I'm having trouble achieving sharp telescope focus

Question: I've been having trouble getting really sharp focus on my telescope. Using Saturn as a reference, with a 20mm eye piece it is very sharp. But when I go to 10mm or 6mm, it gets bigger and blurrier. I re-adjust focus each time I change eye pieces, but the best I can get at the stronger magnifications is still very blurry compared to 20mm. The 20mm will have nice sharp edges on the planet and rings, whereas 10mm and 6mm have fuzzy edges on the rings, and the planet itself.
I'm using the Celestron NexStar Evolution 8 Schmidt-Cassegrain Telescope. I'm in a suburban area so I'm sure there is light pollution, could that be a factor?
Last night I was observing for about an hour, which I believe would be enough time for the temperature of the telescope to match the temperature outside.
I also tried combining the 20mm with a Celestron 2x Barlow lens, just with an idea that maybe it would make a difference, but it was pretty much the same.
Is there something I'm doing wrong?
Comment: When did you last clean the eyeball end of your eyepieces? Check them, both ends.
Comment: In general, only one end of the eyepiece gets smeared quickly - the one facing the eye. If you use the plastic caps and you're careful when handling eyepieces, the other end should stay clean a very long time. But yes, a dirty lens on the eyepiece could soften the image quite a bit. http://televue.com/engine/TV3b_page.asp?return=Advice&id=103
Answer: Your telescope has a 2000 mm focal length, and 200 mm aperture. With the 20 mm eyepiece, you get 100x magnification. With the 10 mm lens, you get 200x. Several things could cause what you describe:
Seeing (turbulence) might be bad. It is common in many places that less than great seeing means everything at 200x and over might start looking blurry. But seeing changes with the seasons, or day to day, hour to hour, or indeed from one second to another. It's random. If you get the exact same results all the time, and the image never improves even after many weeks of trying, then perhaps it's not seeing that's the culprit.
Your telescope might not be collimated. Look in the user's manual. Is there a collimation procedure you need to follow? (EDIT: yes, there is - page 20) A miscollimated scope indeed behaves as you described - seems "sharp" at low-ish magnification, but quickly becomes blurry if you push magnification up.
Point the scope at a bright star and defocus. Are the rings perfectly circular? If not, it might be miscollimated.
http://www.astrophoto.fr/collim.html
Another factor is optics quality. Optics with lots of aberrations cause "mushy" images and a difficulty to find the true focus. High quality optics produce a scope that is very "snappy" when achieving perfect focus, and make very sharp images when seeing is good. There's nothing you can do about this, but the good news is - this problem is not very common nowadays; optics quality, while not always great, tends to be at least passable in many cases. Quality could be evaluated either on the test bench (which requires equipment), or via direct observation of stars (which requires a very experienced observer and takes time).
Finally, have you compared views with another instrument of similar aperture? The image at higher magnification will always seem a bit more soft, compared to lower magnification - even in perfect seeing with a collimated instrument. Also, the contrast always decreases as you increase magnification. This is why there is always an optimal magnification for each view - sometimes higher, sometimes lower, depending on many factors. The best scope in the world will look mushy and blurry if you push magnification up too much.
Can you easily see the Cassini division in Saturn's rings? If yes, then you're probably in okay shape (collimation and seeing), you're just not used to the softness at higher magnification. I have a very high quality reflector (self-made), similar aperture class to yours (a bit smaller), and Cassini is immediately visible at 180x and looks sharp; it's also visible at 255x but the whole thing starts to look mushy and faded. It looks super-sharp and high-contrast at 136x but it's kind of small. This is in perfect collimation, perfect thermal equilibrium, high quality optics throughout the stack (primary and secondary mirrors, eyepieces), and good seeing typical of N. California.
When seeing is bad, Cassini starts to fade no matter what I do.

Does Saturn look equally soft in the center of the image, as well as near the edge? Good optics (mirrors, eyepieces) make a good image everywhere, cheap optics make an okay image in the center and a blurry image at the edge.
My bet is that it's either bad collimation, or you haven't compared the view at 200x with another scope, or both. But it's hard to diagnose things over the Internet.
You should probably get a 15mm eyepiece as well. You'll use it a lot, probably more than the 10mm.

Light pollution is never a factor for: planets, the Moon, the Sun, most double stars. These things are just too bright to care about it, so observing them from the city is fine. What does matter is seeing (turbulence). Also, it's important that the scope is in perfect collimation.
Light pollution only matters for the "faint fuzzies": nebulae and galaxies. These are low-brightness, low-contrast objects, that are difficult to see from the city. Being low contrast, your eye will not perceive a lot of detail anyway, so for these objects seeing doesn't matter.
Congratulations for being aware of, and following, the rule that says you need to acclimate the scope to ambient temperature. A lot of people are not aware of it. At least you're removing one unknown from the equation. It's always a good idea to let the scope "breathe" outside for an hour before you even begin to observe.

One way to be 100% sure that your scope is focused perfectly is to use a Bahtinov mask. Just make (or buy) one to match the scope's diameter, put it over the scope's mouth, point scope at a bright star, and tweak the focuser until all spikes intersect exactly in the center of the star. Then remove the mask - the scope is in perfect focus. This will probably not help in your case, but it's just one way to remove another uncertainty. It also helps with regular observations - I use it almost every time I observe, it makes focusing so much easier.
The mask could be made from a piece of cardboard and a sharp knife in like 20 minutes. It doesn't need to be a marvel of engineering precision to work well.
http://www.deepskywatch.com/Articles/make-bahtinov-mask.html

One thing that is certainly NOT a factor is the central obstruction. You'll often hear on the Internet how Cassegrain instruments, or other scopes with a large central obstruction (secondary mirror) are supposedly less "sharp" than scopes without a central obstruction, or ones with a very small obstruction.
Yes, the obstruction does matter a little, but not to the extent that Internet mythology would have you believe. A Cassegrain instrument, with good optics, in perfect collimation, can be a superb telescope even at a 40% obstruction. It simply performs like a somewhat smaller instrument, that's all (the physics of the central obstruction are very complex, and would require an entirely different discussion on this forum to fully elucidate).

(Added in reply to a comment below)
Miscollimation is quite likely the #1 performance killer of all amateur telescopes. Some well-made refractors are more or less immune to it, but most reflectors need periodic collimation.
It's a good idea to google around for various collimation techniques, there's so many of them. Some require the observation of stars, some require special devices known as collimators. The technique is also different for different types of telescopes (newtonian, cass, etc).
If you own a reflector, it's a good idea to collimate it once in a while, just in case, like changing the oil on a car. Dobsonian telescopes probably require most frequent collimation.
Check Howie Glatter's site - he makes some of the most precise collimators currently available:
http://collimator.com/
Regardless, just learn any method you can and apply it. If the telescope is massively miscollimated, you'll get an improvement immediately.
Not being able to see Cassini is not a good sign. Maybe seeing is persistently bad (it can happen), or maybe the scope is off-collimation. Can anyone else in your area see Cassini these days in a similar aperture, in a scope known to be collimated well?
Surface detail on Saturn is not easy to see, but should be doable with apertures over 100mm. It's like zones of latitude similar to Jupiter's equatorial belts, but very, very pale and faded. You have to keep looking for a while to see it. Don't worry if you don't see that stuff for now.
Jupiter's Great Red Spot is harder to see nowadays. It's been shrinking and fading for decades now. Go on Wolfram Alpha and type in Great Red Spot. It will tell you if it's visible at the moment. Usually it looks more like a dent in the equatorial belts, rather than an actual visible spot.
http://www.wolframalpha.com/input/?i=great+red+spot
Jupiter works best at not so high magnification, otherwise the contrast gets too low. Around 140x ... 180x should give you decent contrast in your aperture.
Comment: `Light pollution is never a factor for ... the Sun ...` Made me smile.
Comment: Thank you for this detailed answer! I didn't even know collimation existed. I think that's the problem - I ordered this telescope online and it was shipped UPS, maybe in shipping the alignment got screwed up. I can never see the Cassini division, and Saturn looks equally soft in the center and on the edges, there's never any surface detail at all. In Jupiter I can never see the spot. The sites you linked all seemed to describe my problem exactly. I have to wait for a clear night to confirm and fix the problem. I'll update you here as soon as that happens - the next week or so doesn't look good
Comment: @MikeWillis added more edits to the post above
Comment: @asawyer It's technically true! :)
Answer: Did you try to slowly "fine tune" your focus when using the 10/6 mm eyepieces? The higher the magnification, the more sensitive focus is to adjustment.


If someone is "playing horse with you" why are they either teasing, ridiculing, or perhaps flirting with you?

Question: Why does a horse and the activity of "playing horse" describe one who is teasing, ridiculing, or even flirting? The survey of College Words and Phrases by Eugene H. Babbitt published in 1900 lists the expression to play horse with and the many meanings of it including to tease, to ridicule, and to flirt. I'm curious to know if anyone has any ideas as to how "playing horse with" became associated with these meanings?
Comment: Is this use still common? I've never heard it used this way in American English. I know of "horseplay", but it doesn't have anything to do with teasing or flirting.
Comment: No, it's not but I enjoy writing and to make my books as realistic as possible I'll research old slang from the decade of which I'm writing and sometimes it's easy for me to understand why a word or phrase has a certain meaning but other times I haven't the least notion! The folks on this site are very helpful with helping me determine etymologies and origins. I shouldn't be surprised if I use this phrase on account of it does sound neat Ha-ha-ha
Answer: Not sure, but may be it originated from the way young horse's frolic and play-fight.Its just silliness and fun with no constructive purpose. So, perhaps it was thought that anything a "fool" was engaged in was similar to "horse play" and was frivolous, And hence the association.


Can't wrap my head around wpf databinding

Question: My scenario:
A wpf form has a textbox and a wpf toolkig datagrid.
When a text is entered in the textbox, my service returns an IEnumerable<TranslationItem> items. I want my datagrid to show the result of this service.
I tried googling around, but I can't get heads or tails to it. I'm only starting to learn WPF and most of the used terms elude me.
What I gather is that I should put my service's result in an ObservableCollection, no sweat.
But then I want to somehow bind it to my datagrid. How can I do that? How will the grid know what columns to generate?
Answer:
What I gather is that I should put my service's result in an ObservableCollection, no sweat. But then I want to somehow bind it to my datagrid. How can I do that?

