Young's modulus for composite auxetic material

Question: If you look at the screenshot you will see a composite auxetic material. How would you calculate the E-modulus for it? Is it as simple as the rule of mixtures, E = V1 * E1 + V2 * E2? Or is it more complex because of its structure?
Comment: Which E modulus? From the picture the composite material is unlikely to be isotropic. It's not very clear to me how the OP's "2D" image relates to the 3D material structure, or how the diagonal "links" connecting the individual components are going to function mechanically.
Comment: @alephzero the elements between the panels are not connected, they have room to move. "Young's modulus, also known as the elastic modulus, is a measure of the stiffness of a solid material."
Comment: So these elements are not connected to the panels, so these elements have 2d of freedom or 3...
Comment: @SolarMike these elements are bound to the two plates on top and bottom, but the arms (cylinders that are not connected to top or bottom) are free to move. I attached a picture of the models.
Answer: Rather than thinking about a Young's modulus directly, the problem becomes easier to grasp if you think in terms of a relationship between the load and displacement, and subsequently between stress and the strain.
If you consider the case where deformations are small, and just consider the stiff direction (tension/compression vertically and not horizontal shear in your first figure), you can apply a vertical load to the structure and compute the vertical deformation.
Let $P_y$ be the applied load and let $u_y$ be the resulting displacement.  For the linear part of the load-displacement curve you will get a relation
$$
  P_y = K u_y
$$
where $K$ is the slope of the curve.
To convert these into stresses and strains, you will need two quantities:

the area ($A_{xz}$) over which the load is applied
the initial length ($L_y$) of the unit cell.

The choice of these quantities, particularly the area, is not obvious and several options are possible.  For example, the area might be:

just the area of the cylinder on which the load is applied
the area of the triangular domain of the unit cell
a square area that tiles space in the horizontal plane, etc.

You have to explicitly define what the area is and justify that choice satisfactorily.  Once that choice has been made, you can compute the stress-strain relation using
$$
  \sigma_{yy} = \frac{P_y}{A_{xz}} = \frac{K L_y}{A_{xz}}\frac{u_y}{L_y} = \frac{K L_y}{A_{xz}} \varepsilon_{yy} =: E_{yy} \varepsilon_{yy}
$$
where your definition of modulus is
$$
  E_{yy} = \frac{K L_y}{A_{xz}}
$$
That's the preferred way of computing a modulus.
You can compare that with your mixture relation (or any other continuum theory) to check what the error in the rule of mixtures is.
Answer: Related: How do I calculate an estimate for the properties of a composite material?
In this case, we actually have an unusual case of the fourth method.  We could look at each individual unit cell of the material and apply a unit force on the structure in the x, y, and z directions.  Each application of unit force would correspond to a certain deflection.  The net deflection divided by the bounding box dimensions of the unit cell would result in a strain, and the unit force divided by a stress would result in a stress.
However, this simple approach does not appreciate the 3D complexity of the problem.  Since we are calculating the response to a force, we are finding the individual relations of the stiffness tensor.  Note that for general anisotropic materials such as this one, we need to find 21 different relationships.  However, afterwards, the stiffness tensor is not usable, we need to invert the matrix to find the compliance tensor.  The elastic modulus would then be the inverse of the first term of the stiffness tensor, but the usable format would utilize the compliance tensor.


Looking for old satellite images

Question: I'm looking to display satellite images of regions on a timeline starting from 2010, preferably updated regularly say every month or so. Are there apis that provide that for commercial use ?
Comment: you mean something like Google Timelapse ? https://earthengine.google.com/timelapse/ What resolution?
Comment: Yes, something I can download images from. For the resolution I don't need anything big, I just wanna see crop fields over time
Answer: If you want to download images from 2010, you can go the Nasa Earthdata portal. There you will find tons of Landsat images. And it looks as though there is a way to access some web services.
https://wiki.earthdata.nasa.gov/display/GIBS/GIBS+API+for+Developers
Answer: LandViewer contains full collections of medium-resolution imagery dating back to 80's.

Sentinel-1 - archive since 2014
Sentinel-2 - archive since 2015
Landsat 4 - archive 1982-1993
Landsat 5 - archive 1984-2013
Landsat 7 - archive since 1999
Landsat 8 - archive since 2013
MODIS - archive since 2012

You mark the location and it shows you all available imagery from various active and past satellites. From there on, you can pick those you need and act depending on your purpose: view, run remote sensing analysis, download images.
I randomly chose Milan as search location in Landviewer, set cloudiness filter to minimum and found over 450 satellite images for it.
Here's the oldest one, taken by Landsat 5 in April 1984:

And this is a recent one from Sentinel-2:

One more thing to add: in case you are looking for old satellite images in order to perform multitemporal analysis - there's this Landviewer feature called Time Series. It allows to build a graph that shows how a certain parameter performed over time.
All images used for calculation can be visualized and rendered on the map with a click. Quite useful if you need to visualize the dynamics of some parameter and identify patterns for current decision making or predicting possible result (e.g. crop yield).
Answer: Google Earth Pro has a historical function that displays changes over time. For the time between 2010 and now, there may be 6 images. The interval lengths between image capture seem random and perhaps too long, but could be a start. Its fairly straight forward to capture images out of Google Earth and geo-reference based on known points.

Looking for old satellite images


Getting ABI interface using solc offline

Question: Suppose I have an inheritance contract (xyz.sol) with three contracts A, B and C such that C is B, I am trying to compile using solc offline in node console. Upon compiling, I have three separate bytecodes and ABI interface generated. For a single contract alone ABI=..... [`:xyz'].interface used to give the ABI alone. Now because there are more ABI present in the output compiled code, it returns an error (undefined). Is it possible to get all the ABI interface together or separate by any indexing method?
Answer: I don't understand this statement:

Now because there are more ABI present in the output compiled code, it
  returns an error (undefined).

I'm not 100% sure "what" exactly returns the error. Depends on what you're doing, I suppose. You should probably only be concerned with the ABI for the contract you want to deploy.
There's an ABI for each contract and it's possible you want to deploy any of the contracts contained in your source. You'll find that B's functions are part of C's ABI.
Consider this even simpler example:
pragma solidity 0.4.18;

contract A {

    function aDo() public pure returns(bool success) {
        return true;
    }
}

contract B is A {

    function bDo() public pure returns(bool success) {
        return true;
    }

}

Notice that the ABI for B contains function aDo() because B is A.
[{"constant":true,"inputs":[],"name":"bDo","outputs":[{"name":"success","type":"bool"}],"payable":false,"stateMutability":"pure","type":"function"},{"constant":true,"inputs":[],"name":"aDo","outputs":[{"name":"success","type":"bool"}],"payable":false,"stateMutability":"pure","type":"function"}]

The compiler is going to give you the ABI for A, but the thing is you don't really need it (or the bytecode for A) unless you plan to deploy A, and you probably don't want to.
In most use-cases, you would only want to deploy the main contract that has inherited the lessor components.
Hope it helps.
p.s. If using Truffle, they take/make the assumption that the source file name matches the name of the contract you will be interested in deploying (B in my example, C in yours), although the source and imports may load it up with many contracts that won't be deployed by themselves.


Why are circular references considered harmful?

Question: Why is it a bad design for an object to refer to another object that refers back to the first one?
Answer: Circular dependencies between classes are not necessarily harmful.  Indeed, in some cases they are desirable.  For example, if your application dealt with pets and their owners, you would expect the Pet class to have a method to get the pet's owner, and the Owner class to have a method that returns the list of pets.  Sure, this can make memory management more difficult (in a non-GC'ed language).  But if the circularity is inherent in the problem, then trying to get rid of it is probably going to lead to more problems.
On the other hand, circular dependencies between modules are harmful.  It is generally indicative of a poorly thought-out module structure, and/or failure to stick to the original modularization.  In general, a code-base with uncontrolled cross-dependencies will be harder to understand and harder to maintain than one with a clean, layered module structure.  Without decent modules, it can be much harder to predict the effects of a change.  And that makes maintenance harder, and leads to "code decay" resulting from ill-conceived patching.
(Also, build tools like Maven won't handle modules (artefacts) with circular dependencies.)
Comment: @Jim - I'm not saying that you cannot remove the circular dependency between the classes.  For example, you could do it by declaring (say) `getOwner()` to return an `Object` rather than an `Owner`.  But, IMO this sort of thing is likely to result in worse problems than the circular dependency you have eliminated.
Comment: Perhaps the classic SQL relationship table could be used to remove the circularity of dependency between pet and owner?  An object holding references to owner/pet relationships and supporting set methods to return subset tuples in either direction (as well as those to add and remove such relationships).

In that scheme the pet and owner classes each depend on this relationship class.  It, however, doesn't have to depend on either of the classes that use it.
Answer: Circular references are not always harmful - there are some use cases where they can be quite useful. Doubly-linked lists, graph models, and computer language grammars come to mind. However, as a general practice, there are several reasons why you may want to avoid circular references between objects.

