destruction will be limited by the SIP system that protects the operating system core. However, still do not run the command with sudo, as you risk ruining the system and deleting your own data.

What will "rm -rf /" actually do?


How to achieve large horizontal aerial propulsion with Marth/Lucina's Shieldbreaker?

Question: I've been playing Smash 4 quite a bit lately, and have seen online that some players are able to use Marth/Lucina's Shieldbreaker to propel them a substantial distance horizontally while in the air. Checking smashboards and the wiki confirms that Shieldbreaker was buffed to have a small amount of horizontal propulsion. My own experimentation that simply using shieldbreaker in the air produces a small amount of horizontal propulsion, however nowhere near the levels I've seen online. Does anyone know how to do this? I tried inputting a direction after starting shieldbreaker, much like Dedede's gordos require, but it didn't seem to do anything.
Answer: Marth/Lucina's shieldbreaker can be held down for longer to create a stronger hit (as demonstrated here). When the shieldbreaker is tapped in midair, you'll receive a small amount of horizontal propulsion. However, if you hold down neutral B in midair and charge the shieldbreaker fully, Marth will gain a large amount of horizontal propulsion. You don't need any other button inputs to get the boost.
Note that once you reach a full shieldbreaker charge, Marth/Lucina will automatically complete the attack (which is where you get the large boost). If you try to let go too soon in midair, you'll only get the small amount of propulsion. Also note that Marth/Lucina cannot jump high enough to perform this boost by the time they reach the ground on a level stage like Final Destination (You'll hit the ground before the charging is complete, so you'll throw a ground shieldbreaker instead).
Answer: Short and simple, hold down B and don't let go.
It's very useful for recovery if you get launched off to the upper corners. You might want to practice at a huge stage before trying it out in battle, because you can't just double jump and then do the shieldbreaker. You have to be really far up in the air to carry it to its full extent.


Length contraction in a boxcar problem

Question: I'm trying to understand length contraction with an eye to the relativity of simultaneity, but things didn't go well. Let's say Alice and Bob both try to find the length of a boxcar moving to the right with speed $v$. Alice is on the boxcar, while Bob is standing still on the ground. As is known to many of us, moving objects are shortened. So Bob would measure the length of the boxcar to be a number $L$ less than the reading $L_0$ taken by Alice. I'm trying to explain how the contracted length arose, by employing the relativity of simultaneity: in Alice's rest frame, Bob is moving to the left; thus, Alice would say that Bob measured incorrectly because he didn't take the readings of both ends simultaneously. In fact, according to my textbook, Alice would complain that Bob measured the front end first and then the back end. Why is that? I can understand the complaint immediately by using the transformation formula
$$\Delta t'=\gamma(\Delta t-\frac{v}{c^2}\Delta x)$$
with $\Delta t=0$ and noting that the prime denotes the boxcar reference. But, is there any other approach that doesn't involve the Lorentz transformation? Thank you.
Comment: "Alice would complain that Bob measured the front end first and then the back end. Why is that?" - For intuition about the relativity of simultaneity, see my very informal answer [here](https://physics.stackexchange.com/a/664875/132945); basically, the two of them don't "see" the same spatiotemporal slice of the boxcar (think of it as a 4D object).
Comment: " is there any other approach that doesn't involve the Lorentz transformation? "  Of course not.  All of relativity comes down to the Lorentz transformation.
Comment: @WillO Perhaps I'm a little clumsy with the words. Many textbooks derive relativistic results, such as time dilation and length contraction, without **directly** resorting to the Lorentz transformation. These authors just don't feed any numbers into the transformation but still arrive at the same results. And that's what I'm trying to do in this boxcar problem.
Comment: Draw a spacetime diagram, as @FilipMilovanović did in the link in the comment. It's useful to think of defining "simultaneity" using "radar methods". It can be to used to argue geometrically that different observers have a different sense of simultaneity for a given pair of events... without actually computing or deriving any algebraic formula [unless one wanted more details].  "A spacetime diagram is worth a thousand words."
Comment: **Thank you, everyone.** I found this problem when I read Griffiths's introduction to electrodynamics. Actually, it's not a homework exercise, but a statement embedded in his solution to a paradoxical problem in Lorentz contraction. I'm wondering how Griffiths concluded Alice's complaint. And, for your information, the professor has **NOT** introduced anything about the Lorentz transformation and spacetime diagrams up to his solution. So I believe he is using some elementary facts instead of invoking any transformation formula or any weird diagram.
Answer: The key to understanding length contraction is to realise that to measure the length of a moving object you have to note the positions of its two ends at the same instant, and then work out the distance between them. If you note the position of one end and then the other, the second end will have moved in the time since you measured the first, so you will get the wrong answer.
The cause of length contraction is that reference frames that are moving relative to each other have planes of simultaneity that are tilted relative to each other. What that means is that events that are simultaneous to Alice happen at different times according to Bob, and vice versa.
When measuring the length of the moving box car, Bob notes the positions of the two ends of the car at a given instant in his frame- as far as he is concerned, he pinned down the two positions simultaneously, so it was a valid measurement. However, in Alice's frame the two events were not simultaneous- in her frame, Bob noted the position of the rear of the train a moment after he had noted the position of the front, which gave the rear of the train time to move forward slightly before its position was noted, thus giving a shortened result for the length of the car.
There is a nice thought experiment you can do to remember how the planes of simultaneity tilt for moving observers. Imagine Alice and Bob stand together, and Alice shines a flash of light to the left and to the right. Alice stands still but Bob walks after the light to the right.
After a nanosecond in Alice's frame, the light will have travelled (roughly) a foot away in each direction. In her plane of simultaneity there are two simultaneous events- namely the light being a foot to her left and the light being a foot to her right. In Bob's frame, however, as he is walking after the right hand flash, at the instant the two flashes are a foot from Alice, the right hand flash is slightly less than a foot from him, and the left hand flash is slightly more than a foot from him. Since the speed of light is the same in all directions, from Bob's perspective both flashes should be equidistant from him at any given instant, so the only way in which the right hand flash can be nearer to him than the left hand flash is if the positions of the two flashes are being noted at different times. From his perspective the right hand flash was a foot away from Alice (and just under a foot away from him) at one moment, then the left hand flash was a foot away from Alice (and just over a foot away from him) at a later moment. The two events that were simultaneous for Alice happened at different tines for Bob.
You should also be able to work out from the though experiment that from Bob's perspective Alice's clocks are getting progressively out of synch in each direction- appearing to be set ahead of time in the direction he is headed, and set behind time in the opposite direction.
Of course, the entire set-up is totally symmetrical. Alice thinks Bob's clocks are out by the same amount in the other direction.
Answer:
"The professor has NOT introduced anything about the Lorentz
transformation and spacetime diagrams up to his solution. So I believe
he is using some elementary facts instead of invoking any
transformation formula or any weird diagram."

In a thought experiment, you might try and use the fact that the speed of light is the same for both observers to come to this conclusion. Suppose Bob has already measured the length of a moving boxcar to be $l_\text{train}^\text{Bob}$, and wanted to confirm this measurement and discuss it with Alice. Bob creates the following setup: two light sources are placed alongside the tracks the distance $l_\text{device}^\text{Bob} = l_\text{train}^\text{Bob}$ apart, and a detector is placed exactly in the middle. When the boxcar aligns with the two sources (when the front and the back pass their corresponding post), a mechanism triggers, and they each emit short light pulse - these signals travel at the speed of light, and hit the detector on each side (they have traveled the same distance). Alice will record her own observations of events, and also determine these lengths in her own frame: $l_\text{device}^\text{Alice}, l_\text{train}^\text{Alice}$

Why this setup? Because Bob wants to know what Alice observes in her frame, and this setup has a number of distinct single-place events that both Alice and Bob will agree upon. An event (which is what we call a point in space and time) is as such agreed upon by all observers. They might assign it different local coordinates, but a thing that happened at a particular place, happened - there's no way around it. It's just that observers might disagree if two distinct events (things that are, in any frame, separated in space and/or time) are simultaneous or not.
So the unique events of interest here are:

the front end of the boxcar is at the same place1 & time as the front
light (and the signal is triggered at this exact moment),

the back end of the boxcar is at the same place1 & time as the rear
light, (and the signal is triggered at this exact moment),

the two signals arrive to the location of the detector (so at a
particular place) at the same time (both observers must agree on
what the detector shows, and the detector indicates simultaneous
arrival). This is considered a single event (single point in spacetime - one set of coordinates), not two distinct, but simultaneous events.

1 They occupy different points in 3D space, but since the effects of special relativity only occur in the direction of motion, we can suppress the coordinate perpendicular to it, and treat the problem as two-dimensional. So, in the 2D picture, the front end of the boxcar, the front light & the emission triggering event all have the exact same coordinates (t, x, y).