The easiest way is to set the ItemsSource property of the DataGrid to the ObservableCollection.

How will the grid know what columns to generate?

The DataGrid reflects the objects in that collection and create a column for each public property it finds. See here for more info.
If you set the ItemsSource property directly then it's not really wpf binding. Here's three links I found useful when I started databinding in WPF.
Bea Stollnitz: What does "{Binding}" mean?
WPF Basic Data Binding FAQ
MSDN: Data Binding How-to Topics
Comment: Looks like there's some good reading in here. Don't have time to check them now, but will return after I have.
Answer: While an ObservableCollection can be used for this, depending on how it is used you won't recive any benifit from it.  The key feature of the ObservableCollection is that it implements INotifyCollectionChanged.  What this interface does is provide a notifcation mechanism to tell the UI that a property has changed.  Since ObservableCollection already implements this, if you bind your DataGrid, ListBox, ItemsControl, etc.'s ItemSource property to a collection of this type it will automaticly update the UI any time an item is Added/Removed/Replaced/Moved/Reset.  Because of this, every time you want to update the collection with a new IEnumerable result set, you will have to first clear the collection, and then add the new results.
However, there is another option that I would recommend over an ObservableCollection in this case. It is to use something called an ObjectDataProvider.  Using this we can avoid the code behind entirely, and it is overall much cleaner.  So we have our service somewhere, in this case in my Window.xaml.cs
public class TranslationService
{
    public IEnumerable<string> Translate(string s)
    {
        return s.ToCharArray().Select(c => c.ToString());
    }
}

Like the service you describe, it takes it a string from a textbox, and returns an IEnumerable.  Now, in the XAML we can use this service and make calls to it.
In the window declerations, we add the namespace for where the service is located:
 xmlns:local="clr-namespace:WpfApplication4"

Now, in our Window.Resources (Or UserControl, or anywhere else) we can reference our service.  Once we have exposed our service as a resource, we can create an ObjectDataProvider that exposes the Translate method we wish to use.
<Window.Resources>
    <local:TranslationService x:Key="MyTranslationService" />
    <ObjectDataProvider x:Key="MyProvider"
                        ObjectInstance="{StaticResource MyTranslationService}"
                        MethodName="Translate">
        <ObjectDataProvider.MethodParameters>
            ""
        </ObjectDataProvider.MethodParameters>
    </ObjectDataProvider>
</Window.Resources>

The ObjectDataProvider is keyed to our Service and calls the Translate method with a String parameter.  Now all we have to do is get it to respond to our text box.
We can do this by making use of some of the Binding properties.  We want our TextProperty in the TextBox to bind to the ObjectDataProvider, so we set the Source property to point to it.  The part of the ObjectDataProvider that we want to bind to, in the Path, is the MethodParameter. Now, we set it to Bind directly to the source of that property, and to only travel one way, meaning that the ObjectDataProvider's method parameter won't update the TextBox's text.  Finally we can set the UpdateSourceTrigger to PropertyChanged, telling the binding to set the source we are binding to, in the object data provider, whenever there is any change to the text.
<StackPanel>
        <TextBox TextChanged="OnTextChanged"
            Text="{Binding Source={StaticResource MyProvider}, Path=MethodParameters[0], BindsDirectlyToSource=True, Mode=OneWayToSource, UpdateSourceTrigger=PropertyChanged}" />
        <ListBox ItemsSource="{Binding Source={StaticResource MyProvider}}" />
    </StackPanel>

All that's left is to set the ItemsSource in the Grid, or a simple ListBox in this case.
Regarding the final part on the DataGrid:
If you are using the WPFToolkit's data grid, it does have an auto generate feature that can be set through the properties, and you can find more info on it here.
Comment: Thanks, I think I can apply this example with little changes. I'm not going to call my service directly as I wouldn't want litterly every keystroke to trigger a call to my service (and thus my translation db), but rather have a little timer buffer in between. But other than that it looks promissing.
Answer: You set the DataSource (or even the DataContext) of the grid to your Observable Collection.
I'm not familiar with that data grid, but most grids have options to either expose all public properties of the type in the Observable Collection as columns, or you explicitly set a column layout in XAML and one of the properties of the column definition is the property of the the object to use for the column data.
e.g. with Infragistics Data Grid
                <igDP:Field Name="OrderSize" Label="Order Size">
                    <igDP:Field.Settings >
                        <igDP:FieldSettings CellWidth="75">
                            <igDP:FieldSettings.EditorStyle>

                                </Style>
                            </igDP:FieldSettings.EditorStyle>
                        </igDP:FieldSettings>
                    </igDP:Field.Settings>
                </igDP:Field>

Name is where you set the property on the object to use.
Answer: your grid can either build the columns directly, or you can specify the column types that you wish. If you watch this video, it'll explain it.  That's for VS2010, but the fundamentals are the same for VS2008--though implementation is just slightly different, as it's not quite as integrated.
As for how to bind, assign the ObservableCollection that holds your items to the ItemsSource property of the grid.


what is difference in ethereum transaction after constatanople upgrade?

Question: I have heard that Constantinople update transverse the consensus protocol of ethereum from proof of work to proof of stake. So they claimed that this update could speed up the TPS(transaction per second).if the above update is working.how can I achieve this?
Answer: The Ethereum network is still 100% Proof-of-Work. Proof-of-Stake will only be introduced in Casper update - still unsure when that will happen. Some plans (https://blockexplorer.com/news/ethereum-scaling-roadmap-casper-plasma-sharding/) indicate middle of this year but I somehow doubt that.
The Constantinople upgrade was mostly a minor upgrade to prepare for future upgrades (such as Casper). The biggest notable change is the reduction of mining reward from 3 ETH to 2 ETH. So there is no direct effect on TPS with this upgrade.


What does exec sp_updatestats do?

Question: What is the use of sp_updatestats? Can I run that in the production environment for performance improvement?
Answer: sp_updatestats updates all statistics for all tables in the database, where even a single row has changed. It does it using the default sample, meaning it doesn't scan all rows in the table so it will likely produce less accurate statistics than the alternatives.
If you have a maintenance plan with 'rebuild indexes' included, it will also refresh statistics, but more accurate because it scans all rows. No need to rebuild stats after rebuilding indexes.
Manually updating particular statistics object or a table with update statistics command gives you much better control over the process. For automating it, take a look here.
Auto-update fires only when optimizer decides it has to. There was a change in math for 2012: in <2012, auto update was fired for every 500 + 20% change in table rows; in 2012+ it is SQRT(1000 * Table rows). It means it is more frequent on large tables. Temporary tables behave differently, of course.
To conclude, sp_updatestats could actually do more damage than good, and is the least recommendable option.
Comment: @dean I'm interested as to your concern about the consequences of this? "could actually do more damage that good, and is the least recommendable option." - It seems the only problem is that it might do things your maintenance plans already do, or be inefficient, but how big of an issue are those if the alternative is a SQL server instance that's unresponsive?
Answer: This is true if you replace full-scan statistics with inferior stats at 1% sampling (or less)
However after a big dataload the most important thing is often the edges of the histogram, and those are updated with sp_updatestats
So don't dismiss its use, especially since it runs very fast. It can save the day on certain DW loads

What does exec sp_updatestats do?


Face recognition Using EigenObjectRecognizer

Question: I am pretty new to EMGUCV . I want to make a face recognition system , I had implemented it but the results are not acceptable. Here is my code for the recognition :
public List<Person> RecognizeFaces(List<Image<Bgr, byte>> faces)
{
        List<Person> RecognizedPersons = new List<Person>();
        MCvTermCriteria termCrit = new MCvTermCriteria(TrainDB.Count, 0.001);

        EigenObjectRecognizer recognizer = new EigenObjectRecognizer(
         this.ToGrayList(this.TrainDB),
         labels.ToArray(),
         7000,  // I changed this argument many times but nothing has changed (1000, 2000, ...
         ref termCrit);

        string label = "";
        for (int i = 0; i < faces.Count; i++)
        {
            label = recognizer.Recognize(faces[i].Convert<Gray, byte>());
            RecognizedPersons.Add(new Person(faces[i],!label.Equals("") ? label : "unknown"));
        }

        return RecognizedPersons;
}

This function takes a list of previously detected faces from an input image and returns a list of Type Person where each person contains an image and a label to the recognized person .
My Question is why the results are not good ? is there is something wrong with my code ? Or there is something wrong with the training set TrainDB , if so , what is the best guidelines to follow when creating the training set ?
I had collected the training set according to this :
1- Applying face detection (using EMGU) on an image that's contain a single person
2- Then I resize the detected face to 200 : W, 200 : H
Some images from my training set :

Some Examples of test images :

List item

My last question .. do Emgu/OpenCv are powerful tools to be used in face recognition ? or there is something else that could be more accurate in results ?
Comment: What is the size of the training set? How many subject are in the training set and how many images are there from each subject? Are the test images in the same pose/lightning conditions/facial expression as the training images?
Comment: @GilLevi The size of the training set is 15 .. I have 3 persons 5 images for each person .. Not all test images are in the same pose/lightning condition but the facial expressions are the same
Comment: Thanks for uploading the test images. For each test image, are you first detecting the face and them apply face recognition or are you running face recognition on the entire input image?
Comment: @GilLevi no sir, first I apply face detection on the input image, then applying face recognition on the returned result
Comment: 5 per person are definitely not enough. 20, maybe.
Comment: @berak I tried that also but the results are very bad
Answer: your training set should have the same size (dimensions) and gray images
List<Image<Gray, byte>>