Data and graph consistency. Updating objects with circular references can create challenges in ensuring that at all points in time the relationships between objects are valid. This type of problem often arises in object-relational modeling implementations, where it's not uncommon to find bidirectional, circular references between entities.
Ensuring atomic operations. Ensuring that changes to both objects in a circular reference are atomic can become complicated - particularly when multithreading is involved. Ensuring consistency of an object graph that is accessible from multiple threads requires special synchronization structures and locking operations to ensure that no thread sees an incomplete set of changes.
Physical separation challenges. If two different classes A and B reference each other in a circular fashion, it can become challenging to separate these classes into independent assemblies. It's certainly possible to create a third assembly with interfaces IA and IB that A and B implement; allowing each to reference the other through those interfaces. It's also possible to use weakly typed references (e.g. object) as a way to break the circular dependency, but then access to the method and properties of such an object couldn't be easily accessed - which can defeat the purpose of having a reference.
Enforcing immutable circular references. Languages like C# and VB provide keywords to allow references within an object to be immutable (readonly). Immutable references allow a program to ensure that a reference refers to the same object for the lifetime of the object. Unfortunately, it's not easy to use the compiler-enforced immutability mechanism to ensure that circular references cannot be changes. It can only be done if one object instantiates the other (see C# example below).
class A
{
    private readonly B m_B;
    public A( B other )  { m_B = other; }
}

class B
{
    private readonly A m_A;
    public A() { m_A = new A( this ); }
}

Program readibility and maintainability. Circular references are inherently fragile and easy to break. This stems partly from the fact that reading and understanding code that includes circular references is harder than code that avoids them. Ensuring that your code is easy to understand and maintain helps to avoid bugs and allows changes to be made more easily and safely. Objects with circular references are more difficult to unit test because they cannot be tested in isolation from one another.
Object lifetime management. While .NET's garbage collector is capable of identifying and dealing with circular references (and correctly disposing such objects), not all languages/environments can. In environments that use reference counting for their garbage collection scheme (e.g. VB6, Objective-C, some C++ libraries) it is possible for circular references to result in memory leaks. Since each object holds on to the other, their reference counts will never reach zero, and hence will never become candidates for collection and cleanup.
Answer: Because now they're really one single object.  You can't test either one in isolation.
If you modify one, it's likely that you affect its companion as well.
Comment: @RCIX - be careful. You've just ruled out graphs, some trees and doubly-linked lists.
Comment: What if they implement the same interface?
Comment: @Pierreten: Even then, it's a bad idea for the same reason.
Answer: From Wikipedia:

Circular dependencies can cause many
  unwanted effects in software programs.
  Most problematic from a software
  design point of view is the tight
  coupling of the mutually dependent
  modules which reduces or makes
  impossible the separate re-use of a
  single module.
Circular dependencies can cause a
  domino effect when a small local
  change in one module spreads into
  other modules and has unwanted global
  effects (program errors, compile
  errors). Circular dependencies can
  also result in infinite recursions or
  other unexpected failures.
Circular dependencies may also cause
  memory leaks by preventing certain
  very primitive automatic garbage
  collectors (those that use reference
  counting) from deallocating unused
  objects.
Comment: how wonderfully 'meta'. this SO page has a circular reference with Wikipedia. This answer quotes Wikipedia, and the Wikipedia page links back here: https://en.wikipedia.org/wiki/Circular_dependency (see "external links" on that wiki page)
Comment: This is more about assembly references than object references.
Answer: Such an object can be difficult to be created and destroyed, because in order to do either non-atomicly you have to violate referential integrity to first create/destroy one, then the other (for example, your SQL database might balk at this).  It might confuse your garbage collector.  Perl 5, which uses simple reference counting for garbage collection, cannot (without help) so its a memory leak.  If the two objects are of different classes now they are tightly coupled and cannot be separated.  If you have a package manager to install those classes the circular dependency spreads to it.  It must know to install both packages before testing them, which (speaking as a maintainer of a build system) is a PITA.
That said, these can all be overcome and its often necessary to have circular data.  The real world is not made up of neat directed graphs.  Many graphs, trees, hell, a double-linked list is circular.
Answer: It hurts code readability. And from circular dependencies to spaghetti code there is just a tiny step.
Answer: Here are a couple of examples that may help illustrate why circular dependencies are bad.
Problem #1: What gets initialized/constructed first?
Consider the following example:
class A
{
  public A()
  {
    myB.DoSomething();
  }

  private B myB = new B();
}

class B
{
  public B()
  {
    myA.DoSomething();
  }

  private A myA = new A();
}

Which constructor is called first? There's really no way to be sure because it's completely ambiguous. One or the other of the DoSomething methods is going to be called on an object that's uninitialized., resulting in incorrect behavior and very likely an exception being raised. There are ways around this problem, but they're all ugly and they all require non-constructor initializers.
Problem #2:
In this case, I've changed to a non-managed C++ example because the implementation of .NET, by design, hides the problem away from you. However, in the following example the problem will become pretty clear. I'm well aware that .NET doesn't really use reference counting under the hood for memory management. I'm using it here solely to illustrate the core problem. Note also that I've demonstrated here one possible solution to problem #1.
class B;

class A
{
public:
  A() : Refs( 1 )
  {
    myB = new B(this);
  };

  ~A()
  {
    myB->Release();
  }

  int AddRef()
  {
    return ++Refs;
  }

  int Release()
  {
    --Refs;
    if( Refs == 0 )
      delete(this);
    return Refs;
  }

  B *myB;
  int Refs;
};

class B
{
public:
  B( A *a ) : Refs( 1 )
  {
    myA = a;
    a->AddRef();
  }

  ~B()
  {
    myB->Release();
  }

  int AddRef()
  {
    return ++Refs;
  }

  int Release()
  {
    --Refs;
    if( Refs == 0 )
      delete(this);
    return Refs;
  }

  A *myA;
  int Refs;
};

// Somewhere else in the code...
...
A *localA = new A();
...
localA->Release(); // OK, we're done with it
...

At first glance, one might think that this code is correct. The reference counting code is pretty simple and straightfoward. However, this code results in a memory leak. When A is constructed, it initially has a reference count of "1". However, the encapsulated myB variable increments the reference count, giving it a count of "2". When localA is released, the count is decremented, but only back to "1". Hence, the object is left hanging and never deleted.
As I mentioned above, .NET doesn't really use reference counting for its garbage collection. But it does use similar methods to determine if an object is still being used or if it's OK to delete it, and almost all such methods can get confused by circular references. The .NET garbage collector claims to be able to handle this, but I'm not sure I trust it because this is a very thorny problem. Go, on the other hand, gets around the problem by simply not allowing circular references at all. Ten years ago I would have preferred the .NET approach for its flexibility. These days, I find myself preferring the Go approach for its simplicity.
Comment: .NET's garbage collection uses a mark-and-sweep algorithm that starts from all known roots (static members, etc.) and marks every reference that is reachable from there. Any references that are not reachable (whether they refer to each other or not) are not marked, and therefore become eligible for collection. Why do you find this to be problematic for circular references?
Comment: can't you just assign the object reference to null when you don't need it anymore?
Answer: It is completely normal to have objects with circular references e.g. in a domain model with bidirectional associations. An ORM with a properly written data access component can handle that.
Answer: Refer to Lakos's book, in C++ software design, cyclic physical dependency is undesirable. There  are several reasons:

It makes them hard to test and impossible to reuse independently.
It makes them difficult for people to understand and maintain.
It will increase the link-time cost.
Answer: Circular references seem to be a legitimate domain modelling scenario. An example is Hibernate and many other ORM tools encourage this cross association between entities to enable bi-directional navigation. Typical example in an online Auction system, a Seller entitiy may maintain a reference to the List of entities He/She is selling. And every Item can maintain a reference to it's corresponding seller.
Answer: The .NET garbage collector can handle circular references so there is no fear of memory leaks for applications working on the .NET framework.
Comment: "there is no fear of memory leaks for applications" I'm not sure if that statement is very true.
Comment: I agree strager; espcecially when it comes to forgetting to detach event handlers
Comment: Application scope memory leaks are possible in .NET; however, the type described here *should* be handled by the GC, as stated, Event Handlers are a different story.


My parents have control of my money and see everything I can do. What should I do?

Question: So I know that you have read the title. But today I come to you pouring out all of my emotional finance trauma. So I am all for saving money. My parents are control freaks and the moment I show any independence they are all for taking it away from me. When I was smaller on birthday relatives, and friends would send me money. I would take that money and save it. I decided to give it to my dad because I didn't like my money being free in my room as I would have people come stay in my rooms during holidays and I didn't want them to steal it. Fast forward a couple months I ask for my money back and my dad says no. H screams at my and says it is no longer your money you gave it to me and that means I am going to keep it. I was upset for good bit of time because they was a huge sale going on for something I had been looking at for months on in. Now I'd turned 13 I asked my parent's if I can get a bank account that way I would be able to put my money in a safe place. I know that meant that my parent's would be able to track my spending and deposits. But at the time I didn't think that would be a big deal since I trust them and they trusted me. Key word I trusted them. Since its covid I can't go to the bank and take out money like I would used to. So when I need physical cash because id didn't want to use my card, i go and exchange my electronic dollars for that cash. My dad saw my bank statements and yelled at me for exchanging dollars. I am scared that he is going to take my account away from me and I honestly wish that I didn't open up the account in the first place. because its electronic he can just be nosey when he wants to. I hate it because I though I was going to get some sort of financial freedom but it turns out whenever I talk business with ,y parents. They don't want me to do anything that means I get independence. They hate that I am growing up and want independence along with my own money. Should I take all my money out so that they don't know when, what, where and how I am spending or should I just wait until I turn 18 to for a separate account without my prying parents eyes.
Comment: He kept the money you gave to him to hold, but he's so far left your bank account alone? I think you've got a fine question, but it would be better if it were cut down a bit.
Comment: This fine question could surely use a copy editor.
Comment: Related: [As a 16 year old, how can I keep my money safe from my mother?](https://money.stackexchange.com/q/112353/10997)
Comment: @BobBaerker - you just know that the reason our OP here wants the cash, is to trade options !! whaa !
Comment: @Fattie - How fast can I turn my nest egg into  $1,000 in the market?  Start option trading with $2,000 !
Comment: How old are you?  If under 18, you must accept this, and plan to get on your own.  (Back In The Day, I opened my own checking account at age 14 at a bank I could ride my bike too.)
Comment: har har har  hee  :)
Comment: You stated your dad said, "it is no longer your money you gave it to me and that means I am going to keep it". Do you mean he is not letting you spend your money until you're older, or is he claiming the money is now his and he could just transfer that money into his own account?
Comment: How about keeping your money in cash instead of putting it in the bank account? And hiding it. Or bringing it with you
Answer: Until you are 18, your money is not your own. If you were a child star of stage or screen, California's Coogan Act could protect your earnings. If you're not a star, the law does not protect you from your parents' emptying your piggy bank.
Answer:
Unfortunately, just suck it up until you are 18.  :-/

You won't believe me now, but when you are ancient (like, 27) you will look back and wish you had "bit your tongue" and just sucked it up, to keep a good relationship with your parents.