From Alice's perspective, the whole setup is moving past her in the opposite direction. The event (1) happens when the light source on the right passes the front of the boxcar, the event (2) happens when the light source on the left passes the back of the boxcar. So from her perspective, the signals are emitted from the locations of the two ends of the boxcar (both emission points are stationary; it's just a very quick pulse, and the origin does not move with the source). The signals themselves are traveling at the speed of light. Note that I haven't said anything yet about when each emission happened.
Now, since the detector is in motion, if the signals were emitted simultaneously (from Alice's point of view), the signal coming from the back of the boxcar would hit the detector first (because the detector is moving towards it), and the signal from the front would catch up some time later.
But the detector was triggered, and it shows that the signals arrived at the same time, in accordance with (3) - both Alice and Bob must agree on this.
This can only happen if, in Alice's frame, the device was shorter than the boxcar, so that (1) triggers before (2) in such a way as to allow the signal emitted from the front end to catch up to the detector just as the detector slams into the signal coming from the back.
(Note that in the image below, the origin of the arrow is an imaginary point of reference; it tells you from where the signal was emitted so that you can see the travel distance. To Bob, the signal appears to originate from the tilted black line. The the tip of the arrow, though, corresponds to the propagation of the wavefront, which is physical.)

This then tells you (a) that Alice and Bob disagree on simultaneity of distinct events, and (b) that both Alice and Bob see the moving object as shortened (Alice sees the measuring contraption as shortened, and Bob sees the boxcar as shorter than it is in Alice's frame):
$$
l_\text{device}^\text{Alice} < l_\text{device}^\text{Bob} \\
\text & \\
l_\text{train}^\text{Alice} > l_\text{train}^\text{Bob}
$$
The back end of the boxcar that Bob experiences as existing at the same time as the front end is in Alice's future. Suppose the back end explodes at the moment the endpoints of Bob's device light up; in Bob's view, the state of the boxcar is that it is a wreck with a hole at the back end. For Alice, who is leaning against the wall at the front end, and who happens to be giving a quick yawn at the moment when the first light source emits the signal, the explosion hasn't happened yet. Even so, Bob sees this same Alice standing and yawning next to a developing explosion. They are literally not experiencing the same time-slice of the world; their simultaneity hyperplanes (what they experience as "now") are tilted in time with respect to one another.

Length contraction in a boxcar problem


Is there any combination of numbers which upon division gives the exact number of P?

Question: In other words, there are (probably) infinite combination of numbers/operations which leads to irrational numbers. So I wonder, if there is one which gives exact number representation of P(π)? Do we need to measure the ratio of diameter/circumference or we can actually get infinitely accurate representation by just doing simple math? Thanks.
Comment: $3+.1+.04+.001+.0005+...$
Comment: See http://en.wikipedia.org/wiki/Pi.
Answer: That depends on what you consider simple math. The most basic sequence you can get (i.e. looks simple) is $\pi/4 = 1 -\frac13+\frac15-\frac17+\frac19+...$.
However, as this one converges painfully slow, no one in their right mind uses it because more effective formulas have been known for ages (for example, I once used $4\arctan {\frac15} - \arctan {\frac1{239}}$). Currently rapidly converging series are used. Read the Wikipedia article on this, as suggested in the comments :)
Comment: Sorry, well... $arctan(x)$ itself is calculated using taylor series $x - \frac{x^3}3 +\frac{x^5}5 -...$, which, I guess, is simple enough math, even if proving the fact that it actually equals this is beyond what you can consider "elementary math" that  anyone knows.


What does Paul mean when he said, "she ... is dead even while she is still alive" in 1 Timothy 5:6?

Question:
1 Timothy 5:6 But she who lives for pleasure is dead even while she is still alive.
Comment: It is a site requirement to identify the translation you are citing. Thanks.
Answer: This is a very common NT idiom.  The word (noun) for "dead" is "nekros" and is used of physical death (obviously) but also spiritual death to describe an unsaved person who is physically alive but spiritually dead.  Here are some examples:

Matt 8:22 - But Jesus told him, "Follow me, and let the dead bury their own dead."
Eph 2:1 - As for you, you were dead in your transgressions and sins,
Eph 2:5 - made us alive with Christ even when we were dead in transgressions--it is by grace you have been saved.
Eph 5:14 - This is why it is said: "Wake up, sleeper, rise from the dead, and Christ will shine on you.
Col 2:13 - When you were dead in your sins and in the uncircumcision of your flesh, God made you alive with Christ. He forgave us all our sins

The same idea is extended to the condition that Christians should have in regard to sin.  See Rom 6:11 - In the same way, count yourselves dead to sin but alive to God in Christ Jesus.  James used the same idea extended to faith in James 2:17, "In the same way, faith by itself, if it is not accompanied by action, is dead."
In 1 Tim 5:6 we have the verb, "tethveken" (= having died) which is the perfect form of "thnesko".  However, in view of the above, the sense is clear.  The woman is physically alive by spiritually dead.
Comment: I don't think that Paul is saying she's not a believer or saved, personally. Paul didn't even say that about the guy sleeping with his [prostitute?] mom. I think Possibility hit the nail on the head with the "unproductiveness" angle. She "might as well be dead" if she isn't serving. -1 (to offset all the upvotes just a tad!)
Answer: I agree with Dr. Peter and would only add that the immediate context tells us in what way she is spiritually dead. Here are verses 3-6:

3 Honour widows that are widows indeed. 4 But if any widow have
  children or nephews, let them learn first to shew piety at home, and
  to requite their parents: for that is good and acceptable before God.
  5 Now she that is a widow indeed, and desolate, trusteth in God, and
  continueth in supplications and prayers night and day. 6 But she that
  liveth in pleasure is dead while she liveth.

These verses are dealing with giving support to those women who are truly widows (V3).  V4 states that if the widow or nephews, that they should support the widow before the church. V5 says that if a woman is truly a widow without any family then she will act as becoming a godly woman. However, V6 states, the woman who lives in pleasure does not honor Christ.
The word pleasure means to be sexually wanton.  See Strong's:
https://www.blueletterbible.org/lang/lexicon/lexicon.cfm?Strongs=G4684&t=KJV
Strong's uses the words "to be voluptuous" and  "be wanton".  Both words support sexual meanings. In addition, the continuing context tells you this is a correct application. Verses 11 and 12 tell you that the younger widows will face damnation (spiritual death) having left off from their faith when they will "wax wanton against Christ they will marry".

11 But the younger widows refuse: for when they have begun to wax
  wanton against Christ, they will marry; 12 Having damnation, because
  they have cast off their first faith. 13 And withal they learn to be
  idle, wandering about from house to house; and not only idle, but
  tattlers also and busybodies, speaking things which they ought not. 14
  I will therefore that the younger women marry, bear children, guide
  the house, give none occasion to the adversary to speak reproachfully.
  15 For some are already turned aside after Satan.

So, she who gives herself over to unrestrained sexual passion, while physically pleasurable, is in fact spiritually dead.
1 Corinthians 6:15-20 (KJV)

15 Know ye not that your bodies are the members of Christ? shall I
  then take the members of Christ, and make them the members of an
  harlot? God forbid. 16 What? know ye not that he which is joined to an
  harlot is one body? for two, saith he, shall be one flesh. 17 But he
  that is joined unto the Lord is one spirit. 18 Flee fornication. Every
  sin that a man doeth is without the body; but he that committeth
  fornication sinneth against his own body. 19 What? know ye not that
  your body is the temple of the Holy Ghost which is in you, which ye
  have of God, and ye are not your own? 20 For ye are bought with a
  price: therefore glorify God in your body, and in your spirit, which
  are God's.
Comment: Would be beneficial to identify the Greek word and then cite a lexicon which supports your assertion.
Comment: Since your answer rests on the word "pleasure" and since you don't support your definition with a lexicon, -1.
Comment: In fact, I just looked it up and it means no such thing. https://www.blueletterbible.org/lang/lexicon/lexicon.cfm?Strongs=G4684&t=KJV  See also https://www.blueletterbible.org/kjv/jas/5/5/s_1151005
Comment: @Ruminator Thanks for the Strong's reference. I added it to my answer as it was the exact reference I used before I answered. Strong's uses the words voluptuously and "be wanton". In context it makes perfect sense to derive the sexual connotation since both words support that definition. Also, verse 11 tells you that this understanding is correct, for it says that a widow will marry after she "waxes wanton against Christ".  "11 But the younger widows refuse: for when they have begun to wax wanton against Christ, they will marry.KJV" I will update my answer to avoid confusion.
Comment: No, the word does not "mean" to be "wanton". And "wanton" does not "mean" "fornicating". Please quote the actual lexicon entry. The idea is living indulgently, not necessarily promiscuously. Actually, I recommend deleting your answer as you seem bent on forcing the incorrect answer despite it being patently false.
Comment: @ Ruminator; done
Comment: It seems to me that the younger widows are the ones Paul is concerned will leave their widowy commitments *and marry*. The older widows are more likely to blow the family fortune gambling or on shoes.  :)
Comment: LOL doesn't seem to be consistent with living on church support!
Comment: That is, I believe, Paul's point.
Answer: The letters to Timothy are personal letters from a church leader to his apprentice, not a public letter to the congregation or church community. It mostly contains housekeeping advice rather than instruction directly to the people.

