Predator and Prey

Question: There are two players trapped in the unit square, $A$ and $B$. $A$ is chasing $B$ and wishes to catch her. Both players can move according to any strategy provided that neither exceeds unit speed. Both players have perfect knowledge of each other's position. For a given $r\ge 0$, if $|A-B|\le r$ then $A$ is said to have caught $B$. For large $r$ (say, $>2$) $A$ clearly has a winning strategy. What about for small $r$? What happens as $r\to 0$, does $A$ always have a winning strategy? What about for $r=0$? I've been thinking about this problem for a while but can't seem to make any major inroads into it.
Comment: Is the problem from a book? If so, which one? If not, where did the problem come from?
Comment: It's from the book of my head
Comment: It's not an easy problem, and I don't yet have a full solution, but it's clear to me that in its current form, the reasoning in Rushabh Mehta's posted answer is not valid (as I explained in my comments to that post).
Comment: For a discussion of the $r=0$ case (albeit on the unit disc instead of a square), see Bela Bollabas' article "The Lion and the Christian, and other Pursuit-Evasion games".  It ends up being surprisingly subtle.
Comment: I've added an answer going in a bit more detail about the solution described in Bollobas's paper.
Answer: The case where $r=0$ and the players are in the unit circle instead of a square is discussed in Bollabas' "The Lion and the Christian, and other Pursuit-Evasion games", where he first describes two incorrect arguments before giving a third argument (due to Besicovitch) that shows that $B$ can win.  The common thread in the incorrect arguments is that they make assumptions about one player's strategy (like the "$A$ always moves towards $B$" and the "$B$ will never leave the border" assumptions in the existing answer here) that are plausible but turn out not to be correct.   Here is a sketch of $B$'s winning strategy.
Suppose that player $A$ starts at the center of the circle and $B$ starts at a point $B_1$ some distance $x$ from $A$.  Choose constants $t_1, t_2, t_3, \dots, t_n, \dots$ such that $\sum t_i=\infty$, but $\sum t_i^2 < 1-x^2$ (for example, by choosing the $t_i$ to be a tail of the harmonic series).  Player $B$ then repeatedly follows the following procedure: If $O$ is the center of the circle and $B_i$ is his current position, then he chooses whichever direction orthogonal to $\overline{OB_i}$ moves him away from the current position of $A$, and moves in that direction for $t_i$ units of time.
The point is that the condition $\sum t_i^2 < 1-x^2$ guarantees that player $B$ never leaves the circle, so $B$ can keep on following this strategy indefinitely.  Meanwhile, in any individual time step $B$ is moving away from $A$'s previous position, so $A$ can't catch them even if $A$ moves directly towards $B$'s future location.
This immediately carries over to the unit square (just inscribe a circle in the square and scale appropriately), but as far as I can tell doesn't say anything about the case $r>0$.
The case $r>0$ is analyzed in Alonso, Goldstein, and Reingold's "Lion and Man: Upper and Lower Bounds", where (Theorem 2) they give a strategy that lets player $A$ get within $r$ of player $B$ in time proportional to $\log(\frac{1}{r})$.  I haven't looked at this proof in full detail though.
Comment: Thanks for your exposition for the case $r=0$, and thanks for the reference for the case $r > 0$. The results confirm what I previously intuited: $\;(1)\;$If $r > 0$, there is a strategy for $A$, by which $A$ can capture $B$, regardless $B$'s choice of evasive strategy.$\;(2)\;$If $r = 0$, there is a strategy for $B$, by which $B$ can forever avoid capture by $A$, regardless $A$'s choice of pursuit strategy.$\;(3)\;$It's not an easy problem.
Answer: For $r>0$, Costello also linked Alonso, Goldstein, and Reingold's "Lion and Man: Upper and Lower Bounds" which gave a strategy for $A$ to win and proved it happened sometime in $O(R/S\log(R/r))$ where $R$ is the radius of the enclosure and $S$ is the maximum speed. $A$ has an easy winning strategy which wins in this same order of time.
Our strategy can not win for $r=0$. It is suggested by this upper bound, but actually it is a provable property of this strategy that if $A$ is moving at equal or less than speed of $B$, at the instance of contact $A$ is moving perfectly head on with $B$. This requires careful intervention by $B$ to happen.
$A$'s strategy is to always move towards the current position of $B$ at some sufficiently fast speed. Unfortunately rigorously analyzing these curves is generally very difficult. Our analysis uses easy upper bounds to avoid this, but we should still be very close to the least upper bound on $T$, the time for $A$ to catch $B$ using this strategy. Particularly for small $r$ where $\theta$ becomes small and our approximations begin increasing in accuracy by orders of magnitude.
Denote $s$ = speed of $A$, $S$ as the maximum speed, $2R$ as the length of the side of the square, $l_0>r$ as the initial distance, and $T$ as the time for $A$ to catch $B$ with this strategy.  We will neglect the transient time for $A$ to fall behind the path of $B$ and assume $A$ starts from such a position. Sufficient $s/S > \sqrt{1-(r/R)^2}$, we will prove
\begin{equation}
T \leq S^{-1}R\sqrt{2}^3 \log \frac{l_0-R\sqrt{1-(s/S)^2}}{r-R\sqrt{1-(s/S)^2}}
\end{equation}
$S$ is a matter of scaling and can be set to $1$. Suppose $s=1$. This forces $B$ to move at unit speed as otherwise $l$ will decrease at a steady rate and $B$ will be quickly caught. If $\theta$ is the angle between the velocities of $A$ and $B$, $dl/dt = -s + \cos\theta$.
\begin{equation}
T \leq \int_{l_0}^r \frac{dl}{dl/dt} = \int_{l_0}^r \frac{dl}{-1+\cos\theta} \leq 2 \int_{r}^{l_0} \frac{dl}{\theta^2}
\end{equation}
where if there is a finite segment of path where $\theta = 0$, we slightly can bend the path of $B$ to change this without changing $T$ or the path of $A$ and otherwise remove singular values of $\theta = 0$. We must find $\theta$ as a function of $l$.
If $B$ plays optimally to maximize $A$, they must in general minimize $\theta$. Also if $B$ turns suddenly, $A$ will close a lot of distance. This implies the following statement, which may not have a simple proof.

The most optimal escape path for $B$ to take against this strategy of $A$ is for $B$ to move in the circle of maximum radius.

$B$ will move in a circle and $A$ will chase $B$ from within this circle. $A$ is moving on an expanding spiral which can be approximated by a slowly expanding circle. The important reason this model describes an upper bound is because the spiral is always intersecting this expanding circle from inwards outwards which makes $\theta$ larger than this model indicates.

Draw two circles $C_B$ and $C_A$ centered at the origin with respective radius $R_B > R_A$. $B$ is moving counterclockwise on $C_B$ and we assume its position is $(0,R_B)$. Extend the tangent $xx_0 + yy_0 = R_A^2$ of $C_A$ to $(0,R_B)$. Then $(x_0,y_0)$ is the position of $A$. Solving,
$y_0 = R_A^2/R_B$ and $x_0 = R_A\sqrt{1-(R_A/R_B)^2}$. Using $\cos \theta = R_A/R_B$
\begin{equation}
l = \sqrt{x_0^2 + (R_B - y_0)^2} = \sqrt{R_A^2 \sin^2\theta + R_B^2 \sin^4\theta} = \sqrt{2}R_B \sin^2 \theta
\end{equation}
Because $\sin \theta \leq \theta$ it follows $T \leq R_B\sqrt{2}^3 \int_{r}^{l_0} \frac{dl}{l} = R_B\sqrt{2}^3 \log \frac{l_0}{r}$. Set $R_B = R$ and scale $s=1 \rightarrow s=S$, $T \rightarrow T/S$ and we've solved the problem when both are moving at equal speed $S$.
\begin{equation}
T \leq S^{-1}R\sqrt{2}^3\log \frac{l_0}{r}
\end{equation}
But $A$ can catch $B$ when $s<1$. We'll recycle our upper bound for equal speeds to find an upper bound for this. The optimal escape path of $B$ is still thought to be a circle. Suppose $s<1$ and forget about $A$ catching $B$. As time goes to infinity $A$ will approach a circle of radius $sR_B$ inside of $C_B$.

Now suppose $A$ pursues a third object $B'$ moving on this limit circle, both moving at $s$, where $B'$ is pursuing $B$ which moves at speed $1$ on $C_B$. In this model $A$ is very nearly pursuing $B$, but because $A$ is not, this model is a larger upper bound on $T$. The tangent from this new circle to $C_B$ has length $R_B\sqrt{1 - s^2}$, and describes the position of $B'$.
Draw a circle of radius $r$ centered at $B$ and one of radius $r-R_B\sqrt{1-s^2}$ centered $B'$. This shows if $A$ comes within $r-R_B\sqrt{1-s^2}$ of this third object, they will be within $r$ of $B$. The initial distance between $A$ and $B'$ is not $l_0$, but by the triangle inequality is at least $l_0 - R_B\sqrt{1-s^2}$.
To reuse our first inequality for $A$ and $B$ to this model for $A$ and $B'$ set $S=s$. Take $R \rightarrow sR_B$. Replace $r$ by $r-R_B\sqrt{1-s^2}$ and $l_0$ by $l_0-R_B\sqrt{1-s^2}$. Set $R_B = R$. Lastly rescale the maximum speed by $S$ so $T \rightarrow T/S$ and $s\rightarrow s/S$. This returns the first equation.
Comment: Thanks for your answer.  As regards your analysis for the case $r > 0$, it may take me a while to fully follow it, and I may lack some prerequisite knowledge. But I would like to understand it, so I'll try to work through it.  It seems that the key insight is the claim that any time $B$ spends "turning" is time where $A$ can reduce the distance, and the rate of distance reduction is an increasing function of the "sharpness" of $B$'s turn. That certainly makes sense at an intuitive level.  Your argument appears to offer a number of other valuable insights, as well. Thanks again!
Comment: Pictures were added. It would be interesting for someone to run a computer simulation to probe the accuracy of this upper bound.