Really you can't officially move out until 18. But realistically when you are about 16, you'll have plenty of freedom and realistically be able to move out if you want to.

Here are the takeaways

It sucks. You really just have to "suck it up".

Once you are about 15-16, it's very unlikely anyone will take your money from you. After all you're only 13 currently, and, that sucks sometimes in various ways.

I would encourage you to just "not worry about it" until then. Just "let it go". Your parents could be a million times worse in a million ways, so, you know. That's life.

Sometimes you just have to accept that "you're gonna lose this one". You know?
Time will pass quickly - don't worry about that!
Comment: Re #2, not necessarily.  Some parents are pretty abusive, and trying to maintain any sort of relationship with them just sets you up for pain.
Comment: @jamesqf On the other hand, not everything that seems restrictive to a 13 year old counts as abuse. Without discounting the possibility of abuse, I support Fattie's general encouragement to factor in the longer-term relationship.


Does CommVault use native SQL Server backup COM objects or the VSS?

Question: We are being told by a parent company that we will be transferring our backups to COMM Vault and are trying to gather some information regarding how the product works as far as whether it is using snapshots or the native SQL Server backup objects.  Can anyone tell me a little more about how COMM Vault backups work underneath?  A couple of us here have gone through the documentation and it appears that snapshots are not the only choice but I thought I would post the question here as well.  Thank you in advance.
Answer: I used CommVault at a previous job. Version 9.4+ did both native and VSS snaps.


How to capture the gas produced by combusting alcohols

Question: I am burning various alcohols and am attempting to capture the carbon dioxide produced. I could burn the alcohols in a conical flask with a cork on the top, with a tube directing the gas into a gas jar. However, due to the small volume of a conical flask, there would be almost no oxygen to burn and the flame would extinguish quickly. Is there any experimental setup I might have access to with high-school lab equipment that allows me to burn alcohols in an environment with plenty of oxygen, while capturing the gas produced? Thanks
Answer: If you look at the overall reaction, there is actually no gas produced, but there is a consumption of gas:
$\ce{CH3(CH2)_n-OH + \frac{3}{2} (n + 1)O2 -> (n + 1)CO2 +(n + 2)H2O}$
So 1.5 mol of oxygen are consumed, while only 1 mol of carbon dioxide is formed, so the volume will be less if the temperature is maintained.
To perform this experiment, you must collect the carbon dioxide in a solution of alkali and then titrate the solution.
The carbon dioxide will be converted to carbonate:
$\ce{2NaOH + CO2 -> Na2CO3 +H2O}$
A constant flow of air must be put into the system and the gas that comes out must be bubbled with a diffuser through the alkaline solution.
The air flow must be slow, so that all the carbon dioxide is washed out of the gas stream.
In the titration there will be two equivalence points, the first one of the excess of alkali and the second one of the carbon dioxide.
This one has a much smaller pH change.
Be sure to make a careful blank of the alkali, because it normally has some carbonate that will add to the value of carbon dioxide.
Comment: Hi, thanks for the help, just a few questions: 1. Why does the solution that the CO2 is bubbled through need to be alkaline? 2. How could I pump a constant flow of air into the system, realistically, in a lab setting? Thank you again.
Comment: Look at the second reaction. The carbon dioxide is converted to carbonate which is not a gas and will stay into the solution. In our lab we have a line of compressed air. There are also small pumps wich give low flows of air. They are simple and cheap
Comment: Is there a reason you can think of that could justify this experiment to determine the CO2 emissions of alcohols over just using their mole ratio in the reaction you provided to find out? Possibly to find the environmental emissions per litre of fuel used as opposed to per mol? Thanks
Comment: If you have an unknown alcohol, you could determine the chain length by doing this experiment, and if an alcohol is mixed with water you could, in principle, determine the content. However, the precision of the results will probably be very low, so it will be kind of orientative only. To analyze water in alcohol it is better to use Karl-Fisher


How to pronounce basic vocabulary

Question: I'm really new at this but I'm a big fan of the Japanese language. I found a list on Quizlet with the most used vocabulary such as to go, to see, etc to learn with their Kanji (I already learned Kana). My question is since there's always a Kunyomi and/or Onyomi pronunciation, which one am I learning right now?
For example, いく (行く)= to go   (this is how Quizlet teaches it). Is いく one way of saying to go or is there like a basic pronunciation or does it depend on the word?
Answer: Kanji change pronunciation based on many factors, and one kanji can have a dozen different prounciations. It's not productive to worry about all them, and instead you should learn complete words.
The same kanji can represent different words with different readings, like 行(ぎょう)-line and 行(こう)-act, in which case the reading and the meaning should be clear from the context. Sometimes, the author of the text will provide furigana (the reading of the word, written in hiragana, positioned on top of the kanji) to specify which reading and meaning is intended.
And in some cases, words can have the same readings but their meanings differ based on the pitch accent. Something to be aware of when listening to native speech.
Answer: To answer your question 行く (いく) you learned is the kunyomi reading, and yes sometimes there are more kunyomi readings and sometimes more onyomi readings for a kanji.
Almost always when you have kanji together they use onyomi instead, like the word action for example,  行動 (こうどう) you use the onyomi of 行 (コウ) and 動 (ドウ). To be clear here I wrote the onyomi readings with katakana.
So I guess in the beginning when you learn words like 帰る、行く、食べる、飲む。they will all use kunyomi, then when things get more kanji heavy you will use the onyomi more frequently.

How to pronounce basic vocabulary


Factoring Technology into Economics models

Question: There has been tremendous improvement in technologies in recent years. My question is: have economists been able to properly evaluate and factor the values of recent innovations in the technology sector? We often hear about various economics indicators such has Manufacturing PMI and industrial productions, but these indicators often fail to mention technology related jobs or the values of recent innovations. Can anyone help me understand how technology is valued?
Answer: Yes, this is pretty standard stuff in economics.
First, the simplest models contain it in a hidden form. Supply curves are derived from production functions, and include (marginal) cost of production. Technology innovation is then reflected in reduced production costs per unit. It can also change the indifference curves of consumers, leading to changes in the demand curve (shifts, slope change, etc.) Labor costs are also affected, thus also changing macroeconomic models.
Second, there are models which focus specifically on innovation. Look up the Schumpeterian models, or (for a more business-oriented than economic-oriented theory) Christensen's work.
Third, some of the more involved models can accommodate the consequences of a specific technology as opposed to a generic assumption of innovation. For example, brick-and-mortar stores are known to max out at some size due to rising overhead, while for online stores, overhead is considered to rise underproportionally to profit, which predicts "winner takes all" markets, which is what we are seeing today. Similarly, when you look at information assymmetry models, you see that modern information technology can remove some asymmetries from specific markets (new ways for principals to monitor agents, allowing price comparison in some markets, eliminating transaction costs or discovery costs thus making intermediars obsolete). You can get a good, if somewhat dated, review of the results of IT developments on economic models in Macho-Stadler and Perez-Castrillo's "An introduction to the economy of information".
I know this is a rather vague answer, but I'm afraid your question is similarly vague.
Answer: Measuring technological progress (especially in specific sectors is relatively simple). The basic idea is that a technological improvement can be understood as an improvement in efficiency, i.e. it can be measured how much the output improves while keeping constant the amount of input or initial resources. For example, even with a simplified production model such as the Cobb-Douglas function:
$$Y_t = A_t K_t^\alpha L_t^{(1-\alpha)}$$
where $K_t$ is amount of "capital" used, and $L_t$ the total labor used, $0< \alpha < 1$ is an adimensional parameter. Then a technological improvement will mean that $A_{t+1} > A_t$, and by using the same amount of $K_t$ and $L_t$ the output will be greater than the conditions with the old technology.
What is more difficult is to predict the technical rate of change and whether there is a clear and direct relationship with economic factors. So much so that many models consider technical progress to be an exogenous shock and essentially unpredictable, other models treat it as a stochastic process $\epsilon_t$. Where technological progress could be modeled approximately as:
$$A_{t+1} = A_t e^{\epsilon_t}$$
There are even models that consider the time in which two technologies coexist and study the transition between them. An interesting model of this type is Ayres (1997), where the transition from an old to a new technology evolves as:
$$z = \frac{1-a e^{-(a+b)(t-t_0)}}{1- b e^{-(a+b)(t-t_0)}}$$
where $0 < z < 1$ is the proportion of use of the new technology ($b \ge a > 0$).


Hot water is not getting hot in kitchen sink only

Question: The hot water valve is open and the water comes out but it stays cold. We shut off the cold water valve and it made no difference (besides obviously when you turn to cold side no water pours). We turned closed and opened hot valve and it did not change the temp, it when closed had no water came out.
The water in the bathrooms gets hot and very quickly.
We just got new countertops so everything was disconnected yesterday.
Answer: It depends on how you get hot water. There may be a "tankless" water heater under the sink, or very close to it, that was turned off during countertop installation. This is likely, because the sink is getting water, but it does not get hot. If that's the case, just find the instructions for turning it on, and check circuit-breakers as well.
These heaters are popular where energy is costly, since they run only on demand.


Can't set up task to run from non admin user

Question: I have Windows server 2008 R2. One non administrator account named for e.x. abc. abc is member of backup operators, power users and schedulers. I have allowed all permissions to C:\Windows\Tasks for schedulers. I checked, that groups backup operators and schedules are in list in local policy so these can log on as batch job. When I try to set up task I get "Task Scheduler cannot create the task. The user account is unknown, the password is incorrect, or the user account does not have permission to create this task". Any ideas?
Answer: For troubleshooting purposes you could attempt to add the account directly in the policy, instead of as a member of a group.  Also, you may want to verify the Group Policy setting  "Network access: Do not allow storage of credentials or .NET Passports for network authentication"


Running SQl Server on the same box as Lotus Domino

Question: Is there any reason why we shouldn't run these too database servers on the one machine?
Answer: There shouldn't be any interference between the two servers that would break things. For example, you need to install both of them on the same machine when running Blackberry Enterprise Server for Domino.
But as lg says, performance may be a concern depending on what you need from them.
Answer: As above performance, also as sql is licenced by the socket you may to ensure that socket is used purely for sql as much as possible. Of course if you've got more than one cpu and affinity set for both apps (don't if that is possible for domino but I guess it is) then this is less relevant.
Answer: Performance, anyway it depends by hardware and by the use of both servers.