Answer: Can you tell us the capture method for faces that you are trying to detect?  Are you simply giving it bitmaps and asking it to recognise?  What you need to do is first do facial detection, you can use the same libary e.g.:
I have put 5 unique faces into my training data (one of each face and label) and it is pretty good at detecting.  So you dont need 20+ images per person, but I guess it will help vastly...
   face = new HaarCascade("haarcascade_frontalface_default.xml"); //You will need an XML training file
   gray = MyImage.Convert<Gray, Byte>();
   MCvAvgComp[][] facesDetected = gray.DetectHaarCascade(face,1.2, 10,
      Emgu.CV.CvEnum.HAAR_DETECTION_TYPE.DO_CANNY_PRUNING,
      new Size(20, 20));

where MyImage is your actual file (as an image object of the same class).  You can read about Haar cascade object detection here: https://docs.opencv.org/3.4/d7/d8b/tutorial_py_face_detection.html
so lets say you only have one face now in your facesDetected array (the second dimension indexes the total number of faces found, when there are mutliple faces in the MyImage):
face = MyImage.Copy(facesDetected[0][0].rect).Convert<Gray, byte>().Resize(100, 100,
Emgu.CV.CvEnum.INTER.CV_INTER_CUBIC);

Now you have a resized, and converted face, you can use.  So, once you have captured a normalized face of all of your training data, save them to an image array, save the labels in a string array and use the following command to recognise the face:
    MCvTermCriteria termCrit = new MCvTermCriteria(TrainingImages.Length, 0.001);
    EigenObjectRecognizer recognizer = new EigenObjectRecognizer(
                       TrainingImages, //as Image<Gray, byte>[] array
                       TrainingImageLabels, //as string[] array
                       3000,
                       ref termCrit);
                    name = recognizer.Recognize(result);
                    // Name will contain the recognised label

This works reasonably well, when you have pre-trained images, in my case, I am trying to detect for unique faces.  This method works like statistics, and I am trying to figure out if the Recognizer is able to return some % level of confidence that I can use to reject the suggested name.  But, I hope this help you!


proof of product rule on conditonal probabilitiy

Question: From the basic product rule on conditional probability, we know the following:
p(x,y) = P(x|y)P(y).
But I cannot understand this formula:
p(x,y|z) = p(x|y,z)p(y|z).
I have tried to prove this as:
p(x,y|z) = p(x|y|z)p(y|z)
But i am confused on p(x|y|z)[don't know this notation exists or not.]. And if p(x|y|z) exists then again confused why p(x|y|z) = p(x|y,z)
Comment: $p(x\mid y \mid z)$ is not good notation, but to the extent it suggests something like "the probability (or probability density) of $X$ given $Y=y$ given $Z=z$" then this might really be "the probability (or probability density) of $X$ given $Y=y$ and $Z=z$", i.e. $p(x\mid y , z)$
Comment: Related: https://math.stackexchange.com/questions/301207/why-is-px-yz-pyx-zpxz
Comment: Thanks Henry...
Answer: Note that $p(x,y\mid z) = p(x,y,z)/p(z)$, that $p(x\mid y,z) = p(x,y,z)/p(y,z)$, and that $p(y,z) = p(z)p(y\mid z)$ by definition. So
$$
p(x\mid y,z)p(y\mid z) = p(x,y\mid z).
$$
Comment: Late message here, but I just noticed that you may have a typo in the first equation with the denominator. P(x,y|z) = p(x,y,z)/p(z)  not  P(x,y|z) = p(x,y,z)/p(x).
Comment: @Arrow, Hi thanks; exactly.
Answer:
But i am confused on p(x|y|z)[don't know this notation exists or not.]. And if p(x|y|z) exists then again confused why p(x|y|z) = p(x|y,z)

The notation does not exist.   The divider is not a set operator; it is a separator between the event (or random variable) and the condition over which it is being measured.   The event can be joint, as can the condition, but there can only be one event-condition-divider in any measure function.
$p(x\mid y,z)$ is a representation for $\mathsf P(X=x\mid Y=y\cap Z=z)$, the probability measure for the event of $X=x$ under the condition that $Y=y$ and $Z=z$.
And such.
$$\begin{array}{lll}& p(x,y\mid z)&=\mathsf P(X=x\cap Y=y\mid Z=z)\\[1ex] =& \dfrac{p(x,y,z)}{p(z)} &= \dfrac{\mathsf P(X=x\cap Y=y\cap Z=z)}{\mathsf P(Z=z)}\\[1ex]=&\dfrac{p(x\mid y,z)\,p(y,z)}{p(z)} &=\dfrac{\mathsf P(X=x\mid Y=y\cap Z=z)\,\mathsf P(Y=y\cap Z=z)}{\mathsf P(Z=z)}\\[1ex]=& p(x\mid y,z)\,p(y\mid z) &= \mathsf P(X=x\mid Y=y\cap Z=z)\,\mathsf P(Y=y\mid Z=z) \end{array}$$
Answer: Sum Rule:
$$p(a) = \sum_{b}p(a,b)$$
As the sum rule holds for any probability distribution it trivially holds for the distribution a|c:
$$p(a|c) = \sum_{b}p(b, a|c)$$
For the product rule:
$$p(a,b)=p(a|b)p(b)$$ but does $$p(a,b|c)=p(a|b|c)p(b|c)$$?
Well presumably, they are equal if $$p(a|b|c) = p(a|b,c)$$ - but does this equality hold?
Yes: Aim to prove:  $$p(a|b|c) = p(a|b,c)$$
$$p(a,b|c) = p(a|b|c)p(b|c)$$
$$p(a|b|c) = \dfrac{p(a,b|c)}{p(b|c)} = \dfrac{ \dfrac{p(a,b,c)}{p(c)}} {\dfrac{p(b,c)}{p(c)}} = \dfrac{p(a,b,c)}{p(b,c)} = \dfrac{p(a|b,c)p(b,c)}{p(b,c)} = p(a|b,c)$$
Comment: For others' benefit, I note that this answer (and the underlying topic) is very similar to that in [this post](https://math.stackexchange.com/a/3085024/9754).
Answer: The said notation does not exist for $x,y,z$ events.
However, the identity could be proved by
$$P(x,y\mid z)=\frac{P(x,y,z)}{P(z)}=\frac{P(x\mid y,z)P(y,z)}{P(z)}$$
$$=P(x\mid y,z)\frac{P(y,z)}{P(z)}=P(x\mid y,z)P(y\mid z)$$
Answer: Doubtless this derivation is included either in the body of the text or as an exercise across a great array of textbooks across numerous fields. For example, #13.16(a.) from Russell & Norvig's Artificial Intelligence, A Modern Approach, Third Edition, says,

Prove the conditionalized version of the general product rule:
  $P(X,Y|e) = P(X|Y,e)P(Y|e)$.

$$P(X,Y|e) = \frac{P(X,Y,e)}{P(e)}$$
$$P(X|Y,e) = \frac{P(X,Y,e)}{P(Y,e)}$$
$$P(X,Y,e) = \frac{P(X|Y,e)}{P(Y,e)}$$
$$P(X,Y|e) = \frac{P(X|y,e)P(Y,e)}{P(e)} = P(X|Y,e)P(Y|e)$$
In the first of the 4 lines in the derivation, above, we have $P(X,Y|e) = \frac{P(X,Y,e)}{P(e)}$. We can think of this, perhaps overly simplistically (but it is a nice, workable mental representation so we can see what is going on here), as $P(A|B) = P(A, B)/P(B)$, simply Bayes familiar rule, where we have replaced A with $X\cap Y$ and e with B. To map my answer back on to the original question replace X with x, Y with y, and e with z. C.f. page 9 of 10, question & answer #6, here as well. Cheers!
Comment: Line 3 doesn't make sense (and implies $P(Y,e) = 1$). Copy/paste error?


Would 'Storm clouds gathering' be considered as an example of personification?

Question: I came across this phrase in school and I became quite confused with whether it should be seen as a personification or not. It may be a collocation but does that mean it can't be a personification? 'Gathering' to me usually entails the getting together of people/ animals (animate) so if it is placed next to something that is inanimate (Storm clouds), do we automatically classify it as a personification?
Answer: Yes, from personal experience I would classify "Storm Clouds Gathering" as a personification. Your evidence to support why it should be a personification is absolutely right.
Comment: So long as Storm does and Clouds does and gathering does not take a capital…
Answer: Gather can be used in multiple ways:

[ no obj. ] come together; assemble or accumulate: a crowd gathered in the square.
[ with obj. ] bring together and take in from scattered places or sources: we have gathered the information.

pick up from the ground or a surface: they gathered up the dirty plates and cups.
...
draw together or toward oneself: she gathered the child in her arms.

Oxford Dictionaries

You seem to be thinking of the one with an object, which is not necessarily a personification ("somebody gathered storm clouds" and "storm clouds were gathered by someone" are not personifications). In your sentence, however, there is no object, which makes it a personification (since the clouds gathered themselves).


How do we clear the console in assembly?

Question: I am looking for a win32 api function that clears the console, much like the cls command
Thanks!
Devjeet
Answer: This is pretty old, but should still work. Conversion to assembly language is left as an exercise for the reader, but shouldn't be terribly difficult (most of it is just function calls, and the multiplication is trivial):
#include <windows.h>

void clear_screen(char fill = ' ') {
    COORD tl = {0,0};
    CONSOLE_SCREEN_BUFFER_INFO s;
    HANDLE console = GetStdHandle(STD_OUTPUT_HANDLE);
    GetConsoleScreenBufferInfo(console, &s);
    DWORD written, cells = s.dwSize.X * s.dwSize.Y;
    FillConsoleOutputCharacter(console, fill, cells, tl, &written);
    FillConsoleOutputAttribute(console, s.wAttributes, cells, tl, &written);
    SetConsoleCursorPosition(console, tl);
}

Comment: Last 2 statements, attrib isn't defined and neither is console. I'm guessing console is screen? But what is attrib? Thanks.
Comment: :O Assembly got voids :D Really, though, conversion is indeed trivial.
Comment: @UkownUser: It looks like the code still had a couple minor of bits left over from the program I was writing when I originally wrote this code. I've edited in code that works standalone. Thanks for the heads-up.
Answer: There is no Win32 API which directly clears the console - you need to use something like FillConsoleOutputCharacter.


How are PCB's programmed?

