Transfer of electron energy to atoms (heating up of matter by absorption of photons)

Question: If an electron absorbs a photon to get exited to a higher energy level, it should either come back to same state or any other lower state by emitting the required photon.  How then can there be a net transfer of energy to the atom?  Heating up means increase in kinetic (vibrational) energy of the atoms.  If the energy absorbed by a photon is re-emitted as a photon then how do atoms extract energy from incident photons?
Comment: You might find it instructive to read about the [Mössbauer effect](http://en.wikipedia.org/wiki/M%C3%B6ssbauer_effect).
Comment: @dmckee---ex-moderatorkitten I really wish you'd post that as answer. Also, more generally, https://en.wikipedia.org/wiki/Nuclear_spectroscopy
(I don't know if that Wiki page existed 7 years ago, though. Hello from the future! Erm...it's a bit scary here right now.)
Answer: Let us clear some misunderstandings:

if an electron absorbs a photon to get exited to a higher energy level,

It is not the electron that absorbs the photon to go to a higher energy level. It is the whole atom, which is represented by a potential well with energy levels filled by electrons up to a point. A photon with the correct energy, i.e. an energy that covers the difference of the level where the electron is and a higher energy empty level, will be absorbed by the whole atom. The electron will decay with a characteristic decay time from the higher energy level to a lower one and a photon will take up the energy again. It can happen that cascades of photons may take up the energy.

it should either come back to same state or any other lower state by emitting the required photon....

Again, it is the system nucleus+electrons that absorbs and emits quantized photons.
In this case, ( of absorption and emission), little kinetic energy is transferred to the atom, from momentm conservation within the Heisenberg Uncertainty Principle.

*BUT HOW CAN IT TRANSFER ITS ENERGY TO NEARBY ATOMS *

I hope it is clear that it does not. It could happen that, for example, the dipole field of an atom interacts with the field of another atom and transfers kinetic energy it has received, but it will be from an almost continuum scatterings, where the photons are off mass shell (virtual).   Black body radiation agrees for large wavelengths between classical and quantum forms. The need of quantization appears at higher energies to avoid the  ultraviolet catastrophy.

THE only way is through electron- atom collisions (absurd).....

No, photons from the continuum can interact with the dipole and quadrupole moments of the atoms and transfer energy there. Infrared photons of the continuum can scatter off the left over fields of the atoms/molecules transferring momentum /kinetic energy and increasing temperature. It is a continuum process, not a quantized one. Relaxation of the vibrational levels of the lattice will release quantized photons.

heating up means increase in vibrational energy of the atoms,

right but also of kinetic energy

then how exactly do they gain it from excited electrons?

They do not, they gain it from the continuum of infrared  photons interactions with the left over fields of the atoms and molecules.
Answer: Essentially what you are asking about is the "photoelectric effect". The intensity of the light does not contribute to energy needed for the material to expel a photon.
The Minimum energy needed to release a photon is given by the work function $\Phi=hf$ where h is Planck's constant and f is frequency.
Here is a flash simulation of what is going on.
What the affect the intensity has is that it will change the number of electrons that are excited by the incident light which is different.


Why did Paul collect and deliver an offering for the Church in Jerusalem?

Question: Paul's collection for the Jerusalem church occupies significant portions of his letters (1 Cor 16:1–4; 2 Cor 8:1–9:15; Rom 15:14–32).  It is so important to Paul he is willing to face hostility (Romans 15:30-31) and is indeed arrested in Jerusalem in part because of it (Acts 24:17).  What compelled Paul to raise funds among his gentile converts for the poor in Jerusalem?  Why did he feel this money would be better spent on the poor in Jerusalem rather than on the poor gentiles surrounding the communities where he collected it?  What did he hope this offering would accomplish?
Comment: I don't think there's anything that says money was **not** given to the "poor gentiles surrounding the communities where he collected it." Rather, so much focus is on Jerusalem because it required more logistical coordination to send large sums of money to Jerusalem. Whereas, if Gentiles surrounding the local churches needed assistance, they only had to go to that local church. Also, during that time, there were few churches in Judea. Either most Christians had been exiled along with the Jews, or the non-believing Jews were persecuting the believing Jews. In such a case, bel. Jews needed help.
Comment: You've answered why it might be mentioned over and above an offering to the surrounding gentile.  But I'm not assuming that money wasn't given to the poor gentiles.  A gift to Jewish Christian some 800 miles away in Jerusalem necessarily takes the money away from otherwise local purposes.  Why then did Paul think it important that they send such an offering to Jerusalem and not to poor Christians in another city, let say like Antioch?
Comment: @H3br3wHamm3r81 But also see 2 Cor. 8:1-2
Comment: Interesting scripture @Soldarnal. Thank you.
Comment: And now the subject of a feature film: for info, ebook, and trailers, see the ["A Polite Bribe" website](http://apolitebribe.com/).
Comment: No church was a stranger to persecution at this time, but it was especially severe and long lasting in Jerusalem. [Here](http://www.biblegateway.com/passage/?search=Acts%208&version=NIV) and [here](http://www.biblegateway.com/passage/?search=Acts%2021:26-30&version=NIV). I think it is to be expected that wide spread poverty among the church would be one of the many side effects of this persecution.
Answer: An alternative explanation could just be rooted in practicality.
The pattern had already been established by the Antiochene church in Acts 11:27-30. Agabus predicted an imminent famine and the church in Antioch. There were many famines during Claudius's reign (41-54), the most severe of which occurred in Judea around 46-47.
Because of the imminent threat, the Antiochene church acts quickly to meet these needs. The Judean famine of 46-47 is the most likely referent for the famine that Agabus predicted, though the church in Antioch would not have been in any better position to assist the main church of Jerusalem. There is evidence of a severe crop failure in Egypt around A.D. 45-46, though it is not directly documented.  However, the hallmarks of the results of such a failure and famine occurred, including mass people migration, and large spikes in default on debt and taxes in the years immediately following this failure.
It was a common practice for the rich to take responsibility for ensuring the grain supply in times of shortage. Therefore, an expected practice for the Antioch community would have been for the leadership to develop a list of the wealthiest Christians in the city, and call on them for support. However, the church responded as a unified body, not relying upon the most wealthy to aid the church in Jerusalem, effectively placing the responsibility of benefaction upon all Christians, and not just the wealthy.  It should be noted, though, that the response was by ability, of which the wealthy would have had more.
It is not inconceivable that Paul was also taking a collection for a greater need in Jerusalem than in the surrounding communities. It also would have shown solidarity with the main church in Jerusalem. Overall, it seems like the pattern of generosity (especially internally) had been set by the Antiochene church. Helping others is not dictated by how much need there is in your immediate vicinity, but is a multifaceted expression of solidarity and shared faith.
Thus, to address the question of why not just let Jerusalem fend for itself because Paul's church plants had their own needs, perhaps the needs of the Achaean churches were less significant than those of the famine-struck region of Judea. It is important to note that questions of attempting to discern the motivations of people whom we cannot directly ask can lead to the wrong kind of speculation. We can attempt to find reasons and circumstances that may have led to the particular decision, but settling on a definitive response will lead to more arguing than should actually occur.
I personally prefer the above practical reason because it fits the chronology reasonably well and it makes sense to me. I'm also not sure there's any evidence of unmet needs in the Achaean/Macedonian churches.
Winter, Bruce W. "Acts and Food Shortages," in The Book of Acts in its Graeco-Roman Setting, ed. David W. J. Gill and Conrad Gempf, 59-78. Grand Rapids: Eerdmans, 1994.
Answer: One possibility is that it came out of the meeting in Jerusalem described in Galatians 2, where Paul writes:

and when James and Cephas and John, who were acknowledged pillars, recognized the grace that had been given to me, they gave to Barnabas and me the right hand of fellowship, agreeing that we should go to the Gentiles and they to the circumcised. They asked only one thing, that we remember the poor, which was actually what I was eager to do.

If you read the rest of Galatians 2, Paul had a bit of a falling out with the Jerusalem church, it could be that this made him even more interested in holding up his end of the bargain in an attempt to reconcile.
Answer: In short, Paul sees his outreach to the Gentiles as a ministry to Israel (Romans 11:12-15).  God promised Abraham that in his seed all the nations (Gentiles) of the world would be blessed (Genesis 22:18; Galatians 3:15).  And Isaiah prophesied

In the last days the mountain of the Lord's temple will be established
  as the highest of the mountains; it will be exalted above the hills,
  and all nations will stream to it. (Isaiah 2:2)

Paul's understands his ministry and particularly this offering as a fulfillment of God's promises.  Through this offering the Gentiles are journeying to Jerusalem to worship the God of Israel and in so doing offering themselves as proof to the Jews that Jesus is the one Christians claim him to be.
In Galatians, the origins of the offering appears as a important bridge between Paul's Gentile ministry and Peter, James and John's ministry to the Jews.  To see this we need to understand the context in which the mention of an offering first appears.
Paul wrote Galatians to defend against Jewish Christians who taught that Gentile believers in Jesus needed to follow the traditions of the Jews. Paul is resolute in his hostility to such a doctrine, eternally condemning any who preach a message other than the one he delivered to them (Galatians 1:8-9). To give context to his opposition, Paul recounts his own history; his former zeal for these traditions, his conversion and his subsequent relationship with the Jerusalem church who are presumably the source of the present conflict.
Paul claims to initailly have been extremely zealous for the traditions of his fathers, alluding to God's commendation of Phineius who killed an Israelite man in the very act of fornicating with a Gentile woman (Numbers 25:1-8).  But a revelation of Jesus and Paul's call to the Gentiles changes all that.  Without consulting anyone (once again presumably leaders in Jerusalem), Paul journeys to Arabia (2:17) and possibly even Mt. Sinai (4:25).  Its only three years later that Paul briefly meets some of the apostles, Peter and James, in Jerusalem.  In all this Paul stresses that his message came from God and not from any man.
14 years pass before Paul feels compelled to consult with these leaders in Jerusalem again.  Paul presents the message he preaches among the Gentiles privately to them in the hope that they will see it from his point of view. Peter and James agree that they should go to the Jews and Paul should continue his outreach among the Gentiles.  The one thing they ask is that he "continue to remember the poor."
This last phrase, "continue to remember the poor," appears to refer specifically to the poor in Jerusalem. Paul gives ample evidence to a serious tension that existed between Jewish Christians in Jerusalem and his ministry among the Gentiles.  That this meeting and agreement occurred "in private" likewise suggests that Peter and James felt apprehensive in giving Paul the right hand of fellowship.  A financial offering from Paul and his Gentile converts would certainly help to smooth out any difficulty that might develop among the believers in Jerusalem.
There are also other reasons to see this phrase as a reference to the poor in Jerusalem.