Should I select 'I live with family' in UK visit visa application since I live with my sister?

Question: I am applying for UK Visit Visa from Canada and my parents are back in Pakistan. Should I select 'I live with my family' option for my home address as I am living in Canada with my sister in her house? Or should I select "other" and type in the details that I live with my sister? Please guide ...
Comment: It probably doesn't matter, as long as the information is all there. But hopefully somebody will have a more detailed answer.
Comment: @DavidRicherby want to expand on this?
Comment: @MarkMayo I don't have any definitive information, sorry.
Answer: Although your sister is family, I think the two options have slightly different meanings. I would interpret "living with my family" as living with one or more family members in a situation in which you are not contributing economically towards rent and expenses. "Other- Living with my sister" could mean that you two share accommodation and you both contribute to paying rent.
All in all I think this question is aimed at providing information to determine your financial situation and possibly even your home ties - paying rent means that you have economic means to support yourself. Therefore, answer with what better represents your situation. Be honest and truthful and do not hesitate to add extra information to detail your situation.


What are some standard ways of computing the distance between documents?

Question: When I say "document", I have in mind web pages like Wikipedia articles and news stories.  I prefer answers giving either vanilla lexical distance metrics or state-of-the-art semantic distance metrics, with stronger preference for the latter.
Answer: There's a number of different ways of going about this depending on exactly how much semantic information you want to retain and how easy your documents are to tokenize (html documents would probably be pretty difficult to tokenize, but you could conceivably do something with tags and context.)
Some of them have been mentioned by ffriend, and the paragraph vectors by user1133029 is a really solid one, but I just figured I would go into some more depth about plusses and minuses of different approaches.

Cosine Distance - Tried a true, cosine distance is probably the most common distance metric used generically across multiple domains. With that said, there's very little information in cosine distance that can actually be mapped back to anything semantic, which seems to be non-ideal for this situation.
Levenshtein Distance - Also known as edit distance, this is usually just used on the individual token level (words, bigrams, etc...). In general I wouldn't recommend this metric as it not only discards any semantic information, but also tends to treat very different word alterations very similarly, but it is an extremely common metric for this kind of thing
LSA - Is a part of a large arsenal of techniques when it comes to evaluating document similarity called topic modeling. LSA has gone out of fashion pretty recently, and in my experience, it's not quite the strongest topic modeling approach, but it is relatively straightforward to implement and has a few open source implementations
LDA - Is also a technique used for topic modeling, but it's different from LSA in that it actually learns internal representations that tend to be more smooth and intuitive. In general, the results you get from LDA are better for modeling document similarity than LSA, but not quite as good for learning how to discriminate strongly between topics.
Pachinko Allocation - Is a really neat extension on top of LDA. In general, this is just a significantly improved version of LDA, with the only downside being that it takes a bit longer to train and open-source implementations are a little harder to come by
word2vec - Google has been working on a series of techniques for intelligently reducing words and documents to more reasonable vectors than the sparse vectors yielded by techniques such as Count Vectorizers and TF-IDF. Word2vec is great because it has a number of open source implementations. Once you have the vector, any other similarity metric (like cosine distance) can be used on top of it with significantly more efficacy.
doc2vec - Also known as paragraph vectors, this is the latest and greatest in a series of papers by Google, looking into dense vector representations of documents. The gensim library in python has an implementation of word2vec that is straightforward enough that it can pretty reasonably be leveraged to build doc2vec, but make sure to keep the license in mind if you want to go down this route

Hope that helps, let me know if you've got any questions.
Answer: There's a number of semantic distance measures, each with its pros and cons. Here are just a few of them:

cosine distance, inner product between document feature vectors;
LSA, another vector-based model, but utilizing SVD for de-noising original term-document matrix;
WordNet-based, human verified, though hardly extensible.

Start with a simplest approach and then move further based on issues for your specific case.
Comment: Note that when doing LSA, typically you use the cosine distance on the LSA projections of the original dataset. Just to clarify.
Answer: Empirically I have found LSA vastly superior to LDA every time and on every dataset I have tried it on. I have talked to other people who have said the same thing. It's also been used to win a number of the SemEval competitions for measuring semantic similarity between documents, often in combinations with a wordnet based measure, so I wouldn't say it's going out of fashion, or is definitely inferior to LDA, which is better for topic modelling and not semantic similarity in my experience, contrary to what some responders have stated.
If you use gensim (a python library), it has LSA, LDA and word2vec, so you can easily compare the 3. doc2vec is a cool idea, but does not scale very well and you will likely have to implement it yourself as I am unaware of any open source implementations. It does not scale well as for each document, a new and separate model has to be built using SGD, a slow machine learning algorithm. But it will probably give you the most accurate results. LSA and LDA also don't scale well (word2vec does however), LDA scales worse in general. Gensim's implementations are very fast however, as it uses iterative SVD.
One other note, if you use word2vec, you will still have to determine a way to compose vectors from documents, as it gives you a different vector per word. The simplest way to do this is to normalize each vector and take the mean over all word vectors in the document, or take a weighted mean by idf weighting of each word. So it's not as simple as 'use word2vec', you will need to do something further to compute document similarity.
I would personally go with LSA, as I have seen it work well empirically, and gensim's library scales very well. However, there's no free lunch, so preferably try each method and see which works better for your data.
Comment: How exactly have you used LSA? It's worth noting that LDA is actually a pretty thin wrapper around LSA (it's pLSA with a dirichlet prior) that has been empirically shown to greatly increase generalization. You would almost certainly see better accuracies with LSA, but that's generally a result of overfitting, which is a very notable problem with LSA. Also, what exactly do you mean by scaling here? doc2vec does not actually require a new model for each document, and for computation there's no notable difference between LSA and LDA, both being very scalable.
Comment: I have not observed over fitting with LSA, and like I said, I have met multiple other people who have seen better performance over LDA. Also, I have seen LSA used in many winning entries in semeval competitions, I have never seen LDA used in a winning entry. That is the academic conference for comparing semantic similarity between documents, so I assume they know what they are doing. Doc2vec, if you are referring to Mikolov's paragraph vector implementation, does SGD on each document separately. So it's very slow.
Comment: @SlaterVictoroff I think that's over stating things to say it's overfitting. LDA is known to be poor for search / information retrieval and recommendation cases, empirically LSA has been shown to work much better and that matches my own experience too as I like to validate these findings against our own data. Versions of Doc2Vec do a gradient descent per document, it depends on the algorithm used in Doc2Vec, as that generally refers to a lot of different algorithms.
Answer: State of the art appears to be "paragraph vectors" introduced in a recent paper. Cosine/Euclidean distance between paragraph vectors would likely work better than any other approach. This probably isn't feasible yet due to lack of open source implementations.
Next best thing is cosine distance between LSA vectors or cosine distance between raw BOW vectors. Sometimes it works better to choose different weighting schemes, like TF-IDF.
Comment: Note my comments below about paragraph vector scalability. This technique looks very promising, but is hard to implement, and does not scale well at all, as you are doing a separate SGD for each document, which is very costly, if I remember the paper correctly
Answer: It is useful to have in you bag of tools the family of locality sensitive hashing algorithms.
This family is not semantic at all.
Actually is consider the text as a sequence of bits.
I find it useful in dirty data sets when the same text appears many times with slight differences.
You can use ssdeep (which is based on Nilsimsa hash) for identifying such documents.
Ssdeep was originally planned for the domain of spam. Spammers often do small changes in the message (add a space) in order to prevent detection by exact signature (e.g., md5).
Since many version of almost the same document in the same data set will cause havoc to statistical methods that will be applied on it, doing such a cleanup can be very beneficial.


How to link to new pages in Magento

Question: This may be a very stupid question. I made a new page in Magento and I want to make a link to the page on my Magento homepage, but I dont know what the link url to use for that page. The url key for my page is over-ons My base url is: 127.0.0.1/html/magento/magento I cant seem to find out how to get to my new page. How do I link this to the home page? I've seen a tutorial on the official magento website itself and it said you have to make a new category but that doesn't seem right for such a small thing. How do I get to my new page?
Answer: You need to add this in the content field of the homepage in CMS->Manage pages
Over Ons

This will build the correct url independent of your base url.
Answer: If you mean putting a link in the main navigation, then unfortunately a Category is probably the only way to do it without a custom module observer.  If you want to do it properly by customization, Marius has an excellent answer here.
Magento's navigation building code is an ancient relic from around Magento 1.0, and mainly just loops through the categories to build tiered out navigation.  You can take advantage of this by giving a Category and a CMS page the same short url, as Magento processes the CMS router after the Category router, meaning that the link to the CMS page is the one that will be honored.
EDIT: Cleared up the wording on why this should be done.
Comment: I don't think the OP asked how to add a link in the navigation, but in case you got it right, I want to challenge your answer. Starting version 1.7 you can add anything to the top menu using the event `page_block_html_topmenu_gethtml_before`. See [here an example](http://magento.stackexchange.com/q/7329/146). The example is in the question not the answers.
Comment: Excellent point.  That would probably be strictly-speaking the best way to do it.  I offered the response because OP mentioned he saw a tutorial mentioning the Category method, and seemed to want to approach this from a "non-customization" (everything done in the admin) angle.  Adding to the navigation via observer is definitely cleaner.
Comment: I cleared up my initial answer to explain it a bit better.  Thanks.