Question: So recently I have been looking into prototyping my arduino project and there are a lot of tutorials online on how to take your Arduino and using a software such as Eagle to make a PCB layout but I was wondering how you would get the Arduino program onto that PCB. Surely in large scale circuit manufacturing they wouldn't have a usb attached so that they upload a program into each and every one of their circuits?
Comment: Large scale manufacturing doesn't involve arduino boards, as this would be too costly. They, however, can use the same microcontroller (Atmega 328). And usually they program with ISP programmers, so it does not require a FTDI chip or any usb/serial converters. And it also doesn't require a (usb-programmer) bootloader.
Comment: This belongs back on EESE where it was originally asked - "Arduino" is effectively nothing more than an *example* in this generic question.
Comment: I don' t find this question that generic; it is AVR oriented since the issue is how to program series of AVR, compared with programming one AVR with Arduino.
Answer: why not?
Well ok ... they might not connect a USB to each one as there are faster ways - flyingProbe/PogoPins on a "bed of nails"
http://en.wikipedia.org/wiki/In-circuit_test

This type of temporary connection speeds up testing & pre-programming considerably.
Each card that is made has to go through some form of test process at the end (flying probe or manual).
https://learn.sparkfun.com/tutorials/electronics-assembly

On boards that require programming or calibration, additional steps
  are taken to load code and verify the output.
Answer: For commercial products where very large quantities are produced, a microcontroller will be "mask programmed" - the (well-debugged) program will be built into the microcontroller as part of the IC production process.
For smaller quantities, the microcontroller may be programmed in a special fixture before being installed on the PC board.  Also, most current microcontrollers have "In System Programming (ISP)" capability, which typically requires a 6 - 10 pin connector on the board, and a special programmer to plug into that board.
I think it is highly unlikely that an Arduino-based program would be used in any commercial product.
Comment: Atmel doesn't mask-program AVRs, but will preflash an image you provide to them.
Answer: Actually, they generally do load a program into each and every one of their devices individually.  They can do a lot more than that as well.  Ever heard of something called JTAG?  For large circuit boards, most of the large ICs are connected to a JTAG scan chain.  Once the board is assembled, they connect a JTAG cable to the scan chain.  Special test software controls the scan chain through the JTAG cable, possibly connected to the control computer via USB.  The software can take control over the I/O pins on the ICs on the scan chain and set test patterns to check for opens or shorts.  It can also perform more complex testing in concert with external test equipment, even including temporarily loading special diagnostic firmware.  Once the test passes, the computer can then initialize ROMs, load firmware, set serial numbers, etc.
This is done one board at a time.  Often, special test and/or programming jigs are built that allow boards to be connected and disconnected quickly, and usually multiple jigs are used in parallel for higher throughput.
For some chips, they can be programmed before installation either with mask ROM or with a standalone programmer.  This is done when volumes are large and testing is not required, or if the devices must be programmed for the automated testing to work correctly.
Answer: cheers Brandan :)
I would suggest for you to make your own programmer. Then you will know a lot about programming and so on. If you want to program a chip on pcb, you have to have output pins of ISP. Throw these pins you will pump your code to Atmega chip.
Software Khazama is "old" and tested one. It really works :) AVR studio is also good one
Hardware USBasp There are ready pcb layouts in eagle. Building your own HW will be a bit more expensive than buying one, but if we count in experience it will be priceless :)
Also you can use bootloader. Check in google there are plenty of tutorials.

How are PCB's programmed?


How does electrostatic attraction work?

Question: We're doing electrostatics in my physics class and have just finished discussing the fact that like charges repel one another, whereas unlike charges attract. I have a vague notion that a charge is determined by an uneven number of protons/electrons. But how do charges work? What are the mechanics of attraction and repulsion, on an atomic level?
Answer: On an atomic level, what you have is charged particles that attract and repel each other based on their electrical charges, through the very same electrostatic interactions you just learned about.
The electromagnetic interaction is fundamental, and it cannot be explained in terms of anything else; you can pretend to explain it through other means but that is ultimately creating circular reasoning. There's a reason that electromagnetism is a fundamental interaction - it just something that happens.
As usual, though, (if you swap charges for magnets), Feynman explains it best, in the classic Feynman interview on magnets and 'why?' questions, so I'm going to shamelessly steal from the transcript:

it's the same electric repulsions that are involved in keeping your finger away from the chair because it's electrical forces in minor and microscopic details. [...] It turns out that the magnetic and electrical force with which I wish to explain this repulsion in the first place is what ultimately is the deeper thing that we have to start with to explain many other things that everybody would just accept. You know you can't put your hand through the chair; that's taken for granted. But that you can't put your hand through the chair, when looked at more closely, why, involves the same repulsive forces that appear in magnets. [...]
It's a force which is present all the time and very common and is a basic force of almost - I mean, I could go a little further back if I went more technical - but on an early level I've just got to tell you that's going to be one of the things you'll just have to take as an element of the world: the existence of magnetic repulsion, or electrical attraction, magnetic attraction.
I can't explain that attraction in terms of anything else that's familiar to you. For example, if we said the magnets attract like if rubber bands, I would be cheating you. Because they're not connected by rubber bands. I'd soon be in trouble. And secondly, if you were curious enough, you'd ask me why rubber bands tend to pull back together again, and I would end up explaining that in terms of electrical forces, which are the very things that I'm trying to use the rubber bands to explain. So I have cheated very badly, you see. So I am not going to be able to give you an answer to why magnets attract each other except to tell you that they do.
And to tell you that that's one of the elements in the world - there are electrical forces, magnetic forces, gravitational forces, and others, and those are some of the parts. If you were a student, I could go further. I could tell you that the magnetic forces are related to the electrical forces very intimately, that the relationship between the gravity forces and electrical forces remains unknown, and so on. But I really can't do a good job, any job, of explaining magnetic force in terms of something else you're more familiar with, because I don't understand it in terms of anything else that you're more familiar with.

How does electrostatic attraction work?


Does a spell attack trigger extra damage from Hexblade's Curse?

Question: If a target is inflicted with Hexblade's Curse, would hitting that target using spell attack (for example Scorching Ray) trigger the additional damage? and if so, would it affect each "ray" with additional damage?
Answer: Yes, it does.

You gain a bonus to damage rolls against the cursed target. The bonus equals your proficiency bonus.

So long as you roll for damage you get the bonus. The bonus applies to each damage roll, so if you roll damage multiple times for a spell (e.g. scorching ray), the damage bonus applies to each roll.
Answer: Yes, it does. But only once per spell, or per attack roll.
In this case, you deal the damage of the weapon or spell and (according to PHB 196, "Damage Rolls") you can then add your proficiency bonus.
In the case of spells that require multiple attack rolls, congrats - you get the bonus per attack.
In the case of spells without an attack roll, sorry you just get to add the bonus once.
Comment: Should be noted, that *magic missile* is an exception, you add Hexblade's Curse damage bonus to each dart.


Will an Honours year grant me a second degree?

Question: I am about to graduate with a Bachelor's degree in Biotechnology (Hons). I plan to further my studies with a Masters in Research. I would like to apply for an Honours year in Medical bioscience or related subject at a different university (perhaps in Australia) to gain enough qualifications to pursue an interdisciplinary Masters Research. Will doing the Honours year give me a second degree? And will I be able to even apply for an Hons year of a different major?
Comment: I'm thinking it depends on the specific school and degree program. Maybe contact the appropriate people at the university you are considering.
Comment: What is a Honors year?
Answer: In Australia, an Honours is considered a separate degree.  Other countries may have no idea what your Honours is.
Usually, once you have an Honours there is no reason to pursue a masters in the same subject.  However, I do not have specific knowledge of bio/medicine.
As for your eligibility, you need to check with particular universities about their policies.  Probably they will consider your marks (grades).


Is there a minimum weight requirement for flying manned aircraft in controlled airspace (B, C, D, E)?

Question: The Colomban Cri-Cri is a small twin engine plane with an empty weight of ~200 lbs and a MTOW of ~400 lbs.
Can I fly this in controlled airspace? What are the requirements for aircraft registration?
Comment: How do you count lighter-than-air craft? They're allowed into controlled airspace, with a suitable clearance.
Answer: If there were any minimum weight requirement for manned aircraft in controlled airspace, the requirement would probably be in 14 CFR Part 91, Section 126 through Section 135. These are the sections which describe the operating rules for various types of airspace. I looked through those sections, and there are no weight requirements.
I also can't imagine why there would be a minimum weight requirement for any class of airspace.
Like most aircraft, the Colomban Cri-Cri must have an airworthiness certificate and a registration certificate (14 CFR 91.203). The regulations describing getting an airworthiness certificate are in 14 CFR 21, Subpart H.
Some aircraft weighing less than about 254 pounds are "Part 103 ultralights" and do not need to be registered. However, this doesn't apply to the Colomban Cri-Cri, because Wikipedia says that it has a cruise speed of 100 knots; Part 103 ultralights must be "not capable of more than 55 knots calibrated airspeed at full power in level flight".
Answer: It's not a weight thing, it's an equipment thing. Need a transponder, ADS-B-Out (starting Jan 1 2020) and 2 way radio.  OR, calling ahead and making arrangements if not so equipped.
I don't imagine flying something that light around big jets would be very much fun, altho some of them have smaller runways for smaller GA aircraft.
Boston, for example:
https://img.airnav.com/aptdiag/w240/00409.gif?v=MZPOGC
Comment: Transponder and ADSB would not be required in Class D or E airspace, and 2 way radio not required in Class E.
Comment: Yes,  but the question did not break them out separately.


How to repair large holes in inner tubes?