James and Peter's request that Paul "continue to remember the poor" indicates that this something Paul is already doing. If Galatians is written prior to the Jerusalem council (Acts 15), than this meeting occurred when Paul delivered aid from the church in Antioch to the famine starved church in Jerusalem (Acts 11:27-30).
Paul claims his offering for the Jersualem church is for "the poor among the saints in Jerusalem" (Romans 16:26).
His audience would presumably understand the shorthand reference since they themselves had been instructed about Paul's collection (1 Corinthians 16:1-2).

Paul's commitment to the poor in Jerusalem does not originate with Peter and James. It's an idea which appears to be fundamental to his understanding of his ministry among the Gentiles.  This can been seen in his letter to the Romans.
Scholars are apt to point out that Paul wrote to the Romans to prepare for a further missionary trip to spain (Romans 15:23-24).  But what we often overlook is that Paul's occasion for writing is more immediately connected with his journey to Jerusalem where he  will finally deliver this gentile offering. (Romans 15:26-32).  And it apparently weighs heavily on his mind (Romans 15:31).
Read in this light, the theme of Jew and Gentile makes a great deal more sense. Romans is a meditation on Paul's gospel and what he hopes to achieve through his ministry to the gentiles.  In Romans 11:13-14 Paul states

I am talking to you Gentiles.  Inasmuch as I am the apostle to the
  Gentiles, I make much of my ministry in the hope that I may somehow
  arouse my own people to envy and save some of them.

It appears highly likely that Paul saw this arousal as coming from the prophetic fulfillment of a later day worship of God among the Gentiles.
Answer: It strikes me that Paul is a Pharisee and well steeped in the OT instructions regarding tithe.
Given the language of his instruction and the context of it, I perceive that Paul is applying the principle of tithe given in the torah and applying it to the Gentiles.
I.  SIMILARITIES IN LANGUAGE AND INSTRUCTION BETWEEN PAUL'S INSTRUCTION TO THE GENTILES AND OLD TESTAMENT INSTRUCTIONS TO THE JEWS PERTAINING TO TITHE:
1) The tithes brought to Jerusalem.
2) Tithe for the priest, the orphan, & widow.
   The collection to assist the poor and the saints.
3) The context of Passover, first fruits and Pentecost, times when tithes would be brought from the harvest
4) the phraseology of laying aside or storing up "according to how the Lord has prospered you"
Exodus 23:14-18
Exodus 34:18-26
Leviticus 2:11-14
Leviticus 23
Numbers 18:21
Deuteronomy 12:5-26
Deuteronomy 14:22-29
Deuteronomy 16:9-17
Deuteronomy 26:1-19
II Chronicles 31:5-6 (a little insight into how this "storing up" looked).
It would be natural for Paul to instruct gentile churches likewise to send support to the saints in Jerusalem.  this was the hub for the Apostles.
II.  NEW TESTAMENT CONTEXTUAL CONSIDERATION:
In Acts 20,  we see Paul in a hurry to get to Jerusalem for the day of Pentecost!
In 21:17 He is in Jerusalem (presumably based on Acts 20, for Pentecost).  By chapter 24 he is explains at his hearing that after many years, he had come to bring alms to his nation (Jerusalem-Pentecost).
I Corinthians 16 is not a chronological historical account; it is a letter. None the less, we see Paul's instruction regarding the collection to fall in the context of Passover & Pentecost:
Passover: In chapter 5:7-8 Paul instruct the Corinthians to purge out adultery and keep the Passover feast with the unleavened bread of sincerity and truth. In chapter 11, Paul writes to the Corinthians about keeping the traditions as He delivered them to them. He speaks of the Lord's supper in the context of Passover, "Therefore when you come together in one place, it is not to eat the Lord's Supper. . . .For I received from the Lord that which I also delivered to you: that the Lord Jesus on the same night in which He was betrayed took bread; (v11-23) (That "same night" was Passover).
In chapter 15 & 16 He speaks of delivering to the I Corinthians "that which [He] received from the Lord, that Christ died for our sins according to the Scriptures, and that He was buried, and that He rose again the third day according to the Scriptures, . . . But now Christ has is risen from the dead, and has become the first fruits of those who have fallen asleep" (15:20, 23; 16:15). (the first fruits are waved before the LORD by the priest on day one of counting fifty in the context of seven Sabbaths).
Pentecost: We then see, in chapter 15, verse 8 that Paul wants to see the Corinthians and would even like to spend the winter with them, but he doesn't wish to see them "now on the way , "But will tarry in Ephesus until Pentecost!"
We have traditionally tended to spiritualize Paul's instruction In I Corinthians about keeping the Passover. Likewise, we have applied I Corinthians chapter 11 to weekly Sunday services with communion; but, in reading the whole of Paul's letter it is not unlikely that these are in the literal context of Passover Firstfruits and Pentecost as is Paul's instruction regarding the collection!
III.  FURTHER THINGS TO CONSIDER:
I have reason to believe that in the New Covenant instances where Sabbaths is plural in the phrase translated "first day of the week," it is referring to the day of firstfruits (day one pertaining to the 7 Sabbaths counting to Pentecost rather than to merely the first day of the week (though it falls on the first day of the week once a year).If that is the case that further contributes to a connection between firstfruits and Paul's incstruction pertaining to the collection.  This increases the likelihood that Paul is applying old Covenant principles of tithe to Gentile believers.
IV.  ADDITIONAL PERSPECTIVE:
Of the 13 or 14 books Paul writes, He speaks of first-fruits only in I Corinthians and Romans, books containing instructions on tithe.  (other than that I find the word only used in James where it occurs once and in Revelation.  The book of Revelation has strong Passover connotations with the Lamb slain and the robes dipped in blood . . . Thus, the usage of the term firstfruits in context of Passover, Pentecost (times firstfruits/tithes were brought to Jerusalem) are noteworthy.
In sum, I see strong correlation between YHWH's instruction to Israel regarding the firstfruits (tithe) and Paul's instruction regarding collections.
Answer: Part of Paul's intention that we may undervalue, if we view it as somewhat transcendental, is his desire to increase the praise from the earth to God, who admittedly deserves all praise. Paul lists praise to God as one desired result of this offering (2 Corinthians 9:12-13).
He contrasts the world's religions with the fact that God deserves eternal praise (Romans 1:25). Christ accepted us in order to bring praise to God (Romans 15:7-9) and Paul says we should accept each other for the same reason. In other words, Christ's 'evangelism,' and by extension Paul's, was to change the recalcitrance of the world into proper praise of God.
Comment: You quoted references to illustrate your point, which is  part of our [Site Directives](http://hermeneutics.stackexchange.com/tour). However, you should consider 'quoting the text itself', using the "quote symbol" located at the top of your toolbar and draw your specific inference from the text itself. That way your audience can see the point you're making and compare it with the text you're making it from. Be encouraged! :>)
Comment: Welcome to BHSE! We're a little different here, please read our [Site Directives](http://hermeneutics.stackexchange.com/tour) as you ask and answer questions. Thank You!
Answer: On Acts 4:32 Says that all the believers were one in heart and mind. No one claimed that any of his possession was his own, but they shared everything they had. Also on verse 34 says that, There were no needy person among them. For from time to time those who owned lands or houses sold them, brought the money from the sales and put it at the apostles' feet, and it was distributed to anyone as he had need that including on verse 36 Barnabas. I think when the amount of believer increased probably they need more income to go forward. then Paul collects the money for their needs.
Answer: I asked myself this question...and Googled it:  "Why did Paul say to the church at Corinth (1 Corinthians 16:1-2) - 'Now concerning the collection for the saints, as I have given order to the churches of Galatia, even so do ye.  Upon the first day of the week (Sunday), let EACH OF YOU lay by him (or her) in store (save up) as God has prospered you (over and above His meeting your daily bread needs), that there be no gatherings when I come...(I won't have to take up an offering when I arrive...start setting it aside now)."
The sole answer to that question is found in Romans 15:26 - Paul says to the church at Rome, "For it has pleased them in Macedonia and Achaia (both in Greece -as are the churches of Thessalonica - the present capital of Greece, and Philippi) - to make a certain contribution for the poor saints at Jerusalem."
Now, Paul is speaking in 1 Corinthians.....the church at Corinth about that same offering.
If you trace Paul's footsteps, you would see that Corinth is in Greece.  South of there is Turkey where Galatia and Ephesus are located, and south of there is Israel and Jerusalem in particular.
From Corinth to Jerusalem is a road trip of 1890 miles.  It would take Paul a long time to travel from Corinth to Jerusalem.
When Paul was speaking to the church at Rome (now Romans 15:27-29), he tells them, "It has pleased them (the churches making the contributions) because, indeed, these churches owe the main church at Jerusalem because they have sent us and others out to them to bring the salvation of the Gospel of Christ to them, so they are thrilled at the notion of sharing their financial blessings (prosperity) with the main church at Jerusalem.  So, AFTER I have completed the task and have made sure that they (Jerusalem) have received this contribution, I will go to Spain - but I will stop by here in Rome to visit with you once again."
Answer: The many references to famine are true but one thing which seems odd is the slowness of response. Famine relief needs to be fast yet this collection takes a long time and Paul is revisiting churches and waiting at least one winter while organising it and moving the silver(?) with a group of men.
I would suggest that this is as much about sharing capital as sharing income.The Jerusalem church seems to have been a training centre for diaspora Jews from the Day of Pentecost on and they impoverished themselves to support the extra numbers who "devoted themselves to the Apostles teaching". There is also the impact of property confiscation mentioned in Heb 10:34. Imprisonment of members also placed a financial burden on the members outside gaol. Paul states in 2 Cor 8:13 that his aim is "equality" which is a step beyond famine relief and would mean having the resources (land, workshops, tools and materials) to produce an income out of which the Hebrew christians might in future be able to help their Gentile brethren.
Answer: On Pentecost the Apostle Peter alluded to the apocalyptic great and terrible Day of the Lord in Acts 2:17-21. Later in the same chapter the believers in Jerusalem started divesting themselves of all their worldly goods (Acts 2:45). For those who were not sincere in their personal sacrifice and who dissembled (Ananias and Sapphira), the discipline was immediate death which caused "great fear" in the Jerusalem church (Acts 5:11). Thus the believers in Jerusalem disenfranchised themselves of their worldly possessions while there were still yet others who later had their worldly possessions seized from them (Hebrews 10:34). The result was the abject material poverty of these Jewish believers. The Apostle Paul therefore sought to minister to their economic needs through the love of the church from other geographic areas.
Comment: I think any treatment of this subject that should at least raise the issue of famine and other hardships referenced in various places. If if that isn't the direct answer here, you should address why it is not since that is the most commonly cited reason. Until you engage and either confirm or disprove the most commonly cited reasons, this comes off as a very weak argument. Would you be interested in [edit]ing to expland on that point?


