I saw an explanation of the transistor working  by Bill Beaty. I absolutely loved it. But I'm not entirely sure why the action cannot be achieved by two diodes connected back to back by a metal piece in between. Here's what I'm thinking, Metal wire obviously has large number of charge carriers in it so any electron in base that accidentally enters the metal will cause a small current at the other end of the wire by pushing an electron into the collector region just like it happens in case of transistor. Where did I go wrong? And where does the mechanism fail exactly?
Another form of the question is: will joining two diodes with wires (pn-np) make a transistor-equivalent?  I read that they are not equivalent, but why?
There is (there's) vs.There are What are the roots of the creeping usage of "there's" for both singular and plural predicates? (This seems to be more common in spoken English.) I have 2 theories.  Perhaps it is because spoken communication our society is becoming less formal by the day. Or perhaps it is a reflection of speakers of other languages having increasing contact with native English speakers.  I am aware of German  "es gibt" and Spanish  "hay" constructions. Please comment.  3/06/14 I ran a couple of Google Ngrams and was surprised to find a greater rise in the use of "there's" in English fiction than in general English language books, since the turn of the century.  This would seem to support my contention that informality is behind the shift. Story about a time-traveler living backwards in a capsule I'm hunting for the title of a short story where a senile old man appears in a capsule in a laboratory.  They eventually realize time is passing backwards for him. After years he helps them build the machine he's in, and he changes places with the him that has been aging normally in the timeline. How to resolve a Java Rounding Double issue Seems like the subtraction is triggering some kind of issue and the resulting value is wrong.  double tempCommission = targetPremium.doubleValue()*rate.doubleValue()/100d;   78.75 = 787.5 * 10.0/100d  double netToCompany = targetPremium.doubleValue() - tempCommission;   708.75 = 787.5 - 78.75  double dCommission = request.getPremium().doubleValue() - netToCompany;   877.8499999999999 = 1586.6 - 708.75  The resulting expected value would be 877.85.  What should be done to ensure the correct calculation? How is Mono magical? I'm learning C#, so I made a little C# program that says Hello, World!, then compiled it with mono-csc and ran it with mono:  $ mono-csc Hello.cs $ mono Hello.exe Hello, World!   I noticed that when I hit TAB in bash, Hello.exe was marked executable. Indeed, it runs by just a shell loading the filename!   Hello.exe is not an ELF file with a funny file extension:  $ readelf -a Hello.exe readelf: Error: Not an ELF file - it has the wrong magic bytes at the start $ xxd Hello.exe | head -n1 00000000: 4d5a 9000 0300 0000 0400 0000 ffff 0000  MZ..............   MZ means it's a Microsoft Windows statically linked executable. Drop it onto a Windows box, and it will (should) run.  I have wine installed, but wine, being a compatibility layer for Windows apps, takes about 5x as long to run Hello.exe as mono and executing it directly do, so it's not wine that runs it.  I'm assuming there's some mono kernel module installed with mono that intercepts the exec syscall/s, or catches binaries that begin with 4D 5A, but lsmod | grep mono and friends return an error.   What's going on here, and how does the kernel know that this executable is special?    Just for proof it's not my shell working magic, I used  (aka sh) to run it and it still runs natively.    Here's the program in full, since a commenter was curious:  using System;  class Hello {   /// &lt;summary&gt;   ///   The main entry point for the application   /// &lt;/summary&gt;   [STAThread]   public static void Main(string[] args) {     System.Console.Write("Hello, World!\n");   } } Do we use the SD of whole population or SD of just one sample to calculate SE of samples means in central limit theorem? I'm learning applications on Central Limit Theorem and got really confused with a few points. According to this , the procedure to apply CLT usually goes like this:   So if SD is the population standard deviation, how are we gonna get it?? Isn't the whole population standard deviation what we eventually calculate by applying CLT and analyzing a sample of the whole population? How come the population standard deviation become a prerequisite??   Please tell me this tutorial is wrong.   I think the SD actually refers to the standard deviation of a sample (of some size n), which we can actually get easily. For example:    sample the whole population with a sample size of n (e.g. randomly select 10000 users from the whole population of 10 billions) calculate the mean of the 10000 measurements. calculate SE = SD / sqrt(10000), where SD is the standard deviation of the 10000 measurements, instead of the standard deviation of the whole population of 10 billions.    This explanation would make much more sense.   Any thoughts? A toplogical space $X$ is said to be second countable   if there exists a countable basis for the topology.      $X$ is separable if there exist a countable dense subset.   Show that a second countable space always is separable.      Give an example of a space that is separable but not    second countable   The first part was easy and i managed to solve it on my own  Assume that $X$ is second countable and let $B_1, B_2, B_3...