Predator and Prey


Full Bitcoin Node Download / Synchronization Takes very long

Question: I want run an Full Bitcoin Node on Ubuntu but it takes 3 Days to download all of the Bitcoin Blocks from 2009-2017. I have 8 MB/s Internet Speed. Why does it take so long, the blockchain is only 150GB max. I have also tried to copy the Node from my old Server to the new one but I get always an error and i must download it again. Why can't I copy it to an new server? I have copied only the blocks and chainstate folders. Can i download it faster?
Comment: Remove the <> characters, they're just there to tell you where to substitute something. And you don't need the double dash.
Comment: try running with `bitcoind -dbcache=`
Comment: I get always this error: bash: syntax error near unexpected token with **bitcoind -dbcache=<5000>**
Comment: sorry its --dbcache, note the double dash
Comment: i get the same error..
Answer: next time try to make it several questions, there is too much to answer...

Why takes so long ?? Its just 150GB Max the Blockchain size...

The full node verifies each transaction for its validity, so that your local copy of the blockchain is safe to be used. More information on bitcoin.org. So this is normal, it is not "just downloading".

... but i get always an error and i must downlaod it again, cant i
  copy it to an new server?

You didn't say s.th. on your error, we cannot guess! If you have the same software, it should be possible though. I have done it many times with core client. Need to sav wallet files before though, just to be sure!

I have copy only the blocks and chainstate folder. Can i download it
  faster?

See, it is not downloading a file, it is connecting to peers asking for tx to assemble "the blockchain" locally. And therefor verification process happens. You can use a pruned mode, which will not store the whole blockchain on your drive, but also this would have to read once all tx. It is faster, but again not done within minutes. Try to read about it on bitcoin.org or the online book of Andreas ("Mastering Bitcoin"). This helps to understand.
Comment: I had copied my data from one SD card in a raspberryPI with SuSE Linux to another, and the copying took a while (+20 minutes), and then I could use it in my second RasPi. So the .bitcoin folder with all it's contents could easily be copied...
Comment: Thanks for this. Its 1:1 the same what i want copy , takes also so long if i copy and paste it?
Answer: a full synchronization for a new node not only downloads, but also validates all blocks and transactions.
if you trust the source of your blocks, you can ask the client only download and not validate the data, to speed up the process.
you could also copy over all metadata about the validation from another node to make your new node believe it has already properly verified the transactions.
another possible reason for a slow sync process is the peer selection, which may have chosen slow peers making it take longer to download (their speed might be significantly slower than yours).
it is also possible that your new node could be using its network bandwidth for other purposes and therefor nor prioritizing the blockchain download.
which of all these cases apply to you cannot be determined from your question, but might provide some good starting points when looking for the culprit.
Comment: And how do you do it? "only download and not validate"


Could reflected sunlight from moons provide heat to darkside of tidally locked world?

Question: I was reading through a site describing terraforming the solar system. What interested me most was the idea of turning jupiter into a small star and what would happen to its tidally locked moons, specifically Io. It describes Io as having a night/day/superday cycle on its dark side due to the other moons and the distant sun. What I'm curious about is if this would provide enough light/heat to the dark side of a similar system to allow some type of terrestrial and human life to exist without hand waving? (all other factors such as atmosphere,weather, gravity,etc notwithstanding)
Answer: Not if it was like our moon. I think you could create an artificial "moon" made of reflective material to heat up the dark side of a tidally locked planet far more plausibly. If we have space travel and the ability to colonize new planets, that seems within the realm of possiblity.
Answer: From moons?  They have to be a lot more reflective than our moon is.  Despite how bright it appears in the sky, it is about as reflective as a asphalt parking lot.
The amount of radiation you get is in ratio of the moon's radius to the moon's orbital radius squared.  E.g. with a moon of radius 1000 miles, and an orbital radius of 100,000 miles then the light/heat will be diluted by (1000/100,000)^2 or about 1/10,000.  And that is with perfect reflection.
Flat mirrors help. You can get some degree of directionality with the reflection.
Closer bodies help.  More of the reflected light is collected by the planet.
Bigger bodies help.  Jupiter takes up a lot more sky for it's innermost planet than the earth does for the moon.
How about a trillion Echo balloons?
Answer: I am not sure for that scenario but I have heard of plans to heat Mars up by using gigantic reflectors made using the same material as rescue blankets. By gigantic I mean hundreds of kilometers on a side. So if this is what we could do with technology, I am not sure if a moon can reflect AND focus enough infrared energy.


How can I disable SIP on IOS 12.x

Question: How would I disable SIP on an Cisco router running IOS 12.x?
Comment: Both Darren and Brett have answered below however it would be better to be a little clearer what you mean with future questions.
Answer: Depending on which version of 12.x you're on, you should be able to do the following:
router(config)#sip-ua
router(config-sip-ua)#no transport tcp
router(config-sip-ua)#no transport udp

This stops the router from listening on port 5060.
Comment: There's also TLS. Cisco actually has this documented in a security bulletin. To be **absolutely** sure, include an acl on internet (insecure) links.
Answer: If you mean how to disable the SIP ALG you can configure the following:
no ip nat service sip udp port 5060

How can I disable SIP on IOS 12.x


Finding the location of the SharePoint Configuration Database in API

Question: I'm building a .NET Health Analyzer Custom Rule which checks a few vital signs of another custom application I have built, however if the application is also deployed on a server that is also hosting SQL Server AND the configuration database, there's a few checks I don't want to run (don't ask why, long story, mostly performance related).  How can I determine via API where the Config database is actually located in SharePoint 2010?
Answer: This was nice quest for me. I will not ask why do you need this and I hope that you don't need warning like: Never mess with SharePoint databases directly!
I didn't found many articles on net covering this specific issue (no wonder when it is bad practice). The only one was 2007 related: Determining the Configuration Database in a SharePoint 2007 Farm. Check it out.
I came up with this piece of code:
    SPDatabase configDb = null;

    foreach(var service in SPFarm.Local.Services)
    {
        if(service is SPDatabaseService)
        {
            foreach(var instance in service.Instances)
            {
                if ((instance is SPDatabaseServiceInstance))
                {
                    var dbInstance = (SPDatabaseServiceInstance) instance;

                    foreach (var db in dbInstance.Databases)
                    {
                        if (db.GetType().ToString().CompareTo("Microsoft.SharePoint.Administration.SPConfigurationDatabase") == 0)
                        {
                            configDb = db;
                        }
                    }
                }
            }
        }
    }

So after few foreachs and few ifs you have SPDatabase object containing config database. I have tested it against my local SP and it works. I will not exclude there can be some issues in different environment and that there is maybe better way to do this.
Did I mentioned it before that even if you just take a look at hardware responsible for SharePoint database you can end up with some unexpected behavior?
Comment: Nice 1, Op should definately keep the warnings in mind, SP DB's are to be considered "on loan" from MS, they are highly optimized and even a simple select on a highload content db can produce unexpected results and may result in your environment becoming "unsupported"...
Comment: Well ok I can explain, what I'm doing is building a custom health analyzer rule that gauges performance.  What I'm doing is running different benchmarks on SQL Server databases, including even more different benchmarks on the Config Database SQL Server.  Everything is outta the box and I'm not tampering in any way with the content db itself.  This is just to help me gain awareness of the overall environment.  After I've tested this I'll mark for an answer or no :-)
Comment: +1 for the effort, very nice


Does a strong exchange rate always mean a cheap holiday regardless of where you are from and going?

Question: As question title states, does going from one country with a much stronger exchange rate, or a moderately higher exchange rate, in comparison to a country you visit always mean you get more bang for your 'buck'?
Answer: If by "stronger exchange rate" you mean you get more units of the local currency for a dollar (or whatever currency your home country has) then no, not at all. In fact, in economics the term "stronger exchange rate" is not used in this context. People sometimes say that currency A is growing stronger compared to currency B, but this only means that the exchange rate is improving, it does not say if the number is large or small.
If you want to know whether you get more bang for your 'buck' then what matters is purchasing power: how many goods in the foreign country you can buy for the currency units you got for your dollar. Assuming an exchange rate of $E$ units of currency X per one dollar, and a foreign price $p^*$ for some good, you can buy $E/p^*$ units of said good. You can compare this number for different countries to see how far your money will go.
The comparison is made somewhat difficult by the fact that the ratio will probably differ for different goods. Economists usually construct price indices to go around this problem. However no price index is perfect, each one is made to suit a distinct purpose. Basically you would have to look at the price of the goods that you are likely to buy during your stay.


White Paper & MS in Mechanical Engineering?

Question: I am a Mechanical Engineering student & want to pursue Master of Science with specialization in Design/CAD. Recently I have come across white papers on website of some companies about their products. My friend told me that publishing a white paper would be beneficial for my MS applications. So I wanted to know:
1. What exactly is the scope of a white paper?
2. What sort of topic/information must one put in the paper?
3. How to publish a white paper (I mean is there a Journal for white papers or any other standard)?
4. How much effect does it have in showing my research aptitude compared to a scientific publication/ research paper?
Answer: In the research field a white paper is not so important. However, if you plan to work in an applied field,a white paper is valued highly because it shows you are aware of the current issues in your field. In general, white papers are issue-oriented.
For example, in the field of architecture "green" building materials are valued over traditional. But why? A white paper could present facts on "green" building materials versus traditional materials in terms of cost and performance. Other factors could also be weighed such as customer preference. Therefore, when the executives of a firm read the white paper it could help them make a major decision on materials they will buy in the future.
See below the definition of white paper:

White papers in business-to-business (B2B) marketing
Since the early 1990s, the term white paper has been applied to documents used as B2B marketing or sales tools. Far more commercial white papers are now produced for B2B vendors than political white papers for governments[citation needed].
B2B white papers are long-form content designed to promote the products or services from a specific company. As a marketing tool, these papers use selected facts and logical arguments to build a case favorable to the company sponsoring the document. B2B white papers are often used to generate sales leads, establish thought leadership, make a business case, or inform and persuade prospective customers, channel partners, journalists, analysts, or investors.
Many B2B white papers argue that one particular technology, product or method is superior for solving a specific business problem. They may also present research findings, list a set of questions or tips about a certain business issue, or highlight a particular product or service from a vendor.
Comment: Please do not sign your posts, especially not with a link to your website included; every post you make is already linked to your account. [Self-promotion is frowned upon on the Stack Exchange network](http://academia.stackexchange.com/faq#promotion). Just stick to the relevant material in each post, and put a link to your website in your profile page if you want.