Netherlands resident - can I work in the UK?

Question: I am Canadian, with a Canadian passport, living in the Netherlands, I am a resident in the Netherlands. Can I work in the UK? And how would I go about sorting that out?
Answer: The answer to this question is pretty much the same as the answer to Working in the Netherlands as a Spanish resident Your being a resident of the Netherlands does not generally entail any right to work in the UK or access to any special procedure to do so, beyond what's already available based on your citizenship and personal situation.
For third-country nationals (i.e. people from outside the European Union), moving to another EU country is more-or-less like moving to a completely unrelated country (with a few limited exceptions like family members of EU citizens, EU blue card holders, etc.). In this particular case, the UK isn't part of the EU blue card scheme so even this (very small) group of Dutch residents doesn't have any particular advantage.
Beyond that, there are a number of different work visas for the UK and a few other things you might qualify for (e.g. ancestry visa) so you might want to ask a follow-up question with more information about your situation (type of work you are considering, whether you have a concrete job offer or not, for what type of contract, your age, your education and experience, etc.)
You might also find the Gov.uk "Do I need a visa?" widget useful to find more info about all this.
Comment: Thank you for your response, Gala. I really appreciate it. I expected that moving to/working in the UK would be the same for me as if I was going from Canada instead, but I just thought I'd ask.
I'll look over your links and post further questions/comments if they arise.
Cheers!


What is the song that plays when Muramasa charges at Ichigo?

Question: I heard several times but can't find the song. It plays in season 13 in episode 253, starting around 9:58, right when Muramasa charges at Ichigo after turning into a hollow.
Answer: It's Shadow's Masquerade.
The first usage of the soundtrack is in episode 88 of the Season 4: Bount arc. The music itself is the track #16 from BLEACH Original Soundtrack 2.


The measure problem in the anthropic principle

Question: The anthropic principle is based upon Bayesian reasoning applied to the ensemble of universes, or parts thereof, conditioned upon the existence of conscious observers. That still leaves us with the problem of determining the prior probabilities. This is the famous measure problem. How is this measure determined in the current cosmology literature?
Comment: Since everyone seems to be in agreement, I'm reopening this and clearing the comments.
Answer: One approach is there are no a priori probabilities, but every probability is a function of the prior knowledge. See e.g.

Carlton M. Caves, ''Resource Material for Promoting the Bayesian View of Everything'', http://info.phys.unm.edu/~caves/thoughts2.2.pdf

BTW, what does mean that "There is $p=\frac{3}{4}$ for the creation of (a very different universe from ours) and  $p=\frac{1}{4}$ for the creation of our universe". For me it makes totally no sense, as you can put any probabilities u like (justifying it or not) and it won't change anything.
And going into calculations of phase-space of consciousness beings sounds like a crackpottery (or at least - an unjustified philosophical claim), unless it gives any experimental prediction (or simplifies any theory).


Solubility of ortho- and para-nitrophenol in benzene

Question: Which one between o-nitrophenol and p-nitrophenol is more soluble in benzene? And what is the reason? If the solvent had been polar, hydrogen bonding would have provided a lot of explanation, but what about non polar solvents?
Comment: Same thing$ $...
Answer: It turns out that the opposite of what happens in polar solvents takes place when a non-polar solvent is used. At the same temperature, o-nitrophenol is more soluble in benzene than it's m and p isomers.
Sidgwick et al.1 did a study of this and obtained the following results. (Note that the solvent they used was toluene and not benzene, but they are similar enough that the same principles would apply.)

EDIT: The original version of this answer wrongly interpreted the graph and stated that o-nitrophenol would be less soluble in benzene than the other isomers. That was an error on my part. Thank you to Mathew Mahindaratne for pointing it out!
References:

Nevil Vincent Sidgwick, William James Spurrell, Thomas Ellis Davies, "CXXXII.—The Solubility of the Nitrophenols and Other Isomeric Disubstitution Products of Benzene," J. Chem. Soc., Trans. 1915, 107, 1202–1213. doi:10.1039/CT9150701202.
Comment: @Aniruddha Deb: Please see my comment to OP's question under y answer. According to chart, solubility should be higher for 2-nitrophenol than that of other two at all temperatures. Accordingly, would you correct your answer.
Comment: @MathewMahindaratne I edited the answer. Thank you for pointing it out :)
Comment: Good reply. But yours and 2nd one contradict each other?!
Answer: The nitrophenols have completely different physical behavior based on the position of nitro group:
$$
\begin{array}{c|ccc}
\hline
\text{Compound}  & \text{Melting point} & \text{Boiling point} & \text{Water solubility at } \pu{25 ^\circ C}\\
\hline
\text{2-Nitrophenol}  & \pu{43-45 ^\circ C} & \pu{215 ^\circ C}  & \pu{2 g/L} \\
\text{3-Nitrophenol}  & \pu{89-95 ^\circ C} & \pu{278 ^\circ C}  & \pu{13.5 g/L} \\
\text{4-Nitrophenol}  & \pu{113-114 ^\circ C} & \pu{279 ^\circ C}  & \pu{16 g/L} \\
\hline
\end{array}
$$
This different behavior is due to having intramolecular hydrogen bonding:

Due to these intramolecular H-bonding on 2-nitrophenol, its $\ce{OH}$ group is not readily available to form a hydrogen bond with solvent water. Hence 2-nitrophenol is sparingly soluble in water while 3- and 4-nitrophenol are soluble due to intermolecular H-bonding with water.
Similarly, because 3- and 4-nitrophenols contains much more intermolecular H-bonding than that of 2-nitrophenol, they have large difference in boiling and melting points than that of 2-nitrophenol (need extra energy to break intermolecular H-bonding; The intramolecular H-bonding increases the volatile nature of the molecule comparatively with their isomeric siblings that cannot have intramolecular H-bonding, thus effecting the relevant boiling points).
The intramolecular H-bonding effects oppositely in the solubility in nonpolar solvents such as benzene. Compounds involved in intramolecular H-bonding (kind of chelation) become non-polar (not many interaction forces intermolecularly). As a consequent, these compounds are soluble in nonpolar solvents (oposite to only sparingly soluble in water) whereas their meta and para isomers are less soluble in non-polar solvents but more soluble in water due to intermolecular H-bonding (such as water).
Comment: Good answer but your answer tells that Ortho nitrophenol will be more soluble than para and meta and it contradicts with the first answer!?
Comment: Not really. For example, check percent composition at $\pu{40 ^\circ C}$ at all three isomers in @Aniruddha Deb answer. That was about 90% for 2-nitrophenol. To get the same composition, you need to increase the temperature to about  $\pu{90 ^\circ C}$ for 3-nitrophenol and  $\pu{105 ^\circ C}$ for 4-nitro-isomer. Aniruddha Deb's interpretation is incorrect.


How secure is FileVault 2 while the computer is in sleep mode?

Question: How secure is Apples disk encryption FileVault 2 when someone has physical or network access while the computer in sleep mode or is running a screen saver? Are there ways to circumvent FileVault 2 when the computer is not turned off?
Answer: Just a quick addition to the previous answers; if you are still worried someone would get the hold of your encryption key from RAM during standby, one could enable a power management feature of OS X called "DestroyFVKeyOnStandby", as mentioned here (same link as Richard Belisle), page 37.
From man pmset:
destroyfvkeyonstandby - Destroy File Vault Key when going to standby mode.
    By default File vault keys are retained even when system goes to standby.
    If the keys are destroyed, user will be prompted to enter the password while
    coming out of standby mode.(value: 1 - Destroy, 0 - Retain)

The full command would be sudo pmset -a destroyfvkeyonstandby 1.
This would enable destruction of the FileVault key during standby for all -a power modes. That is UPS -u, battery -b and charger -c (wall power).
Comment: Note that according to http://support.apple.com/kb/ht4392, standby mode is different than sleep and only occurs under certain limited conditions.  You may need to use `sudo pmset -a destroyfvkeyonstandby 1 hibernatemode 25` to get the full benefit of this setting.  This will, however, lead to slower sleep/wake times.  Also it will reportedly sometimes cause a kernel panic on wake.  See also http://apple.stackexchange.com/questions/39294/what-user-behavior-is-necessary-to-make-filevault-2-maximally-secure
Comment: Actually, `hibernatemode 3` (the default for laptops) should work fine.  There is a delay before standby (default 3 hours) which can also be changed using `pmset` (see [the man page](https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man1/pmset.1.html)).
Answer: Yes, OS X is still vulnerable to the Cold Boot Attack, because the encryption keys are kept in memory while the machine is powered on (i.e., from you enter your password on boot until your machine is completely powered off). This is a problem for all software full-disk encryption tools, and not specifically related to FileVault 2.
Depending on the version of OS X that you are running, your machine may or may not be vulnerable to DMA attacks with tools like inception. Versions >= 10.7.2 disables FireWire DMA when the machine is locked.
Comment: So basically you simply need to get to the lock screen to ensure that your computer is safe from attacks?
Comment: @Pacerier: According to augurar's comments on other answers, you will have to take steps to clear the memory upon lock screen in order to protect against a "cold boot" attack. Lock screen should be sufficient against "DMA" attacks from OS X 10.7.2+ though.
Answer: Apparently FileVault 2 is secure against a DMA Attack if the screen isn't unlocked, since 10.7.2 (so make sure you're running Lion). My guess is that on sleep the keys are encrypted with your password, rather than just left in memory.
I'm assuming that it also means it is protected against a Cold Boot Attack too.
The only sources I could find on it is this blog post.
Comment: It is not protected against a cold boot unless you take steps to clear the memory on sleep.
Answer: In older versions of Mac OS X, an attacker with physical access to the machine could plug in via Firewire (or Thunderbolt) and use DMA attacks to gain access to memory.  This would let the attacker slurp out your password and thus defeat FileVault 2's protection.  However, this vulnerability was fixed in Mac OS X 10.7.2.
Later versions of Mac OS X have largely eliminated this vulnerability.  The attack is still possible if a user is logged in and the machine is unlocked.  However, when the screen lock kicks in, the OS enables extra protections that prevent this attack.
References:

"OS X Lion disables DMA when the user is logged out/screen is locked. Attacking will only work while the user is logged in, or if user switching is enabled. The user switching trick only works for  versions before 10.7.2, where the vulnerability is patched."  http://www.breaknenter.org/projects/inception/

Also, setting a firmware (pre-boot) password may help.
Answer: I found an article with the general security analysis of FileVault: http://eprint.iacr.org/2012/374.pdf
It references other security research that answers my question.
Comment: Could you give a summary of the article in your answer?