How to link to new pages in Magento


Mobile game textures sizes

Question: I am developing a Windows Phone 7 game using XNA 4.0 which I plan to later port to iOS and android using Monotouch and Monodroid. My question is how to determine what texture size should I include? This is because the in-game objects that utilize the textures dont care about the actual texture size and just re-size them. Windows phone by its own exists in multiple resolution variants. Should I pick the highest resolution emulator and then measure the sizes of my in-game objects, and then re-size their respective textures to be the same?
Comment: You should profile the game on an actual device or devices you are targeting, and then determine how good can you make the texture without sacrificing performance.
Answer: Going from the emulator to an actual mobile device, keep in mind that the mobile device's performance characteristics will not match.
In your particular case, keep in mind that texture-fetch on a mobile GPU is relatively slow. Therefore you want to minimise the amount of texture data that you are accessing for each pixel. Ideally you want the texture to be drawn at 1:1 scale on screen.
For example, if you have a large texture that you scale down, then a small rendering region is going to have to load up a large amount of texture data. (Even if texturing only uses a few pixels from the texture, if they're far apart, unused pixels in between will get loaded into cache.)
So, basically, try and store textures in GPU memory as close to their on-screen size as possible. Or use mipmapping, which will automatically select smaller versions of your textures when needed (and also gives you higher quality when scaling down). Trilinear filtering (interpolating between two mip levels) requires more texture fetch - but is still better than substantial downscaling.
As for the maximum supported texture size, WP7 has a maximum texture size of 2048 (XNA Reach profile), as does iOS (from iPhone 3GS onwards, before it was 1024). The actual size you make your textures is more of a artistic consideration - and a consideration of how much memory you can afford.
Comment: So the most practical approach seems to just design for the highest resolution emulator and set the texture size 1:1, since mobiles don't vary all that much, at least for WP8, and there will rarely be a situation where a texture will be half its size on one mobile compared to another (that's what mipmaps are good for, relatively extreme cases). sounds good?
Comment: Sounds good :) Just to make it clear: if you never scale below 50%, there's no real benefit to having a mipmap.
Answer: Max texture size for WP7 is 1k x 1k and from what I remember there is only 1 physical resolution available for WP7 480x800 so really if you are looking at different texture sizes it would only be for the other platforms. Windows Phone 8 introduced larger resolutions and the capability of utilizing much larger textures. If you want the maximum detail for each platform then you will likely wind up providing different rez variants for each distinct screen resolution in order to minimize the amount of scaling that is done.
Comment: Considering windows phone 8 alone, it sound very inefficient to include each texture more than once... It still seems to me that designing for the max res emulator and scaling down for others is the best way. But when it comes to android & iPhone, the max resolution goes pretty far (galaxy note 2 is huge).
Comment: Are you sure it's 1k? [This table](http://blogs.msdn.com/b/shawnhar/archive/2010/03/12/reach-vs-hidef.aspx) says that it's 2k.
Comment: According to your table it seems it should support 2k x 2k but that wasn't my experience when I was developing initially for my WP7. It took awhile for me to track down that the emulator didn't have a problem with the 2k textures but the device did. Perhaps it was a problem with previous versions of the operating system that were ironed out later. That being said it should be even easier to support WP8 + WP7.

Mobile game textures sizes


How does one get tomcat to bind to ipv4 address?

Question: I recently installed tomcat via an installation script from the apache solr typo3 community and spent the last 3 days trying to figure out why it won't work until by chance I noticed that when I queried the process listening on the port via "lsof -i", it was bound to the ipv6 protocol.
I have googled everywhere and most say that setting address to 0.0.0.0 in the tomcat connector resolves this issue, others say setting JAVA_OPTS="-Djava.net.preferIPv4Stack=true".
I have tried the former which doesn't work but the latter I am unsure of where to put it. One solution I read somewhere suggested to put it in setenv.sh but I can't find this file in my tomcat installation. I would appreciate any help at the moment regarding this.
The tomcat version is 6.x and the OS is ubuntu 11.10.
Thanks
Comment: What ips/ports is it listening on, via lsof -i?
Comment: @becomingwisest 8080.
Comment: You are correct - setenv.sh file does not exist out of box. You need to create setenv.sh file in your CATALINA_BASE or CATALINA_HOME bin directory. The startup scripts check if you created the file, if you did create customization (setenv.sh) file - startup script calls it, otherwise just ignores, and moves on.
Answer: Many suggested updating catalina.sh startup script. Yes, that solution would work, but catalina.sh script is not meant to be customized/updated. All changes should go into the customization script instead, i.e. setenv.sh.
NOTE: TOMCAT_HOME/bin/setenv.sh doesn't exist by default, you need to create it. Check the catalina.sh script and you will see the startup script checks if setenv.sh exists, and executes if it does.
So, I suggest you create new TOMCAT_HOME/bin/setenv.sh script with a single line:
JAVA_OPTS="$JAVA_OPTS -Djava.net.preferIPv4Stack=true -Djava.net.preferIPv4Addresses=true "

Comment: I would also use CATALINA_OPTS instead of JAVA_OPTS, especially if you have other JVM options you want to pass into JVM on Tomcat startup. If you use JAVA_OPTS, the same options will be passed into Tomcat shutdown, probably not what you would want/expect. Use CATALINA_OPTS instead :)
Comment: I'm choosing this as the answer now since most people having this issue today would most likely be using a newer version of tomcat.
Answer: Ok I finally solved it. I was directed to try this and Henk's solution. Neither of which seemed to work with the remote virtual server. I'm guessing the fact that because I'm on a shared kernel space so the provider prevents this. In any case I added: JAVA_OPTS= $JAVA_OPTS -Djava.net.preferIPv4Stack=true -Djava.net.preferIPv4Addresses to the catalina.sh startup script and that seemed to have fixed the issue of binding tomcat to ipv6.
Comment: So you're on a VPS with a shared kernel... D'oh!
Comment: With tomcat 8, this option didn't work for me, but nevenc's answer did work.
Comment: This seems not to work anymore with tomcat 8.
Answer: The correct syntax for modifying catalina.sh would be:
JAVA_OPTS=" $JAVA_OPTS -Djava.net.preferIPv4Stack=true -Djava.net.preferIPv4Addresses=true "

Answer: If you used this installer: "Apache Solr for TYPO3", you can change the address in the file server.xml. The default points to localhost, so look for 127.0.0.1 and change it into the IPv4-address you want. Don't forget to restart Tomcat6 for the changes to take effect.
UPDATE, 20120521
See my comment below on how to disable IPv6 on Ubuntu 11.10.
I have successfully tested this on a Virtualbox-VM on my Mac. The address for the connector port 8080 has been changed from 127.0.0.1 to 0.0.0.0 in server.xml.
Then disabling IPv6 makes the "tcp6" to go away, so it's binded to an IPv4-only address.
Before / with IPv6 enabled:
# netstat -anp | grep 8080
tcp6       0      0 :::8080                 :::*                    LISTEN      1972/java

After / IPv6 disabled:
# netstat -anp | grep 8080
tcp        0      0 0.0.0.0:8080            0.0.0.0:*               LISTEN      2045/java

Comment: Tried this also and that didn't work netstat shows that despite the process being bound to an ipv4 address it's still looking for an ipv6 a la this return: tcp6       0      0 X.X.X.X:8080       :::*                    LISTEN. which I think is odd but the problem has also been replicated on my friend's vm which is hosted on a mac.
Comment: And when IPv6 is disabled? Here is a howto for Ubuntu 11.10: http://pario.no/2011/12/09/disable-ipv6-on-ubuntu-11-10/
Answer: While probably not the preferred method, I have observed that disabling IPv6 at the kernel level will convince Tomcat to open an IPv4 bind.
Answer: Assuming Java Options have been set to disable IPv4:
JAVA_OPTS=" $JAVA_OPTS -Djava.net.preferIPv4Stack=true -Djava.net.preferIPv4Addresses=true"

Using Tomcat SSL with APR, the only way I could get Tomcat to bind to ipv4  was to add this to the connector config:
address="0.0.0.0"

server.xml looks like this:
<Connector port="8443" protocol="org.apache.coyote.http11.Http11AprProtocol"
               maxThreads="150"
               SSLEnabled="true"
               scheme="https"
               compression="off"
               connectionTimeout="1190"
               address="0.0.0.0"
               >

Comment: This does not work.
Comment: Did you use the preferIPv4Stack Java Opts arg when starting?
Comment: Yes. I had to disable IPV6 in redhat to fix this.
Answer: Debian 8 navigate to  using your favorite editor on /etc/default/grub ; look for the section GRUB_CMDLINE_LINUX_DEFAULT="quiet"** with then add ipv6.disable=1, as seen below
GRUB_CMDLINE_LINUX_DEFAULT="ipv6.disable=1 quiet"
Save and exit.
In the same directory use your favorite editor on /etc/default/tomcat8 then look for the section with JAVA_OPTS= which will be commented out, add the following below that line. JAVA_OPTS=" $JAVA_OPTS -Djava.net.preferIPv4Stack=true -Djava.net.preferIPv4Addresses=true"
Save and exit
At the command prompt type update-grub , if you have sudo use with sudo, then restart tomcat8  service tomcat8 restart
You should be on IPv4 now.
Please in future posts include complete paths and file names.
Thank You


Can i make all the functions in my smart contract as constant?

Question: What if i make a smart contract that does modify the state, but the all its functions are constant.(it will still compile) It means that i can use my smart contract for free without paying for the gas. Wouldn't this be a lose to the miners?
How does Ethereum handles this situation?
Answer: Yes, you can. Calling constant function doesn't create transaction. Your client just computes result locally.
However, constant functions can not change state (because they're constant)
Answer: You cannot change contract state without a blockchain transaction.
The reason it still compiles is because the Solidity compiler does not yet figure out with 100% accuracy which functions modify contract state. When I try to compile it, I do get this compiler warning:

Warning: Function declared as view, but this expression (potentially) modifies the state (...)
Answer: Well, If you can write such kind of code and deploy it ... Yes, it will work, but, the use case for such a contract that can only return static data form any source will be extremely limited.
And as far as I know, suppose you create such a contract, it cannot return anything(dynamic data related to something) as it has nothing inside the contract(say, mapping or arrays or structs).
here is an example:
<code>pragma solidity >=0.4.22 <0.7.0;

contract sample {
 address user;

 constructor () public{
     user = msg.sender;
 }

 function returnUser() public view returns(address){
     return user;
 }

}
</code>
Think about it... range of use case of such codes?


downloaded software in Conda environment does not show in ls command

Question: I installed gzip package in Conda environment, then  to make sure the software was there, I used "ls" command, it was not in the list. Tried to install again, system says all the packages have already installed, Why? How do I check the list of software installed in the environment? Thanks!
Answer: Just type
conda list gzip

OR
conda list | grep gzip

OR if its installed in a given environment
conda activate myenv

or else just activate myenv system depending followed by either of the first two commands.
It is better to use conda-navigate a very nice and under used GUI, and would work in this particular instance. The details are found in an earlier post I made on esentially the same question here together with a pretty picture.
ls will only work if you type ls ~/Anaconda3/bin | grep gzip , but depends on the installation, e.g. you might have installed miniconda.
Having said all that conda is very cool, but whatever you do NEVER use pip or pip3 from this moment onwards EVER, it messes up your conda and you'll need to env your way out of it.