Why does an A4 canvas look larger on the screen at a higher resolution?

Question: In Photoshop (and some other drawing programs), when setting the canvas size in mm or inches for printing intent, the canvas appears larger on the screen at a higher resolution. A4 @ 300 ppi appears larger than A4 @ 72 ppi, and the exact same brush size appears smaller on the former than on the latter as a result. Why is that? I guess because the pixel size is fixed, the mm or inch metrics have to change accordingly to the resolution, but does that change occur only on the screen, or does it also affect the dimension of the print in any way?
Answer: It's because . . .
A4 at 72ppi is 595px x 842px
A4 at 300ppi is 2480px x 3508px
The number of pixels in the width/height is determined by number of inches multiplied by the PPI.  A4 is 8.27" x 11.69"
So basically, an A4 at 72ppi has a smaller canvas size in pixel dimensions than one at 300ppi. So if your brush is a specific size, it's going to look bigger on a smaller document.
Edit, to clarify:
All that matters in raster images ultimately is the number of pixels.  You can think of a higher PPI as like a conversion factor used to cram more pixels into a smaller space - i.e. more pixels per inch.  The PPI alone doesn't actually affect the number of pixels. It only does so when used in combination with a physical size unit, measured in inches, when used to work out the size of a document in pixels, and the size it will be output at in print.
For images that are to be displayed on a computer screen, the PPI is irrelevant. Images are generally displayed at the native screen resolution of the device being used, assuming no zooming/scaling is taking place.
Comment: YOu should probablyt add that "Computers dont actually care about you mm values. All they and you should care about is number of pixels. And in tradition of computer graphics 100% means show me the pixels" Or something like that.
Comment: @joojaa I suspected that too. If the final print will retain the same dimension, the difference in appearance on the screen should be irrelevant.
Comment: @joojaa, I've made an edit to clarify that point.
Answer: On 300 ppi a 72 pixel brush will take 1/4 of an inch.
While in 72 ppi it will take one inch. So Photoshop show that difference accordingly.
The length and width of your artwork is fixed. It's either a4 or Letter or else. The amount of pixels change in the file itself. While your screen resolution is set. So an A4 300 ppi will have 2478 pixel in width. While your screen may have 1440. Zooming out to see whole page require program to recalculate what you see. While 72ppi would take 595 pixels. One pixel in artwork using only pixel of your screen can be done without zooming in or out.
SO: It would only affect final size of print IF your printer could choose paper size depending on PPI with stiff set of pixel per inch. SO it would do exactly opposite of what resolution is used for.


Lost Bitcoin-qt password

Question: I have a friend (yes it's actually a friend and not me) who lost his password for their Bitcoin-qt wallet and can't access is Bitcoin.  The wallet is installed on the hard drive of a MacBook and he doesn't know any of the characters   Does anyone know how we can get this coin back without his wallet password?
Comment: If the password is more than a handful of characters long, and your friend has absolutely no idea what some of the character can be. Unfortunately no one can help you. Does he have the private key though?                                     However, if he knows some of the characters of the password. Then famous Dave can help, his link https://walletrecoveryservices.com
Comment: He doesn't know any of the characters.  I assume he's out of luck but I'm curious if anyone has any creative solutions.
Comment: Does he know anything about the password? Length? characters or numbers or both? Parts of the password?
Answer: "Creative solution" would be to forget about his wallet and to join Large Bitcoin Collider project, he might "recover" another lost wallet one day.
Also, if he had 1000 BTC or more in his wallet, some hypnosis-guru might help him to remember his password.
Answer: You keys are stored in file wallet.dat
You can use brute force for wallet.dat

Lost Bitcoin-qt password


Remove member from #general channel on Slack

Question: I tried but there seems to be no way I can remove a member from general channel on Slack, nor can the member himself quit. Any way around there? It's okay if I can delete the channel altogether.
Answer: The #general channel is kind of special in Slack. While you don't have full control over this channel, there are a few things you can do if you don't want to use it the way its intended by Slack:

Rename it to something less important (and then name another channel general)
Restrict post permissions (e.g. by restricting to owners only)

That way you can ignore it and use other channels as your #general channel instead.
I would not delete it (and frankly not sure if you even can), because some Slack apps might rely on its existence.
Comment: @Deepak Sharma If this answer solved the question, please consider marking it as solution by clicking on the checkmark. Thanks!
Answer: Agreed with Erik, seems like you can't remove an user from # general even though you're the owner, in my case a particular user has left my company & we're unable to remove him from # general & # random . So if you remove that user from workspace rather than in every channel, then automatically they'll be removed from those two channels (you don't see their name). You can do that this way :

click on workspace on top left in slack desktop app --> Administration --> Manage members
  --> click on left three dots --> Click on Deactivate account
Comment: This worked like a charm for me
Comment: Perfect. Just what I was looking for
Answer: Might be an old question but I've just managed to do this...

Login to Slack.com using a browser.
Go to Settings & Administration > Manage Members.
Scroll down and find the users you want to remove.
Click the 3 dots to the right of their name.
Select Deactivate Account.
The member/s will be removed from the workspace and all channels.
Answer: We have some staff members who were furloughed and will be coming back, assuming this isn't the end of the world. We don't want them "working" so we are removing them from all channels that are work related (so they can still see our "recipes" channel and our "movies" channels, for example), and because of that I do not want to remove them from the whole Space. The only solution I had was to make work channels "private," and to tell the remaining staff to not post anything in #General until everyone is back.
Answer: If the user is a full member they will be in general channel by default and there is no option to remove.
However, if you update the user to a single or multi channel guest you can remove them from the general channel!
Hope this helps people in the future.


What is the best reason to object a pure theory-theory of mind reading (one that relies exclusively on a folk psychological theory)?

Question: The theory-theory contends that when we attribute beliefs to others in order to explain and predict their behaviour (mind read) we use a a folk psychological theory. What is a good reason to object the use of a pure theory-theory?
Comment: I would say that a standard objection should be that it's not clear what TT is really proposing. If you look at the SEP page, you'll see what I mean from the intro and the list of [questions in section 2](https://plato.stanford.edu/entries/folkpsych-theory/#TheoTheo);
Answer: The theory-theory (or theory of mind) is generally considered a successful (even indispensable) theory throughout history, such as contemporary functionalist Jerry Fodor. The pure TT hinges on the fact that the attribution of mental states to another mind is justified only on folk psychological conceptual level theory without any use of lower-level imaginative simulation. No wonder Fodor endorsed it since he's a strong proponent of the hypothesis of a private internal language of thought (LOTH).
However, a major argument against TT from Simulation theorists (such as Robert Gordon and Alvin Goldman) argue that folk psychology is not a theory, but rather depends on internal simulation of others, and therefore is not subject to Popperian falsification in the same way that most scientific theories are. Also from ST POV when we explain other's intentional states and actions we don't draw on any psychological theories instead we employ our own decision-making capacities and imagination skills, which is fundamentally different from explanation of natural sciences generally.
Another argument from cognitive Eliminativism argues that the common-sense laypeople understanding of the mind is mistaken, and that the neurosciences will one day reveal that the common mental states such as "intend", "believe", "desire", do not refer to anything real. Because of the inadequacy of natural languages, people mistakenly think that they have such beliefs and desires.

Eliminativists argue that discrete and combinatorial characteristics of psychological theory have no place in the neurosciences, which speak of action potentials, spiking frequencies, and other effects which are continuous and distributed in nature. Hence, the syntactic structures which are assumed by folk psychology can have no place in such a structure as the brain.


Can a corp have more than 1 'Chairman Hiro' rezzed at the same time?

Question: Chairmain Hiro: Can a corp have more than 1 'Chairman Hiro' rezzed at the same time?
Comment: @corsiKa, actually Chairman Hiro is a unique asset, so only one can be rezzed (or activated) at a given time.  See my answer for more details.  Also, you should learn Netrunner - it is a fantastic game :)
Comment: I don't think the question is self explanatory. It's self defining. It doesn't explain what you're asking. I don't play netrunner, but when I look at this card, I don't see anything about it that would indicate why it would be limited to a singleton. Can you explain why you have reservations about whether or not you can have multiples, and perhaps the situation that prompted the question?
Comment: I agree with Twitch_City, Android: Netrunner is a fantastic game, more people should learn and play!
Answer: Chairman Hiro is a unique asset (there is a diamond next to the card name designating this status).  While multiple unique cards may be installed simultaneously, only one may be rezzed at a time.
So, if you have two Chairman Hiro's installed, and one is rezzed and the runner runs on it, you could conceivably rez the second Hiro which would cause the first one to trash itself before the runner accesses it (so long as you do this during a legal paid ability window).
Comment: Ah the diamond. That's the key - that's the information that will help future visitors! :)
Comment: Maybe you could add the relevant section of the rule book to your answer to improve your answer further. Nice to see A:NR gets some love here on this site :-)
Comment: Thank you so much Twitch_City; for some reason I am not able to upvote your answer, but I appreciate it a lot. Thank you! =)
Answer: From the Android Netrunner rulebook:

Unique Cards
Some cards have a unique symbol (◆) in front of their title. There can be only one unique card of the same title active at a time. If a card with a unique title becomes active, any other card that shares its title is immediately trashed. This trashing cannot be prevented.

Chairman Hiro has the diamond in front of his name, marking him as unique. You can have multiple copies installed at a given time, but you can't have more than one of those copies rezzed.


Why is my Samsung Galaxy S still on Android 2.2.1?