How does the Certificate Authority ensure that the CSR came from the right server?

Question: My understanding of Certificate Signing Request (CSR) is that it contains the public key and details of the requesting server. The CSR is also signed by the server using its private key so that the CA can verify that the owner of the private key actually did send the request. How does the CA verify that the certificate request came from the correct server. Can't anybody generate a public key/ private key pair and send a CSR.
Answer: Short answer: it's in the CSR.
Longer answer:
When you generate a CSR from a key pair, you provide the CA with information as to what properties the certificate should have (for what domain(s)/names, etc.).
There are different ways how CAs can verify that the CSR does in fact come from the legitimate owner of the domain(s) the CSR is for. Different CAs use different methods and for regular (non-OV/non-EV) certicifates this usually boils down to show that you have control over the domain(s) by for example

sending an email to a usually reserved email address such as hostmaster@domain with a short lived secret that has to be provided back to the CA,
requesting that the user generates a specific challenge entry in DNS,
requesting a specific challenge file from a well known directory from port 80 of the domain(s) in question.

There are however problems that arise from this sort of testing, as it is not completely conclusive: When an attacker is a MitM for the CA, this can go horribly wrong. Yet, CAs usually double check such things using different ISPs.
Comment: @AhmadKarim quick and short answer: It's not possible automatically when there is no checkable trust anchor. In those szenarios, what often happens is that the CA uses the S/MIME Certificates you can get with control over an E-Mail account. The E-Mail-Account would be the trust anchor then.
Comment: what if it's an OpenVPN scenario, where a client who's certificate is expired, creates a CSR from its private key and sends it to CA. In such a case how would CA  know that CSR is coming from a legit client and not anyone else?. As in this unanswered question  https://security.stackexchange.com/questions/191459/how-to-auto-renew-openvpn-client-certificates-using-easyrsa


Can I change my oil filter 2 weeks after putting new oil in?

Question: I recently done an oil change on my Audi A4 Avant 1.9tdi. At the time I didn't have the tool to open the oil filter so i couldn't change it. I've bought the took and am going to put a new oil filter in (2 weeks later). My question is, do it need to change the oil again or is it ok to just change the oil filter? A mechanic said I have to change the oil also as it could be contaminated with debris from the old oil filter.
Answer: When you change your oil, you never get all of the old out.  There will be some left in the sump and on all the internal surfaces.  Not changing the filter just means there will be a little more old oil left than normal.
Change the filter now as you suggested.  If you want to you could change the oil and filter a week or so earlier next time.
Answer: I would go ahead and change the filter.
I used to run a high quality & expensive oil for 20K and change the oil filter every 5k - the filters were so cheap in comparison - even quality ones.
Comment: Agreed. Go ahead and change the filter and not worry about  the oil. Its virtually brand new and fresh enough to keep it.
Answer: Just be careful about where it's placed. The usual position on the side of the engine means that, at the very least, you'll want to have parked it overnight and let everything cool and settle first, and still put a catch tray underneath just-in-case. If it's lower down, it might be prudent to empty all the oil out into a clean tray (5 litre capacity minimum) before hand, swap the filter, then pour it back in. Otherwise you could have a huge mess and a lot of wasted oil on your hands, and end up putting half as much clean stuff back in anyway.


Why are Q values updated according to the greedy policy?

Question: Apparently, in the Q-learning algorithm, the Q values are not updated according to the "current policy", but according to a "greedy policy". Why is that the case? I think this is related to the fact that Q-learning is off-policy, but I am also not familiar with this concept.
Answer: Short answer
The Q values are updated using a greedy policy because, in the Q-learning algorithm, the $\max$ operator is used to determine the target, which is denoted by
$$\color{green}{R_{t+1}} + \gamma \color{blue}{\max_{a}Q(S_{t+1}, a)}$$
Intuitively, the $\max$ operator is used because we assume that the target policy (the policy associated with the optimal value function that we want to learn) takes a greedy action, which is defined, in this context, as the action associated with the highest Q value: $\color{blue}{\max_{a}Q(S_{t+1}, a)}$ means that we are selecting the $Q$ value, associated with the (next) state $S_{t+1}$, which corresponds to the action $a$, such that $Q(S_{t+1}, a)$ is the highest (with respect to other possible actions from $S_{t+1}$).
Note that the $Q$ function receives as input a state and an action. So, for each state $s$, we have an action (among all possible actions, $a_1, a_2, \dots$, which we can take from the state $s$), denote it by $a^*$, such that $Q(s, a^*) > Q(s, a_1)$, $Q(s, a^*) > Q(s, a_2)$, etc. In the expression $\color{blue}{\max_{a}Q(S_{t+1}, a)}$, we are basically selecting $Q(s, a^*)$ for $s = S_{t+1}$.
More explanations
To explain all the components of this target, I will explain the Q-learning algorithm. Hopefully, after this explanation, you will be able to understand why Q-learning uses the greedy policy to update the Q values and why it is an off-policy algorithm (which is, IMHO, a quite unintuitive and confusing term to describe what off-policy actually means).
How does Q-learning work?
Here's the Q-learning algorithm

The Q-learning algorithm estimates a value function, known as the Q-function, associated with a policy $\pi$. Intuitively, it does that by simulating an agent which takes actions in the environment, observes the impact of those actions on the environment in terms of the received rewards and the new states where the agent ends up in after taking those actions. Meanwhile, during this exploration of the environment, it attempts to estimate the optimal $Q$ function (i.e. the value function associated with the optimal policy, which, if followed, will give the agent the highest amount of reward in the long run, aka return).
Q-learning proceeds in episodes. So, initially, you need to pass the number of episodes as input. You can think of episodes as iterations (like in any iterative optimisation algorithm). However, in the context of RL, an episode is a little bit more specific: the start and end of an episode are associated with specific states of the environment: the episode starts when the agent is in a starting state $S_0$ (which can be sampled from a probability distribution over $S_0$, if there is more than one) and ends when it is in a terminal state.
In the pseudocode above, at the beginning of each episode, we initialise $t=0$, where $t$ represents the time step of a specific episode.
Inner/Episode loop
We then have the following loop, which terminates when the agent reaches a terminal state:

So, at each episode, we run the loop above. The block of code inside this loop contains the main logic of the Q-learning algorithm.
Behaviour policy
On each iteration of this inner loop, the agent chooses an action $A_t$ (the action at time step $t$ of the current episode) using a policy (which is known, in this context, as the behaviour policy, which should ensure that all states are sufficiently visited, in order for tabular Q-learning to converge). In this case, the $\epsilon$-greedy policy is used.
How does this $\epsilon$-greedy policy work?
If you look at the pseudocode above, $\epsilon$ is initialised at beginning of each episode. In the pseudocode above, $\epsilon$ can change from episode to episode, but assume, for simplicity, that, at every episode, it is a fixed small number (e.g. $0.01$). The statement

Choose action $A_t$ using policy derived from $Q$ (e.g., $\epsilon$-greedy)