$ be a countable basis. Construct the set $A$ by taking one element $x_n$ out of every $B_n$. Then $A$ becomes dense because every open set $U$ contain a $B_n$ which in turn contain the element $x_n$ in $A$.    Again, like in the question i posted yesterday, i had trouble to come up with an example but our professor gave an example during the last lecture but i dont fully understand it:  "For an example of a separable but not second countable space let $X$ be the real line provided with the cofinite topology, i.e. where a set is open if its either empty or if its complement is finite.   Then every infinite set is dense and hence $X$ is separable.  But for an arbitrary countable family of open sets $U_1, U_2...$ write $U_i = X$\ $A_i$, where $A_i$ is finite, there is an element $x \in X$\ $(\cup_i Ai)$, because $A_i$ is countable,  such that the open set $X$\ $\{x\}$ is then not able to contain any $U_i$. Consequently $X$ is not second countable."  We asked him to explain it in more detail but his english is poor and nobody understood anything...:)       Def: A subset $A$ of a topological space $X$ is dense in $X$ if for any point $x$ in $X$, any neighborhood of $x$ contains at least one point from $A$.   $1.$ "Then every infinite set is dense"  Take $A$ to be one of these infinite sets. If $x \in A \subset X$ then it trivially fulfills the definition. But if we take a $x \not\in A$ why is it not possible to find a neighborhood in these finite sets that does not contain an element in $A$?  edit: its obvious if it is only open neighborhoods, open sets in the topology, but that is not specified in the definition.  $2.$ I am not fully understanding this either  "But for an arbitrary countable family of open sets $U_1, U_2...$ write $U_i = X$\ $A_i$, where $A_i$ is finite, there is an element $x \in X$\ $(\cup_i Ai)$, because $A_i$ is countable,  such that the open set $X$\ $\{x\}$ is then not able to contain any $U_i$. Consequently $X$ is not second countable."  Is it possible to express this in a simpler way? Alternatives to Dmitri Bertsekas' Introduction to Probability and Sheldon Ross' A First Course on Probability I have Ross' A First Course on Probability and Bertseka's Introduction to Probability book. However these two books do not exactly give me what I am looking for. The problem is  Bertsekas' book is introductory, while Ross' book explains expression $E[\ E[X\mid Y] \ ]$ just by briefly saying it is a function of $y$ if I remember correctly. I need a probability book which will make me feel comfortable with the symbols and letters rather than ready made formulas. I want to be able grasp when a random variable is a constant or when it is not. I want  to be able to read and understand equations involving both random variables and constants and when are interpreted as a rv. when they are not etc. For example block cipher AES-128, Key size is 128bit and it is used to make a 10 round key which is total 320bit.  Question 1. If i use another Key schedule algorithm in AES, then security decreased or remain same? In other words, if the key schedule algorithm of AES has some relationship with AES encryption itself, then it will be decreased, or it will be same if the new key schedule algorithm is safe enough.  Question 2. Why we expand key? brute force will take O(2^128) time regardless of the key schedule is strong or not. what is the advantage of making key expend? I was curious about this when I was learned about 3DES. Which developed because the former DES has weak key length 56bit so vulnerable to brute force attack, i was wondering why doesn't they just remove key schedule process and use the whole round key as a key. Of Half-elven and mortal life Can the jQuery UI Datepicker be made to disable Saturdays and Sundays (and holidays)? I use a datepicker for choosing an appointment day.  I already set the date range to be only for the next month.  That works fine.  I want to exclude Saturdays and Sundays from the available choices.  Can this be done?  If so, how? Non-blocking Synchronous AJAX How to install a "pdf printer"? Is there a way to install a printer that prints to pdf?  There's a lot of discussions about this in the web foruns, but none seens to work. How to evaluate $\int_{0}^{+\infty}\exp(-ax^2-\frac b{x^2})\,dx$ for $a,b>0$ Deleting an element from an array in PHP Create taxonomy terms on the fly Avoid trailing zeroes in printf() I keep stumbling on the format specifiers for the printf() family of functions.  What I want is to be able to print a double (or float) with a maximum given number of digits after the decimal point.  If I use:  printf("%1.3f", 359.01335); printf("%1.3f", 359.00999);   I get  359.013 359.010   Instead of the desired  359.013 359.01   Can anybody help me? Serializing and Deserializing Expression Trees in C# Is there a way to Deserialize Expressions in C#, I would like to store Expressions in a Database and load them at run time. How to get text under mouse in any open window I am looking for an application which can do the following:   Resize one or more images Compress images Rotate and flip images Rename multiple images using a progressive number or a prefix/suffix Convert an entire PDF file to a bunch images Extract an image from a Windows .ico file Convert images to DPX, EXR, GIF, JPEG, JPEG-2000, PDF, PhotoCD, PNG, Postscript, SVG, TIFF, and other formats   I am running Ubuntu GNOME 15.10 with GNOME 3.18. How do I figure out who's been banished to the penalty box recently?