The widow who is really in need and left all alone puts her hope in
  God and continues night and day to pray and to ask God for help. But
  the widow who lives for pleasure is dead even while she lives. Give
  the people these instructions, so that no one may be open to blame.
  Anyone who does not provide for their relatives, and especially for
  their own household, has denied the faith and is worse than an
  unbeliever. 1 Timothy 5: 5-8

The distinction is made between those widows who need to be supported because they have no other means, and those who expect to be supported by the church simply because they are widows. The instructions that Timothy has been advised to give to the people are two-fold:

the onus is on families (not the church) to support their widowed relatives as part of their commitment to the faith and Jesus' teaching;
a widow without family, who is expecting to be supported by the church, is expected in return to accept the role of 'widow' to the church in the same manner that a widow would serve her family:

bringing up children, showing hospitality, washing the feet of the
  Lord's people, helping those in trouble and devoting herself to all kinds of good deeds. (1 Timothy 5:10)

The 'widow who lives for pleasure' in this context would be one who expects to be supported by the church so that she can serve herself - so that she can live her own life, for her own pleasure. To say that she is 'dead even while she lives' refers in some way to Paul's teachings, that caution against living according to what the flesh desires:

Those who live according to the flesh have their minds set on what the
  flesh desires; but those who live in accordance with the Spirit have
  their minds set on what the Spirit desires. The mind governed by the
  flesh is death, but the mind governed by the Spirit is life and peace.
  Romans 8: 5-6

But it's important to recognise that a widow who had no family was considered of no use to anyone. With no childbearing years left, no children to raise, no means to support herself and no husband to serve, such a widow was simply a burden on society. To put it crudely, she was considered 'dead even while she still lives' - a body waiting to die.
A widow could avoid this status by living in accordance with the Spirit within the church community, who will support her as a valued member of the body of Christ. The advice to Timothy is that she shouldn't get that support by simply being a widow and then living according to the flesh - according to her own desires.
Comment: Very nice analysis Possibility, thanks. I learned a lot. +1
Answer: The above answers are terrific!  I would just like to add a comment by Warren Wiersbe, "the PASTOR'S PASTOR" (Greg Laurie):
"A widow the church helps should not be a self-indulgent person, seeking pleasure, but a godly woman who hopes in God and has a ministry of intercession and prayer. See Luke 2:36-37 for an example of a godly widow."
The Wiersbe Bible Commentary the complete New Testament NT in one volume


Does a philosophy scholar in the modern world need to study ancient philosophy?

Question: I am a philosophy enthusiast and I study philosophy on my own. I wanted to know whether a philosophy scholar in the modern world needs to study the ancient philosophical ideas, such as those by Socrates, Plato, Aristotle, etc.? In some subjects such as political philosophy, those ideas seem to be old and irrelevant to our modern days.
Comment: Edit to my comment (again) because it may give the wrong impression. It's not so much that you must read about, say, Aristotle, but you likely will. Making larger and larger circles around any philosophical topic to understand it better will lead you to ancient Greece sooner or later. And probably you end up spending time there more than expected, of course guided by the best secondary literature available today.
Comment: It's worth saying, political philosophy is not the exception you suggest. It requires a great deal of history, & the impacts of thinkers like Rousseau & Locke on that will be *essential*. I found this account of the importance of philosophy on Einstein in honing his thinking, salutary: https://www.britannica.com/story/what-einstein-meant-by-god-does-not-play-dice
Comment: I wasn't going to say anything, but I believe the almost universal opinion expressed here needs to be challenged. Although it is good to have a basic idea of what Aristotle, Plato, and the pre-Socratics had to say, none of it is really relevant to most modern philosophy and unless you find it appealing, I wouldn't dedicate too much time to any philosopher predating Descartes.
Comment: @David Gudeman, https://el.m.wiktionary.org/wiki/%CE%B5%CE%BD%CF%84%CE%B5%CE%BB%CE%AD%CF%87%CE%B5%CE%B9%CE%B1
Comment: If he wants to be real scholar, yes. People today aren't smarter than yesterday.
Answer: Look at the following three statements:

Euclidean geometry is basically about straight lines.

Newton saw the motion of a particle without any net force acting on it as moving in a straight line.

Einstein saw this particle moving on a straight line on a curved surface.

Here, we can see the most modern ideas traced back to that of earlier ideas, to ideas of the most ancient epoch. Newton said, that the only reason he could see further is because he stood on the shoulders of giants. And it is because that they are giants that we remain in dialogue with them.
Answer: It depends on which tradition you are talking about.
In the analytical tradition, like it is prevalent in the US or UK, it is sometimes not needed at all. That does by no means mean that you don't have to know about these philosophers or their ideas. You can learn these via handbooks and the like though. For example, one of my senior lecturers in the UK never actually read much before the 1800s. He talked and read a lot and learned about earlier philosophers from others though. On the other hand, this depends on the university. Many more prominent universities use ancient philosophy as standard text even across all studies, like for example the ivy league.
In the continental tradition, like it is prevalent in continental Europe, it is much more common that it is expected that you have actually read all the canonical philosophers' main works and some secondary literature on them. For example, one of my German professors didn't actually care much about older philosophy first and finished his undergraduate studies without reading anything written before 1960, which he was proud of. At that point, his supervisor told him that he has to study the roots of these ideas if he truly wanted to understand and develop the contemporary ideas further. In general, we argue a lot more based on primary literature.
In other words: from graduate studies on, it is expected you have studied ancient, and in general all the canonical philosophy at some point in continental Europe. In the UK or the US, you also have to be acquainted with the traditions in some way but there is much more emphasis on discussing and studying contemporary literature.
Answer: There is a mode of philosophy, which goes straight to modern ethical conundrums, and to thought-experiments. And it is notable ancient philosophy is rarely referenced in deciding what priority a self-driving car should give in who's life to prioritise in an accident, eg this paper.
Modern tools like game-theory can help us avoid simply telling people what they should do, and look at social dynamics, eg Is the tyrannicide perpetrated by William Tell morally legitimate?
I'd also argue a great deal of ethical thinking is summarised by the Golden Rule, probably the most culturally universal ethical principle. And that through the idea of intersubjectivity, inviting others into our perspective and sharing their perspective, we can do a great deal to reduce the need to cover the details of things like Kant's Categorical Imperative. Discussed here: Is the Categorical Imperative Simply Bad Math? :)
But you have to ask, you must ask, what is philosophy? How is it different from religion, from science, from mysticism? What are we doing, when we say we are doing philosophy? This kind of demarcation of disciplines, and defining of terms, is key philosophical work. I argue here, that we have to understand what socratic dialogue is, and that academia must be understood at least in major part as founded on it:
What is the difference between western and other philosophies?
Philosophy fits into an older and wider tradition, the cultivation of wisdom. Understanding the history of philosophy, can greatly help in seeing the need and the mechanisms, for understanding how wisdom helps us be better at solving dilemmas, by acting from the integrated centre of pur concerns:
Wisdom and John Vervaeke's awakening from the meaning crises?
You don't have to become a specialist in the history of philosophy to have your opinion taken seriously. But it really helps to have a short-hand, a shared tool-box of ideas, to have more productive and concise discussions. I discuss here how language and shared-intelligence, arise from shared-modes-of-life like reading the same books or having the same kinds of classroom discussions, and that through these we can create salience landscapes and effective terminology which help encode and transmit intelligence: According to the major theories of concepts, where do meanings come from? A great deal of detail can be dropped, but I think a broad-brush understanding of developments, like Vervaeke's series, is invaluable.
And if you want to get into the thorniest debates, like free-will, consciousness, the nature of time, or the status of mathematics, you will need substantial terminology and background reading just to get at what the contentions are, although most of the reading will be modern thinkers.
Comment: I find your analysis totally misleading.
Comment: @IoannisPaizis: In what sense?
Answer: Unlike science, which has a more linear progression, philosophy is cyclical. Very old ideas can gain new relevance, and long dead philosophers often argue with--and sometimes get the better of--current ones. So while ancient science is largely a historical curiosity, ancient philosophy is very much a live subject.
I personally follow a version of Bretano's classification system (https://www.google.com/books/edition/The_Four_Phases_of_Philosophy/YPz2Na2LlYQC?hl=en&gbpv=1&pg=PA2&printsec=frontcover) in which philosophers can be classified as follows:

Skeptics: Socrates, Chuang-Tzu, Hume, Sextus Empiricus, Descartes (1st Meditation), Ecclesiastes, Zizek, Derrida
Mystics: Plato, Lao Tzu, Sufis, Kierkegaard, Descartes (3rd Meditation)
System Builders: Kant, Hegel, Aristotle, Descartes
Didacts: Hammurabi, Moses, Confucius, Covey, Singer