means that, with probability $1 - \epsilon$, the greedy action is chosen, and, with probability $\epsilon$, a random action is taken.
What is the greedy action in this case?
In this case, the greedy action is the action, in the current state $S_t$, which is associated with the highest Q value (given the current estimate of the Q value). It is exactly the same action as the action $a^*$ (as I explained above). The difference is that, in this case, we choose $A_t$ using the $\epsilon$-greedy policy: so, most of the time, we choose the greedy action, but, sometimes, we can also choose a random action.
The agent then executes the just chosen action $A_t$ in the environment, and it observes the impact of this action on the environment, which is determined by how the environment responds to this action: the response consists of a reward, $R_{t+1}$, and a next state, $S_{t+1}$.
To recapitulate, the agent chooses an action using the $\epsilon$-greedy policy, executes this action on the environment, and it observes the response (that is, a reward and a next state) of the environment to this action. This is the part of the Q-learning algorithm where the agent interacts with the environment in order to gather some info about it, so as to be able to estimate the Q function.
Q-learning update
After that, the agent can update its estimate of the Q function using the following update rule
$$\color{orange}{Q(S_t, A_t)} \leftarrow \color{red}{Q(S_t, A_t)} + \alpha ([\color{green}{R_{t+1}} + \gamma \color{blue}{\max_{a}Q(S_{t+1}, a)}] - \color{red}{Q(S_t, A_t)})$$
where $S_t$ is the current state (of the current episode) the agent is in, $A_t$ is the action chosen using the $\epsilon$-greedy policy (as described above), and $S_{t+1}$ and $R_{t+1}$ are respectively the next state and rewards, which, collectively, are the response of the environment to the just taken action $A_t$.
So, how is the estimate of this $Q$ function updated?
First of all, I would like to note that, if you look at the beginning of the pseudocode above, $Q(s, a)$ is initialized arbitrarily for all states $s \in \mathcal{S}$ and for all actions $a \in \mathcal{A}$: it can e.g. be initialised to $0$. $Q(s, a)$ can e.g. be implemented as a matrix (or 2-dimensional array) $M \in \mathbb{R}^{|\mathcal{S}| \times |\mathcal{A}|}$, where $M[s, a] = Q(s, a)$, $|\mathcal{S}|$ is the number of states in your problem and $|\mathcal{A}|$ the number of actions.
Furthermore, note that the symbol $\leftarrow$ means "assignment" (like assignment to a variable, in the context of programming). So, in the update rule above, we are assigning to $\color{orange}{Q(S_t, A_t)}$ (which will be the next or updated estimate of the Q value for the current state $S_t$ and the just taken action from that state $A_t$) the value $\color{red}{Q(S_t, A_t)} + \alpha (\color{green}{R_{t+1}} + \gamma \color{blue}{\max_{a}Q(S_{t+1}, a)} - \color{red}{Q(S_t, A_t)})$. Let's break this value down.
$\color{red}{Q(S_t, A_t)}$ (on the right side of the assignment) is the estimate of the Q value for the state $S_t$ and action $A_t$ before the assignment. So, we are summing $\color{red}{Q(S_t, A_t)}$ and $\alpha (\color{green}{R_{t+1}} + \gamma \color{blue}{\max_{a}Q(S_{t+1}, a)} - \color{red}{Q(S_t, A_t)})$, and then we assign it to $\color{orange}{Q(S_t, A_t)}$ again.
$\color{green}{R_{t+1}} + \gamma \color{blue}{\max_{a}Q(S_{t+1}, a)}$ is what is often called the target. Q-learning is a temporal-difference (TD) algorithm, and TD algorithms update estimates of the value or action-value functions based on the difference between the current estimate, in the case of Q-learning it is denoted by $\color{red}{Q(S_t, A_t)}$ (on the right side of the $\leftarrow$), and a "target". So, in the Q-learning algorithm, $\color{green}{R_{t+1}} + \gamma \color{blue}{\max_{a}Q(S_{t+1}, a)}$ is the target. We can roughly think of it as "the value that $\color{red}{Q(S_t, A_t)}$ should have been". So, in a certain way, we are performing supervised learning, where $\color{green}{R_{t+1}} + \gamma \color{blue}{\max_{a}Q(S_{t+1}, a)}$ would be the ground-truth label and $\color{red}{Q(S_t, A_t)}$ the current estimate, and so $[\color{green}{R_{t+1}} + \gamma \color{blue}{\max_{a}Q(S_{t+1}, a)}] - \color{red}{Q(S_t, A_t)}$ would be the error (or loss): in fact, it is often called the TD error. However, note that this is not really supervised learning, because $\color{green}{R_{t+1}} + \gamma \color{blue}{\max_{a}Q(S_{t+1}, a)}$ is not a ground-truth (it is partially an estimate, because of the part $\gamma \color{blue}{\max_{a}Q(S_{t+1}, a)} $, and it partially a ground-truth, because of $\color{green}{R_{t+1}}$).
To recapitulate, $\color{green}{R_{t+1}} + \gamma \color{blue}{\max_{a}Q(S_{t+1}, a)}$ is the target, $\color{red}{Q(S_t, A_t)}$ is the current estimate, and $[\color{green}{R_{t+1}} + \gamma \color{blue}{\max_{a}Q(S_{t+1}, a)}] - \color{red}{Q(S_t, A_t)}$ is the TD error. We are thus summing the "error" (weighted by the hyper-parameter $\alpha$, which is, in this case, often called the learning rate) and the current estimate $\color{red}{Q(S_t, A_t)}$ in order to produce the new estimate $\color{orange}{Q(S_t, A_t)}$.
In the target, you can see that we are multiplying the $\color{blue}{\max_{a}Q(S_{t+1}, a)}$ by $\gamma$. This is a hyper-parameter (a parameter which often needs to be chosen by the programmer before the algorithm is executed), known as the discount factor. It controls the contribution of $\color{blue}{\max_{a}Q(S_{t+1}, a)}$ to the target: that is, how much of $\color{blue}{\max_{a}Q(S_{t+1}, a)}$ we want to include in the target. Recall that I've just said above that the target is composed of the reward $\color{green}{R_{t+1}}$ (which is a ground-truth or real-world experience, because it is directly received from the environment) and $\color{blue}{\max_{a}Q(S_{t+1}, a)}$ (which actually uses an estimate of the Q function, that is, it uses $Q(S_{t+1}, a)$). So, $\gamma$ controls the contribution of an estimate to the ground-truth.
As I said at the beginning of this answer, $\color{blue}{\max_{a}Q(S_{t+1}, a)}$ can be thought of as the $Q$ value associated with the next state $S_{t+1}$ (which was observed by the agent after he has taken the action $A_t$) and associated with the action $a$, such that $Q(S_{t+1}, a)$ is the highest among all other possible actions from state $S_{t+1}$. In other words, $\color{blue}{\max_{a}Q(S_{t+1}, a)}$ can be thought of the estimate of the Q value associated with the next state $S_{t+1}$ and the greedy action taken from that same state.
Q-learning is off-policy
Note that, when we update the value function, the agent is not really taking actions in the environment (the only action taken is $A_t$, and it was taken, using the behavior policy, before the update!). Nonetheless, people often call Q-learning an off-policy algorithm because

It uses the $\epsilon$-greedy policy to interact with the environment (aka a behavior policy). In this case, actions are really taken, and the responses of the environment are really produced, observed, and used to update estimates of the $Q$ function.

It uses a target that is based on an estimate which is greedy (i.e. it uses $\color{blue}{\max_{a}Q(S_{t+1}, a)}$).

Given that Q-learning uses estimates of the form $\color{blue}{\max_{a}Q(S_{t+1}, a)}$, Q-learning is often considered to be performing updates to the Q values, as if those Q values were associated with the greedy policy, that is, the policy that always chooses the action associated with highest Q value. So, you will often hear that Q-learning finds a target policy (i.e. the policy that is derived from the last estimate of the Q function) that is greedy (so, usually, different from the behavior policy).
Comment: Small note: start and end of episodes do not necessarily have to be associated with specific states. In theory we can have a probability distribution over initial states $S_0$, although in practice the most common case certainly is that the starting state is always the same. It is relatively common that in practice that there are many different states that can all end the episode, though I believe that is already consistent with what you wrote :)
Comment: @nbro: Thanks for your answer. I have a couple of questions to it as several things are unclear. 1) How can we actually calculate the max Q( S(t+1), a)? You write "We can roughly think of it as "the value that Q(St,At) should have been"". What is exactly meat by that? How can we know, what value it should have been? 2) Can you give me an example on how the Q-values are updated in each iteration (or a link where an example is explained in a comprehensive way)? 3) You claim that Q-learning used e-greedy policy for taking the actions and greedy policy for updating. Can other policies be used?
Comment: 4) If we initialise all Q(s, a) to 0, will then not every max Q (s(t+1), a) be also 0 in each iteration? 5) If we always use a greedy policy to udate the Q values, I would assume that the chances are quite high that we will be stuck in a local (but not global) optimum. Why can we for example not also use the e-greedy policy for updating the Q values?
Comment: @PeterBe I am not sure I understand your first question. $\max Q( S(t+1), a)$ is just the maximum state-action value function with respect to $a$. For example, say you have a function $f: \{1, 2 \} \rightarrow \{10, 5\}$, and define it like this: $f(1) = 10$ and $f(2) = 5$, then $\max_x f(x) = 10$ and $\text{argmax}_x f(x) = 1$. To answer your second question, remember that, during learning, $Q$ is an estimate of the true value function $q^*$. $\color{green}{R_{t+1}} + \gamma \color{blue}{\max_{a}Q(S_{t+1}, a)}$ is an estimate of the true value $q^*(S_t, A_t)$.
Comment: To answer your 3rd question, we don't know $q^*(S_t, A_t)$. @PeterBe This post is already too long and I already spent too much effort on it, so I don't plan to give you a step-by-step explanation of Q-learning, because that was not even the question originally. I explained Q-learning here, but I should not have done that. I recommend that you ask your other questions in separate posts, but make sure to search for answers on our site first, because I think some of your questions are already answered by my post above and other posts on the site. **Please, ask only one question per post!**


US citizen working for a US company. I want to live in Honduras

Question: I am a US citizen, and I work for a US Company. I want to live in Honduras because I work on the internet . Can this be done even on a temporary basis? I have been away from my wife for 3 months and I want to be able to spend time with her in Honduras and work for my current US company online.
Comment: Is your wife a citizen of Honduras?
Answer: One important consideration is that just because you say "I work on the internet", that doesn't mean that the Honduras government won't treat that as "working".
I know this is the case in Thailand (where I live as an expat), where any kind of work that is done while in Thailand counts as "working in Thailand".
While this might seem to be a trivial unimportant detail to the person doing the work, as far as the government is concerned you might be liable for income tax. So "working on the internet" while in a country, can constitute "working" in that country, and mean that you need to pay income tax in that country.
I don't know that Honduras is like this - it might not be. But be aware that it is distinctly possible. And while you might think it's not something important, they could easily see it as tax evasion. Make sure you are aware of the local laws before attempting to work in a foreign country. Work permits and tax might be involved.


Resize openSUSE 12.3 GPT ext4 partitions in a RAID1 disk array

Question: I have a software raid configured (/dev/md2) which has assigned the partitions sda3 and sdb3 from two 3TB disks (sda and sdb).
The partition types are GPT not LVM and file system is ext4.
Now, the root partition is taking 1TB and the home partition (sda4 and sdb4 on raid /dev/md3) is taking 1.8TB, I wanted to resize the root partition to take only 250GB and assign the free space to the home partition, so that it ends up being 250GB for root in md2, and 2.5TB for home.
For doing this, I read that using mdadm and resizing the array volume would achieve that, but the problem is that it only resized the md2 device, but the free space in sda3 and sdb3 is unused, and I have no way to assign it to sda4 and sdb4 so I can use that extra space in md3, so I have:
At the beginning:
/dev/md2 with ext4, mounting / with quota of 1000GB
    sda3: 1000GB with ext4 using the full 1000GB
    sdb3: 1000GB with ext4 using the full 1000GB

/dev/md3 with ext4, mounting /home with quota of 1800GB
    sda4: 1800GB with ext4 using the full 1800GB
    sdb4: 1800GB with ext4 using the full 1800GB

I want:
/dev/md2 with ext4, mounting / with quota of 250GB
    sda3: 250GB with ext4 using the full 250GB
    sdb3: 250GB with ext4 using the full 250GB

/dev/md3 with ext4, mounting /home with quota of 2550GB
    sda4: 2550GB with ext4 using the full 2550GB
    sdb4: 2550GB with ext4 using the full 2550GB

And ended up with:
/dev/md2 with ext4, mounting / with quota of 250GB
    sda3: 1000GB with ext4 using only 250GB
    sdb3: 1000GB with ext4 using only 250GB

/dev/md3 with ext4, mounting /home with quota of 1800GB
    sda4: 1800GB with ext4 using the full 1800GB
    sdb4: 1800GB with ext4 using the full 1800GB

To achieve this, I followed the instructions in this website: http://www.howtoforge.com/how-to-resize-raid-partitions-shrink-and-grow-software-raid
So, I restarted in rescue mode, and ran the following commands as root:
#configure everything to edit the raid devices
modprobe linear
modprobe multipath
modprobe raid0
modprobe raid1
modprobe raid5
modprobe raid6
modprobe raid10
cp /etc/mdadm/mdadm.conf /etc/mdadm/mdadm.conf_orig
mdadm --examine --scan >> /etc/mdadm/mdadm.conf
mdadm -A --scan

### Shrink md2 from 1000GB to 250GB
#check the raid disk
e2fsck -f /dev/md2
#resize the file system from 1000GB to 245GB to leave room for the partition to shrink without issues
resize2fs /dev/md2 245G
#resize the partition from 1000GB to 250GB
mdadm --grow /dev/md2 --size=262144000
#resize the file system to fit the whole 250GB
resize2fs /dev/md2
#do a disk check again
e2fsck -f /dev/md2

