Cannot remove hashset in java
Why is a ConcurrentModificationException thrown and how to debug it
Foreword  This question was inspired by initial mistakes in . I wanted to explore the strange circle with $A&gt;\pi r^2$ and got lost into geometrical jungle.  A  is usually described by it's height $h$, radius of the base $a$ and radius of the sphere $r$. Besides we have a relation $a^2+(r-h)^2=r^2$ so the cap actually only has two degrees of freedom.  When considering a case of circle on a sphere (like the Earth), it's the surface of a spherical cap, but we can't see the height or other classic parameters of this spherical cap. Instead we encounter parameters like area $A$, perimeter $C$ and diameter $d$. I will try to write formulae for these: $$A=2\pi rh$$ $$C=2\pi a$$ $$d=2r\theta$$  Where $\theta$ is introduced with $\sin\theta=\frac{a}{r}$ or $\cos\theta=(1-\frac{h}{r})$.  As the cap only has two degrees of freedom, it should be a good problem with unique solution if only two values are known.  A-C Problem  The simple example: $A$ and $C$ are given. I show this only as example before the other tasks.  We can easily find $a=\frac{C}{2\pi}$ and then solve:  $$A=2\pi rh = 2\pi r (r-\sqrt{r^2-a^2})$$  to obtain  $$r=\frac{A}{2\sqrt{\pi(A-\pi a^2)}}$$  Parameters $a$ and $r$ are now known and it is straightforward to find $h$, $d$, $\theta$ and anything else.  C-d Problem  $C$ and $d$ are given.  First step once again is easy: $a=\frac{C}{2\pi}$. The knowing of $a$ suggests that we might prefer the $\theta$ definition with the sine. We obtain: $$d=2r\theta=2r\arcsin\frac{a}{r}$$  That's where I got stuck. Is there an easy way to solve this? Is there a hard way to solve this?  I tried solving numerically with Wolfram Mathematica and it went into complex numbers. When solving the geometrically equivalent $\sin\frac{d}{2r}=\frac{a}{r}$ I got some roots, but the search seemed very unstable and not finding the root closest to initial value. Besides it was finding a lot of different roots while I am pretty sure that there should only be one $r$ for each pair of $d$ and $a$. What would be the right formulation to solve this problem numerically?  A-d problem  $A$ and $d$ are given. This seems the hardest one. We obtain a system of equations:  $$A=2\pi r h$$ $$d=2r\arccos\left(1-\frac{h}{r}\right)$$  (The first equation suggested solving the problem in $h$ and $r$).  Is this system solvable analytically? Or numerically? My experiments with Wolfram Mathematica never converged.   Of course, I could express $h$ or $r$ from the first equation and leave only the second in one of these forms: $$d = 2r\arccos\left(1-\frac{A}{2\pi r^2}\right)$$ $$d = \frac{A}{\pi h}\arccos\left(1-\frac{2\pi h^2}{A}\right)$$  But I have no idea how to solve these either. I have a simple query:  select * from countries   with the following results:  country_name ------------ Albania Andorra Antigua .....   I would like to return the results in one row, so like this:  Albania, Andorra, Antigua, ...   Of course, I can write a PL/SQL function to do the job (I already did in Oracle 10g), but is there a nicer, preferably non-Oracle-specific solution (or may be a built-in function) for this task?  I would generally use it to avoid multiple rows in a sub-query, so if a person has more then one citizenship, I do not want her/him to be a duplicate in the list.  My question is based on the similar question on .  UPDATE: My function looks like this:  CREATE OR REPLACE FUNCTION APPEND_FIELD (sqlstr in varchar2, sep in varchar2 ) return varchar2 is ret varchar2(4000) := ''; TYPE cur_typ IS REF CURSOR; rec cur_typ; field varchar2(4000); begin      OPEN rec FOR sqlstr;      LOOP          FETCH rec INTO field;          EXIT WHEN rec%NOTFOUND;          ret := ret || field || sep;      END LOOP;      if length(ret) = 0 then           RETURN '';      else           RETURN substr(ret,1,length(ret)-length(sep));      end if; end; I am trying to prove:     $a\mid b \land b\mid c \Rightarrow a\mid c $   $a\mid b$ means that a divides b if there is an integer k, that $b=k\cdot a$  Please give me a hint on how to start, because I have no idea. Will my 320W PSU handle a Asus GTX 750Ti I've got a 320W PSU as far as I know and it's a Chinese PSU because I've seen the Chinese letters on it. I just had a question if I can upgrade my pc to Asus GTX 750Ti 2 GB OC.  My pc specs are:   4GB DDR3 RAM Intel Core 2 Duo E8400 3.0 GHZ 250GB HDD Onboard graphics card I've always wondered why the number of protons in the Universe exactly matches the number of electrons. They are such different particles with totally different cross sections. So, first of all, is this true? And, secondly, how could it be that they have been produced in the same amount after the Big-Bang? I've recently read the Yahoo manifesto . They recommend to put the JavaScript inclusion at the bottom of the HTML code when we can.  But where exactly and when?  Should we put it before closing &lt;/html&gt; or after ? And above all, when should we still put it in the &lt;head&gt; section? Solve $\sin(z) = z$ in complex numbers Show that $\sin(z) = z$ has infinitely many solutions in complex numbers. Little Picard theorem should help, but using big Picard theorem is undesirable. Thanks a lot! The meaning of imaginary time What is imaginary (or complex) time? I was reading about Hawking's wave function of the universe and this topic came up. If imaginary mass and similar imaginary quantities do not make sense in physics, why should imaginary (or complex) time make sense?    By imaginary I mean a multiple of $i$, and by complex I mean having a real and an imaginary part, i.e., $\alpha + i\beta$, where $\alpha, \beta \in {\mathbb R}$. Why does a query run slower in a Stored Procedure than in the Query window? Why does camera this tracking fail completely even when the solution value is very low? The attached animated gif shows some frames of a video with a mesh box placed at each tracking marker used to successfully track (solution error &lt;.4) an 800 frame chunk of video.   