Out of these four types, the didacts and the system-builders are the ones most tied to a given time-period. Their work is often best studied historically. But the mystics and skeptics (despite Bretano dismissing them as the worthless half of the cycle) are perennial. Their work--properly understood--is as relevant and as current now as when it was first written.
Answer: Yes, modern scholars interested in philosophy are well advised to study ancient philosophy too, and to study also philosophies in non-European tradition.
The philosophical method presents arguments and argues with others about their arguments. Other people come from different points of view. In particular when they lived in other time periods and/or in other cultural traditions. But it is always necessary to question the own point of view, it is difficult to find out my personal prejudices. By symmetry, it is necessary to question what others consider as self-evident.
When one concludes

Certain approaches from antique or medival philosophy are outdated,

then it is a challenge and a good exercise in philosophy to present arguments for this conclusion.
Aside: Indeed, I consider several approaches from former philosophers outdated resp. wrong.
Answer:
All terminology regarding philosophy, thinking and sciences is rooted in the ancient Greek language.
If you don't study the historical evolution of knowledge, I doubt that you can make sense of anything, other than what you are told.
If you go deeper in what human civilization is, you will understand that everything thought-of is always present. (unless it is forgotten)
You cannot rely on one person to tell you what another person said, or thinks, or feels. You must verify on your own.
A meaning can be derived only by reference to experience; extending your experience to the past, makes the scope of the meaning larger.
Answer: Yes. Unless you think that a botanist should just study the flower and ignore the roots. The fundamental questions that philosophers ask today are the same as those that the ancient philosophers ask. Old, obviously, but relevant.
Comment: Those old roots that still hold today, but altered by every epoch.
Answer: Is it essential to study all of it in detail? Definitely not. Can it be useful and interesting to have some familiarity with it? Sure. Be selective and use your common sense. Much ancient and even more recent philosophy is now well past its shelf-life. No one believes that everything is made of four elements, for example. But some older ideas still resonate today, and if you know nothing at all about them, you will soon wish you did if only because they are cited endlessly by everybody else. There are so many resources on the internet now that you can pick up the key knowledge in no time. Part of the difficulty I experienced in approaching philosophy without any background knowledge is that there are many conventional uses of words within philosophy that differ from everyday usage, and sometimes the only way to get a proper understanding of their shades of meaning is to put some effort into understanding the historical context in which they arose. My recommendation would be not to make a point of studying old philosophers up-front as a kind of foundation, but read-up selectively as and when you find a need to.
One final point. Some of the old ideas seem very strange now. Why and how they seem strange can in itself be a sobering focus for reflection, since there is every chance that what we smugly consider to be our up-to-date thoughts will seem equally wrong-headed in another millennium.


Do energy Gels help in long Tennis matches

Question: I do a lot of long distance running and I normally take a gel if I am racing and it goes longer than 2 hours. (Like a Marathon) My question is for tennis would I get the same benefit? For example, a tennis match could last 2-3 hours. Would taking a gel every 40-50 minutes help mitigate cramping and give me some energy later in the match?
Answer: Well, it really depends on the duration of the match ; however it would definitely help and trigger your energy.
 Since you also run while playing tennis, the effects of the gel should be similar.
I would not mind using before the match .


What is the difference between ITIL and PMP?

Question: It seems I'm dragging you along with me in the IT field, believe me, it's not premeditated. My question - a little broad that I admit - is: when is ITIL used and when is the PMP used? and why rather the PMP and not the ITIL? and vice versa?
Comment: I guess you mean PMBoK instead of PMP.
Comment: ITIL is about operations (related with IT services). PMBoK is about projects.
Comment: @SergeyKudryavtsev Yes PMBok
Comment: PMBoK can be very useful in IT services
Answer: This question can go in a lot of different directions, so I will attempt to be brief.
 ITIL, as you're probably aware is Information Technology Infrastructure Library; and PMP is a Project Management Professional. Though one can be certified in both, normally when one uses "PMP" they are referencing the certification. Are you asking when would the certifications be used?
 I think what you're asking is when to use the practices within each framework (ITIL or PMP). And the answer is that these are not mutually exclusive. ITIL is a standard around IT services (SLAs, Continuity Management, Security Management, etc), and PMP is a standard on how to manage projects.
The projects occur within an across an IT organization which utilizes ITIL.
I hope this helps!
Answer: In simple words you need PMP to survive during project creation and ITIL to make it survive after delivery.
If you need more details then you need to spend some time looking for the answer to know the processes for each framework.
Answer: A PMP is good across all industries where a project management professional is needed. Whereas an ITIL cert is specific to the IT industry. Both are valuable certifications to have and will serve you well. If you plan on remaining in the tech industry it would be beneficial to get ITIL as well as PMP. If you are wanting to branch out into other industries then just go for the PMP.


Sitecore Commerce Installation Error at GetIdServerToken

Question: I am installing sitecore commerce in development environment, encountered some errors prior to this error, all those are fixed, now i am getting this error. seems some issue with the Identity Server. any help would be much appreciated. i can give more details if required.
Answer: You mentioned you had problems before this with installation, please can you post those here? You can also check the steps below:

Check logs in Identity server C:\inetpub\wwwroot\SitecoreIdentityServer\wwwroot\logs.
Check that connection string points to the correct username and password.
Update Trusted connection to False.
Answer: Before performing the installation again for the same instance, you need to check the below steps:

Ensure that there is no custom AllowedCorsOriginsGroup in <InstallationFolder>\SIF\IdentityServer\Sitecore.Commerce.IdentityServer.Host.xml (they're added automatically in prior installations)

Since you're running the same script again to reinstall the Sitecore XC, You must remove the reference to the "ModuleHabitatImages" from the Includes section of the <InstallationFolder>\SIF\Configuration\Commerce\Master_SingleServer.js file.

## Remove this for second-time installation
"Module-HabitatImages": {
                "Source": ".\\Configuration\\Commerce\\Common\\Common.InstallWdpModule.json"
            },
## you can remove this to fasten up the second time installation
"Module-AdventureWorksImages": {
                "Source": ".\\Configuration\\Commerce\\Common\\Common.InstallWdpModule.json"
            }
## Also, you can remove this to fasten up the second time installation:
"Module-PowershellExtensions": {
                "Source": ".\\Configuration\\SitecoreUtilities\\InstallModule.json"
            },
"Module-SXAFramework": {
                "Source": ".\\Configuration\\SitecoreUtilities\\InstallModule.json"
            },

Do the IIS Reset.

After performing all the above steps, do the installation again.


What does "We're all living in America" mean?

Question: Recently I've heard "We are all living in America" in the text of America from Rammstein. Does this phrase make any sense or is there "are" just to fit in the rhytm?
Comment: "We all live in X" is a dry statement of fact.  "We are all living in X" (or "We're all living in X") is more poetic and metaphorical.  Rammstein isn't saying that they actually live inside the boundaries of US territory.
Comment: "are" is the main verb in the sentence.  It wouldn't make any sense *without* "are".
Comment: @stangdon I would say "We all LIVE in America." to talk in present time
Answer: It just means "We all live in America." but the Continuous aspect emphasizes the activity of living (each day we live, work, eat, etc.) rather than the state (America is where we live).
Based on the lyrics, the meaning seems to be saying that all the world is participating in America's culture, maybe a cynical way of admitting how much America influences the rest of the world.
It's also possible that the difference in grammar is due to Rammstein's native tongue being German, as German doesn't use the Continuous "ing" form, but I think that it's just as likely that they understand the difference and are using it as I indicated in my first paragraph.  We are living in America, day by day, participating in American things, regardless of where we live geographically.


Do I need to plug internal cable routing ports?

Question: I have just ordered a OneUp Components 42T sprocket and a Race Face 30T Narrow/Wide single ring so that I can convert my 3x10 drive train into a 1x10.  This means that I'll be removing the front derailleur all together.  I have a carbon frame with internal cable routing so once I've removed the shifter cable there will be open "ports" in the frame.  Is it important to plug these ports?  What might happen if I don't plug them?  There is one where the cable enters at the top of the frame, and one under the bottom bracket where the cable exits.
Comment: Great work going to 1x10, is there a way you can leave the cable in the ports in case you decide to go back to 3x10 (or whatever the future holds) or sell the bike? From what I've read it's a PITA to re-cable through internal ports.
Answer: I would plug them. If left open you will have the chance of water ,dirt, etc, inside the frame. The water and grit will eventually make its way to the bottom bracket. Even if the bearings are sealed it will still get on the threads of the bearing shell or threaded inset potentially making it difficult to remove.
Comment: @Vorac - Don't seal the frame up tight.  There need to be vent holes, at least, and in some cases the cable holes are serving this purpose.  Try to "seal" to prevent water entry but not totally block airflow, especially with the lower port.
Comment: And even sealed bearings won't last forever if suspended in water, especially if there's salt involved from winter riding.
Comment: Water will make it's way in regardless, if the bike is used much in the rain (or left out in the rain).  The water needs a way to get out.
Comment: @DanielRHicks, then what is your advice?
Answer: If it were me, I'd do it ,but I don't think it's necessary. Frames of all materials will have drain ports in them. My carbon Focus road bike,for example, will have water pouring out of. Frame when held vertically after a very wet commute, this is how it was designed. Metal frames have holes to allow welding gasses to escape, water can drain from these. So,plug them if you can, but if not, don't worry about it!
Comment: When I posted this question I also contacted Rocky Mountain to see what they had to say.  Your answer is pretty much what they said.  It would be best to plug the top one (they even make a rubber plug specifically for Element frames) but the one under the bottom bracket is less important and should have some gaps to allow for drainage.