### Grow md3 to include md2 750GB free space
#grow md3 to the max free space
mdadm --grow /dev/md3 --size=max
#run a disk check
e2fsck -f /dev/md3
#resize the file system
resize2fs /dev/md3
#check the file system again
e2fsck -f /dev/md3

But then it turns out that for actually achieving the resize, I need to fail and remove the partitions from the raid device, repartition each disk, add back to the raid device and sync.
So I tried instructions from this web site:
http://www.zedt.eu/tech/linux/resize-grow-mdadm-raid1-device/
And also tried from this serverfault answer:
How to resize two RAID-1 partitions?
I've tried doing all that, resize with parted, but nothing, parted sends errors stating that it doesn't recognize the file system of the partition, I've read somewhere else that parted doesn't support ext4 but I don't care, I already resized the file system using resize2fs, so all I need to do is resize the partition.
Also on some samples, I see parted print output to print file system type and all, but when I print mine, I don't get any of that information:
sda and sdb disk output:
Number  Start   End     Size    File system  Name  Flags
 5      1049kB  2097kB  1049kB                     bios_grub
 1      2097kB  12.9GB  12.9GB                     raid
 2      12.9GB  13.4GB  537MB                      raid
 3      13.4GB  1113GB  1100GB                     raid
 4      1113GB  3001GB  1888GB                     raid

As you can see, I get no info on file system or name, and I get errors if I want to resize partition 3 (sda3 and sdb3), and likewise I cannot grow partition 4 to acommodate the free space.
Can anyone help me on how can I resize my disks so that my raid devices can take the full space?
Thanks!
Comment: Did you try yast?
Answer: I had good result using this approach:
http://www.howtoforge.com/how-to-resize-raid-partitions-shrink-and-grow-software-raid
Regards
Comment: Whilst this may theoretically answer the question, [it would be preferable](http://meta.stackexchange.com/q/8259) to include the essential parts of the answer here, and provide the link for reference.


how make document library custom upload form in office 365/Sharepoint 2013

Question: I am working on office 365 and  want to make a custom upload form for document library to upload the document and also want to add two custom field on this form to save along with document upload.
  Is It possible using sharepoint designer and infopath ?
Please provide the solution ASAP, THANKS IN ADVANCE.
Comment: judging by the number of downvotes, you should probably try to spend a bit more time describing your scenario and what you are trying to accomplish.
Answer: The only way you can do this is by creating an Auto Hosted / Provider Hosted App or sand-boxed solution which will contain a custom upload form.
You will need to override the default upload form URL with the new URL where your custom form resides using JQuery
Answer: You can create content types and associate these site content types with the document library either through PowerShell/manually.  After creating site content types, you can add the custom site columns you have created. These site columns are added to your document library and it will get saved along with the document after uploading the document.


Calculations for stock market target price?

Question: I'm looking at various stocks on finance websites and noticed that some of them don't have a 1-year Target price. How can I calculate estimated target price of a stock myself based on the company's fundamentals/analysis, PE ratios, and other variables? I'm looking for a number of different formulas that can give me various estimated target prices.
Comment: If there was such a thing as being able to accurately determine the price at which you should sell, wouldn't everyone be doing that, creating a self fulfilling proposition?  Knowing that price point, short sellers would then come in at that price, driving it back down to where it all began. Wash, rinse, repeat.  That's not going to happen  because such analysis is based on differing opinions of what metrics  are important. That's the reason that you're getting various estimated target prices.
Comment: Wasn't what I was asking. I was asking HOW to estimate target prices. Formulas that analysts use to calculate their target prices.
Comment: Investment banks hire a multitude of quants to analyze every aspect of the companies they follow in order to forecast  earnings for  the coming years.  From that they extrapolate target prices.  And even then, they don't get it right. Good luck doing better than they do.
Answer: In answer to your question, some financial analysts start with a DCF Discounted Cash Flow model.
https://en.wikipedia.org/wiki/Discounted_cash_flow
It sounds like the company has no financial analysts covering them.
Another formula that's good to understand is NPV Net Present Value.
That said, I've only tried doing my own DCF analysis a couple of times just to gain an intuitive understanding of the concepts.  The exercise took me several days of work.
When I listen to corporate earnings calls or read their transcripts, many companies provide a "financial model" to the analysts that cover the company.  (Companies do not make these models publicly available.)  Maybe shoot a quick email to investor relations asking for a model.
At the very least, they should be able to provide you with a few pie-in-the-sky  numbers.

total addressable market size for existing product/service
total addressable market size for future product/service
a long term pie-in-the-sky goal for Gross Margin and Net Margin

You could then plug these numbers into a DCF model, comparing it to the "risk-free interest rate of return".
There's a 5% chance they will reply to your email, and if they do, it will say we can't tell you anything.
Answer: Target price = current price*1.1
Many predictions on those websites are not more accurate than this.


Is it reasonable or even possible to have a minor, major seventh chord?

Question: To play a major seventh, we play the seventh degree of the scale plus one half step. I've never really understood why it's called a major seventh, so maybe this can be cleared up for me here. My main question, however, is whether or not this addition can be applied to a minor chord. That is, if I had a D minor, and I wanted to add the seventh note plus one half step, my notes would include D, F, A, and C#. Is this traditional?
Comment: Yes, it's called a [minor major seventh chord](https://en.wikipedia.org/wiki/Minor_major_seventh_chord).
Comment: The seventh is the "major seventh" interval, and it's played over the minor triad, hence "Minor-major seventh chord".
Comment: Why's it called a major seventh?
Comment: Could it be that English native speaking don't know or reflect the latin meaning of major and minor as these are borrowed words?
Answer: The 7th scale degree plus a half step brings you to the tonic. Think key of C. Seventh degree is B. B and C are a half step apart. So I think that may be part of your confusion.

Major triad with a major seventh is a "major 7th" chord.
Major triad with a flat 7 is a "dominant 7th"
Minor triad with a flat 7 is a "minor 7th"
Minor triad with a major 7 is a "minor major 7th"

In C:

C E G B: Cmaj7
C E G Bb: C7
C Eb G Bb: Cm7
C Eb G B: CMin(Maj7)

Note that the "C7" means C dominant 7th.
Answer: In a minor-major seventh chord the triad is minor (like Dm) but the seventh added is a major seventh above the chord root.
When naming seventh chords it's the quality of the seventh used in the names.
Another case is the diminished seventh chord. The base triad is diminished, but most importantly the seventh is a diminished seventh above the root.
The naming isn't totally logical and consistent: ex. dominant seventh chord, half-diminished seventh. Just learn the exceptions.

...possible to have a minor, major seventh chord?

Yes.
Don't get confused by the tones of the key signature and the major seventh used in the chord. It just requires an understanding of how minor key music works.
The D minor key signature uses a C natural, but the minor-major seventh chord will be spelled with a C#. Using this tone which isn't in the key signature is no problem. In minor key music the 7th and 6th scale degrees get raised or lowered depending on the harmonic context.
A really well known example of a minor-major seventh chord is the song My Funny Valentine. Take a look at the song and you will see the minor-major seventh chord is formed by a chromatically descending line from the tonic down to the sixth scale degree. The whole time that line descends the bass not holds the tonic. So, that is a line descending D C# C B over a D bass.
When the D F A C# chord is played you could call it a proper minor-major seventh chord, but you could also call it a kind of passing motion, a mere decoration of a plain Dm chord.
An unambiguous example would be Bernard Hermann's Psycho.
When you ask is the chord possible, I think you want to keep in mind how the chord is formed in the music and whether it's a bona fide chord or something decorative.
Answer: Others have already described the technicalities of the chord, but I wanted to add a bit more about its use in the wild.
When I used to teach the 7th chords in my classroom, my favorite go-to example of the chord was the Dr. Evil Theme from Austin Powers.  It partially outlines the chord twice, and then does a full statement at 0:25-0:26.
It's something of a special effect chord for high tonal instability.  It's not always as stark and dramatic as the example I just gave you.  You can also find it used in impressionistic music from time to time.
Most commonly, you'll find ghost appearances of the chord occurring spontaneously from time to time with the major 7th as a passing tone between 8 and 6.  These would typically not be analyzed as a Major-minor seventh, however, since the 7 is really just a passing tone.
Answer: Yes, it's possible.   Cm(maj7) is C Eb G B.    The 'minor' part of the name refers to the basic triad which is Cm.
It's not only possible but very common!


Does all wood glue need to be removed from a cement floor to use thinset to seat ceramic tile?

Question: Does all wood glue need to be removed from a cement floor to use thinset to seat ceramic tile? We've chiseled it smooth with an air tool. If the old wood cement was water-based will thinset set?
Answer: I was mixing thinset on my porch last year and some dropped on the cement.  My son started wiping it up and I said "don't bother" and he was a bit surprised.  Went to the porch a week later with a putty knife and popped the thinset off with a few taps of a hammer on the putty knife.  Moral of the story being thinset kind of binds to concrete but not really.  It "sticks" and if you use some bonding agents it might do a little better than stick.  But it doesn't "bond" to cured concrete.
Another example is in my area a lot of bathrooms were poured concrete over subfloor - about an inch - then thinset and tile.  I have demo'ed 50-100 of these bathrooms.  I just tap a hammer along the tiles like I am beating a drum.  The vibrations will cause the thinset to "unstick" and I can have an average bathroom floor detiled and dethinsetted in 2 hours while leaving the original concrete in almost perfect condition.
So whether you husband is applying the thinset to the concrete or concrete with a little wood glue on top, he should expect the same amount of "stick".  The only way I could see the wood glue interfering is if it were still loose or became loose during the install, and even then this doesn't seem to be a huge obstacle.  Thinset doesn't need to have a permanent bind to the subfloor to perform well.  In fact if it did it may perform worse and crack at certain points.  The important thing is the thinset binding with itself.