Help Identifying this Shower Faucet Cartridge

Question: I am "attempting" to fix our shower faucet and in feeling ambitious, I decided to take a wrench to the broken knob. To my pleasant surprise, the "cartridge" just popped out and I thought to myself "ok cool, just pop another one in and I'll be good". Unfortunately, there are no identifying marks on the cartridge or any of the hardware associated with the shower. I have spent hours looking online and I still have no idea what brand, let alone model it is. Any ideas would be greatly appreciated. I am looking for the blue/white cartridge in the attached photo.
Comment: Sometimes the handles/knobs give a clue as to the manufacturer. Could you please post pictures of the trim/knobs/handles from your actual valve?
Answer: I'm confused, you have two cartridges in your hand and all the cartridges are in place in the shower valve. Is it a different shower?
If there are no markings anywhere on the valves or handles, You've got two choices: go to your home store and look on the plumbing racks for an identical cartridge (bring the old one with) or go to a plumbing supply store and they should be able to match it. Those guys usually know their stuff. It will be hard to find one online without actually being able to line it up with the old one. Good luck and stay safe out there.
Comment: No that picture is the only picture i have of my system that I have been able to find online. unfortunately, the website that I found it on says he just found as a generic photo.
Answer: Di Vapor Victoria Plum series
https://www.divapor.com/spares/showers/diverter-cartridge/100mm-two-way-diverter-cartridge-for-victoria-plum-shower-valves.php
Answer: If anything you could get away with  a Samuel heath thermostatic cartridge (brown)


Architecture and Capacity planning for FAST search farm

Question: We are building our new fast farm and have plans to put up one column of Physical servers for FAST indexing and crawling. We will be crawling around 15 million items including SharePoint sites, exchange public folders, custom sql databases and file share documents. I know if it was just SharePoint sites, the formula is 20% of your total content DB size. Is there some similar formula with which I can calculate the physical memory required on my index and crawl boxes. As per MS (http://www.microsoft.com/download/en/details.aspx?displaylang=en&id=23576) they recommend to use 1 TB HDD size per 5 million items. But that doesn't seems to be the case. Any recommendations?
Answer: As data vary so much from installation to installation your best approach is to index perhaps 100,000 items which are representative of your corpus, and extrapolate the disk needed from that.
Also read over the scenario listed at http://technet.microsoft.com/en-us/library/ff599526.aspx which have numbers for raw data and index size, which might can carry over to your installation.
Comment: No caching on the WFE's. The crawler meta data is stored in the crawler db, but 1/5 or so of what SP search stores there. The rest is stored on the FAST servers.
Comment: Thnaks for the link and info. So that test results are somewhat in-line with what MS recommends, 1 TB HDD for every million items. However as far as I know there is a concept in FAST that the items index gets cached on WFE's for faster retrieval. Is there a formula or calculation by which we can calculate HDD requirements on WFE if its just gonna host FAST search site?


Salesforce: Is there any way to set System time in test class

Question: So I have a trigger and a schedule class. The trigger will fire upon update on an object and schedule a job which will execute the scheduled class. There is some logic in the trigger related to non-working hours, weekends and holidays. It will usually schedule the job 1 hour after insert (due to business requirement). but if insert happens over the weekend, afterhours, or holidays. It will schedule the next business day 8am.
My problem is my test class does not cover everything. I insert the object in test class, and depends on what time I run the test, it will cover different part. Like if I run during business hours, the weekend, holiday logic doesn't get covered, etc.
My question is there anyway in test class we can set the Datetime.Now to any date and time we want? thanks very much
Answer: No, there's no way to set system Datetime.Now, but you can wrap the Datetime.Now call on a known variable and replace that to test your methods.


How to create a business page in Facebook

Question: I have a personal page on Facebook and now want to start a business page. My own name is also the name of the business, as is my website. When I try to create a business page, Facebook says the name is already in use. How can I get round this?
Comment: If the name is already in use (whether this is used by your own account or someone else) then I imagine you are going to have to choose a different name (or variation of).
Answer: If I guessed right, You are talking about the page Address not the name.
If you want to create a page in same Address you created before, you will have to change previous page's Address to other, and same time change the business page's Address to the one you wish to add.


Why does every neuron in hidden layers of a multi-layer perceptron typically have the same activation function?

Question: Why does every neuron in a hidden layer of a multi-layer perceptron (MLP) typically have the same activation function as every other neuron in the same or other hidden layers (so I exclude the output layer, which typically has a different activation function) of the MLP? Is this a requirement, are there any advantages, or maybe is it just a rule of thumb?
Answer: As you stated, it's popular to have some form of a rectified linear unit (ReLU) activation in hidden layers and the output layer is often a softmax or sigmoid (depending also on the problem: multi-class or binary classification, respectively), which provides an output that can be viewed as a probability distribution.
You could generalize this further to blocks of different activation functions within the same layer. This is something I've thought about, haven't done, but imagine has been attempted. In some sense, the idea here would be to allow for a subsection of the network to develop a representation that may not be feasible otherwise. These different representations within the same layer would then be unified by subsequent layers as we move closer to the output.
Comment: regarding your last paragraph that's what I'm thinking about. Is there ever a benefit to have a layer where each Perceptron has a different activation function?
Comment: I updated the answer, maybe it addresses the question a bit better. I think the larger point is that anything is possible; neural networks are highly flexible models afterall. But the motivation for doing so is crucial – what would you hope to gain by having a block of neurons with alternate activations in the same layer? Why is this gain not possible without it? I suspect that answers that motivate such an architecture are hard to find. Further, as far as I'm aware, these aren't features that are readily available in existing software libraries.
Comment: @Greenstick I suspect with some problems where characteristics of the objective function are not obvious that experimenting with a mixture of activation functions might afford a better approximation to the function trying to be learned.
Comment: @user8714896 This is in effect done. Certain activations (e.g. PReLU) take in parameters that have their own associated weights (multiple params could exist for a layer); the neural net then learns how to parameterize these activations to minimize the objective function. You lose some interpretability and (maybe) motivation doing this (e.g. the biological motivation discussed above), but the nice part is that you don't have to precisely *know* the activation ahead of time – the neural net figures it out. Hope that helps; if this is in line with the answer you're looking for, please accept it.
Comment: This answer would improve considerably if you could do a little bit of research to find some research papers that show that, indeed, different activation functions can be used for different hidden neurons.


Affix Wood Shelving Unit to Studs

Question: I have a freestanding wooden shelving unit I want to secure to the studs on the wall as they're leaning forward. However, I'd only be able to secure 1 side since the studs measure ~23" apart whereas the shelving unit is 26.5" apart. What can I do to properly secure both sides onto the studs?
Answer: Add another board across the back and screw through that into the two studs.
If you would want to locate the shelf with a stud in the middle, then use a metal angle or a wooden piece under or on top of the top shelf and fasten that to the stud. One attachment in the middle of the shelves would hold unless you are in a seismic zone and there are specific requirements.
Be sure that when you tighten the screw it does not pull the shelving unit off one or two of the feet. You might need to put shims behind the wood you are using to secure the shelf.
Comment: Agreed you don't want to attach it to the wall at one of the sides as it may be prone to swiveling.

Affix Wood Shelving Unit to Studs


What can I do about lead paint on fieldstone foundation walls?

Question: I have an old field stone basement (1870's) that is crumbling but not just the mortar, the stone itself. We have tuck pointed the mortar areas with some success. Our problem is: the previous owner worked in a shipyard in the 50's and painted some of the walls with blue lead paint. The paint has disintergrated the actual stone into a sandy mess. Any solutions for this problem would be welcome.
Comment: I've never heard of paint disintegrating rock. What kind of rock? If it's something like limestone, it's perhaps more of a water issue than a paint issue. Either way, if it's all crumbling, might be best to bring in a structural engineer/foundation expert.
Comment: Be careful with creating dust of lead paint, especially if you have any children, pets or pregnant women in the household.  Lead dust can be extremely harmful to them.
Answer: The mortar is crumbling because of moisture seeping through the walls from the outside, not from the paint. That is a common problem with fieldstone foundations. See questions about how to remove crumbled mortar and what kind of mortar to use.
I would be very surprised if the fieldstone itself is actually crumbling. If so, you need a structural engineer that specialized in old foundations. Can you please post a picture? (If the system won't let you, upload it to imgur.com or somewhere else, and someone here can edit the picture into your question.)
Working with the crumbled mortar and peeling paint will probably require protective equipment because of the lead paint. See this related question. Bring in a specialist if you're unsure about how to protect yourself.


Do I need to turn off answering machine before Shabbos?