Triangle Texture Packing Problem

Question: I'm trying to create a texture atlas where each triangle face is laid separately from each other (i.e. it is not typical mesh parameterization). In order to do this, I would have to pack the triangles into a 2D texture image. Can someone suggest a good algorithm to do this efficiently, both timely and spatially?
Comment: Don't forget to updvote and accept my answer if that helps you :). Welcoem to gamedev.Stachexchange.
Answer: Note: this algorithm is my "empirical approach", I did no research paper on the topic, so chances are high that a better algorithm exists. Simply since there's no simple to retrieve algorithm it is easier for me to create my own. I assume the following stuff:

Triangles can be rotate inside the atlas
Triangles can't be morphed inside the atlas

You start by taking the triangle with the widest surface and by placing it in the middle of your atlas.
At each iteration you seek the next bigger triangle, and you attach it  trying to match as much as possible one of its edges with unconnected edges on the atlas.
void AddTriangleToAtlas( Atlas atlas, Triangle triangle){

    var edges = atlas.Edges;
    Edge atlasEdge = null;
    Edge triangleEdge = null;
    float previousScore = 0;

    foreach(var edge in edges)
    {
        foreach(var triEdge in triangle.Edges)
        {
             var match_score = edge.Lenght / triEdge .Lenght;
             if(match_score > previousScore && match_score<=1)
             {
                 if( atlas.SimulateAdd( triangle, triEdge , edge))
                 {
                     // Simulation went OK! (the triangle do not intersect other triangles)
                     atlasEdge = edge;
                     triangleEdge = triEdge;
                     previousScore = atch_score;
                 }
             }
        }
    }

    // adds the triangle to the atlas, rotating it to match edges,
    // but preserving size, surface and shape.
    // NOTE THIS UPDATE THE EDGES LIST OF THE ATLAS
    // (one edge is removed, 2 are added)
    atlas.Add( tringle, triEdge, edge);
}

Each triangle you add, open 2 edges, so the computational cost of this algorithm is not very cheap, however you can improve it greatly by some euristics:

You can keep a list of edges sorted by Lenght, so you can search only the firsto 20/30 edges.
You can check, whenever the placement simulation fails, if it is failed by a good margin (in example the triangle was intersecting 50% of its surface with pre-existing triangles, in that case you can remove the edge from the list to avoid it is re-used for a simulation).
edges near texture limits are automatically discarded

Chances for optimization are many. The algorithm in itself is not very complicated, however you need the following stuff before starting to develop it:

An algorithm to rotate triangles and to place it over another image
Another algorithm to check intersection between triangles (actually it can be faster, especially for small triangles, to just rotate them and then see if you are going to write to a pixel that is already occupied or not).

By using the above euristics you could basically ends up in an algorithm that is O(n log n + p) where n is number of triangles to be placed and p is number of pixels (total surface).
Without euristics the algorithm is O(n^2 log n+p)
Comment: Thank you for your answer. Did you use any padding to the triangles to prevent seams?
Comment: Absolutely yes, use at least 2 pixels padding, preferrably 4 or more if you plan to use them at step angles or from distance. I use 8 pixels padding to get no seam with height-mapping shaders (and I have to use a soft heightmap since a greater weight in height requires even more padding).
Comment: Another question: when rendering, you need to make duplicate vertices to provide texture coordinates, am I correct?
Comment: Yes, exactly, otherwise you won't be able to texture them properly

Triangle Texture Packing Problem


Expand $(\vec{A}\times \nabla)\times \vec{B}$ using tensorial notation

Question: I was given a task to prove
$$(\vec{A}\times \nabla)\times \vec{B} = (\vec A \cdot \nabla)\vec B + \vec A \times \operatorname{rot} \vec B - \vec A \operatorname{div} B$$
using tensorial notation i.e. Kroneker delta and Levi-Civita symbol. Here are my wrong calculations:
$$\epsilon_{ijk}A_j(\nabla \times B)_k = \epsilon_{ijk} A_j (\epsilon_{klm} \frac{\partial}{\partial x_l} B_m) = \epsilon_{kij}\epsilon_{klm}A_j \frac{\partial B_m}{\partial x_l} = (\delta_{il} \delta_{jm} - \delta_{im} \delta_{jl} A_j \frac{\partial B_m}{\partial x_l}) = A_j \frac{\partial B_j}{\partial x_l} - A_l \frac{\partial B_m}{\partial x_l} = A_j \frac{\partial B_j}{\partial x_l} - (A \cdot \nabla)B$$
It is not even close to what I wanted. I do not understand how I can ever derive three terms from initial expression. Please, help me out.
Comment: I'm not yet sure how to do it with the tensorial approach, I need to ponder it a bit, but, I hope you can see from my current half-answer why there are three terms. You get one from raw derivatives acting on $\vec{B}$ then the terms like $\partial_2A_3$ act on $\vec{B}$ under a product rule hence producing two terms; $2+1=3$.
Comment: I think we need to antisymmetrize on the $\vec{A} \times \nabla$ term as to pick up the product rule. Unfortunately the usual formula $\vec{A} \times \vec{D}_k = \epsilon_{ijk}A_iD_j$ assumes that $\vec{A}$ and $\vec{D}$ have commuting components. Of course in this context that is false as $\vec{D}= \nabla$ is an operator so... we have to respect that.
Answer: Let ${\bf e}_i$ be unit vectors then $${\bf A}\times \nabla = [ \epsilon_{ijk}A_j\partial_k]{\bf e}_i$$
By the same formula it follows that $$({\bf A}\times \nabla)\times {\bf B} = \epsilon_{i'j'k'} (A\times\nabla)_{j'} B_{k'} {\bf e}_{i'} = \epsilon_{i'j'k'}[\epsilon_{j'jk}A_j\partial_k]B_{k'}{\bf e}_{i'}$$
Now by using the property $\epsilon_{ijk}\epsilon_{imn} = \delta_{jm}\delta_{km}-\delta_{jn}\delta_{km}$ we get
$$({\bf A}\times \nabla)\times {\bf B} = [\delta_{i'k}\delta_{jk'}-\delta_{i'j}\delta_{k'k}] A_j\partial_kB_{k'}{\bf e}_{i'} = [A_{k'}\partial_{i'}B_{k'} - (\partial_{k'}B_{k'})A_{i'}]{\bf e}_{i'}$$
The last term is simply $-{\bf A}(\text{div }{\bf B})$. Rewriting the first term it becomes
$$A_{k'}\partial_{i'}B_{k'}{\bf e}_{i'} = A_{k'}[\partial_{i'}B_{k'}-\partial_{k'}B_{i'}]{\bf e}_{i'} + A_{k'}\partial_{k'}B_{i'}{\bf e}_{i'} = ({\bf A}\times\text{rot }{\bf B}) + ({\bf A}\cdot \nabla){\bf B}$$
Putting it all togeather gives the desired result. The derivation of
$$({\bf A}\times\text{rot }{\bf B})\equiv {\bf A}\times (\nabla \times {\bf B})  =  \ldots  = A_{k'}[\partial_{i'}B_{k'}-\partial_{k'}B_{i'}]{\bf e}_{i'}$$
follows the same steps as above using the $\epsilon\epsilon = \delta\delta-\delta\delta$ formula.
Comment: @Dankevich What I did there was basically just to write $X = (X-Y) + Y$, i.e. I added and subtracted the same term (motivated by the answer). The $'Y'$ term I subtract is just $(A\cdot\nabla)B$.
Comment: Oh, why is it $A_{k′} \partial _{i′} B_{k′} e_{i′}=A_{k′}[\partial _{i′} B_{k′} − \partial _{k′} B_{i′}]e_{i′}$.

Why did you split it into two parts and changed subscripts? Thank you in advance.
Comment: Now I finally got it! Yay!
Answer: Ok, so the question is what is meant by $\vec{A} \times \nabla$. Notice:
$$ \vec{A} \times \nabla = \hat{x_1}(A_2 \partial_3-\partial_2 A_3)+\hat{x_2}(A_3 \partial_1-\partial_3 A_1)+\hat{x_3}(A_1 \partial_2-\partial_2 A_1)$$
when you feed this operator $\vec{B}$ under the cross product you have to think about product rules for the terms with partial derivatives to the left and component functions to the right.
It is not at all the same to start with $\nabla \times \vec{B}$. Even without the operator, the cross product is not associative. I haven't answered your question yet, but this is too long for a comment and I thought it would help for now.
Comment: Haha, I understand you, but I simply can not write it like this, it will not count. And I still do not get how to do the first step formally. How do I deal with brackets in such cases, like in example I gave and in $\nabla \times (\vec A \times \vec B)$, for instance? It is rather not obvious:)
Comment: I'm sorry, I have trouble with this one. I need to stew on it a while, perhaps someone will swoop in and solve it before I finish this.... I haven't found the right way to write the operator expression in the Einstein notation just yet.