Question: I have tried by using samsung kies but it says there are no updates. i have even tried to update it within the phone (software update). Other people are on android 2.3 but i am still on 2.2.1 and it won't update
Comment: That's what I've thought. So T-mobile had to pick the latest update, adapt it, and roll it out. I could imagine they ignore such an "old device". Try contacting their service, asking for the update. If they refuse, an alternative would be to have it unlocked (here T-mobile certainly won't refuse), root it (this they won't do, you must do that) and flash a custom ROM. This way you might even get 4.x onto the device.
Comment: Let me make a guess: The SGS was released 2010, has already 3 successors (S2, S3, S4) -- I wouldn't wonder if it would have fallen out of support. But according to [GSMArena](http://www.gsmarena.com/samsung_i9000_galaxy_s-3115.php), official support ends with 2.3. Could you provide a few more details on your device, e.g. if it's branded? Maybe it's your carrier blocking the update (as he had to include his adjustments, i.e. the branding stuff, again otherwise -- which would mean additional work for an "outdated device").
Comment: i bought it from T-mobile on a contract
Answer: As you've got your device with a contract, this added a special barrier to updates. Short explanation how that goes:

Google (or rather the AOSP, Android Open Source Project) releases a new Android version. This is usually immediately available to Nexus devices -- but...
Device manufacturers need to adapt it to their hardware (drivers etc.) and, in many cases, their special "skinning" (HTC Sense, Samsung Tourchwiz, Motorola Blur, etc), which again at least delays availability of this update
Carriers using their special branding (boot logo, special apps, and more)

So if there's a new Android version released by the AOSP does not necessarily mean it becomes available for all Android devices. It might get stuck already in step 2 if a manufacturer decides to no longer support a device (in your case that happened after the 2.3 release, so 2.3 is the latest "official version" available to the device) -- and even if it passes this step, but you've got a branded device, it might get stuck in step 3 (which most likely is what happened to you).
The only way out if got stuck in steps 2 or 3 is checking for custom ROMs. As that "getting stuck" also affects developers, they often decide to build their own ROMs. Many times this is done by porting a ROM from compatible devices, adjusting the missing stuff (or leaving some gaps, so there might be things not fully working, which then is pointed out in the instructions).
So if you want to go the "do-it-yourself" way (after having t-mobile unlocking the device, if needed), check How do I root my Samsung Galaxy S? for how to root your SGS, and Where can I find stock or custom ROMs for my Android device? for fitting ROMs. The "how-to-flash" usually is described on the page you can download the ROM from.


strange log message in cpp-ethereum eth-log

Question: I see a strange message in my eth-log:
[35m⚙^[[97m ◌ ^[[35m07:26:46 AM.561^[[0m^[[30m|^[[34meth^[
[0m Paying 63 szabo^[[0m from sender for gas
( ^[[34m21000^[
[0m gas at 3 Gwei^[[0m )
(sorry for the format, its copied from vi)
it appears often, what could it mean? I have no account associated to my eth. So how that line makes no sense for me,
so far, everything else is ok in syncing process and I'm close at highest block
Answer: seems it was a kind of attack,  but never appears anymore


What is Fermcap-S and what does it do?

Question: Lately I've been doing some very large boils -- 7 gallons in a 7.5 gallon kettle. To prevent boilovers, I've been using Fermcap-s. It's great, and I've had no boilover problems, but I have no idea how Fermcap works or what's in it. What is this stuff? Other than preventing boilover and excessive kreusen, does it have any effect on my beer?
Comment: Here is a link to FDA outlaying the CFR for dimethylpolysiloxane. http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/CFRSearch.cfm?fr=173.340
Answer: Fermcap-S is a silicone (Dimethylpolysiloxane) based emulsion that works by breaking surface tension. It has no effect on the finished product if used in the boil at designated dosage. If used in the fermenter, I have read that it will increase residual bitterness by ~10%. This is likely due to the compound binding to the yeast cells before the dissolved bittering compounds do, thereby leaving more bitterness in the beer. I have not verified this myself.
Stick to recommended dosage, or you will exceed FDA guidelines for silicone content in your beer.
This thread at BeerAdvocate covers the topic: Fermcap-S (login required)
Comment: [interesting...](http://en.wikipedia.org/wiki/Dimethylpolysiloxane)
Answer: The FDA has recently decided that Fermcap S should be filtered before you drink your beer.  Fortunately, there's Fermcap AT which is fine to use without filtering.
Here's some info from Birko, which makes a product very much like Fermcap..

"Brewers should not use silicone-containing antifoam for unfiltered
  beers. The FDA allows active silicone to be used up to 10
  parts-per-million (ppm) but stipulates that the silicone must be
  removed prior to packaging by either filtration or centrifugation. In
  the case of unfiltered beers, use a food grade, non-silicone antifoam.
  We sell a food grade, canola oil based antifoam that works well for
  this purpose and has an added benefit of being yeast-friendly at the
  same time. Look for my article on antifoams in the brewery in the
  July/August issue of The New Brewer. Please contact me directly if you
  would like to discuss this or any other matter further.
Cheers!
Dana Johnson Brewery Technical Representative BIRKO Corporation
  Henderson, Colorado www.birkocorp.com:
Comment: Here is [the regulation](http://www.gpo.gov/fdsys/pkg/CFR-2012-title21-vol3/pdf/CFR-2012-title21-vol3-sec173-340.pdf) that limits occurrence of the active ingredient in Fermcap to 10 ppm.
Comment: Could you link to something that describes this decision?
Comment: I'll try to find it, but it may take a while.  IIRC, it was on the probrewer forum.
Comment: Here's info from a guy at Birko, which makes a product very much like Fermcap...
Comment: So a guy that's selling a competitor to fermcap says don't use fermcap. This is not the most convincing argument. Still, I'll probably give Fermcap-AT a try if I can find some.
Comment: He's speaking of his own product, which is silicone based like Fermcap.
Comment: In the text you posted he's saying "We sell a food grade, canola oil based antifoam." That's not silicone based.
Answer: If a drop of this product were, liberally speaking, 1 mL, then 1 mL / 18.9 L (or 5 gal) = 1/18,900 = 5.291 x 10^-5.  And (5.291 x 10^-5) x 1,000,000 is equal to 52.9 ppm.  This is above the allowable level stated by FDA.  However, if a drop of this stuff is actually much less (eg, water is  approx 50 to 70 µL per drop) than 1 mL, say 0.10 mL, then 52.9 ppm/10 would be almost two times below the allowable level at 5.3 ppm.   I think it would be safe without filtering before packaging the beer.
Comment: See Denny's answer above, which states that the silicone (up to 10ppm) should be filtered, it doesn't say that under 10ppm is ok and can be left in the beer. However, the information quoted seems to have appeared from nowhere and searching for the first sentence shows it's frequently quoted and without attribution to source - so credibility is not certain.
Answer: Don't use fermcap in your kettle . You are better of playing with your temp setting for your boil . You want a slow roiling boil in your kettle . I have been a brewer for over 25 years at a large brewery . Gaining control of your temp setting in your kettle will improve your beer quality over the long term and more consistent flavor

What is Fermcap-S and what does it do?


Software to merge template PDF to many PDF files using a batch file

Question: I need to create a PDF blank template dispatch note with terms and conditions printed on the back of each page.  I need to then merge the template (front of the page) document with many individual PDF dispatch notes and print or email them.  I need to run this job in batch in a job scheduler.
Thank you for your time.
Answer: This is the sort of job that PDF TK Server is perfect for.
It is a free, command line utility for Windows, Mac or Linux.
PDFtk Server can:

Merge PDF Documents or Collate PDF Page Scans
Split PDF Pages into a New Document
Rotate PDF Documents or Pages
Decrypt Input as Necessary (Password Required)
Encrypt Output as Desired
Fill PDF Forms with X/FDF Data and/or Flatten Forms
Generate FDF Data Stencils from PDF Forms
Apply a Background Watermark or a Foreground Stamp
Report PDF Metrics, Bookmarks and Metadata
Add/Update PDF Bookmarks or Metadata
Attach Files to PDF Pages or the PDF Document
Unpack PDF Attachments
Burst a PDF Document into Single Pages


Is Palladium an exception?

Question: I have been taught in school that atoms cannot have more than 8 electrons in the outer shell. Palladium atom's electron configuration is 2,8,18,18. Why isn't it 2,8,18,17,1 like the case of Platinum 2,8,18,32,17,1 or Nickel 2,8,17,1 and they are all in the same group?
Answer: As suggested by M.Sameer I convert my comment into an answer:
Dear M.Sameer: It seems that you are missing that the $n+\ell$  Madelung rule is not an exact result derived from first principles, but rather a rule of thumb, that holds for, say, approximately 95 percent of all the elements, with important exceptions, cf. this wikipedia page. Nevertheless, there are semi-rigorous theoretical arguments why such a rule of thumb should hold to a good approximation.
Answer: all d-block elements will have their last electron in 'd' orbital.
thus even if it is in the 5th period it has no 5th shell as it first fills the 'd' block in 4th shell
Comment: That isn't true. The $5s$ orbital (usually) has a lower energy than the $4d$, so electrons will start filling the $5s$ orbital before they start filling the $4d$ orbital.

Is Palladium an exception?


Watch only Wallet help

Question: i need help. I'm totally New with bitcoin. Last time i would buy something and when i pay i've got an sms-code for bitcoins with that i've to make the pay. I send this to a watch only Wallet. But that was wrong.  What i can do to send it to the right wallet.  I've no pc, only my Handy with the blockchain and bitcoin wallet APP.
Comment: So you sent bitcoin to another watch-only address, do you own that address? If not, you can't spend the funds.
Comment: No. It was the Wallet from the support of bitcoin.org
Comment: Yes i know the adress. What i have to do?
Comment: I asked if you own the address, there's a big difference. If you do not own the address, you don't have the corresponding private key to spend it
Answer:
I send this to a watch only Wallet. But that was wrong. What i can do to send it to the right wallet.

Bitcoin transactions are irreversible. You cannot cancel or reclaim them. Therefore you cannot now redirect those bitcoins to the right wallet.
Watch-only addresses are controlled only by whoever knows the corresponding private-key.
If you know who owns the private-key to the watch-only address, you can ask them to make a new payment to you of the same amount you sent them. You have to rely on their charity.

Related?