Question: Do I need to turn off answering machine before Shabbos? I'm talking about where the message comes to your machine, not to your provider like by mobiles. Would this be the same by having a phone turned on and someone calling, or receiving an SMS?
Answer: Rabbi Frand mentioned this case in one of his weekly shiurim. He said that someone in the United States can call Eretz Yisrael on Erev Shabbos and leave a message on the machine (or send a FAX) as long as the person in Eretz Yisrael does not read the FAX or listen to the message. Many people turn the answering machine off in order not to accidentally hear the message. However, this is not required. It is better to just set it up not to play the message as it is being received (or turn down the volume) before Shabbos.
After shabbos, one listens to the message in the same way that one opens the snail mail or logs on to the computer to get the email that came in over shabbos.
Comment: What's Rabbi Frand's source?
Comment: Unfortunately it was some years ago so I do not remember his source. I will have to try to check his books or google his torah.org divrei torah
Comment: I was just wondering, because I know a lot of people leave their answering machines on and listen to the message in case of an emergency.


Charge On parallel plate capacitor?

Question: Suppose if we supply two plates of parallel plate capacitor with charges $Q_1$ and $Q_2$.What would be the charge of the capacitor?
As the field inside plate should be zero and hence using gauss law I can understand that charges on inner surface should be equal in magnitude and of opposite nature.Further charges on outer surface would be $\frac {Q_1+Q_2}2$ but what do we call the charge of capacitor in these cases?
Comment: I'm not totally clear on what you're asking.  Usually we talk about a capacitor with charge $+Q$ on one plate and charge $-Q$ on the other plate.  Are you asking about the case where the two charges are not exactly equal and opposite, and the capacitor has a net charge?
Comment: @rob I want to ask what if the charges on the two plates of parallel plates capacitor is not equal.what will be the charge of the capacitor in this case?
Answer: First, to be clear, when we speak of a charged capacitor, we almost never mean electrically charged but, rather, 'charged' with energy in the same way we speak of a charged battery.
If the plates of a capacitor with capacitance $C$ have equal and opposite electric charge $Q$, the capacitor is electrically neutral but stores an energy
$$U = \frac{Q^2}{2C} = \frac{1}{2}QV$$
where $V$ is the potential difference between the plates.
If the plates of a capacitor have unequal charge, there is now energy stored in more than one capacitance.  There is the capacitance that exists between the two plates (the mutual capacitance) as well as the capacitance of each plate to the environment.
Let $V_1$ be the potential of plate 1 with charge $Q_1 = Q$.
Let $V_2$ be the potential of plate 2 with charge $Q_2 = -Q + \delta Q$.
The energy of system is then
$$U =  \frac{1}{2}Q_1V_1 + \frac{1}{2}Q_2V_2 = \frac{1}{2}Q(V_1 - V_2) + \frac{1}{2}\delta QV_2 = \frac{Q^2}{2C} + \frac{\delta Q^2}{2C_{20}}$$
where $C_{20}$ is the capacitance of plate 2 to the environment.
Answer: For an example of a real system where this occurs, consider the capacitor in a high-pass filter.  Here's  a diagram from Wikipedia:

If $V_\text{in}$ is a constant, the capacitor will charge up, but $V_\text{out}$ will discharge through the resistor.  In that case one side of the capacitor is at ground.
However if $V_\text{in}$ oscillates, it takes some time for the capacitor to charge up, and so an oscillating signal appears at $V_\text{out}$.  There is a phase shift between $V_\text{in}$ and $V_\text{out}$, so for much of the oscillation neither end of the capacitor is at the ground potential.  This is exactly the situation that you're talking about, except perhaps that you weren't imagining a transient net charge.
Alfred Centauri says, correctly, that there will be an "extra" capacitance between the net charge on $C$ and the ground plane of the circuit.  However for all but the most sensitive of measurements this "stray" capacitance can be neglected.  Typically stray capacitances have values of a few picofarads, while intentional capacitors may carry nanofarads or microfarads.
In analyzing a real circuit, then, we always describe the "charge on the capacitor" $Q$ as if the capacitor had zero total charge, so that the voltage drop across the capacitor is $V = Q/C$.
Answer:
From your conclusion, which is correct, $$\large q=\frac {Q_1+Q_2}2$$ and thus as charge is conserved the charge on inner surface of plate $\mathrm{I}$ is $$Q_1-q=\frac {Q_1-Q_2}2$$ and similiarly on inside of other plate $\mathrm{II}$ is $$Q_2-q=\frac {Q_2-Q_1}2$$
Note that these two are just equal but negative in charge, which poses a similiarity to a "normal capacitor", when the outside charge is zero and the inside charges are equal and opposite in sign; The charge of the capacitor can be said as $$\left|\frac {Q_2-Q_1}2\right|$$

Charge On parallel plate capacitor?


Can a Jew sell a sefer Torah to a non-Jew? Why or why not?

Question: A question was raised whether a non-Jew can run a Judaica store.  My thought on that was it depended on what types of judaica he sold.  I doubted that a sofer would sell him mezuzah parchments, tefillin or sefer Torahs.  Also, I saw a video of a church using a sefer Torah in its services (opening it up from end to end and wrapping it around congregants; oy!) Is there a halachic prohibition to selling a sefer Torah to a non-Jew?
Answer: If you look at the Halachos of Mezuza:
see Rambam (sefer ahava - Hilchos Tefillin, Mezuza, v'Sefer Torah - Chapter 5:11)
you will find that it is forbidden, when moving out of an apartment, to leave a Mezuza there if a gentile will be moving in. The reason given is that he may come to desecrate it.
Also, see Ramah (Yoreh De'ah 291:2)
Who states that it is forbidden to give a mezuzah to a gentile to affix on his door.
We may be able to infer from here the Halachos of selling a Torah to Gentiles. If we are worried they will desecrate it, it is probably not allowed. (Wrapping it around the congregants is surely a desecration!)
Having said that, I have heard from an Orthodox Rabbi, that selling a Mezuzah to a gentile can be permissible today if we are not worried they will come to desecrate it. (there are plenty of Gentiles out there who are aware of the spiritual benefits of Mezuza and would be happy to affix one on their door)
Comment: related discussion about mezuzot and non-Jews http://judaism.stackexchange.com/q/17808/759
Answer: Just adding to the comment of @shnozolla that the daf of the SA continues
(כך השיב מהרי"ל) ונראה לי דמ"מ במקום דאיכא למיחש משום איבה ושירע משום זה לישראל שרי (כנ"ל):
Maharil: But it seems to me that this applies in a case where there is reason to be concerned that in that particular place there will be hostility [towards Jews if one does not give the mezuzah to the idol worshiper], and that the idol worshiper will treat Jews poorly because of this, it is permissible [to give a mezuzah to an idol worshipper].


How to define that how many users are login at given time in jmeter

Question: if I have 1000 user which will login in 10 sec but load testing fails means 1000 users are not login in 10 sec then how we define how many numbers of user are login(count) means is there in Jmeter is any report or graph which shows the proper report like 600 users are login 400 are unable to login or anything like this.
Answer: Well, it is something you can figure out from Aggregate Report, you have Error % there

However I would recommend increasing your load gradually, this way you will be able to tell for sure how many concurrent users your application can support without errors. Increase ramp-up period or use one of custom Thread Groups available via JMeter Plugins project.
In regards to monitoring, you can either generate an HTML Reporting Dashboard which provides i.e. Active Threads Over Time and Response codes per second graphs or use the JMeter Plugins equivalents:

Active Threads Over Time
Response Codes per Second

Aforementioned (and other) plugins can be installed using JMeter Plugins Manager


How can positive and negative mindsets influence interpersonal relationships with others?

Question: I want to know how positive and negative mindset can influence interpersonal relationships with others.  For example: if you have a negative mindset, would people not want to relate with you?
Answer: The lexical approach may assist. The lexical hypothesis states that, 'All aspects of human personality, which are or have been of importance, interest or utility, have already become recorded in the substance of language'(Cattell, 1943) and, 'When an idea is important, people are likely to have a word for it … the more important something is, … the more words there are likely to be'(Miller, 1996). The atlas of personality, emotion and behaviour uses a lexical approach to visualise words using the axes of affiliation and dominance (Mobbs, 2020).
Relevant synonyms are:

positive: assured, beneficial, clear, complete, confident, constructive, convincing, decisive, effective, persuasive, practical, pragmatic, proper, real, regular, sanguine, satisfied, sure, unreserved, useful, wholehearted, worthwhile.

negative: adversarial, adverse, damaging, derogatory, destructive, dismissive, disparaging, hostile, inhospitable, prejudicial, untoward.

At face value, people with the traits associated with positive are more likely to attract others.  The traits associated with negative are more likely to dissuade others from associating with the individual.
The visualisation of positive and negative according to the atlas approach is shown below.

References
Cattell RB. The description of personality: basic traits resolved into clusters. J Abnorm Soc Psychol. 1943; 38: 476–506. https://doi.org/10.1037/h0054116
Miller GA. The science of words. New York: Scientific American Library; 1996.
Mobbs AED (2020) An atlas of personality, emotion and behaviour. PLoS ONE 15(1): e0227877. https://doi.org/10.1371/journal.pone.0227877
Declared interest: I am the author of the atlas paper.


Why does idling RPM increase with altitude in a jet engine?

Question: The Snecma-Larzac 04-C6 idling RPM ranges from approximately 53% on the ground to 85% at high altitude. Why is that?
Answer: For one there is less resistance to the fan and compressor due to the decreased air density -> more rpm out of the given power. On the other hand, the mass flow needs to be high enough to sustain the operation of the engine -> both lead to a higher rpm...
Comment: There is also the ram air effect in flight at altitude.
Answer: Ground idle and in-fight idle are different.  In-flight idle would move aircraft on the ground and would require a bit of brakes to keep the aircraft stationary. Additionally, landing roll would be longer. Therefore, ground idle activates on most jet aircrafts after touchdown. This activation is usually occurs after a small delay, taking into account that pilot may be doing a touch and go.
Of course there is also density effect on both ground and in-flight idle rpms. Both increase with altitude to provide some engine stall margin.
Comment: Would "ground idle" be different for an airfield at MSL vs one at MSL + 5000'?