Why is the melting point of propane lower than that of both methane and ethane?

Question: I've seen their properties in my textbook and multiple online sources, but I can't seem to find a straight answer as to why. Excluding propane, the trend is increased melting point as the molecule gets bigger, but propane is the weird exception here.
Answer: Ok, let's first look at the trend...
$$
\begin{array}{c|c|c|c}\hline
\text{Chemical} & n \text{ in }\ce{C_nH_{2n+2}} & \text{m.p. / K} & \text{m.p.}(n) - \text{m.p.}(n-1)\text{ / K} \\
\hline
\text{Methane} & 1 & 90.7 & \\
\text{Ethane} & 2 & 90.4 & -0.3 \\
\text{Propane} & 3 & 85.5 & -4.9 \\
\text{Butane} & 4 & 133\mathrm{-}139 & +50.5 \\
\text{Pentane} & 5 & 144.1 & +8.0 \\
\text{Hexane} & 6 & 177\mathrm{-}179 & +36.9 \\
\text{Heptane} & 7 & 182.2\mathrm{-}183.0 & +4.6 \\
\text{Octane} & 8 & 216.0\mathrm{-}216.6 & +33.7 \\
\text{Nonane} & 9 & 219.0\mathrm{-}220.0 & +3.2 \\
\text{Decane} & 10 & 242.7\mathrm{-}243.9 & +23.8 \\
\hline
\end{array}
$$
Using the mean for the alkanes with melting point ranges given (e.g. 136 for n-butane and 178 for n-hexane), it isn't clear there is any "great" linear trend. In fact it seems that the even number n-alkanes are on one curve and the odd n-alkanes are on another.
There is a "general trend in that with increasing MW the MP increases. However the exceptions would seem to be both methane and ethane, not propane. Also it is obvious that a simple correlation with MW alone is not sufficient to characterize the trends. All in all I think one must be careful not to read too much into such trends without a very careful analysis.
Comment: In general even number of $\ce{C}$ atoms in a chain are much more accommodating to a lattice than odd number of carbons. This is why the increase in boiling point from odd $\implies$ even is much more higher than even $\implies$ odd.
Answer: The phase transition towards solid state involves crystalization, i.e. organization of the molecules into the long-range repetitive patterns.
The formation of crystal lattice is reasonably simple for balls (methane) and rigid sticks (ethane), but becomes problematic for flexible triangle (propane), which needs to be properly aligned. Propane is the first of alkanes, which have important internal degrees of freedom, which have to adjust during the solidification.


SNR estimation prior to demodulation?

Question: What are the standard ways to measure SNR, particularly before demodulation/synchronization?
Answer: Perform a 2 dimensional cross correlation against a noise free replica of the signal you expect to receive with delay offset on one axis and frequency offset on the other, and find your maximum correlation. Normalize that to get $\rho$ and from $\rho$ you can get SNR. Do this after channel equalization, otherwise you will get the SNR degradation due to ISI as well.
For further details, see:
Noise detection
and
How can I find SNR, PEAQ, and ODG values by comparing two audios?
If the SNR is high and the noise is well behaved (thermal noise floor, no band selective interference present) then you can make a reasonable estimate directly from a spectrum plot such as the example I show below, where the SNR in this case was 40 dB as is clear from the distance between the signal and the noise. This is signal power to the total noise power that is in the signal bandwidth.
Comment: Interesting — if you don't know anything about the signal I  not really sure how you can distinguish it from noise—- but usually you know something about the signal that you can exploit at which point it is a matter of being clever about that (usually some sort of correlation is involved)
Comment: Yes that makes sense, but what if you don't know the signal you will receive? Only its approximate/average bandwidth?
Comment: Can you limit your question to only being high SNR's?
Comment: Maybe, I don't have an explicit application but just a general curiosity. For example: in high SNR applications you can have your adaptive synchronization routines increase their convergence rate. In low SNR applications you want your adaptive elements to change slowly(be more noise immune). So the question is how do you detect SNR without having synchronized the system beforehand?

SNR estimation prior to demodulation?


How to prevent spotlight from searching external hard drives

Question: I have an external hard drive connected to my Macbook Pro that I always leave on. When I use spotlight to search for something, it always searches this drive. I don't want it to do this because usually the drive is in standby mode, so it takes a while to power up when I use spotlight to find something, and usually what I'm trying to find isn't on that hard drive. How can I prevent spotlight from searching this drive?
Comment: possible duplicate of [How disable mac snow leopard creating .Spotlight-V100 and .Trash folders in USB Flash Drivers?](http://superuser.com/questions/89556/how-disable-mac-snow-leopard-creating-spotlight-v100-and-trash-folders-in-usb)
Answer: System Prefs --> Spotlight --> Privacy.
Add the drive(s) you want to exclude.
UPDATE:
For a scripting approach try this: Disabling Spotlight in OS X Leopard for the removable drives
Should work for Snow Leopard too.
Comment: The only drawback is that if you unmount the drive you will have to re-add it again in the future.
Comment: No that's not correct.  If you add it to the privacy pane, that computer (and others) will ignore it for spotlight in the future.  The spotlight pane writes a . file to the root of the drive, which other systems will recognize.  At least that's the way it works in OS 10.6 and I believe in 10.5...
Comment: @Benjamin Schollnick - You mean the hidden .Spotlight-V100 folder? You are correct but at the time of writing that comment I had not fully researched the behaviour of Spotlight indexing.
Comment: I was thinking of the .metadata_never_index  file...
Answer: Assuming you don't want Spotlight to index the drive either, create a file,

.metadata_never_index

on the root of the drive.  It will no longer get indexed, and, no index, no search.  From this tip from Mac OS-X Hints.


What is the meaning of "mean-field"?

Question: In lots of Bayesian papers, people use variational approximation. In lots of them they call it  "mean-field variational approximation". Does anyone know what is the meaning of mean-field in this context?
Answer: I found some intuitions that might answer this; based on the definition of "mean-field" at Wikipedia,
mean field theory (MFT also known as self-consistent field theory) studies the behaviour of large and complex stochastic models by studying a simpler model. Such models consider a large large number of small interacting individuals who interact with each other. The effect of all the other individuals on any given individual is approximated by a single averaged effect, thus reducing a many-body problem to a one-body problem.
So basically approximating the inference and learning problem, using independence assumptions and decomposition into several products, brings the notion of "mean-field" approximation.
Answer: I believe the mean-field approximation used in mean-field variational Bayes is the assumption that the posterior approximation factorizes over the parameters
$$q(\mathbf{\theta}) = q_1(\theta_1) q_2(\theta_2) \dots q_n(\theta_n)$$
Comment: Is it obvious why this factorization is called "mean-field"?
Comment: It gets this name from it's origin in Physics. In a system with many components that interact, the many interactions are approximated with a single average interaction - i.e. the mean field. The analogy to Bayesian Variational Inference is that a complicated posterior can be simplified if you assume all the individual parameters are independent.
Comment: @aaronsnoswell why does the independence of particles imply that the many interactions are approximated with a single average interaction?
Answer: Mean-field approximation is a way to simplify the variational Bayes procedure. MFA makes it possible to use coordinate ascent to find the approximating function. See https://github.com/idnavid/misc/blob/master/variationalbayes_doc1.ipynb

What is the meaning of "mean-field"?


Remotely running a program

Question: I have Windows Server 2008 and more computers in the domain. I need to run a program from the server on all the other computers, for example to install Kaspersky on the target computers. How can I achieve that?
Answer: You may install programs using Group Policy Objects and MSI files of a program distribution. Deploying an MSI through GPO.
You also have an option to deploy any remote control software e. g. UltraVNC or schedule programs execution using Task Scheduler.
This question is better suited for serverfault.
Answer: I just heard of PDQ Deploy today. Perhaps it could be what you are searching for.
Answer: While some software can be deployed via Group Policies, most can't. You should investigate tools such as psexec, which will allow you to run pretty much any software on a target machine.
Answer: In regards to your specific example, Kaspersky Antivirus installs through a deployment console which is installed on the server. The console itself will allow you to deploy the client to as many or as few machines as you like.
It's called the Kaspersky Administration Kit. Alternatively it has a wizard for configuring Kaspersky which will give you an MSI for deployment using GPOs in Active Directory.
Answer: Sorry that this is kind of a necro, but i used powershell to get around this.
Dependencies :
ActiveDirectory Module (Is included in Microsoft RSAT)
psexec.exe from sysinternals.
<code>Import-Module ActiveDirectory
$servers = Get-ADComputer -Filter * -Property * | where { $_.Name -like "EB*"} | Select Name

foreach ($srv in $Servers){
    $server = $srv.Name
    C:\Sysinternalssuite\psexec.exe \\$server -u DOMAIN\USER -p PASSWORD -h -d #EG: gpupdate.exe "/force" or #powershell.exe "enable-psremoting -force"
}
</code>
Answer: The best way to do this is with a configuration management tool. In Microsoft's world, System Center Configuration Manager is typically the tool of choice for this, but there are competing products from third parties. This can be used for a range of tasks from operating system deployment, to patching, to compliance reporting, to software deployment.
You can script something in PowerShell or leverage GPO, but both have their limitations.