Will I need ESTA and I-94

Question: I am a UK citizen and will be flying to Toronto in September and spending 2 weeks in Canada before traveling by train from Montreal to New York for 4 days and then flying back to the UK form New York.  Will I need to apply for an ESTA and a I-94 for my trip to the US?
Comment: @ΦXocę웃Пepeúpaツ EU citizenship does not confer eligibility for the visa waiver program.  Most EU member states' nationals are eligible, but some EU member states are excluded from the VWP.
Comment: @ΦXocę웃Пepeúpaツ Even if you don't need an ESTA, you need to stop at the border and fill in some forms (and get fingerprinted I believe).
Comment: so EU citizen can just enter USA by train/Bus??? no visa no nothing???
Comment: ESTA is required for air and cruise ship travel only https://help.cbp.gov/app/answers/detail/a_id/1132/~/citizen-of-visa-waiver-country%2C-wants-to-drive-into-u.s.-from-canada-or-mexico%2C
Comment: Some new information has come to light, so I have edited my answer.  After your trip (whether with or without ESTA and an advance I-94) please come back and let us know what happens.
Answer: To enter the US by land, you don't need ESTA, but you do need an I-94.  You can get one at the border, but your trip will go more smoothly if you get it in advance.  For reasons I do not understand, you can only do that if you have ESTA authorization.
One source for this assertion is Citizen of Visa Waiver Country, wants to drive into U.S. from Canada or Mexico, or arrive at a port of call in the U.S. by cruise ship, a webpage of US Customs and Border Protection (CBP):

If I am in Canada or Mexico and want to drive to the U.S., do I need to apply for ESTA?
If you are a citizen of a country that participates in the Visa Waiver Program (VWP), and you enter the U.S. by land from Mexico or Canada, you are only required to complete the paper I-94W form at the land border crossing.  ESTA is currently required for air and cruise ship travel only.

(However, as noted at the I-94 site linked above, instead of completing the paper form at the border, you may apply in advance online.)

Note that there is currently a proposed regulation at the "final rule stage" to extend the requirement for ESTA authorization to travelers arriving at land borders.  It appears that this is expected to go into effect in October 2019, but it could be the reason for the requirement to have active ESTA authorization in order to apply for an advance I-94.

As an aside, I've recently seen some carelessly worded copy on the website of some airline or other that said that you need a valid ESTA to fly out of the US.  This is completely untrue.  ESTA authorization can even expire on the date when you enter the country.  See, for example Does my ESTA need to be valid for my entire stay in the United States? from CBP:

Do travelers need to update their travel authorization if it will expire before they depart from the United States?
No, An ESTA is only required to be valid on the day you enter the United States. The ESTA does not need to be valid for the entire time a traveler is in the U.S. If the ESTA expires while you are in the U.S., it will not affect your admissibility or the amount of time you are permitted to remain in the U.S.
If an ESTA travel authorization expires before the traveler leaves the United States, the traveler is not required to apply for another. The traveler will need to apply for a new travel authorization through ESTA, however, for future travel.

See also Does a non-American need an ESTA to leave the USA?
Comment: You can only get an I-94 in advance if you're entering the US with a visa, not VWP, as it asks for your visa details. Or when using a border crossing card if you're Mexican.
Comment: @BritishSam if you use a VWP-eligible passport, you can also complete the advance I-94 form using ESTA authorization ("You selected a Visa Waiver Program country. You must have a visa or an approved travel authorization via Electronic System for Travel Authorization (ESTA) in order to submit your application").  I will edit the answer accordingly.
Comment: You're right, thanks for clearing it up! So it's pay $14 and save time basically.

Will I need ESTA and I-94


Euro or USD acceptance in Mauritius

Question: I note a booking with Booking.com informs that the room charged is given in Euro. Does that imply that Euro (and perhaps USD) is generally accepted in Mauritius?
Comment: Booking.com usually also mentions whether the room has to be/can be paid in said curency. Mostly they mention it in your curency (or the one they think you use) as a friendly help.
Answer: Check this http://www.tripadvisor.co.uk/Travel-g293816-s601/Mauritius:Banks.And.Money.html friendly article for example:

In Mauritius the currency of choice is the MUR, do not be taken in that Euros or dollars are widely accepted. They are not and if they are accepted by an establishment such as your hotel you will be given an unfavourable rate of exchange. This means you will have lost money.
Do not change money in your home country unless you want to get a lower exchange rate. The rate in Mauritius is much better. When you arrive at the airport in Mauritius you will see a host of exchange bureaus at the arrival halls. Just bring your own currency and change it over there for a better deal.
Answer: Namaste.
As i was informed at hotels and airport the usd and euro in conversion  bear huge loss. So get it exchanged outside of these two event places. Its better either carry  MUR  or any other currency like EUR /USD.
have safe journey.
Comment: It's not clear how this answer adds anything to the existing three years old accepted answer.

Euro or USD acceptance in Mauritius


What happens if I run fsck on /dev/sda instead of /dev/sda1?

Question: When I have run fsck in the past I have always run it on what I thought was a filesystem partition, ie. fsck -yf /dev/sdaN, where N is the partition number on disk A. A friend of mine recently ran fsck on his brand new disk and it reported a bunch of errors using fsck -yf /dev/sda. I am curious, what is the difference between running fsck on the whole disk (/dev/sda), verses running it on just a single partition (/dev/sda1)?
Comment: You should recieve some kind of "useless message" like "/dev/sda is in use" like this guy: http://askubuntu.com/questions/239316/dev-sda-is-in-use-from-livecd
Comment: @nwildner That was not the result seen in this case. It seems like `/dev/sda` was truly not being used, as my friend was in some sort of recovery shell which made itself available before the drive was mounted.
Comment: It should stop as it does not recognize any fs at all as it is not to be meant to inspect partition tables.
Answer: When fsck runs, it should first try to locate the superblock of a filesystem to begin traversing the filesystem's structure in order to validate it. Since the /dev/sda device corresponds to whole drive, the first portion of the disk will likely contain the partition table or Master Boot Record and fsck will not be able to locate the superblock for a supported filesystem (unless you get something that somehow matches a magic number for a known filesystem). I would expect it to exit in error or provide inaccurate output (as your friend experienced). However, I have not yet performed this experiment myself.
Answer: It won't work, if the filesystem was installed in a partition (e. g. sda1).
fsck and its brethren are tools for performing maintenance on filesystems (hence the name: filesystem ccheck), not of block devices.
It is, I suppose, theoretically possible to put a filesystem directly onto a block device by way of something like mke2fs -j /dev/sda, but this is very, very bad practice.  But if you do, the filesystem would then live directly on sda, and then to fsck it, you would indeed just point fsck at the device.  Do not do this.  Partition tables are there for a reason, and it's not just to take up a nominal amount of space for a table of contents.
Answer: If the device contains a supported filesystem, fsck should work. The device can be an entire hard drive like /dev/sda, or a partition, or a flopy disk (anyone remeber what those were?) like /dev/fd0, or even a file on another hard disk like ~/hdimage.img.
For floppy drives the entire drive is normally used, so there is no reason to partition, so the filesystem is made on /dev/fd0. Conventionally hard drives are divided into partitions and file systems are built in the partitions. However if you want to use the whole drive as one partition and you do not need to boot from it, you could make a filesystem directly on the device  (mke2fs /dev/sdb), mount it and copy a lot of files to it, then fsck /dev/sdb should work. This has been called a "superfloppy" configuration. I think the old iomega 100MB disks were also normally formatted this way.
A disk doesn't even need to have a filesystem- it could have a single file. I often make tar files the right size to fit on a DVD, burn them to the disk. The disk is then not mountable, but tar -xvf some/where/myfile /dev/sr0 extracts myfile from the disk.
Comment: Come to think of it it may be possible to make a superfloppy bootable,  the same way floppy drives are.


Can lifeboat passengers claim the necessity defense if they are charged with murder for cannibalism?

Question: Suppose a ship at sea suddenly develops problems and the passengers are put into life boats. One boat carrying 20 people gets lost. After 8 days without food and water, the passengers decide to kill one of the passengers and eat him. They kill a 7 years old boy and feed on him. When they get back to shore, they are charged with murder. If the passengers plead necessity as a defense, will they succeed?
Comment: https://en.wikipedia.org/wiki/R_v_Dudley_and_Stephens
Comment: In what jurisdiction?
Comment: Welcome to LSE. What a great question. If you want more detail and some nice pictures, Brian Simpson's, *"Cannibals at Common Law,"* summarizes his book *"Cannibalism and the Common Law."* The wiki article is pretty much all cribbed from his book.
https://chicagounbound.uchicago.edu/cgi/viewcontent.cgi?article=1002&context=crosskey_lectures
Comment: @SJuan76 Why don't you add a sentence or two about what would happen and post that as an answer? (Feel free to add a cite to the article I posted about.)
Comment: It is not a complete defense at English common law. It is in some U.S. jurisdictions.
Comment: need that tag for permissible murdering.
Comment: @SJuan76 Thank you soooo much. The link you provided was exact answer of my question.
Comment: Murder and cannibalism provided the setting for one of the *classic* commentaries on American jurisprudence, Lon Fuller's, "The Case of the Speluncean Explorers." Those who want to know more about how judges decide cases might find it an interesting read:   https://pennstatelaw.psu.edu/_file/TheCaseOfTheSpelunceanExplorers.pdf    https://en.wikipedia.org/wiki/The_Case_of_the_Speluncean_Explorers
Comment: @ohwilleke Can you please name one U.S. jurisdiction in which such an act would be tolerated?
Answer: The stated facts are an almost carbon copy of the facts in R v Dudley and Stephens (1884) 14 QBD 273 DC so we know the answer - No

The court ruled that cannibalising the boy was not urgently necessary. Even though the cabin boy would almost certainly have died of natural causes, the sailors killed the boy intentionally and were guilty of murder. There was some degree of necessity arising from the threat of starvation but, at any moment, a ship could have sailed over the horizon to save them as, indeed, the three were rescued. Since they could never be sure that the killing was actually necessary from one minute to the next, the defence was denied.

Specifically, cannibalism is not the crime; murder is.
Necessity requires:

an urgent and immediate threat to life which creates a situation in which the defendant reasonably believes that a proportionate response to that threat is to break the law.

In practice, necessity is only a defence to relatively minor crimes, like breaking traffic rules in a race to a hospital or breaking a window to escape a burning building - the consequences of the breach have to be relatively trivial in relationship to the consequences of adhering to the law.
Comment: We know the answer in 1843 in England. We don't know the outcome in modern jurisdictions. (England statutorily adopted a necessity defense later, and U.S. courts and statutes have recognized the possibility).
Comment: @ohwilleke I'm not sure it's a *statutory* defence in England, is it? It seems to rely on [case law](http://www.e-lawresources.co.uk/Defence-of-Necessity.php).
Comment: @AndrewLeach You might be right. I'm not an expert on English criminal law.
Answer: A typical Model Penal Code based provision of the choice of evils defense is as follows:

Part 7 - Justification and Exemptions From Criminal Responsibility
§ 18-1-702. Choice of evils
Universal Citation: CO Rev Stat § 18-1-702 (2016)
(1) Unless inconsistent with other provisions of sections 18-1-703 to
18-1-707, defining justifiable use of physical force, or with some
other provision of law, conduct which would otherwise constitute an
offense is justifiable and not criminal when it is necessary as an
emergency measure to avoid an imminent public or private injury which
is about to occur by reason of a situation occasioned or developed
through no conduct of the actor, and which is of sufficient gravity
that, according to ordinary standards of intelligence and morality,
the desirability and urgency of avoiding the injury clearly outweigh
the desirability of avoiding the injury sought to be prevented by the
statute defining the offense in issue.
(2) The necessity and justifiability of conduct under subsection (1)
of this section shall not rest upon considerations pertaining only to
the morality and advisability of the statute, either in its general
application or with respect to its application to a particular class
of cases arising thereunder. When evidence relating to the defense of
justification under this section is offered by the defendant, before
it is submitted for the consideration of the jury, the court shall
first rule as a matter of law whether the claimed facts and
circumstances would, if established, constitute a justification.