Reg: Watch only Address

Watch only Wallet help


Pros and cons of security images

Question: Over the past year or so, I've noticed that all of the banking organizations I work with that have or had "security images" as part of their login process have removed or are planning to remove that feature. Such organizations include Ally Bank, Bank of America, and Amazon Store Card by Synchrony Bank. I thought the purpose of that feature was to make login more secure, so why are they removing the feature? Is there some newly-discovered theory or some such that has made companies feel that this is a feature that should be removed?
Comment: Actually, many institutions have removed site images due to pending litigation by patent trolls.
Answer: One of your link refers to SiteKey (BOA): actually there is an interesting study on it conducted by Harvard University and which I recommend you to read The Emperor's New Security Indicators: An evaluation of website authentication and the effect of role playing on usability studies.
First, this technique was seen as a  defense-in-depth strategy intended to mainly help  foil  phishing attacks. Nevertheless, studies as the one we mentioned, show this technique is a worthless measure as it is less effective than one might think.
Actually, the above empirical study, for example, proofs that  users have been attacked with two types of attacks: phishing attacks and MITM attacks (SSL stripping).
The study above was performed on 3 groups of users:

Groupe 1: usres perform banking tasks using their credentials on their own banking web site
Groupe 2: users perform banking tasks every Sunday afternoon
Groupe 3: similar to Group 2 but users were taught additional instructions to behave securely.

Researchers (who played the role of an attacker), offered login pages with security images where https:// is substituted by http://: all (the three groups)  users typed their credentials.
When the attackers removed away the authentication images were removed and replaced them with warning messages such as This service is being upgraded, 99% of the users (including the ones who have taught security behaviors) typed their credentials.
Conclusion: as security images are not fulfilling what they are supposed to prevent, some online applications decided to use something else instead.

Pros and cons of security images


Can frequent engine lugging lead to blown head gaskets?

Question: I've heard that prolonged driving at low RPMs (AKA "lugging") can cause head gaskets to blow.  Is that true and, if yes, can you provide a lowdown of the causality?
Comment: Is this really a duplicate? The other question doesn't seem to address head gaskets specifically, nor do its answers.
Answer: I've thought about this a bit and don't see how lugging a motor could blow a head gasket.
I do see, however, how lugging can do damage to other areas of an engine.
That being said, lugging a motor does not give it good volumetric efficiency due to the low RPM's.
Look at this way.  An intake track at high RPM's creates an effect where the mass of the air flowing at a high rate 'overfills' the cylinder with an air fuel charge.  The velocity of the airflow compresses the charge a bit and give the cylinder greater volumetric efficiency where you are stuffing more volume (at the current barometric pressure) into the cylinder than if it was just opened up to the atmosphere in non-running state.
This higher volumetric efficiency would create a larger explosion of air fuel with more force to potentially damage the head gasket.  Not that it would necessarily, this is just in theory.
So, with less than desirable air fuel charge during lugging I am unable to see how it could, specifically, head gasket damage.
Comment: I would expect lugging would lead to excessive pressures, which would consequently increase the amount of force trying to separate the head from the block.  I don't know quantitatively how much effect the excessive pressure would have, but it would certainly seem like it could lead to premature gasket failure in at least some engines.


Android Studios Adb server problems

Question:
I formatted the desktop and i'm try to install the Android Studios version 3.2.1 installation setup was completed but when try to run the project .It show error log like this Unable to detect adb version, adb output:I try to install the SDK manager manually. i try to update the tool with stable latest version 3.3.2 when run this project again show like this . So kindly help out from this issuse
Answer: Hardly It takes 2 days to find the answer and fixed.
Error Reasons happened  for me
**OS some .dll files missing*
*Os get erupted**
I faced this type of error in my PC .So check and download the images file properly and install it.
Now i currently working on the Android studios for the project. I do no the exact reason for the error . But tired this way to Solve this error .
If you want to install the Android studios you have to check  the Android Debug-Bridge is running or not .
It has two methods
Method 1:
ABD is part of the Android SDK and is made up of three components: a client, a daemon, and a server.  Daemon is the server establishing the bridge for Emulator and USB type .
You have to download the Latest SDK manager from the developer.android.com in that there is a folder called platform-tools go inside the folder and see the adb.exe file.
Now open the Command prompt and make the directory where the SDk manager you download.
Example refer the screenshot below
D:cd sdk-tools-windows-4333796\platform-tools(this example)
then u see like this
D:\sdk-tools-windows-4333796\platform-tools>
then u check for the ADB server is running or not
D:\sdk-tools-windows-4333796\platform-tools>  adb start-server (enter into cmd prompt just hit enter)
after few second  it will be display i like this

daemon not running; starting now at tcp:5037
daemon started successfully

above message say about the server is starting .
simply go to the Task manager in the you able to see the adb.exe in the process
then Install the Android studios it will work fine.
Method 2:
Directly you install the Android Studios .After the completion of installation process.
Go to the terminal inside the Android studios just type the command  adb start-server
After few second  it will be display i like this

daemon not running; starting now at tcp:5037
daemon started successfully

Android Studios Adb server problems


Core Data Navigation with Relationship

Question: My Core Data model looks like this:
alt text http://fwdr.org/lg4q
The players relationship of the entity Game is a to many relationship and will hold all the player objects related to that Game entity.
I have a UITableView that looks like this <-- link .
The Table View displays all the games, how would I, when selecting a Game show/push another UITableView to display all the players in the players relationship.
I have already attempted this but to no avail, I think I was on the right track but was plagued by to many errors which I was unable to resolve.
Answer: Just create another UITableViewController subclass, wrap it into navigation controller and push. Make sure to use NSFetchedResultsController to populate your table views. You should pass Game object to second table view controller in order to build predicate for it.
Some code snippets:
Wrapping view controller into navigation in table view controller's tableView:(UITableView *)tableView didSelectRowAtIndexPath: method.
    TagsViewController *tagsViewController = [[TagsViewController alloc] initWithMode:TagsViewControllerModeSelect]; // You can view plain style, I've just created my initializer
    tagsViewController.unit = self.unit; // You can pass the Game here
    [self.navigationController pushViewController:tagsViewController animated:YES];
    [tagsViewController release];

Here is fetched results controller lazy initializer
- (NSFetchedResultsController*)fetchedResultsController
{
    if(fetchedResultsController != nil)
        return fetchedResultsController;

    NSEntityDescription* entity = [NSEntityDescription entityForName:@"StudyUnit" inManagedObjectContext:managedObjectContext];

    NSFetchRequest* fetchRequest = [[NSFetchRequest alloc] init];
    [fetchRequest setEntity:entity];

    NSString *predicateString = [[DefaultsManager sharedInstance] filterMode] == kFilterModeShowAll ? @"explanation != '' AND show == YES" : @"ANY tag.show == YES AND explanation != '' AND show == YES";
    NSPredicate *predicate = [NSPredicate predicateWithFormat:predicateString]; // Here goes your predicate to query players for particular game
    [fetchRequest setPredicate:predicate];

    NSSortDescriptor* sortByWordDescriptor = [[NSSortDescriptor alloc] initWithKey:@"subject" ascending:YES];
    NSArray* sortArray = [[NSArray alloc] initWithObjects:sortByWordDescriptor, nil];
    [fetchRequest setSortDescriptors:sortArray];

    NSFetchedResultsController* controller = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext sectionNameKeyPath:nil cacheName:@"Hmm"];
    controller.delegate = self;
    self.fetchedResultsController = controller;

    [fetchRequest release];
    [sortByWordDescriptor release];
    [sortArray release];
    [controller release];

    return fetchedResultsController;
}

Then just implement NSFetchedResultsController delegates, send performFetch message and voila!
UPD:
Assuming your game object is generally NSManagedObject declared in the second controller:
NSManagedObject *game;

Then you can build predicate something like this:
NSEntityDescription* entity = [NSEntityDescription entityForName:@"Player" inManagedObjectContext:managedObjectContext];
//...
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"game == %@", self.game];

This is just theoretically, can't test it sorry ^^
Comment: @Joshua: Assuming you have(gonna have) the same mechanism for the Game controller and you are using `NSManagedObject` for entity, then in `didSelectRowAtIndex` you can retrieve selected game object by `playerViewController.game = [fetchedResultsController objectAtIndexPath:indexPath];`
Comment: Ah, I see where you're going. I understand the first snippet but with the second snippet with `predicateString ` and `predicate` how do I query for those particular players? Other than that I understand exactly what you are doing, thanks very much!
Comment: Cheers, almost all done. ;)
Just unsure what to do with this code, `tagsViewController.unit = self.unit;` I've changed it to `playerViewController.game = self.unit;` but I am unsure what to replace `self.unit` with.
Answer: In the second tableView since you already have the game object, you really do not need another NSFetchedResultsController.  You can get the players directly from the game object that gets injected into the second view controller and using the relationship display the players.
A second NSFetchedResultsController would only be necessary in this situation if you expected the players to change while the user is viewing that second controller.  Otherwise just walking the relationship is sufficient and will use less memory/cpu.
Comment: You still would not need one because **YOU** (as in the view you are in) are adding them.  A `NSFetchedResultsController` is designed to notify **YOU** that something other than **YOU** has changed the data **YOU** are looking at.
Comment: I see what you mean, but I will need the `NSFetchedResultsController` as I will be adding players within the second controller. Thanks anyway
Comment: Please see my further question on this about accessing `NSManagedObject`'s in a relationship without a `NSFetchedResultsController`. http://stackoverflow.com/questions/3457876/getting-nsmanangedobject-from-a-relationship-without-nsfetchedresultscontroller

Core Data Navigation with Relationship


Why does a balloon spiral in the air instead of moving in a straight line?