Remotely running a program


What is the difference between Repository Pattern and Facades Pattern?

Question: I've always used the repository pattern in my applications. But I have seen that many people use facades instead of the repository for naming convention, but the operation is the same, I Think. Why is there this difference? There are a real difference between them or not?
Answer: A facade is more general than a repository.
A facade can apply to anything that is not persistence based, whereas a repository is supposed to encapsulate access to an underlying source and make it look like an in memory data source.
Facades are for creating a simple interface for some other, complicated interface.
Comment: @MirkoPastorelli: a repository is not designed for communicating with the data layer, it is designed for wrapping the communication with a data layer and "abstracting this away" from the user of the repo. (so one can, for example, provide a mock repository for testing purposes which provides only hardcoded test data and has no other dependencies). From this point of view, a repo is indeed nothing but a specific form of facade.
Comment: @MirkoPastorelli - it can. An example of a facade would be to give a single entry point for code that actually has to deal with several subsystems.
Comment: So the repository is designed for communicate with data layer and facades is more generic than this. for example a facades can perform other operation but the structure in designed like a repository, right?


What is the earliest example of a friendly Western dragon?

Question: I note that the friendly Western dragon trope becomes common in the 20th century. My question is whether there are any earlier examples than Kenneth Grahame's "The Reluctant Dragon" (1898).
Comment: @Spencer Ditto with *The Dragon Tamers* (1899) by E. Nesbit.  [TV Tropes](https://tvtropes.org/pmwiki/pmwiki.php/Main/DelightfulDragon) cites "The Relucant Dragon" as the trope codifier and earliest appearance.
Comment: I was going to go for "Farmer Giles of Ham", but this is already after the example you gave.
Comment: The unstated cultural context here is that [dragons in older European/Western folklore tended to be cast as evil and/or violent](https://en.wikipedia.org/wiki/European_dragon). By contrast, dragons in Far Eastern cultures (China, Japan, etc.) were associated with good fortune and "luck".
Comment: The source for that Tv Tropes claim is from Wikipedia as far as I know. It feels like there should be earlier examples of said trope, especially since Grahame uses it in a satirical manner, which would normally imply the existence of earlier straight uses.
Comment: Since my first question seems to have a full answer, my next question is how is there a 47 year gap between the first example and "The Reluctant Dragon". That can't be correct.
Answer: In The Marvellous and Incredible Adventures of Charles Thunderbolt, in the Moon by Charles Rumball (1851), there is a friendly Green Dragon. Rumball wrote under the  pseudonym "Charles Delorme" (1, 2) and the book may be listed under either name. Its full text is scanned at Google Books.
During a storm, Charles Thunderbolt's ship is wrecked and the remains are transported to a strange sea. He tries to eat a goose which accidentally flew into a hot part of the sea and became roasted, but the goose transforms into a dragon and carries him away. He finds that the dragon contains interior spaces including a fully-stocked pantry and bedroom. The dragon has many wings, green scales with golden tips, amber blood, a head "of all the colours in the universe", retractable teeth, the power of teleportation, and makes "a continual noise within his head, like to the roaring of engines" because of the powerful workings of his brain.
More on point to the question, the dragon is a "jolly monster" who guides Charles through various otherwordly encounters, frequently sharing jovial remarks and the odd cigar. They also meet several other tame dragons used as mounts by mercenaries, as well as less friendly ones, and the dragon is a frequent part of Charles' heroic deeds.
Eventually, Charles becomes a king and institutes a knightly order, "the Order of the Green Dragon", in honour of his friend - who is thereafter called "Sir Green Dragon". The dragon is named minister of war, and "so well did he execute the functions of his office that peace reigned under his auspices for seven long years". Charles finally returns to England, saying a fond farewell to his old friend, who promises to return for a visit.
Comment: I just read the book today and it is clearly an extremely early use of a friendly Western dragon, quite plausibly a codifier based on how the dragon's friendly nature is emphasized, not just shown. Thanks for the answer!


Magento 2: Admin Speed

Question: We are building our first M2 site. We noticed that the administration area is running slower than we would expect. This especially causes headaches when trying to create the product catalog. The difference between a 2 second product search, view or edit and a 6-8 second can make catalog management quite a grind.
Just to make sure, I had reasonable expectations, I set out to find some 'demo' Mage 2 implementations for comparison. I found 3 accessible admin interfaces for demo installations.
I ran the several tests on both our test installation and the demos that we found. The results are as follows:

Admin dashboard load: 2-3 seconds
Admin products catalog list load: 5-6 seconds
Admin product load (configurable): 6-9 seconds
Admin product save (configurable, including reloading it following): 9-12 seconds
Admin order listing load: 4-6 seconds
Admin order load: 3-6 seconds

Most of these tests (including our internal test) were on a basic LUMA install with the preset products/orders provided in the seed data.
Before testing, I completed the following optimisations on my test installation:

All caches, indexes on
Production mode on
Redis configured
Logging off
PHP7 used
Enabled flat categories and products
Removed all extensions
Turned on Varnish
Set up memcache

Two questions:

Are my findings on par with expectations? If not, how long should I expect these tests to take if I have a well optimized installation?
What suggestions do you have to optimize admin performance beyond what I have already performed?
Answer:
Try re-indexing all the indices and make sure you have proper permissions for your pub/static and var/ folders.
Make sure your Redis cache is configured properly.
Answer: First, I have the same problem. And I do not like it at all. I can optimize the front end but the back end is very slow.
Three things that seemed to help :

Turning on Opcache and FPM for PHP on our server
I don't know what this is exactly - but we moved from "worker" to "event" for Apache
I set the php memory limit to 4000MB

It changed admin performance from what was like 3 deep breaths to two deep breaths.
I was considering going to lightspeed and litemage - but I dont think it will do anything substantial.
I am moving our cart from something else to magento 2.1 and I am afraid that people will not let me move because the order processing will suck.

Magento 2: Admin Speed


What happens to aerofoil downwash with Flaps extended vs retracted and how does it affect the Tail down force?

Question: In my logic: Flaps extended will cause a greater Angle of Attack, to maintain Straight and Level Flight, AoA will be reduced, due to the increased "Surface", this allows to fly at lower Angle of Attack, tis means lower induced Drag causing downwash to decrease? However why does this result in an increased Tail Down Force and how does the downwash of the wing affect the tail force?
Comment: See last paragraph of my answer-- the nuance here is, we have to consider whether the question is about what happens to the tail downforce upon flap deployment *if the tail configuration (including elevator position) remains exactly the same*, or what has happened to the tail downforce after flap deployment *after the aircraft has been re-trimmed to hold some given parameter constant* (such as airspeed) -- or both.
Answer: A flap works increasing camber, angle of attack and surface of the airfoil.
Anyway, in order to compare the change of the aerodynamic characteristics in respect to the basis airfoil, everything is packed inside the relevant coefficients: if before deploying the flaps the wing has a NACA 65012 airfoil, an AoA of 2° and a surface of 100m², after the deployment of the flaps the wing still has a NACA 65012 airfoil, an AoA of 2° and a surface of 100m². Anyway $C_l$, $Cd$ and $C_m$ they all have changed and, in particular, they all have increased, as visible in the following pictures taken from this report:

Here we see the trends of $C_l$, $Cd$ and $C_m$ for several flap angles. The black line is the one for the basis airfoil. The plot for $C_l$ is shifted upward toward higher lift; the plot for $Cd$ is also shifted upward toward higher drag; and the plot for $C_m$ becomes more and more negative i.e. more and more pitch nose-down.

why does this result in an increased Tail Down Force?

In order to compensated for the:

lift increment $\rightarrow$ speed and AoA are decreased;
drag increment $\rightarrow$ thrust is increased, albeit the previous decrement in speed and AoA partially offset this;
higher nose-down pitch $\rightarrow$ tail downforce is increased.

Plus, flaps are deployed slowly and in steps.
Downwash is proportional to lift which, in turn, equals weight. Not changing the weight during the deployment of the flaps, downwash doesn't change too, at least globally: there's a local increase of lift (and therefore downwash) on the flapped surfaces of the wing and a local decrease on the non-flapped surfaces, but globally it remains the same.

how does the downwash of the wing affect the tail force?

If the higher downwash shed from the flapped surfaces impinges on the horizontal tailplane, its AoA has to be adjusted not only to compensate for the higher wing's $C_m$ but also for the changed local airflow. Note that these two effects are one the opposite of the other.
Answer: In the specific case of radio-controlled model gliders with the wings mounted at about the same level as the horizontal tail, I've found the following:

Deploying flaps that only cover the inboard portions of the wing, i.e. the portions of the wing in front of the horizontal tail, causes the aircraft to pitch up to a higher angle-of-attack.  If the aircraft was initially trimmed to fly near best-glide or min-sink speed, then after flaps are deployed, a nose-down stick (elevator) input may need to be maintained to prevent the aircraft from stalling, unless the aircraft is re-trimmed.