Bluetooth Speaker Controls Too Loud and in Too Few Steps

Question: I bought a set of Dali Zensor 1 AX speakers which has a Bluetooth connection. Playing over the 3.5mm cable works perfectly and the volume can be nicely adjusted from the phone. The phone has 31 volume steps, with 0 being muted and 30 being 100% volume.
However, when I connect the same phone via Bluetooth, there are only 4 steps of volume control:

0 on phone: muted
1 on phone: 33% speaker volume
2 on phone: 66% speaker volume
3 on phone: 100% speaker volume

Steps 4 to 30 are all there but because the volume is already at 100% on step 3, they do nothing.
Having only 3 controls is extremely unpleasant as it's unlikely the volume I want to listen to is one of those 3 and to make matters worse, most often when I connect to the speakers it's higher than step 3 and just blasts whatever I play at 100% for a few seconds while I try to turn it down.
I've tried connecting an iPhone to it via Bluetooth and it works perfectly fine. I've also connected multiple Android phones (OnePlus 5T and OnePlus 6T) and they both show the problem.
How can I get my Android phone to spread the Bluetooth audio volume control across all the steps rather than just the lowest 4?
Answer: After a couple of hours of searching I found the fix:

Enable Developer options if you haven't yet (instructions can be found online).
Open Developer options.
Enable the Disable absolute volume option.
Enjoy listening to music at whatever volume you like.

Here are the details for this option:


To what extent were original papyri used as sources in the translation of the NIV New Testament?

Question: I am wondering to what extent original papyri were used in the translation of the NIV New Testament? I know there are some large papyrus fragments such as p45 and p66 and I am wondering how much the translators of the NIV referenced them directly.
Comment: What do you mean by "original"? It's generally accepted that we don't have the original manuscripts. Clarification might be helpful.
Comment: It would be unusual for a translator to look directly at an image of an old manuscript to make a decision, and unheard of to make a trip to a library to handle the physical manuscript. Instead, they will read a Greek text with footnotes saying which manuscripts support a particular reading. The SBLGNT refers to which readings agree with the wording as understood by the NIV translators.
Answer: The editors of the 2011 NIV write:

The Greek text used in translating the New Testament is an eclectic
  one, based on the latest editions of the Nestle-Aland/United Bible
  Societies' Greek New Testament. The committee has made its choices
  among the variant readings in accordance with widely accepted
  principles of New Testament textual criticism. Footnotes call
  attention to places where uncertainty remains.

I believe that at this time the 27th edition of the Nestle-Aland Greek New Testament would have been the latest.  Metzger's Textual Commentary on the Greek New Testament lists the following papyri as the most important witness texts for this edition:

P20 - 3rd c.
P23 - 3rd c.
P29 - 3rd c.
P38 - c. 220
P45 - c. 250
P46 - c. 175-225
P48 - 3rd c.
P50 - c. 163-172
P66 - c. 200
P69 - 3rd c.
P75 - c. 175-225

Since the editors state that they based their translation on the Nestle-Aland text, they would have at least considered the above in their translation.  A full list of the available papyri can be found here.
You specifically asked about papyri, so I took your question literally.  All of the above texts are fragmentary and sometimes don't even include complete chapters.  The only complete Greek New Testament texts from antiquity (as far as I know) are the codices, listed here, the earliest of which date from the 4th century.
(There is, of course, no guarantee that the age of a text is any measure of its "authenticity" in the sense that it is faithful to some "autograph".  There is no way to know whether there are some still earlier texts that are lost.  The Nestle-Aland text makes educated guesses about what might be in the supposed "autographs" by considering all of the available manuscripts.  The actual text used by the Greek Orthodox Church today follows fairly closely what is found in the majority of the available manuscripts, which varies widely in places from the Nestle-Aland "Critical Text".  Some of the Orthodox readings are quite different from what is found in the NIV).


New tag being created for a single java class

Question: Going through the suggested edit review queue there were a couple question that were edited by a user with a tag edit to add simplefilevisitor.  I know there are tags out there like map and vector but those are general language agnostic tags where this is just about Java.  Should we be creating a new tag just for a single Java class?
Comment: At least for [the question](http://stackoverflow.com/q/22907653) which I saw, it's worthwhile to note that the tag is even irrelevant to the question because it's not about `SimpleFileVisitor`, it's about calculating a progress bar value with an indeterminate total. *"Tags should help to describe what the question is about, not just what it contains."*
Comment: Was this done by the same user in this question? http://meta.stackoverflow.com/questions/295565/do-we-need-a-coreldraw-vba-tag What is going on?
Comment: @BSMP No.  It was done by the creator of the first question in the tag.
Answer: I would say "No, nuke the tag."
Along with the [coreldraw-vba] tag BMSP linked back to. Otherwise, what other simple classes can get their own tags?
Maybe we can create a tag for System or NotificationManager. Before we know it we will have tagged the whole API.
Comment: Yes, tags for individual classes, methods or properties are 99% of the time not useful for categorizing (there are a few exceptions).  What tends to work better for tags are whole APIs or concepts.
Comment: @Manu he's determined he added them back!
Comment: I went ahead and rolled back all the edits.
Answer:
Should we be creating a new tag just for a single Java class?

There's already many out there, especially for the Swing classes (JFrame or JTable for example).
So, if the use of a particular class tends to invoke a lot of questions, then why not?
Comment: "Does the downvoter care to add to the discussion" generally down-vote on meta should be treated as "I don't agree with/like this opinion" and there is no obligation to explain it, just like with up-vote (you don't need to explain why you agree with some opinion).
Comment: That's a rhetorical question that only forces me to say "wusn't me". I'd say if you found such tags, they should be reviewed for burning rather than opening the floodgates. They're Java Swing questions.
Comment: +1, "*if the use of this particular class tends to invoke a lot of questions*" is just the correct policy on tag creation. And while I had been quick with a "no" because I've never heard of this class, it does in fact seem to have provoked [a few dozen questions](http://stackoverflow.com/search?q=simplefilevisitor+is%3Aquestion).
Comment: Does the downvoter care to add to the discussion?
Comment: @Pshemo - fair enough.


Annotation functionality using Okular on Ubuntu 10.04

Question: I am having a problem with Okular when dealing with annotation functionality. I am able to do annotation (write note, etc..) on PDF document but then once it is saved and reopened using Evince or Okular itself, it will not appear as the annotated version of it (The note written does not appear). Is it a bug in the Okular application or have I overlooked something?
Answer: See this question which inadvertently answers your question.
Okular doesn't store the annotations on the document itself, rather it stores them in an XML file, in a location specified below. Unless the format in the XML is recognised by another reader - Evince - then you are out of luck.
In your case, if Okular itself cannot find the annotations, make sure you have the directory (~/.kde/share/apps/okular/docdata).
Note that Okular isn't itself a PDF reader, rather it uses 'backends' - see this link
Comment: I got the this directory "~/.kde/share/apps/okular/docdata", but how do I open it? If I use gedit for xml is not really convenient?


What Makes Stars Hot?

Question: Simple question, but can't seem to find the answer anywhere. It can't be nuclear fusion because nuclear fusion occurs as a result of the heat. And it can't be because of gravity because it's believed that the core of black holes are nearly absolute zero. So what makes stars hot?
Comment: Who believes that the core of black holes is nearly absolute zero?
Comment: Turns out to be a conceptually very interesting question with a non-obvious answer.
Answer: Stars do not get hot because of nuclear fusion, they become hot enough to sustain nuclear fusion and this process maintains their temperatures. Nuclear fusion actually stops a star getting hotter.
Protostars (before nuclear fusion) get hot because of a well known statistical relationship between the gravitational potential energy of a gas and the internal kinetic energy of the particles that make up the gas. [In an ideal gas, the kinetic energy of the particles is directly proportional to the temperature of the gas.] This is known as the virial theorem, which says that twice the summed kinetic energy of particles ($K$) plus the gravitational potential energy ($\Omega$, which is a negative quantity for a bound object) equals zero.
$$ 2K + \Omega = 0$$
Now you can write down the total energy of the system as
$$ E_{tot} = K + \Omega$$
and hence from the virial theorem that
$$E_{tot} = \frac{\Omega}{2},$$
which is also negative.
If we now remove energy from the system, for instance by allowing the gas to radiate away energy, such that $\Delta E_{tot}$ is negative, then we see that
$$\Delta E_{tot} = \frac{1}{2} \Delta \Omega$$
So $\Omega$ becomes more negative - which is another way of saying that the protostar attains a more collapsed configuration.
Oddly, at the same time, we can use the virial theorem to see that
$$ \Delta K = -\frac{1}{2} \Delta \Omega = -\Delta E_{tot}$$
is positive. i.e. the kinetic energies of particles in the gas (and hence their temperatures) actually become hotter. In other words, the gas has a negative heat capacity. But a hotter temperature usually means more radiation is produced and if the energy losses continue, then so does the collapse.
This process is ultimately arrested in a star by the onset of nuclear fusion. This replaces the radiative losses with nuclear energy and the star attains a quasi-equilibrium that lasts as long as it has nuclear fuel to burn.
Comment: @Tanenthor "Astronomy Stack Exchange is a question and answer site for astronomers and astrophysicists. It's built and run by you as part of the Stack Exchange network of Q&A sites. With your help, we're working together to build a library of detailed answers to every question about astronomy." It is *lack* of detail that is notable about many answers on Astronomy SE.
Comment: It's a comprehensive answer, but possibly too complicated
Answer: Before the nuclear fusion in the core starts, the heat of the star comes from the contraction of the original nebula. When the matter comes closer together, the potential energy of it decreases, just like when you drop a rock. Energy is however constant, so it has to go somewhere. That "somewhere" is the heat in the newborn star.
Comment: So you're implying that the birth of a star is somewhat violent and not gradual or am I just interpreting it wrong?
Comment: @ReadySetPawn Nope, I have not said anything about how long the contraction phase lasts.
Comment: @ReadySetPawn  while it's a different question, yes the birth of stars can be very violent and temporarily a lot brighter than when the star settles into it's main sequence.   Jupiter, for example is 1/75th the mass of the lightest red-dwarf stars, but the heat from formation is sufficient that Jupiter still emits 4 times the energy that it receives from the sun.   The energy and heat created when enough matter to form a star coalesces under gravity is impressive.

What Makes Stars Hot?