Question: When an air-filled balloon is released without its opening tied up, it moves in a circular path rather than a straight line. Why is that?
Comment: I wonder if that would still happen if the balloon was released in a vacuum ?
Comment: interesting thought
Answer: A friend sent this as his explanation and it seems quite satisfactory to me:
For a balloon to fly in a straight line, the direction of the jet of expelled air would have to be in line with the balloon's centre of mass and its centre of drag – the point where the forces resisting the balloon's forward motion are symmetrical
If these two centres don't coincide, the centre of drag should be behind the centre of mass, otherwise stability is compromised. The reason darts and arrows have flights is to keep the two centres in line and ensure drag is at the rear of the moving projectile.
If the balloon's line of thrust does not pass through the centre of mass (which is almost certain) but is in the same plane as the line joining the centres of mass and drag (which
is unlikely), the balloon would travel in a circle in that plane, although the pull of gravity will ultimately force it down to the ground, especially as the air driving it forward expires. However, because these lines generally do not intersect, thrust from the balloon's opening comes at an angle to the plane of the circle, pushing the balloon into the helical, screw-like motion you saw when carrying out the experiment. The thrust of the balloon and the air resistance to the balloon will not cancel each other out in such a situation and so a turning moment is exerted.
Comment: I think you answered your own question. If the center of mass (CM) and the center of drag (CD) are not coincident, the body will try to rotate so that the CM precedes the CD. This is the case in rockets, airplanes, automobiles, boats, even shopping carts. For directional stability they need to be nose-heavy.
Comment: I think that would also answer my supplementary question - in vacuum there would be no drag so the centre of drag criterion wouldn't apply, and the balloon should fly straight.
Comment: @twistor59: in vacuum it would probably fly straighter, but the centre of mass is unlikely to be in line with the thrust, especially if the nozzle moves chaotically as I suspect it does, so the balloon would still not fly straight. You could fly it in a vacuum and borrow my cardboard tube of course :-)
Comment: @twistor59: In a vacuum, the thrust vector would have to go through the CM. Otherwise it would create a torque and the rocket would spin. Real rockets that go in space don't go straight by themselves. They either mount the motors on gimbals so they can steer the thrust vector, or they use auxiliary thrusters to maintain direction.
Comment: Yes, I was forgetting that the tube could be pointing in an arbitrary direction relative to the rest of the balloon
Answer: If you make a cardboard tube, put this into the ballons nozzle and then let go you'll find the balloon goes in a mostly straight line. It probably won't go exactly straight because the balloon probably isn't exactly cylindrically symmetrical, but it will go a lot straighter than without the cardboard tube. I recall doing this in primary school long before I'd even heard of physics let alone learnt any.
Bearing this in mind, I would guess the usual erratic flight is because the ballon nozzle is flexible and the airflow through it is turbulent, so the nozzle orientation changes randomly as the air flows out through it.


Are mosquitoes attracted to blue lights in particular?

Question: I've been confronting the fact since childhood that mosquitoes are attracted to lights, especially blue lights. Now I know that all insects including mosquitoes are attracted to bright light but I never got it verified that if they are specially attracted to blue light or not. Is it true or just another urban myth?
Answer: It is not an urban myth. UV and blue lamps are good insect attractants. They have been used for many decades in insect traps, and these too work for mosquitoes. These lamps are routinely used in ecology at insect sampling sites. In some cases, such as with diurnal moths, yellow lamps can also be good attractants. The reason why UV is applicable is because insects are equipped with three photoreceptors, namely the UV-, blue-, and green-sensitive photoreceptors. Below you can see the spectral sensitivity curves of the genus Apis, which is typical for many insects.

As to why they are good attractants is probably variable across species, because it would depend on the neurological encoding of this information. For instance, some species may use this information to determine night and day, or may be superstimuli for nectar as used by plants to advertise themselves for pollination. Or it may simply be for some other reason.
Comment: Nice answer. +1. However, it could address the OP's question about *mosquitoes* more directly. I'd suggest updating with some specific evidence about mosquitoes if you can find something.


Bad thoughts about Allah and insults

Question: i have been having bad thoughts about Allah S.W.T and the Prophet S.A.W and it hurts me a lot especially when i'm praying and then I feel as if there is no need to pray because of the sin I've just committed.I feel sad and angry with myself and  devils are chained in the Ramadan so i think it is myself insulting Allah because Shaytan is chained , how would he say these things in my head.I try to forget it but it always comes and I feel as if i don't deserve this world and i feel as if ill go to hell because of this evil thoughts i have.I really need help and i feel as if its only me having this problems in my family and i do not want to disappoint them.Please help me
Comment: Also, I might add that many scholars believe devils still have the power to whisper even though they are chained up.
Answer:
"إن الله تجاوز لأمتي ما حدثت به أنفسها"
"God has condoned my nation(Muslims) for what they thought to themselves"

thinking about stuff in your brain isn't a sin, in fact, in some cases, it might be praised to do so. Remember this rule in your brain, without thinking whether a god exists or not, it isn't considered all that "belief".
Sometimes you might think about bad stuff that are actually bad(not useful, useful is like questioning yourself does god exist and thinking of reasons)
and in these cases, you ask God to make the devil go away and try not to think about it, but it isn't sinning.
It is a good thing you feel a bad feeling for thinking about your God, but thinking you shouldn't pray(or do anything for Allah) is the biggest sin and is how devil leads people to the pit of hell! God said:

"وَالَّذِينَ إِذَا فَعَلُوا فَاحِشَةً أَوْ ظَلَمُوا أَنفُسَهُمْ ذَكَرُوا اللَّهَ فَاسْتَغْفَرُوا لِذُنُوبِهِمْ"
"And the ones(mentioning good doers right before) who, when they commit any indecency and wrong against themselves, instantly remember Allah and implore forgiveness for their sins"

Whenever you feel you dont deserve to pray to him or "what is the point" know exactly that it is from the devil to get you astray!
Now to a very important misconception, not all devils get chained in Ramadan, in fact, only the "bosses" of the devils get chained or the ones that are of huge evil(like Satan himself) but normal demons don't, there are Muslim demons, and even non-Muslim demons, the ordinary ones don't get locked up, and even if they did, for every human being since he is borned there is a devil that is stuck to him everywhere he goes, the only time he leaves him is when the human himself dies(causing the devil to die as well) the only exception was Prophet Mohammad(PBUH), God helped him convince his devil and made him a muslim! so don't get that sad, there are devils even in Ramadan, after all, they are a race! Just like human beings are, God wouldn't just lock up a whole race each year for a whole month!
In conclusion, whenever you get bad thoughts try praying for God or reading some Quran and ask God to make the devil go away.

Bad thoughts about Allah and insults


When did Litecoin become "GPU-friendly"?

Question: I've heard recently here that Litecoin became more GPU-friendly. When did this happen, and what were the changes to the protocol to accommodate that change. Also, is there any reason for the change provided by the project developers?
Answer: Nothing in the litecoin protocol changed, only a tool to mine litecoins using GPU was independently developed: the Reaper and caused difficulty to improve to make CPU mining much less efficient (compared to electricity price). Still, the rise of diffculty is not so dramatic as was the case for bitcoin and scrypt algorithm has proved its worth. CPUs can still mine 1-3 LTC/day at the moment.
Comment: Is this still true?  I've been CPU mining for quite some time and haven't received any coins.
Comment: Yes I think so.  My AMD Phenon 965 can do about 22MH/s, which amounts to 2 coins a week.  http://www.litecoinpool.org/calc?hashrate=24&difficulty=78.30955148


How is it Google Maps works without data?

Question: I ran out of data for this month so disabled it. Google maps still works in map view when I'm following my route. How is this possible? Is it that GPS can still work without wifi/data and as long as Google Maps has the map it just moves the indicator along?
Comment: GPS is mainly independent from mobile data (except for satellite info). How would navigation systems on your car work without mobile data? It is exactly as you described: Google Maps caches the map and the indicator moves along your GPS position.
Answer: The GPS sensor itself is a separate entity. It can report your current position with driver software. Google Maps doesn't need a data connection for that.
So where is the map data coming from? Apps cache data. If you've been through the area already, or if the map data has been downloaded, Google Maps will do its best with the map data it has. It won't be able to update you on traffic conditions, as those have to be grabbed in real time, nor will it work for an area for which you don't have cached map data.


How best to accelerate vast accumulation of merits?

Question: If the requisites for attaining the path of a stream-enterer is to accumulate merits and having favourable conditions, what are the best ways to gain merits and having all the necessary conditions to achieve this?
Answer: Ways to earn merits:

Practice the Buddha's teaching.
Practice Dana.
Practice daily meditation and share the merits with all beings.
Support the Sangha or better yet, become a Bhikkhu/Bhikkhuni.

With every wholesome deed one gets closer to Nibbana.
With every unwholesome deed one gains distance to Nibbana.
Answer: Perform the Ten Meritorious Deeds..
1.Charity.(Dana)
2.Morality.(Sila)
3.Mental culture.(Bhavana/Meditations)
4.Reverence or respect.
5.Service in helping others.
6.Sharing merits with others.
7.Rejoicing in the merits of others.
8.Preaching and teaching the Dhamma.
9.Listening to the Dhamma.
10.Straightening one's views.
In addition to that,cultivate the factors for stream entry.
The practices leading to stream entry are encapsulated in four factors:
1.Association with people of integrity is a factor for stream-entry.
2.Listening to the true Dhamma is a factor for stream-entry.
3.Appropriate attention is a factor for stream-entry.
4.Practice in accordance with the Dhamma is a factor for stream-entry.
Also here are some things i've observed from personal experiences:
Giving to someone who has greater sila or meditates generally increases one's merit.
Practising metta increases merit (for me).
Giving or performing any good merit towards a noble person increases one's merit substantially.
Answer: Accumulation of merit will only give favorable conditions. If you are born and able bodied person and the ability to access the dhamma this purpose is already served. If you miss this opportunity the conditions perhap many not be as favorable again for a long time. So practice hard to achieve the final goal. This effort will not be in vain as if you miss the goal in this life your most likely be in a more favorable position to practice and also your past practice will help with help mastering the teachings and technique easily.


starting problem citroen berlingo van

Question: I have a 1999 LX 1.9 citroen berlingo mk1 panel van(M49)
For quite a while the ignition has had to be turned 3-4 times, before the circuit responds, and fires the starter, the igntion sw wiring , and starter solenoid have been replaced, but not the sw itself.
 Yesterday i went to the vehicle, and on turning the igntion switch on, the orange heater plug light came on for a flash second, there was an audible click from under the bonnet nearside, and although the engine turns over, it refuses to start, i have checked all fuses, and cleaned all visible earthing points, any suggestions or help would be appreciated.
Answer: Hard to offer something solid here.
You could use old faithful process of elimination, and disconnect the wiring from the switch, and manually hotwire it. This would prove the switch is contacting adequately.
Failing that, are you sure the battery is ok? Maybe check the battery clamps are tight, clean, and battery posts free of acid corrosion.

starting problem citroen berlingo van


Is authorship justified for performing an experiment 7 years ago, which is published? We just used samples from that experiment for a new analysis