Deploying flaps that cover the entire wingspan causes the aircraft to pitch down to a lower angle-of-attack and gain airspeed, at least in cases where the wingspan is large compared to the span of the horizontal tail, unless a nose-up stick (elevator) input is maintained, or the aircraft is re-trimmed.

With a high-wing full-scale airplane such as a Cessna 152 or Cessna 172, lowering the flaps (which only cover the inboard portions of the wing) cause the aircraft to pitch up to a higher angle-of-attack and a lower airspeed, especially at high power settings.  If the aircraft was initially trimmed to fly near best-glide or min-sink speed, then after flaps are deployed, a nose-down stick (elevator) input may need to be maintained to prevent the aircraft from stalling, unless the aircraft is re-trimmed, especially at high power settings.  In this case, it's clear that the flaps are creating a stronger downwash over the horizontal tail, especially at high power settings, which more than offsets the increased nose-down pitching moment associated with increasing the camber of the wing.
So in all these cases we have a "balancing act"-- increasing the camber of the wing causes the wing to create a stronger nose-down pitching moment, but also increases the downwash over the tail, which creates a nose-up pitching moment.  The "balance" tends to be tipped toward the former factor if the span of the tail is small compared to the span of the portion of the wing covered by the flaps.  However a strong propwash tends to tip the balance toward the latter factor, if the flapped portion of the wing and the horizontal tail are both in the propwash.
If you are asking about the tail downforce after the aircraft is re-trimmed as needed to maintain the original airspeed, then you can be sure that the tail downforce is increased (or tail upforce is decreased) after the flaps are deployed, regardless of any other factors.  This is due to the way that the wing's pitching coefficient and pitching moment increase when the camber of the wing is increased.  But since the tail often lies in the downwash of the flapped portion of the wing, this simple observation alone isn't sufficient to tell us whether the elevator would have to be re-positioned higher, or lower, or neither, to deliver the appropriate increase in tail downforce (or decrease in tail upforce).
Comment: Not really a *complete* answer but still intended to convey *some* relevant useful info--


While flashing , is it necessary to unlock the bootloader?

Question: If need to be unlocked tha bootloader.  How can i unlock the bootloader ? For lava pixel v1 android 6.0
Comment: It has mediatek chipset. This issue of unlocking bootloader or not doesn't arise for devices with MTK chipset.
Answer: Lava has a Mediatek chipset (MT6582), you don't have to unlock the bootloader because MTK devices don't have it.
You'll need SP Flashtool and a ROM (with scatter file) and you're done.
You may want to see this thread on XDA Developers forum  on flashing a Lava Pixel phone.


Settings.php in read only mode?

Question: When does drupal settings.php become read-only? I experienced the drupal encountered an error when I was just creating a view. Cannot see the error logs from the admin side. Thought will enable the php error reporting in settings.php and it is shown as read-only? Why does it happen? How can I find the error causing this issue?
Answer:
When does drupal settings.php become read-only?

Towards the end of the installation process, when the install scripts have finished writing all they need to write to the file.

Why does it happen?

Because leaving that file writeable represents a security risk.

How can I find the error causing this issue?

It's not an error, so there's no cause to find.
Comment: Are you sure that's not happening after a `git pull` @Kevin?
Comment: It is a installation in local. I have edited it before as well. But once the website started showing encountered an error message, I noticed that settings.php file became read-only
Comment: That isn't something Drupal can do, you'll have to investigate the problem as a general file system one. Maybe someone flipped the permission bits in your git repo and you updated, maybe you set the file to ready only and forgot, maybe someone got onto your computer and did it, maybe you have a virus, and so on
Comment: Doesn't clearing the cache and similar system calls ensure the directory and file is read only?
Comment: I don't think so @Kevin - there's a status report item for it, which wouldn't make a lot of sense if the system just auto-corrected itself along the way
Comment: Odd. I have some 7.x sites where the file permissions will go to read only on me.

Settings.php in read only mode?


How can prokaryotes repair double strand breaks by homologous recombination if they're haploids?

Question: Title says it all, I must be going crazy.
Answer: Several answers to this question:

Much of the time what bacteria are doing to repair DSBs is actually non-homologous end-joining, which does not require strict homology. This is the more likely mechanism earlier in the cell cycle when copy number is more likely to be ~1.
If cells have completed DNA replication (later in the cell cycle), there are at minimum 2 copies of all DNAs present in the cell. These can be used for homologous recombination.
Bacterial chromosome copy number can be overreplicated in some situations, such that DNA replication is not synchronized to the cell cycle and is more than normal for a cell. In these situations copy number can be >2.


Conservation of Linear Momentum w/ Rotating Body

Question:
I am a high school physics student and I have a question on part (a) regarding the conservation of linear momentum. It says to use the conservation of linear momentum to solve for the velocity of the rod after collision, but I find that kind of skeptical since the distance from the center of mass is not taken into account. Shouldn't the velocity of the rod after collision be dependent on where the rod is hit (the center of the rod as opposed to the end)? I think that it should take distance from the CM into account since most of the initial momentum could get translated into angular momentum rather than linear momentum if the rod is hit at the end, but I could be wrong. This is assuming that there are no external forces and the rod is on a frictionless horizontal surface.
Comment: The linear velocity of the center of mass of the rod will just depend on the linear momentum of the mass that hits it. Linear momentum and angular momentum are different physical things, so you can't convert between the two.
Comment: Depending on trajectory of the hitting object, it will transfer different amounts of kinetic energy, both linear and angular momenta to the rod, that would then have these different amounts of these respective quantities. Note that any moving object has a nonzero angular momentum, unless it is on a linear trajectory passing the coordinate system origin.
Comment: P.S.: .. unless the the position related and rotation related momentum cancel each other, of course.
Comment: Thank you guys for the help!
Answer: The velocity of the rod is fully determined by the change of velocity of the hitting object and related momentum exchange.
As Aaron correctly says, momentum and angular momentum are 2 independent, non interchangeable quantities.
The distance is taken into account.
If the object would hit the rod at other than L/6 height, the return velocity would not be -v/2.
The final velocities of both object depend on where the rod was hit.
The maximum of total translational energy occurs, if the rod is hit at the middle.
In such a case, no energy is spent on its rotation.
The minimum of the total translational energy occurs, if the rod is hit at the end.
In such a case, the big part of energy is spent on its rotation.
Note that for angular momentum conservation, all motion must be considered, not just rotation.


Will a Cedar tree survive if a portion of bark is missing?

Question: Will a cedar tree survive if a portion of its bark is removed?  I found someone had removed a strip (2'x5') from a tree on my property.  I was told it was going to be made into a cedar hat.  I am worried that the tree may not survive the winter.
Comment: What we are concerned about is the shape/position of the missing piece compared to the rest of the trunk. If, for example, a  strip of bark is removed *around* the tree, the sap, water and nutritients can't flow up/down any more and the tree is bound to die. If flow of nutritients is only slightly inhibited, it depends on the wound - and if your tree can heal quickly or gets infected. Not much different than a badly scraped knee in humans. But for an estimation, we need pictures. You may edit your post any time, even after your conference. Welcome to the site.
Comment: Could you add a picture? It will really depend on how much of the circumference of the tree was stripped of bark.
Comment: unfortunately i can't.  i'm ready to take off for a conference.  the strip is 24 inches across the trunk and 72 inches taken from the bottom to the top.  so it was taken from the north side of the tree.  thanks michelle.
Answer: The bark is one thing and the cambium is another. The bark protects the cambium. There are different layers of bark.  Assuming he just removed the outer layers of bark there was probably no harm done. Do not try to cover this wound as the tree will do just fine protecting itself.  The only part that Indians used was the fibrous outer layers to make clothing, building materials, etc.  There are harder, finer layers of bark left to protect the vacular cambium. Even if he damaged the cambium on one side of the tree the tree will usually be able to cope just fine.  Think about all the elk that rub the bark off, feral pigs that pull lots of bark away, beetles that bore beneath the bark and lay eggs and the larvae eat the cambium.  If the cambium damage encircles the trunk, then the tree will die. Please send pictures!!  Close ups of 'wound'...thanks!
Answer: The tree should survive (of its own accord) in all scenarios but being girdled (meaning a ring of bark was removed completely around the tree bole) below all of its foliage.
Living tissues of trees, including the roots, are fed by the foliage. Photosynthates are transported downward in the inner bark or phloem. Water and mineral nutrients are transported upward through the wood or xylem. So, the roots survive as long as they are supplied with photosynthate.
On the other hand, if girdling  removes the food supply for the roots, they will only survive by reconverting stored starches into metabolizable sugars. Once these are exhausted (in about two years), mineral nutrients will no longer loaded and water adsorption will effectively cease - the entire tree then dies.
Answer: Sounds like your tree was harvested. This has been done for generations by the indigenous peoples. If done properly the tree will survive. As a matter of fact keeping the tree alive is a very important part of the tradition of cedar harvesting.
Here is a youtube link for more info:
https://youtu.be/A6KS4J8QyNQ