The point is to illustrate just how poorly the camera tracker calculated: when the box is at a marker placed at the top of a faraway building on the left, it is actually visually much larger than when it is at the tracking marker closest to where the real camera was. I went to great lengths to be sure that the camera's sensor size and focal length are correct, and manually placed and followed all markers to be sure they were accurately tracking.   There is not a lot of parallax motion over the 800 frames, but there should have been enough to at least know that the point closest to the camera moved more than the point furthest away. Sometimes it seems to work, sometimes it doesn't. Does anybody know how to improve this? I've got a capture the flag arena. One issue we keep running into during play, is that of clutter.  We have /gamerule keepInventory false, so the pitch gets rather cluttered with armor, weapons and building materials, such that it becomes difficult to pick the flag up, due to full inventories.  I've discovered the following command to remove all drops from the arena:  /kill @e[type=Item]   However, this removes all the floating items from the ground. I'd like, if possible, to preserve the flag from being destroyed.  I have this command for detecting the flag:  /testfor @e[type=Item,x=40,y=1,z=40,dx=40,dy=27,dz=80] {Item:{id:"minecraft:banner",Damage:4s}}   But I'm not sure how to combine the two in a way that it will destroy everything except for the flag.  Any ideas? Profile changes in Q&A are not pushed to Area 51 What's the simplest way to call Http GET url using Delphi? There's a web services I want to call in my application, I can use it with importing the WSDL or by just use "HTTP GET" with the URL and parameters, so I prefer the later because it's simple thing.  I know I can use indy idhttp.get, to do the job, but this is very simple thing and I don't want to add complex indy code to my application.  UPDATE: sorry if I was not clear, I meant by "not to add complex indy code", that I don't want add indy components for just this simple task, and prefer more lighter way for that. Does it mean neither a muslim nor a pakistani anymore? Or it means muslim and ex pakistani. I will be very thankfull if you people kindly elaborate this for me. How to add custom C++ keywords to be recognized by Minted? I have defined several structures and types in a C++ project. I have to write some documentation for this code, and I use  to typeset my code.   I would like to be able to add some keywords to the minted database, so minted will also recognize these and give them the appropriate color. (I would like to have them colored in the same way a type, eg. double would be colored.) Can someone please verify my proof?     Is the collection $\tau_\infty = \{U:X-U$ is infinite or empty or all of $X\}$ a topology on $X$?   No. Let $X = \mathbb{R}$.  Clearly, $\{x\} \in \tau_\infty$ for all $x \in X$.  Set $\displaystyle{B = \cup_{x \neq 0} \{x\}}$. Clearly, $X - B = \{0\}$, but $\{0\} \notin \tau_\infty$. Since $\tau_\infty$ is not closed under arbitrary unions, it does not define a topology on $X$. I have a question about notation specifically square brackets $[$ and round brackets $($.  My textbook doesn't explain any of this and I cannot find a reliable source online to confirm the difference.  So my question is: What is the difference between round brackets and square brackets in terms of notation in Field Theory? For example, I see $F(x)$ and $F[x]$ in my textbook and I've always assumed they were the same thing. But apparently they're not. Is there ever a time they're the same?  I wanted to know the difference, it may be a silly question but it's something I want to make sure I understand. I am thinking about a wizard who has a rat familiar (from the  spell), and has the  spell prepared. Can he use his familiar's senses to teleport into another room (where the familiar is hiding) by looking through the eyes of the rat and casting misty step?  The wizard could use his action to switch to the familiar's senses and then use his bonus action to cast the spell. This would greatly reduce the "blind and deaf" time he experiences from doing this. When overriding equals in Java, why does it not work to use a parameter other than Object? I ran into an interesting behavior recently.  It seems that if I override .equals() to take a parameter other than Object, it doesn't get called.  Can anyone explain to me why this is happening?  It seems to violate my understanding of polymorphism in OOP, but maybe I'm missing something.  Here's much simpler code that shows what I'm seeing:  public class MyClass {   private int x;   public MyClass(int n) { x = n; }   public boolean equals(Object o) { return false; }   public boolean equals(MyClass mc) { return x == mc.x; }   public static void main(String[] args) {     List&lt;MyClass&gt; list = new ArrayList&lt;MyClass&gt;();     list.add(new MyClass(3));     System.out.println("Contains 3? " + list.contains(new MyClass(3)));   } }   When this is run, it prints "Contains 3? false".  It looks like the equals(Object) function is called, even though there is another that would work.  By contrast, if I write equals like this the code works as expected:  public boolean equals(Object o) {   if(!(o instanceof MyClass))     return false;   MyClass mc = (MyClass)o;   return x == mc.x; }   Why isn't it figuring out which version of the function to call based on the type of the parameter? How can I mount the /system directory rewritable or read-only on my Android phone?