Question: I find that any hole, cut or tear larger than a pinprick is impossible to repair with normal patches because they are too soft. Even a 2-3mm slit will cause the repair patch to balloon out and burst at that point. Is there a way around this?
Comment: Are you inflating the tube inside or outside of tire?
Comment: To clarify advice from @lawndartcatcher , put a dollar BILL between the inner tube and tire, i.e. paper money. Do not put a Canadian Loonie or other dollar COIN in there. Paper money is a common old school emergency tire "boot", the latter term not to be confused with parking boots used to lock car wheels in place. See Park Tool "TB-2 Emergency Tire Boot" - basically you can use a a piece of paper money to improvise one of those.
Comment: You could also try throwing a dollar in between the tube and the tire - it may give you enough support to get you home.
Comment: Also consider getting self sealing tubes http://www.wiggle.co.uk/slime-self-healing-road-inner-tube/ http://www.wiggle.co.uk/slime-lite-self-healing-road-inner-tube/
Answer: I don't think that you will have a problem with the patch ballooning out as long as the tube is inflated while fully mounted with rim and tire. To my experience there just won't be any room to balloon to.
If you fear that this will be a safety risk, I would recommend a new inner tube when the old one has such a big hole. When you are on a ride and have no spare tube at hand, even with a 2-3 mm slit a standard patch should be enough to get you home.
Comment: Patch cement also needs time to cure, so carrying a spare tube is a great solution and well worth the weight (better than carrying your bike home.)
Comment: You shouldn't have a problem with normal Rema patches covering a 3-mm cut. This sounds like an error applying the patch. The thickness of the patch isn't an issue (patches are already thicker and tougher than innertubes).
Comment: I always carry a spare tube, but i also always repair tubes and use them as long as possible . What i am looking for is a way of doing repairs which are a bit heavier duty than a repair patch. Are there thicker patches out there? is there a way of making patches out of old inner tube?
Answer: First of all: do not bother trying to patch too big holes.
If it is just too big, just trash the tube.
It is cheap enough and it's better than struggling and trying 10 patchings, wasting a whole patch kit all just to end up with it leaking and having a flat tire in the middle of nowhere.
Now, I have already successfully patched a hole as big as 5 mm. It was that big that the tube could not even be inflated to find the leak, it was not a leak, it was just an opening...
For that I used of of the bigger patches that are included in most kits. That worked just fine.
Apart from that I learned that my biggest mistake of patching over time was not to let the cement dry long enough. 5 or 7 minutes seems about fine before applying the patch. And do not hesitate to put it on a large surface to make 100% sure 100% of the patch surface is on a treated surface.
This is actually easy to overlook especially on narrower tubes (such as those for 23mm and less tyres). And when applying the patch, make sure to press it hard on the tube, and especially the edges: these are the critical parts that you want the most to stick to the tube since they will be the first to "suffer" when facing an inflation, and if an edge loses adherence, it is the whole patch that is compromised.
I also had the feeling that the "feather" edges attach better handle better the expansion when inflated than the straight edges.
Comment: When I apply a patch I generally use a tire iron or the corner of the patch kit box to rub down the patch and make sure it's fully adhered.
Answer: Two points:
1) Tubes are pretty cheap.  Better to replace one with a large hole rather than attempt to patch it.  (Though 2-3mm cut should be easily repairable with a standard quality patch.)
2) If you have trouble with the tube "ballooning out" then it's not being properly supported by the tire.  You must have a cut in the tire that (if it doesn't render the tire damaged beyond repair) requires a durable "boot".  A tube is intended to be completely supported/contained by the tire and should not "balloon out" even if infinitely thin and pliable.
Answer: Nobody's said it explicitly, so I will: ordinary patches are completely fine for slits and holes a couple of millimetres across.
Your description of the patches "ballooning" suggests one of two things. Either you're inflating the tube to moderately high pressure outside the tyre, or there's a significant hole in the tyre that the tube is coming through.  In the first case, the fix is "Well don't do that, then". Once the patch glue has dried, you can inflate the tube until it just starts to expand, to check that there aren't any other holes, but you don't need to go any farther than that. In the second case, you need to repair the tyre, too.
Having said that, I'm a bit confused by anything ballooning at all. Patches are usually less stretchy than the tube itself so, when you pump up a patched tube outside a tyre, it's usually narrower at the patched area, not wider.
Answer:
Try cut a giant patch from old tube.

To expand on this suggestion, it is possible to make your own patches.
 You'll need:

A dead tube with a hole that needs a patch larger than anything you have available
A second dead tube as a material donor
Sharp scissors
And all the usual parts for patching a tube, including vulcanising fluid (ie glue)

Here's the tube I was working with.  Its a 28mm 700c "wide" tube, but that's not really important.

You can see a patch had failed and got torn off, which took a strip of rubber out of the tube.

From your material donor tube, cut a square of material.  This will be your patch.  So it needs to be big enough to be the patch.

On the inside, notice that there is a white powdery coating.  This is an anti-stick powder, and you need to remove it by washing.  In this photo the left square is shinier and the right patch is as-cut.  You should also nip off the corners from your patch to reduce lifting later.

Abrasion - in the above image, the left patch is shiny.  We don't want that, we need it to be dull and scratched up.  So use your buffing tool to abrade the surface, just like you do with a normal puncture.   Downside, this needs to be enough damage to remove the shininess, but not so much it weakens the patch with deep scratches.  No photo sorry.   Then do the same on the tube, all around where your hole is.
Gloop - Add vulcanising fluid around your tyre's hole, like normal.  The area will have to be at least as big as your patch.  Then do the same on the underside of the patch. Get right out to the edges, and don't touch the fluid with your fingers.  Perhaps an old screwdriver or pokey tool can help hold it down.  Also, don't allow the patch to roll up on itself.

Go away.   Seriously - leave the patch and tube to "set up" for a minimum of 5 minutes, 10 minutes is better, and could be 15 minutes if its cold.
Locate patch on tube, centralised over the hole, and press down with fingers.  Then roll the patch down all around the tube.  I had two holes here to cover, the initial one that needed patching and the big hole torn.

Inflate and test.

Here it is with maybe two pump strokes.  Pressure isn't a good guide at these low areas, and the tube will stretch rather than increasing pressure.

Here it is again after 5-7 strokes of the track pump.

At this point I went away for dinner, and came back to find this:

The patch job failed, possibly because I was inflating the tube outside of a tyre so it stretched, which stresses the very edge of the patch.

Aside - I didn't have a good candidate tube for this so I cheated and cut the donor section from the other side of the tube, then knotted the ends which looks like this:
Comment: @Criggie. Nice. I tried it once (commuter) and my result was similar to the photo's. Perhaps it depends on the quality of the rubber and vulcanizing glue.  I have had too much misery with punctures, which all but stopped when I started running Marathon tires. And nowadays I can afford to replace tubes and tires when they get old.
Comment: Referring to affordability of tubes, I just want to note that cost may not be the only reason for patching punctured tubes. I used to routinely replace them until it occured to me this is just plain wasteful, as in literally making too much waste, and one of the worst kinds of waste. So instead of (indirectly) polluting the very environment I enjoy riding through with loads of rubber, I just patch 'em up these days.
Comment: I got fed up with @Gui's answer in the review queue, and didn't want to loose the point so its being answered in more detail here, using an old tube I had.
Comment: Hello, and welcome to Bicycles Stack Exchange. This does not really answer the question, and would better be suited as a comment. This question is also seven years old and has several good answers. There are plenty of unanswered questions, perhaps you would like to have a look at them.
Comment: Okay, now it's just spamming the front page with late answer.
Comment: Hmm. This seems more like proof that you cannot make a good patch out of an old piece of tube. Patches typically have an extremely soft side, perhaps this is partially vulcanized rubber. The vulcanizing "glue" seems to soften that up more.

Also, I've found sanding to the point that the tube is dull vital.

Finally, in my experience patches of large holes *always* fail over time. Patch material is much thinner that the tube.
Comment: @Michiel  " Patch material is much thinner that the tube"  sounds like the basis of another question.  Go for it!   Separately, I have recently patched a 2" MTB tube that had a similar homemade patch and it has been on that tube for at least 5 years without failing, though MTB tubes run at a lower pressure than a road tube.


What is Trusted Credentials in mobile phones?

Question: My phone is encrypted and i was just playing with the configuration and just found a Trusted Credentials that got me curious on what it is and specially in one called Government Root Certification Authority (Government Root (CA) or Goverment(CA)) and i'm wondering what is that.
Also i'm wondering what kind of vulnerability this may put on my phone and if i should  disable some of those credentials.
Answer: Trusted credentials are nothing else than CA's your device is trusting. This is basically a truststore of your phone. Disabling them may lead to a lot of problems with applications that depend on certificate-based authentication (most of them really). When you trust a root CA you'll also trust the certificates issued by it. This is very important for accessing different API's and webservers. I would not touch those settings if you don't know anything about the public key cryptography and implications of trust.
Answer: Trusted Credentials provide a way for your phone to practically guarantee that the websites it talks to are actually who they say they are.
So the "Government Trusted Credential" doesn't mean your phone trusts the government. It means that when your phone talks to a website who claims that it is the government, your phone can independently check that its true.
Trusted Credentials are created and distributed by Certificate Authorities (CAs). Your phone's vendor/manufactuer will take commonly used credentials that are published from trusted CAs and hardcode them into the OS. You should also be able to optionally disable/delete the listed Trusted Credentials or add your own.
Comment: `It means that when your phone talks to a website who claims that it is the government, your phone can independently check that its true` - rather `...your phone can independently check that the government says so`. Of course, trusting a website that uses a certificate ultimately means trusting the root ca.


1960s F&SF short story - Insane Professor

Question: This short fantasy appeared in F&SF during the '60s. A harried university professor (I can't remember just what he was bothered about) comes under increasing pressure both at home and at work. A student the professor doesn't know appears periodically throughout the story; he wants the professor to read his dissertation on death. The professor doesn't have time to read the report (and he never does) but notices a peculiar odor of decay in the air whenever the student appears. At the end of the story the professor goes completely crazy: he bends over and stares at the moon through his spread legs. (The student really creeped me out.) Any help will be appreciated.
Comment: If it appeared in F&SF that makes it on topic.
Comment: What about this story (other than its publishing locale) makes you think that it's on-topic? Does it contain any fantasy or sci-fi content?
Comment: I think there is the implication that the student is some form of reanimated dead.
Comment: Reminds me of [another short story I've been trying to identify](https://literature.stackexchange.com/q/1859/17). I wonder if @user14111 can nail that one too :-)
Comment: @Randal'Thor I've already looked for that one, I have your question bookmarked. It sounded vaguely familiar, I might have read it but I'm not sure.
Answer: "The Master's Thesis", a short story by David Madden, first published in The Magazine of Fantasy and Science Fiction, July 1967, available at the Internet Archive.
A strange young man named Philip Hockaday Fonville has signed up as Prof. Swinnard's Master's student and is already writing his thesis:

"Now, young man," he said, turning Current History right side up and looking for something, anything that he might have misplaced, "suppose we get this whole affair in perspective."
"I'm having it typed up now, sir."
"What's that?"
"The first chapter."
"Come now," Swinnard said, grinning, trying to find some way to attack the problem. "We must be serious."
"Sir, how much seriouser can I get? Up all night gnawing on candy bars to keep up my strength while I struggle with this thing."
"You are actually in the writing phase of — "
"Oh, it's going to be ready before the deadline — way before. Typed up in three neat copies, bound and everything. Neat and crisp."