Question: 7 years ago, a mouse experiment was performed, and samples are still available for further analysis. This mouse experiment is already published with a certain readout. Now, we publish a whole different story, and one supplemental figure stems from the samples of that old experiment, with a new analysis. The new analysis was not performed by the experimenter of the mouse experiment 7 yrs ago. Still the experimenter is a co-author on the new manuscript. Is that valid? I always thought in such a case the paper should be cited in which that experiment was performed, and then of course the person who did the new analysis on the old samples should get co-authorship. But not the experimenter, since credits for performing that experiment were already gained in the other publication?
Comment: Irrespective of ethical rules and proper authorship, [see the answer of Peter Jansson](https://academia.stackexchange.com/a/184744/75368), I strongly recommend not getting in to a fight with your supervisor. It leads nowhere good. Don't try to win the battle if it costs you the war.
Comment: I'm confused. Who is suggesting she might be an author of the current paper? is it the original author? A supervisor? Or just a quandary in your group?
Comment: Was the raw data from the original experiment published alongside it?
Comment: Do you know that the original author had no participation in the conceptual development of the new paper? In some fields a brief conversation with them might fully justify authorship.
Comment: Just curious: if the given person is a good friend of your supervisor, how do you know they have not discussed topics related to the new paper? I am discussing ongoing research all the times with friends, and often have useful feedback from them. There are circumstances where I would consider coauthorship for signifficant, insightful discussion or idea from people who have never stepped in my lab (especially that it is free and zero effort on my side). Are you sure you want to die on this hill?
Comment: It sounds like you are using the actual samples collected 7 years ago, and not just performing an analysis of the results published 7 years ago.  Is that the case?
Comment: I think what seems a bit unclear here is the "work" required to obtain the samples. Is this months of work to obtain this samples and they are specific to something (say mouses that went though a specific process) or is this a few hours of work and you could have taken *any other labs*  mouses. In other words, is the *preparation of the sample* "academic work", i.e. rather unique and takes time?
Comment: Yes I do, I am first author on that manuscript. The only thing she did is performing the experiment 7 years ago, so she contributed to sample acquisition. It has nothing to do with the current study, as the data analysis on those samples from the old experiment was done by someone else (who is therefore rightfully a co-author).
Comment: its my supervisor who is also corresponding author on that current paper. and she is good friends with that co-author whose contribution I question.
Comment: The performance/establishment of the experiment is published. And how the samples (which we analyzed again for a different readout for the current study) were prepared.
Comment: Thats true, and while I expect they have discussed at some point the paper (I guess she read it for feedback) I just know that she gave zero "significant" input before writing the manuscript as we all work in the same group and there were lab meetings every week over years where it was possible to give input.
Comment: I do not want to die on this hill. I value peoples opinion here a lot. I just thought it was more common to do what is fair and expected some rules that apply to publish, rules I could apply for argumentations. If the situation is like this, I probably won't fight as its not worth it.
Comment: Yes. Its not a re-analysis. I just took the samples (frozen mouse tissue) and did a whole new analysis on them.
Answer: The co-authorship is not milk: there is no expiration date.
Comment: There is nothing controvercial here. Basic sanitaria. There is no need to explain why one needs to wash hands after each use of a bathroom, and do the explanation after each use.
Comment: Controversial answer! On one hand, it is great when pithy answers cut through all the extraneous facts; on the other, it can be difficult to understand the reasoning. In any case, we will let the voters decide; please continue the discussion [in the chat](https://chat.stackexchange.com/rooms/135957/discussion-on-answer-by-markvs-is-authorship-justified-for-performing-an-experim).
Comment: It looks like you have made your decision and now use SE to justify it. Sorry,  I cannot help.  You are risking a lot. Is this paper worth it? Bye!
Answer: Data reuse is the norm in several fields and encouraged to replicate results and build on findings. Many journals require that data be made publicly available upon publication. It may also be a funding requirement. If all you are doing is reusing data from a previously published work, it would seem surprising that co-authorship could be expected.
Nature Data is an example of a journal whose sole purpose is the reuse of data. Obviously, co-authorship cannot be demanded from reusing data published here. In addition this stack academia question asks about co-authorship for data reuse with helpful responses with regards funding and data availability.
Whilst @markvs answer is both true and incredibly quotable, I would say building on or completing the unpublished work of somebody is not the same as building on their published work. In the first case, co-authorship would seem entirely fair; whilst citation would seem most fair in the second case.
However, I ultimately agree with what various people have said: it isn't worth getting into a fight over. It may be better to just get the PhD and use this as an 'annoying supervisor' story to swap with other academics.
To paraphrase a great expression, 'annoying supervisor' stories are not milk: they have no-expiration date.
Comment: I would like to add that @markvs deserves to be cited for my  joke at the end, but in my opinion does not deserve co-authorship of my answer.
Comment: But in this case the data does not seem to be reused from published work. the raw data was never published, but some observations/aspects about them were. So I think this might actually fall closer to using unpublished data than published. To quote one of the comments "The performance/establishment of the experiment is published. And how the samples (which we analyzed again for a different readout for the current study) were prepared."
Comment: Data reuse is something entirely different than the analysis of physical samples. Physical samples (by their vary nature) are in limited supply and cannot be made publicly available.
Comment: "building on or completing the unpublished work of somebody is not the same as building on their published work" - this is exactly how I feel. I couldn't have phrased better what I actually want to say. Thanks, its good to hear that I'm not the only one thinking that way.
Answer: While I think that authorship is not warranted in this case, you would be wise to submit (meekly) to your supervisor. The answer of Peter Jansson suggests why authorship is unlikely warranted.
But fighting with your supervisor over one paper isn't worth the pain it can cause if it spoils a relationship that you need to get your career started. There will be other, future, papers provided that you don't get sabotaged by a vindictive supervisor who seems to be exploiting students for an external friendship.
Protect your own long term interests. Live to fight another day. Sorry to have to say these things, but students have little recourse to such abuse.
Answer: I agree that this is a bit of a gray area. By the strictest definitions, it's quite possible they do not qualify for authorship. However, your supervisor thinks they do, and it really doesn't cost you anything to include them.
I also think you're likely underselling the contributions of the person who performed these experiments. Collecting data is...well, hard. Even when you are using well-reported basic techniques that seem to be used in labs everywhere, there is a lot of effort to bring any technique "in-house".
The person performing the experiments 7 years ago likely shaped how the data was recorded, the specific procedures undertaken, the choice of reagents and other materials. They may have made your entire project much easier than you realize. In a casual survey of other graduate students in biology, most of us agreed that all the data in our theses, collected over typically 5-6 years, probably could have been done in just 6 months if we had known what we were doing from the start. The other 4-5 years was just figuring it out and working out the kinks. All these things can be thought of as intellectual contributions that don't see the immediate link to your current paper, but your supervisor does. Just citing the older paper doesn't entirely capture the weight of these contributions. It's quite different if their work was done in another lab, and you're citing their overall methodology but still needing to do the work to replicate it in a different lab.
Authorship guidelines typically require authors to have some other contributions, though, besides just collecting data. At a minimum, they are expected to review and approve the final manuscript. My overall advice is that you should insist that they do this added step to be included as an author, rather than arguing they should not be an author by not providing them this opportunity.
It's also worth recognizing that in biology, there is no dilution of your work by including other authors. Often, the middle authors on a paper have made very minor contributions to the final work, limited to one figure of the paper, one verification step in the methods, etc, but nonetheless the paper would be incomplete without their addition. Having one more middle author will not reduce the "credit" you get for publishing the paper as first author.
Comment: This answer hits all the points: the question author is unlikely to be able to judge objectively, they lose very little from acquiescing, fighting with their supervisor over a questionable issue will sour relations necessarily, and, critically, mouse work is *really hard* and should not be undersold.
Comment: In any case, does the OP want an advisor who supports crediting her former group members when the group uses their samples or not?  When OP leaves there also will be projects where he has left samples or results that may be further tweaked into minor supporting pieces in future group pubs. Just because a group member leaves and the current members don't remember him isn't a reason to treat his old samples as 'public domain' within the group.  I think the advisor is showing by example how to do things right. (+1  @BryanKrause answer because it notes that the OP is underselling the prior work)
Comment: I do not know where you get the assumption from that I underestimate mouse work and do not appreciate the work done.
Comment: +1 for pointing out that it can be very hard to judge "old" intellectual contributions. In particular, someone may do an experiment as part of their project and as a side line develop an idea of an additional line of research which may be followed with those samples or data. Thus having an intellectual contribution to the outline of a project that someone else get to work on years later. This would be one scenario where OP doesn't know about an intellectual contribution that has actually happened, but their supervisor does.
Comment: The supplemental figure seemed to require the existing samples prepared by the former student ( so it seems to me the the supplemental figure was not based simply on reanalysis of public data in prior paper.) I assume that asking some other group to send their old samples for reanalysis would include discussion of co-authorship.
Comment: @Carol Probably it would, yes.
Answer: One way to view this issue is to apply the "Vancouver rules" (e.g. as described by the ICMJE,  International Committee of Medical Journal Editors. These recommendations outline what is required to become a co-author on a scientific manuscript. Note the "AND" in the list of requirements. It is clear that not all agree with these recommendations but it is also clear that authorship is given for the wrong reasons in many cases. In your case the authorship could be considered a "honorary or guest authorship" (COPE, Council of Scientific Editors) but would be perfectly normal if the person(s) fulfil(s) the expectations of an author, i.e. provides some intellectual input to the work to be published.
So in the end the case may seem clear but not easy to resolve if co-authors do not agree on the rules.
Comment: Here, would it have been appropriate to give that person the opportunity to fulfill these roles? It is tricky to use the criteria of contributing to the manuscript if the person was not allowed the opportunity to contribute to the manuscript.
Comment: The ICME rules are _terrible_ and deny junior scientists just contribution for their work. No-one should be using or promoting them.
Comment: @Amanda, Please read and try to understand the underlying message of the links I provide. Being a "fan" or not is not an issue, it is whether or not you think the recommendations should be be followed or not from an ethical perspctive that counts. It IS an ethical question in the end.
Comment: @Dawn indeed, the ICMJE rules say that if someone would qualify for authorship under other criteria they must be given the opportunity to meet those related to contributing to and revising the manuscript.
Comment: okay I understand that. But in my case, she did not give any intellectual input. But I am sure that the corresponding author and my supervisor who put this person on the paper, would just claim she gave intellectual input. So it seems like I can only lose here? I always thought there would be specific guidelines for such cases (as citing the paper with the previous experiment and then acknowledging the person, or sth similar)
Comment: @Amanda: It is not a gift. This is a rule in Academia which exists for thousands of years.
Comment: She was allowed to contribute actually, over the course of years. She just didn't.
Comment: I guess I'm just not a fan of gifted authorships ;)
Answer: recontextualizing the problem
Let's say there are three researchers, Alice, Bob and, Carol.  Bob and Carol each design experiments about how a treatment affects mice.  They are looking at two aspects of the same treatment, so the control and treatment groups are the same.  Bob's experiment uses lung tissue, and Carol's uses brain tissue.  Alice is an expert in running this treatment protocol, so she deals with the mice and collects all of the tissue samples.  She gives the appropriate samples to Bob and Carol and lets them take over from there.
Bob and Carol each do the rest of their experiments totally on their own with no collaboration from anyone (even each other).  Then each writes a paper.  At this point I think most people would argue that Alice deserves to be a co-author on both papers, due to her direct contribution.  It doesn't matter if the samples Alice collected sat in a freezer one night or one week or one year.  Alice's work was instrumental to Bob and Carol completing their experiments.
In another scenario Bob doesn't exist.  Anything Bob would have done, Alice did.  Most people would argue that Alice still gets to be a co-author on Carol's paper, due to her direct contribution.  If those samples Alice collected didn't exist, Carol couldn't have gotten started.