A law review length analysis from 2008 can be found here and this one in 2006 is also a leading article on the topic.
It is hypothetically recognized under U.S. law, but rarely applied in circumstances like the ones described.


Is the sharpe ratio calculated taking the standard deviation of the portfolio or of the excess return?

Question: Does the formula consider the standard deviation of the excess return: $$\frac{−_}{{(−_)}}$$ or that of the return: $$\frac{−_}{{()}}$$
Answer: The first equation, using the excess return in both numerator and denominator is more theoretically correct. Importantly it generalizes to any two returns, not just $r$ vs $r_f$ but $r_1$ vs $r_2$ for any two returns.
And Wm. Sharpe discusses this general case in his 1994 JPM article, linked
However in the common case where $r_f$ is the short term risk free rate, its movements over time are so small that it makes little difference to the value. And therefore in practice  many people use the second form that you quoted, with the denominator being the standard deviation $\sigma(r)$ of the fund alone. This is so common that it is a de facto standard in the industry.
Answer: The standard deviation of a constant (rf) is zero. Therefore, either way its the same value.
Comment: Why is $r_f$ assumed constant? True, in many simplified scenarios it's just taken to be const, but it's not necessarily so both in theory and in practice.
Comment: As other user said, the short term movements of $r_f$ are so small that makes little to no difference, this is the common practice. Anyway, you can get either model rf as a short-rate model or take it from the market (in backtesting) -> $ r_{f,t}$


A condenser mic gain tuning on a mixer

Question: I am working with a pair of condenser microphones. My mixer's gain knob should be turned on about half of single step (almost "minimum") to adjust the sensitivity each of one. Is there a trick, how I can change the gain knob level "zoom" to have a chance make wide turns for gain tuning?
Answer: If I understand correctly, you are currently using your mixer's gain knob very close to it's minimum gain position, and you'd prefer to be in a middle position.
This means that the output level of your microphones is close to the max input level of your mixer's inputs. Possible workaround (depending on your specific equipment) :

There might be an attenuator switch on your microphones, that would
lower the output signal of your microphones.
There might be an
attenuator switch or potentiometer on your mixer's mic input, that would also lower
the signal incoming into the mic preamp.

This issue might also reveal that you are close to the max acoustic pressure that your microphones can handle without distortion.
Answer: Most Condensor Microphones have a "pad" switch (usually -15 or -20 db).  That will decrease your input volume, allowing for you to turn your gain up and have greater ability to fine-tune.  Also, most mixers have a "pad" switch or button as well, and this will do the same thing.
However, I have never encountered a condenser microphone producing such a strong signal that I literally have no need for a preamp... which makes we wonder if you have another gain stage somewhere in-between your mic and your mixer?  because it sounds like your up at line level before you even hit your mixers preamp.
Answer: I have had this issue as well. I built my own microphones using condensor mics. The circuit was not my own and with it´s original design values I found that it was overdriving my mixer, A soundcraft Signature 16 channel.I could not go above the minimum gain setting at all. I eventually had to modify the circuit values so as to attenuate the output. End of Problem. Most Condensor mics have their own internal preamp and use Phantom power unless it has it's own battery supply. So to be honest you may have to put an attenuator inline with your mike or find a way to modify the circuit values so as to reduce the signal to your mixer. I have recently used them to record a performance of my daughters playing the piano and cello. The sound was pretty good. Only thing I found the Piano a bit sharp, but I think it is due to the piano tuning more than anything else. The original circuit could pick up whispers across a room of 8 meters, that is too sensitive for me (it may be fine for outside recording, but indoor needs less sensitivity). Also a good tip is cover the mice with a sponge. Something that is not too dense. It helps to reduce sensitivity without loosing sound quality. I am now planning on making a small one for use on my daughters viola and also improve one for the cello.
Comment: Is there a chance to listen to the recorded performance?
Comment: If I read your words correctly, the best way not to re-solder the mixer is to connect the mic through an in-line attenuator just like `Sescom SES-MULTI-PAD 6-Position Variable Attenuator` with possible values: -5dB, -10dB, -20bB, -40dB, and -50dB. The huge attenuation will increase the range of possible gain adjustments. Is it exactly what you mention in the answer?

A condenser mic gain tuning on a mixer


What is the difference between buy and purchase

Question: When we use the word "to buy" and when the word "to purchase"? I do not understand the difference between these words.
Comment: Google is your first friend, Wikipedia is the second, and **dictionaries and thesaurus's are the third**! So, my first internetic friend gives me this: http://english.stackexchange.com/questions/212060/difference-between-buy-and-purchase
Comment: If you're going to carp, it's probably best to avoid things such as " thesaurus's " in your carping...
Answer: They are essentially the same. To purchase is perhaps a bit more formal.
In regular conversation with friends and colleagues, "I bought that". I'd use purchase more when writing formal reports, or talking to your boss "The company purchased 25 reams of paper."
Answer: If it comes to the sense of obtaining something by paying money, both "purchase" and "buy" are usually used interchangeably  as a verb.  Nevertheless, the use of "buy" is more common in daily conversational English than that of purchase. The use of purchase is a bit formal.
In some contexts, however, it sounds more natural to use buy instead of purchase such as I will buy him the book/let me buy you a drink.
Similarly, when you use these words as a plural noun, it's more natural to use purchase in some contexts such as she opened the bag of her purchases/her purchases were so heavy that she had to take a taxi.  Besides, you often say in informal English when you buy something at a low/high price "it's a good/bad buy". "It's a good/bad purchase" sounds a bit weird.
Comment: That last point you make is very subtle. I would say "that was a good buy" if I intended to convey "I obtained the object for a lower price than I expected", and I would say "that was a good purchase" if I meant "I have enjoyed owning this object". That is, "good buy" is about *cost* and "good purchase" is about *value*. I never noticed that I make this distinction before; I wonder if others do as well?
Comment: Eric Lippert,  Thanks for your comments that I have just seen.  I think your comments make sense.  We can say "it's a wise purchase".
Answer: They mean exactly the same thing 99.9% of the time, and typically you can freely use purchase where you'd use buy and vice versa.
One exception: There at least one expression (possibly regional) involving buy that would sound weird if you used purchase, for example:

A: He just told me that she's sick and won't be coming in today.
B: I don't buy that.  She probably just is hungover from too much partying last night.

Here, "I don't buy that" means "I don't believe that" - using purchase here would not work.
The saying "bought it hook, line, and sinker" - meaning to have believed someone's words later discovered to be deceptive - also would not work with purchase.
Comment: Another idiom that would sound weird with "purchase" instead of "buy" is "buy me some time"
Comment: "Can't *purchase* me love." - The Beatles
Answer: The difference between buy and purchase is that:
Buying is the act or process of buying goods for immediate consuming, while purchasing is the process or act of buying goods for the purpose of resale.
Comment: could you provide links confirming the information you are giving?


Bose-Einstein Condensate bonding

Question: How would a material bond with a BEC? Would it have valence electrons? If you were to have a couple hundred atoms in BEC, would they bond with a couple hundred other atoms, a lower amount that how many atoms, or just one? How does that works?
Answer: BEC usually have very low densities ("dilute"), with $\sim 10^{13}$ particles cm$^{-3}$ vs $\sim 10^{22}$ particles cm$^{-3}$ for a usual solid.
This is because BEC atoms do bond, they form molecules. A solid is the real ground state of the material, a BEC is just a metastable state that decays to a solid by creating molecules via three-body recombination events. These involve 3 atoms, so as to conserve momentum and energy, otherwise not-allowed in 2 atom processes of identical particles, and go as $a^4 n^3$, where $n$ is the density and $a$ the scattering length (quantifying the scattering length).
So low densities $n$ are needed to even have a metastable BEC in a lab. Denser (more $n$) and more interacting (more $a$) bond more quickly and form a solid faster.

Bose-Einstein Condensate bonding


Can anyone identify a story I read once, about a boy who communicates with creatures in another dimension?

Question: I read this in the 70s, I think it was in a collection although it might have been in a pulp mag like Analog.  This was a short story about a boy who lives in a rural area, (possibly Maine?) who when he wanders in the forest and fields encounters "others"--beings who inhabit our universe but "use" it in a different way.  He has an unhappy life and often talks to these beings about his problems, and they listen sympathetically but don't offer much in the way of advice.  The gist of the story itself is that these beings are planning something soon that will "unmake" our aspect of the universe.  They say regretful goodbyes to the boy, and he goes and tries to tell the adults in his world.  The story ends with the boy hearing some sort of celebration by these beings, and the sounds of our world starting to disappear.
Comment: Welcome to the site. You have a good start here. If you could take a look at this [guide](http://meta.scifi.stackexchange.com/a/9337/68872) to help jog your memory and [edit] in any more details, that would be great. Every little bit helps us.
Comment: Technically Analog had changed from a pulp format to a digest format by the 1970s.  https://en.wikipedia.org/wiki/Pulp_magazine
Answer: Is this the novella "Chains of the Sea" by Gardner Dozois?
It was originally published in 1973, but I read it in "The Visible Man", a collection of Dozois's short fiction from 1977.
The boy is named Tommy.  The aliens are called The Thant.

"What'd you want?" Tommy finally said.
"We've come to say good-bye," the Thant replied.  "It is almost time
for you to be made not."

It ends as follows:

Tommy was very quiet in school the next day.  He sat silently in the
back of class, with his hands folded together and placed on the desk
in front of him. Hard slate light came in through the window and
turned his hands and face gray, and reflected dully from his dull gray
eyes.  He did not make a sound.
A little while later, they finished winding down the world.
Comment: This was exactly it!  Thanks!
Comment: In fact, it was in this very book.  Amazon mistakenly lists this as "by" Silverberg, but he just wrote the introduction.  https://www.amazon.com/Chains-Sea-Robert-Silverberg/dp/0440014182