Fonville gives Prof. Swinnard the first chapter to read, which he loses:

That night he realized he had forgotten to read the chapter, that he had, in fact, left it on the table in the Chinese restaurant. He telephoned, and the dishwasher answered. Yes, he had. It was in the garbage, and the garbage was on the truck. Yes, he could read a little English. One word on the second page, "Death," followed by a name in three parts. Swinnard thanked him and tried to sleep.

A peculiar odor:

Fonville pulled a chair close to the desk until their knees almost touched. Swinnard noticed what tremendous feet the boy had. His galoshes made his own feet look large, but they were small alongside the boy's. Fonville settled himself, but first, Swinnard wanted to know what that odor was.
"Smell, sir?" The boy got up and looked all around the room, sniffing in each corner, sticking his head out the door into the hall, and peeping over the window sill into the yard under the bushes. When the boy sat down, Swinnard realized that the odor was stronger.

The ending:

Standing on the peaceful lawn under the full moon, he knew that, despite the chill in the air, this was what he wanted to do. He ducked under the clothes line. The summer sound of the screen door was still in his ears. He bent over toward the curving grass and reached to grip the backs of his legs. Strands of his thin grey hair brushed the grass. His glasses slid down over the bridge of his nose and dangled from one ear as he peered between his legs, upside-down. The earth turned black, the moon melted, making the sky one white radiance, and in the soles of his feet, Swinnard felt the distant beat of hoofs.
Comment: User14111: You have come through for me again. Thanksyou.


iPhone Dev: Get more data functionality in twitter iPhone clients?

Question: I'm building an app (not necessarily a twitter client) and I'm trying to figure out how developers create the buttons above and below a table view where a user presses them to either reload newer data or reload older data into a table view. Does anyone know of any tutorials out there that does this or know of an easy way?
Answer: If you want fixed buttons, you can just make your table view not use the full screen and add the buttons in the space. If you want the buttons to scroll with the table view, you can add a header or footer view to the table and put your buttons inside that.
Answer: Check the Three20 project. I believe there's a tableview there that does that.
It's actually not that hard to add inline buttons to a tableview. First you check and see if there's actually more data to show. If so, you want to add 1 to the number of rows returned from the datasource. When asked to draw that last row you return a cell that contains "Press for more" caption as well as a hidden spinner instead of the standard cell  that shows your normal data.
If the user presses that last button the table view handler turns on the spinner then fires off a network event. Once the network request completes the data is processed and added to the same tableview datasource that was used to draw the first table.
Now all you have to do is tell the tableview to reload itself and the new data will show up. If you want to limit the amount of data shown you can prune out N number of items from the head of the datasource before redrawing so the memory-use stays manageable.


What kind of spider?

Question: I'd much appreciate help figuring out what kind of spider this is. I'm in Western Washington, in the northern suburbs of Seattle. Its outside, on the eaves of the house. Particularly wondering if a yellow sac spider.
Answer: Interesting!  The size, shape and posture, as well as the visible patterning on the body, are all reminiscent of Dolomedes tenebrosus, the common terrestrial fishing spider here in the east.  However, there are a couple of odd things here:
1.  Dolomedes is a genus only found (as far as I know) in the eastern half of the US,
2.  The abdominal color pattern is a bit off from what I would expect (dark flank marks at the front of the abdomen, especially), and
3.  That looks like an egg sac the spider is sitting on top of, as if it was guarding, but fishing spiders (as far as I know) do not make egg sacs like that - they make ball-shaped egg sacs which they carry around with them using their fangs to hold them.
The thing is, nothing else rings a bell with me - the only other large wandering hunter that would be anything like that in the Pacific NW, that I know of, is the Giant Crab Spider Olios giganteus, which doesn't look much like that, although it does make a silken retreat/egg sac protection.
I confess that I am stumped, although I can confidently say that it is not a Yellow Sac Spider.
However, I can suggest two good options for identification:
the Burke Museum: https://www.burkemuseum.org/collections-and-research/biology/arachnology-and-entomology/contact-form
and Bug Guide: https://bugguide.net/node/view/6/bgimage
both of which can often do identifications from photos alone (although with spiders, this can sometimes be tricky).
Good luck!
Answer: This is not a Cheiracanthium (a yellow sac spider) as those are fairly homogenously-colored with no discernable markings or speckles.
This is a Running Crab Spider of genus Philodromus. Here is a picture of Philodromus Spectabilis from Washington state:

(Source: https://inaturalist.org/observations/140737985 identified by two arachnologists )
The crab-like stance places this in the crab and running-crab superfamily Thomisoidea and the fact that the second legs are its longest legs separates this from the normal crab spiders, placing this is the Philodromidae (running crab spiders) as regular crab spiders have their front-most legs longest.
There are no Giant Crab spider (Olios sp) this far north in the US.
There are other genera of running crab spiders in Washington, but they are distinguished by a darker cardiac mark and/or significantly different leg proportions.
It is not always possible to identify spiders to species level with photos, but P. Spectabilis is a close match. There are over a dozen species of Philodromus found in Washington state.

What kind of spider?


Benchmarks command error

Question: need help with benchmarking: the command cargo build --release --features runtime-benchmarks completes without any problem, but then I try  ./target/release/fs-node benchmark pallet --chain dev --execution wasm --wasm-execution compiled --pallet pallet_nft --extrinsic "*" --steps 100 --repeat 40 --output ./pallets/nft/src/weights.rs --template ./pallets/roles/src/frame-weight-template.hbs and I get  this :  Error: Input("NotPermitted") any idea about the source of this error?
Thanks in advance.
Comment: Maybe your shell interpolates the `--extrinsic "*"` to something else. Try leaving it empty by `'--extrinsic='`, or try a specific extrinsic for testing.
If that does not help, please share the code so I can try it as well.
Comment: It only does it with a particular pallet (nft pallet). the code is here: https://github.com/Fair-Squares/fair-squares/blob/96-pallet-onboarding/pallets/nft/src/benchmarking.rs
Answer: In your case the error originates from your pallet logic itself.
The NotPermitted error is defined here.
In your benchmark you call the mint extrinsic with this account: create_account::<T>("caller", 0) which is not permitted to mint, therefore the error.
The can_mint function in the Permissions trait guards that, you can either mock it by whitelisting that account or use an account that is permissioned.
With mocked Permissions it looks like this for the mint extrinsic but the others need fixing as well.
Pallet: "pallet_nft", Extrinsic: "mint", Lowest values: [], Highest values: [], Steps: 100, Repeat: 40
Raw Storage Info
========
Storage: NftModule Collections (r:1 w:0)
Storage: Uniques Asset (r:1 w:1)
Storage: Uniques Class (r:1 w:1)
Storage: Uniques CollectionMaxSupply (r:1 w:0)
Storage: Uniques Account (r:0 w:1)
Storage: NftModule Items (r:0 w:1)

Median Slopes Analysis
========
-- Extrinsic Time --

Model:
Time ~=    48.62
              µs

Reads = 4
Writes = 4

Min Squares Analysis
========
-- Extrinsic Time --

Model:
Time ~=    48.62
              µs

Reads = 4
Writes = 4

Two more things just for your convenience:

You can run the benchmarks in a test mode where just one iteration is executed. This compiles much faster than the normal benchmarking. The test mode uses the mocked runtime and the real benchmarks use the node runtime.

cargo t --features=runtime-benchmarks -p pallet-nft

In the long run you should try to get rid of the without_storage_info annotation. Just delete them and fix the compile errors. In the future these annotations need to be removed to enable PoV benchmarking.
Comment: Thank you!! It totally makes sense!! and thank you for the additional advices!!!

Benchmarks command error


Is the DLC physically included with the Legendary Edition of Skyrim, or is it just a code?

Question: I have an Xbox 360 and I am thinking about purchasing the Skyrim Legendary Edition. I was wondering if the included DLC would be on discs, or if there is just a code provided?
Answer: If you buy the disc version of Legendary Edition for 360, it's on disc 2.
Answer: In the Legendary Edition of Skyrim there are two physical discs where you can download the DLCs and there isn't a code. If you don't want to go through that hassle you can just buy it digitally through the xbox store which is probably what I would recommend making it easier on you.
Answer: Assuming you purchase a physical edition, then the DLC and patch 1.9 are included on the discs. You can additionally choose to install none, some, or all of the DLC.
If you buy a digital edition, you'll obviously have no discs at all.