Different fields have different standards of authorship, and different individuals have different ideas of how one meets those standards.  People disagree about these things all the time.  In this case I think it helps to look at things from Alice's perspective.  In my own work I tend to err on the side of inclusivity, when disagreements arise.  In the end it will make no difference to your career if you are first author on a paper with five co-authors or a paper with six.
Comment: I don't really understand why your situation is significantly different to the first one with Alice, Bob, and Carol. I'd still say Alice deserves credit. And possibly Bob (depending on what was done to the tissue)
Comment: Well in this scenario I agree. But: in my case the situation is different. They publish as you describe. Then a fourth person, lets name her Karen, takes the lung tissue, analyzes it for a different readout/idea, and publishes that. Who of the other three authors gets co-authorship on Karen's paper?
Answer: If you are feeling pressure to include the original author as an author of your paper and you are considering going along with it, one way to handle the ethics is to actually reach out to that author for collaboration. If they provide even a minor academic contribution (i.e. other than assistance that is purely clerical or administrative), you may be ethically obligated to include them as an author.


The emptiness of the deserts

Question: I have been running around on my Java 1.16.4 world, and I have explored every chunk around the spawn in a 5000 blocks radius, but I have not found any desert. Is it possible that a minecraft world is generated with deserts further from spawn? Should I keep exploring or is there a problem with my world?
Comment: There are no know problems with deserts.  You could use a software like Chunkbase to find the nearest one.  Otherwise, keep exploring!
Comment: @valkyrie_pilot That's an answer, not a comment.
Answer: It's entirely possible - the biome map is generated with accordance with temperature map, which often spawns very large distances. You may see thousands of blocks of taiga, snow plains, ice spikes, frozen ocean, extreme hills, snowy taiga, mountains, giant tree taiga, with occasional birch forest, and not a sniff of a desert, a jungle, a mesa or anything like that, because the whole expanse is determined 'cold' and only generates cold biomes. It's very rare to see a cold biome neighbor a warm biome. Or you can travel for thousands of blocks and never see a splotch of snow outside tips of mountains.
2000 blocks isn't much. Scout further, exploring mostly to warmer biomes - savanna, mesa, jungle, swamp, plains, regular (mixed/oak) forests.
This is a map of a server, nothing very special about the seed choice. The area is about 10,000 x 10,000. Observe the upper-right quadrant of the map. That's a 5000x5000 area without a single desert. Lower left also has hardly any. Upper left - savannas, deserts, mesa, and just a splotch of snow in the far corner.
Answer: There are no know problems with deserts. You could use a software like Chunkbase to find the nearest one. Otherwise, keep exploring!

The emptiness of the deserts


Is Dan Davis from World Record one of the first Ones before Neo?

Question:

The Architect says something about the Neo's predecessors but gives no explain about them. Was Dan Davis one of the first Ones? He also learned to see the code, woke himself up, and bent the space of the Matrix (for example, he seems to fly a bit). But was he really the One or simply, as the movie states, someone with exceptional perception?
Comment: He didn't wake up. That was entirely the point
Comment: To be clear about one point, the previous One is not known to any existent Zionese in the Matrix films. With every new One, Zion (whatever that constitutes at the time) is wiped out and a few humans are selected to live outside the Matrix, until that dynamic reaches a breaking point. The Matrix gets reset. The only person who could possibly know about this pattern is The Architect, and Neo once he's told. Given what we know about the Matrix version Neo was a part of, Dan Davis probably exists in the same version as him.
Comment: I dissent from your idea, I think he did wake up, but he did not exit from the pod he was inside
Comment: Thats a good view o justify he wasn't
Answer: Dan isn't the One. He's simply someone who gains exceptional perception, resulting from his enormous mental exertion. As explained at the start of the short, he's not one of those who reject the Matrix (like Morpheus and Trinity) and he's certainly not one of the Oracle's 'potentials'. He's something else.

VO: Only the most exceptional people become aware of the Matrix. Those that learn it exists must possess a rare degree of intuition,
  sensitivity and a questioning nature.
However, very rarely, some gain this wisdom through wholly different
  means.
This man is one of those few.

We see a very similar set of circumstances in the Matrix Webcomic "Sweating the Small Stuff". A small time hoodlum named Dez, fuelled by a combination of good drugs and bad stress begins to sense that something is amiss.
Comment: Whoah! its a nice explain, I agree that not everyone aware of the matrix is a potential, (as when Morpheus teach Neo the first stages of a simulation), but Im not totally sure Dan is not.
Answer: No. The function of the One requires that the One meet with the Architect and make the choice between rebooting the Matrix or causing the extinction of the human race (by allowing both the Matrix and Zion to be destroyed). All of Neo's predecessors met with the Architect and chose to reboot the Matrix. Dan Davis did not escape from the Matrix much less meet with the Architect to make the choice of the One, so he could not have been one of Neo's predecessors.
Although Dan Davis can bend the rules of the Matrix, so can any red pill (Morpheus, Trinity, Cypher, etc.). The One is able to bend and break the rules of the Matrix more than other red pills, but this is only a difference in degree. The most unusual behavior exhibited by Dan Davis was a short self-substantiation (when he saw his pod in the real world without the use of a red pill), but he's not the only one to have done it -- the Kid also self-substantiated but the Kid was not the One because Neo was the One at that time.


Best way to seal off the rear of an air conditoner to prevent dirt and bugs from entering?

Question: I'm trying to figure out if these is feasible firstly. I have a frigidaire 8,000 BTU window air conditioner for my bedroom and I would like to know if the rear of the A/C, ie. (where the heat sink, exhaust fan are located on the back - the half of the A/C where it is on the outside of a window not the inside) can be sealed off to prevent water damage/bugs/dirt from entering in. I'm not sure why this part is exposed to the elements because it needs to be cleaned. Any idea if this area is possible to seal off with something like clear tape?
Comment: NO, it is designed to be open. The fan needs to pull copious amounts of outside air from the open louvered sides and blow it across the condenser, it's needed for effective heat exchange.
Comment: Do you mean seal around the window to keep bugs from coming in the house? It sounds like you want to seal up the AC vents, which is not okay... but you should have a tight seal between the AC and window.
Comment: Sealing around the window is fine. That is taken care of with the materials provided with the AC. I'm mentioning the back area that is majority (ie. 70-80% of the AC) is exposed outside.
Answer: You cannot seal the condensing coils. The back is where the heat exchange occurs that allows your AC unit to get rid of the excess heat. You can add a external screen or filter media to reduce dust & bug buildup but this will require regular cleaning. If your system has an vent option to bring in outside air only a small amount of outside air is brought in through the evaporator coil and cooled. Window units should have a slight tilt down in the back to make sure the condensate can drip out the back of the unit this is normal. The slight tilt allows the condensate to drip out the back and any rain water that is on the unit to drain and not enter the home.
Comment: So the heatsink in the back still has small slits of space and is still exposed completely outside. How would sealing off the sides of the vents/louvers affect heat exchange?
Comment: The slits on the side is where the air is drawn in for the condensing coil. If the air flow to these is reduced the unit will over heat. I have not had very good luck with newer R134 units that have overheated. Once these overheat even with a complete removal of the Freon and recharge they don't work well. If that happens on the small units like yours we replace them.
Comment: Interesting. Why do you think that even if it overheats it won't work well after?
Comment: I am not sure if the newer compressors are the problem because the pressures are higher. The Mill lost 6 or 7 last year that were less than a year old (very warm location) I tried to recharge 3 and none of them worked well compared to R22 they do fine with a simple recharge.


Normal Fuchsian subgroup of $PSL(2,\mathbb{R})$

Question: I've been working with Fuchsian groups and from geometrical motivations finding a cocompact normal Fuchsian subgroups of $PSL(2,\mathbb{R})$ would have intresting properties for my research.
It is known that $SL(2,\mathbb{R})$ has no connected normal subgroups other than the its centre { Id,-Id }, however it might have discrete normal subgroups. I been working with quaternion generated cocompact Fuchsian subgroups which are in addition purely hyperbolic, all its elements have Trace > 2, and I couln't find any normal subgroups of $SL(2,\mathbb{R})$ there, I suspect that I have to include some elliptic elements to improve my chances.
To add up it possible for $PSL(2,\mathbb{R})$ to have discrete normal subgroups? how about cocompact normal subgroups?
Note: Notice that I'm not talking about normal subgroups of Fuchsian subgroups.
Answer: $PSL_2(\mathbb{R})$ is simple, so it has no non-trivial proper normal subgroups.