Brightness of colors

Question: I have 1W lights, red and green. If I turn the red light full power I get 1W red light, if I turn green light full power, I get 1W green. If I want to mix the lights and get yellow light, which has approximately the same brightness as red and green had, should I turn both red and green full power? But this means I will emit 2W of power. Should I turn 0.5W red plus 0.5W green? How does this work?
Comment: Eye response is highly non-linear to both brightness and color. What are you trying to achieve?
Comment: I need to get same perceived brightness of red, green or yellow.
Comment: @Ruslan You are right, I should only have used two 'a's in waaay.
Comment: Human eyes are waaay less sensitive to red light than they are to green or yellow. So try 1 watt for the red and then tinker with the green power until you get what looks like equal perceived brightness. Then try them together and maybe try cutting the power to each by half. You probably don't need to be too precise, the eye's ability to judge brightness is horrible.
Comment: @zeta-band your "waaay" is just a twofold difference. Luminous efficacy of 610 nm monochromatic red light is half that of 555 nm green. It's the same order of magnitude.
Answer: First, to do a fully theoretical calculation of the power you need from your red and green lights to achieve the same brightness of yellow, you need to know spectral power distributions of your red and green lights. At the very least you need to know their dominant wavelengths. This is important, because e.g. red can be the orangish-red 610 nm (dominant wavelength of the #FF0000 red on a typical sRGB monitor) or deeper red 640 nm (typical red laser pointer). These have different luminous efficacy: luminous efficacy of the former is $0.5$ while for the latter it's $0.18$.
Once you know your spectral power distributions (or dominant wavelengths), you can calculate the CIE 1931 $XYZ$ color coordinates of your lights. Since your target color is yellow, you need to mix the colors so that you get the $xy$ chromaticity coordinates of something like $(0.44, 0.55)$, which correspond to 570 nm light. Actual chromaticity coordinates may be smaller if your lights are considerably non-saturated (i.e. pale red/green) — you'll need to choose a point on a line between your red and green colors on the chromaticity diagram, and take its coordinates.
Once you've determined the $X, Y, Z$ coordinates of your colors, you can play with the mixtures to determine the $x, y$ coordinates of resulting color. You just need to add them like
$$X_{\mathrm{mix}}=\alpha X_{\mathrm{red}}+\beta X_{\mathrm{green}},$$
and similarly for $Y$ and $Z$, and the $x$ and $y$ coordinates are simply defined as
$$\begin{align}
x&=\frac X{X+Y+Z},\\
y&=\frac Y{X+Y+Z}.
\end{align}$$
A quick calculation with a monochromatic 610 nm red and monochromatic 550 nm green colors gives me 75% green and 25% red to achieve close to 570 nm yellow light. If you instead take 82%:27% of green:red (so that the sum is more than 100% or 1W), the resulting brightness will be the same as of each 100% red or green separately.

Brightness of colors


Do I need to go through customs, immigration and security for a connection in Singapore?

Question: I have flights from Manila to Singapore and from Singapore to Bangkok, both on a different airline and ticket. Both flights will be in Terminal 1 and have a 5-hour gap in between. Do I still need to go through and exit immigration (for Manila/Singapore and enter again (for Singapore-Bangkok)?
Comment: I don't have one.
Comment: Depends a bit on the check in procedure of the 2nd airline. Check in desks and kiosks are outside of  immigration. If you can check in online, you are good. Some airlines have customer service desk or kiosks airside, so you can check in there. Some may allow you to check in with the gate agent.  If you need to physically show up at a check in counter, you will have to go through immigration.
Comment: Will you have any checked luggage?
Answer: Singapore has sterile transit available in all terminals, so if your bags are checked through or you have hand luggage only, you will not go through Customs or Immigration.
However, you may go through security on arrival at the gate, and you will go through security at your departure gate.  Singapore laws apply if you're caught with any contraband, up to and including the death penalty for drug trafficking.
Comment: @jcaron It's a fact alright, I lived in Singapore for years and used Changi weekly.  They profile, flights from Indonesia in particular tended to get scrutiny -- not every flight, but maybe 1 in 5?  Yes, the check is at the gate when disembarking, and yes, it can be a bit of a pain, although it usually only adds a few mins.  At Changi all security is at the gate, so they're quite efficient at it.
Comment: Didn't know there were situations where you have to go through security on arrival. Is that a fact (I suppose only for flights from some specific destinations?), or just a supposition? Given the fact you don't go through security when entering the transit/departures area from outside, this would be quite weird. Not to mention the hassle of processing a planeload of people at the quite small security checkpoints at the gates (at a rate which is in theory higher than upon departure).
Comment: I only have a hand luggage.
So, if I understand it correctly, I don't have to go to the immigration (fr MNL-SIN flight) to have an exit; and I can just proceed to my next flight?
Comment: You seem to have created a duplicate account. If you log into the account with which you posted this question, you'll be able to comment on jpatokal's answer. But the answer to your question is yes, you don't need to go through immigration.


Where can I find the three word walls for the Phantom Form shout?

Question: Wyrmstooth adds some cut content back into the game including the Phantom Form shout the Greybeards use when teaching the Dragonborn Unrelenting Force. Where can I find all three words of this shout?
Answer: Fiik: This one can be found in Wyrmstooth Barrow, in the lower levels. I don't remember the exact location, but its behind the draugr fighting the bandits.
Loh: This word is inside of Haetar's Cave, shortly before you are about to leave the cave.
Sah: Fort Valus, inside the Tomb of Volum, which is a secret passage in the northeast (unsure!) of the castle.


Expand 1st Group and collapse 2nd Group in list view

Question: I have a list view Grouped by Status column. Status Contains Active and closed values.
now i want the default view to open with, Status:Active group to be Expanded and Status:Closed group to be collapsed.
Answer: Although this post is quite old, this answer might help the ones who are searching for this functionality.
In a list view grouped by Status column, where Active status should be expanded and Closes Status should be collapsed: Using IE developer tools or Mozilla Firebug, locate the (html element) image ID for the collapse-expand image (the '-' image). You'll find this img tag element to be enclosed within an  tag. This  tag has a javascript function for onclick event as shown below:
javascript:ExpCollGroup('1-1_','img_1-1_');return false;

Note that the image ID may differ based on your page. Also, Note that the expand collapse functionality is achieved by calling the javascript function 'ExpCollGroup'. All you have to do now is to script this function call in a CEWP added to your list view page (ignore the 'return false' part). Go ahead and add a content editor web part to your list view page and ensure that this CEWP is below the LVWP. Paste the following code in the source editor window of the CEWP:

I had tried this solution on MOSS 2007 site, but I guess this solution may work for SP2010 also.
Answer: Are you familiar with jQuery? There is a click() function that does just that:
http://api.jquery.com/click/
So the idea would be to identify the first expand button and click on it. Something like this:
$("img[src*='plus.gif']").click();
Again, this seems like a heavy option, and just creating two views would be easier.
Answer: You don't have this option out of the box.
I would simply create two grouped views on the same page, one filtered for status active, and the other filtered for status closed.
Another solution would be to create the view as collapsed, and write a script that simulates a click after the view has loaded.
Answer: Using the ID will not work as is changing all the time, I'd use instead the name of the Category and in your case get find the related status and match category names. In my case I'm sending the name of the category in the URL to expand a category :
(You can edit the page View or add a content editor web part)
function getParameterByName( name,href )
{
  name = name.replace(/[\[]/,"\\\[").replace(/[\]]/,"\\\]");
  var regexS = "[\\?&]"+name+"=([^&#]*)";
  var regex = new RegExp( regexS );
  var results = regex.exec( href );
  if( results == null )
    return "";
  else
    return decodeURIComponent(results[1].replace(/\+/g, " "));
}
var category = getParameterByName( "category",document.URL);

$(".ms-commentexpand-iconouter").parent().parent().each(function(){
    if($(this).text().toLowerCase().indexOf(category.toLowerCase()) != -1)
        $(this).find("a").first().click();
});

Answer: Use the dataform webpart to achieve above functionality.. Check on blog:-
http://bborole.blogspot.co.uk/2013/08/expand-only-first-group-and-have-rest.html
Comment: Please do not provide link only answers, especially not to your own blog post.
Comment: Hi @Bhushan & welcome to our forum. When providing answers to posts please provide context when linking to another site. There are two reasons for this: first we like to keep all the knowledge in the forum itself, secondly external links tend to die, leaving others seeking the answer later on without a good answer. For guidance on providing good answers read ["how to answer"](http://sharepoint.stackexchange.com/help/how-to-answer). Also: if you are going to link to your own blog, please provide full disclosure and avoid [overt self promotion](http://sharepoint.stackexchange.com/help/behavior).


About vertices of the convex hull of any finite set of points in $\mathbb R^n$

Question: Let $S$ be a finite subset of $\mathbb R^n$ , we know that $x \in S$ is a vertex of $Conv (S)$ , the convex hull or convex polytope of $S$ , iff $x \notin Conv\Big(S$ \ $\{x\}\Big)$ ; then is the no. of vertices of $Conv(S)$ finite ? Is it true that $Conv(S)=Conv (V)$ , where $V$ is the set of vertices of $Conv (S) $ ?
( I can obviously prove the one side $Conv(V) \subseteq Conv(S) $ as $V \subseteq S$ )
Answer: Hint: Suppose $y\in\text{conv}(S)$, then we want to show $y\in\text{conv}(V)$. By definition there are appropriate $a_s$ for all $s\in S$ such that $y=\sum a_ss$. Moreover, for each $s\in S$, there are appropriate $\lambda_{s,v}$ for all $v\in V$ such that $a_s=\sum\lambda_{s,v}v$.
Why is this true? How does it help?


Deploying Object fields in Big Object

Question: Does anyone know whether it is possible to deploy custom or standard object into big object without manually writing those entire object fields and permission set file?
Thanks in Advance
Answer: It doesn't look like possible. There are no document which mention this.
Also there are many difference between existing object and big objects. For example there naming conventions is different. From docs:

Object names must be unique across all standard objects, custom
  objects, external objects, and big objects in the org. In the API, the
  names of custom big objects have a suffix of two underscores
  immediately followed by a lowercase "b" (__b). For example, an
  external object named "HistoricalInventoryLevels" is seen as
  HistoricalInventoryLevels__b in that organization's WSDL.

Also You must use the Metadata API to define or add a field to a custom big object. You can't do it through the UI. So you might need to manually add them in permission set.
Permissionset/profile files — Create a permissionSet or profile file to specify permissions for each field. These files are not required, but is required to grant access to users. By default, access to a custom big object is restricted.
Answer: Since Spring '19 it's possible to create deploy Big Objects from Setup. Before that release, it was only possible through Metadata API.

Create Custom Big Objects from Setup
A big object stores and manages a massive amount of data natively on the Salesforce platform. You can archive data from other objects or bring massive datasets from outside systems into a big object to get a full view of your customers. Big objects provide consistent performance, whether you have 1 million records, 100 million, or even 1 billion. Previously, creating a big object required using Metadata API. But now, if you aren't comfortable using the API, you can create a big object in Setup, define its fields, and build the index that defines how the big object is queried.

Deploying Object fields in Big Object


How to compile Mach 3?

Question: I am interested in the mach operating system these days. However, I found it seems pretty difficult to build it on a modern Linux system, like Debian Wheezy.
So I wanna know if there is any way to build this and boot it?
Any ideas and advises are welcomed :-)
BTW: It seems that the Mach operating system is built by ODETools(odemake and something else). What is that? It pretty hard for me to find anything of this odetools.
Answer: Try GNU Mach building page. It's microkernel which GNU Hurd is running on; I don't know if it's the same as you want to, but it can help anyway.

How to compile Mach 3?


verilog code to perform {w=(3*(p+t-1))/t} with look up taples

Question: I want verilog code to perform  {w=(p+t-1)/t} with look up taples .
The lookup table is to be realized as a ROM
where:
 p is 3 bits
t is 2 bits (and not equal to 0)
Comment: Is there a question in there somewhere?
Comment: i want know how to do that with look up taple realized as rom
Comment: Do you know how to make a lookup table?
Answer: Get a pencil, get a paper. Write down all of the combinations of p and t then calculate w by hand. I'll do one for you...
p=101 t=11
{w=(101+11-1)/11} =>  {w=(5+3-1)/3}  => w=7/3=2.333 = 0b10.0101...
Now do this for all possible inputs
You never specified what kind of signal you need to output so I'll let you think about that.
Then populate a table with all possible outputs and an index. Use logic to find the right output based on the input.
Homework is work and you need to do the work, not me.


Following EAV in my Modules

Question: Is it necessary to follow EAV with the models I create for my modules?  If not always, when should I and when shouldn't I?
Answer: No it is not necessary, especially if you're using your own tables and your own schema for data storage; and that schema is fixed (not end-user-defined).
EAV was used as the basis of Customer, Products (and, once upon a time, Sales) because the schema (business needs) for every install of Magento is potentially  different - e.g. some may want to gather a customer's Pet Name during registration, while others would want a product attribute called Flavor.
In short, define your application's scope and the data storage schema - then use Magento's built-in Resource models (ORM) to provide access and CRUD capabilities.
Comment: Credit and invoice were flattened along with other sales entities...?
Comment: Don't forget customer address and categories! :-)
Comment: And Credit Memo, Invoice and Shipment... and (EE) RMA Item...
Comment: LOL I thought we were compiling the full list of once upon a time. Le sigh.

Following EAV in my Modules


Why is the normal contact force horizontal on an inclined ladder?

Question:
There is only one force acting on the ladder which is its weight and it acts vertically downwards. Then why does the normal contact force from the vertical wall act horizontally on the ladder? There must be a horizontal force acting on the wall to exert a horizontal force on the ladder. What causes the horizontal force on the wall and what is it called?
Comment: I have updated my answer to give you, what I think is, a more definitive explanation as to why there needs to be a horizontal reaction force at the wall. Hope it helps.
Answer: Think about how a ladder stands up in real life. Would the ladder stay in the orientation shown in your image if the ground were ice? No! The reason? Friction.
The friction force, represented by $\vec{F}_{ff}$ in the figure acts to prevent the ladder from sliding to the right.
There are actually 5 forces acting on this ladder:

$\vec{F}_g$: the gravitational force (aka the "weight" force), which pushes the ladder toward the ground
$\vec{F}_w$: The normal force of the wall on the ladder, which prevents the ladder from falling into the wall.
$\vec{F}_{fw}$: The friction force of the wall on the ladder, which prevents the ladder from sliding down the wall
$\vec{F}_f$: The normal force of the floor on the ladder, which prevents the ladder from falling through the ground.
$\vec{F}_{ff}$: The friction force of the floor on the ladder, which prevents the ladder from sliding to the right.
Comment: I think your explanation in the first paragraph, while a correct intuitive explanation, does not explain why there is a need for a horizontal friction force when the only external forces acting on the ladder are vertical. I think that is what the OP is asking.
Comment: @BobD finally somebody who gets it ! Everyone taking it as granted without explaining it !
Answer:
There must be a horizontal force acting on the wall to exert a
  horizontal force on the ladder. What causes the horizontal force on
  the wall and what is it called?

Actually, there must be a horizontal force acting somewhere on the ladder to require an equal and opposite normal reaction force on the wall for equilibrium. That horizontal force acting on the ladder is the friction force at the base of the ladder. So what your question really boils down to is, why is there a friction force at the base of the ladder? @Bunji has given you an intuitive explanation. The following is in terms of the gravitational force acting on the ladder.
To answer that question note that any force can be resolved into mutually perpendicular components. Therefore $F_g$ can be resolved into two components, one acting down and parallel to the ladder, $F_{g}sinα$, and one perpendicular to the ladder, $F_{g}cosα$. At the base of the ladder, this force down and parallel to the ladder has a vertical downward component acting on the ground and a horizontal component acting on the ground to the right. Per Newton's third law these forces have equal and opposite reaction forces as shown in the free body diagram of the ladder at the base. One of those is the horizontal friction force acting to the left. For equilibrium you then need a horizontal reaction force on the wall for the sum of the horizontal forces on the ladder to be zero.
All of the above is intended only to explain the reason for a normal reaction force at the wall. Given that you now have 4 unknown reaction forces and  one known force, $F_g$. So solve for the 4 unknowns, you need 4 equations. From here you should be able to identify the needed equations if you realize that the sum of the moments where the ladder contacts the ground and the wall have to be zero for equilibrium.
Hope this helps.
Answer: I feel like there is something missing in this diagram, which is torque. In reality, there is a torque on the ladder, due to gravity, which causes it to want to rotate counterclockwise around the point where it touches the floor. This torque is "responsible" in some sense for the force of the top of the ladder against the wall (and the counterbalancing force of the foot of the ladder against the floor's friction.)
I don't see any torques in your free body diagram, although I do see an angle "alpha" at the base of the ladder, which is suggestive that maybe there should be some. If you haven't covered torque yet, this is not a great problem to try to work through.
Comment: There is nothing missing in the diagram. The torque, or moment, that the gravitational force has about the points where the ladder contacts the ground and wall are taken into account when summing the moments about those points and setting them to zero for static equilibrium. The locations where the ladder contacts the wall and floor offer no moment reaction (like a hinge support on a simply supported beam has no moment reaction).
Comment: I think this answer is the only sound one, though it could be digged a bit further
Answer: Normal forces are always perpendicular to the direction of possible slipping since they do no work. Zero work means they must be perpendicular to any displacement or motion.
Since the ladder can slip downwards at the top, the only possible direction for the normal force is horizontally.
You can think of normal forces as enforcers of a specific constraint. In this case the ladder cannot interpenetrate through the wall. So a force that resist pushing into the wall must be perpendicular to the wall.
Answer: Normal forces act perpendicular to the surface in contact. The force acting in the ladder is actually somewhere in between the horizontal and vertical forces shown. Those are just the components of the normal force.
Answer:
The vertical force due to the weight creates a torque around the contact point between the ground and the ladder. At this point, nothing is generating a horizontal force. If the wall wasn't there, the ladder would simply rotate around the contact point (with or without friction).
The wall prevents the ladder from rotating by applying a horizontal force (assuming there is no friction on the wall).
Since there is a horizontal force, to achieve equilibrium, it must be countered by another force. The only possibility is the frictional force between the ground and the ladder. Therefore, if there is no friction there, the ladder will slide and fall.
Answer: Actually if you see the diagram , body is in rotational inertia, and rod is not perfectly vertical or horizontal, thing which work here is frictional force between rod and surfaces, friction always try to maintain the inertia, here the friction which acts is static friction.
Comment: Hmmm.. I don't recall statics problems ever talking in terms of "rotational inertia" which is defined, as I understand it, as the measure of an object's resistance to change in rotation. The ladder is in static equilibrium and therefore does not, by definition. undergo rotation. If there is no rotation there is no resistance to a change in rotation. To prevent rotation, the sum of the moments anywhere on the ladder have to equal zero.


Nikon D3100 error state - says Flash required and will not focus

Question: Had this camera around 7 years and recently it just won't focus, with the question mark flashing and when I press to see the error it says "Subject is too dark, cannot adjust exposure, use the flash". The shutter release now when held down doesn't result in the camera adjusting the focus so I'm stuck really not able to take photos like I was doing before. I've tried adjusting settings and modes and the lens. Thanks for your replies.
Comment: What do you see in Live view?
Comment: What happens when you follow the error textand use the flash?
Comment: Are you shooting in low light? Have you tried shooting a brightly lit scene that doesn't need the flash?
Comment: That is in Live View that I'm experiencing this issue. Changing the lighting doesn't affect the situation either, the camera just won't adjust focus with no lens movement. I can take photos yes with or without flash but there is no focusing it just takes it. Thankyou.
Answer: please check if you have accidentally switched the focus mode to MF, both in the i menu and on the lens itself. That could cause the AF disengagement. Also, if you/(someone else who uses the camera) changed the focus mode from shutter button half press to back button autofocus (BBAF), this behaviour would be noticed.
In order to confirm if it is indeed BBAF setting, press the AE-L/AF-L button on the back of your camera and see it it focuses now. If it does, then the issue is BBAF is enabled. just go to settings, controls and reassign what AE-L/AF-L is assigned to. Change it from AF-ON to something else and everything should work like normal.
IF BBAF is not enabled, then try using a different lens and see if the issue is resolved.
Ignore the flash required message for now. First deal with the AF issue and then report back if it has been solved. Once this is done, point the camera in live-view mode at a brightly lit contrasty surface(like a white/yellow car on a black road or a black gate in a white wall) and then try again. Now, the dark scene message should disappear.
Comment: Great response, thankyou. I got it to work by switching focus to A on the lens itself and also AF-A in the i Menu. It would not work at first in M mode on the dial ( which is what I use for close-ups ) and I had to go outside and try in other modes and suddenly it starting to engage the AF and now back inside it works on closeups in M mode. Thankyou, I am still a novice despite having the camera for years.
Comment: @MichaelNashvili, On all stackexchange sites, it is etiquette to **mark as solution** if it worked.
So that way, whoever is searching for the solution need not go through all the answers to find the solution.

If my answer worked, please mark it as solution.
Comment: @MichaelNashvili another suggestion is to refrain from using AF-A. It is convenient but troubleshooting your shots gets a lot trickier if you are using AF-A. It is also very temperamental and not as reliable as AF-S or AF-C. Whilst taking macros, if using a tripod, and stationary subject, use AF-S else use AF-C.