Create instance of derived class from Type variable alone
How to create a new object instance from a Type
Why is the harmonic oscillator so important? I've been wondering what makes the  such an important model. What I came up with:   It is a (relatively) simple system, making it a perfect example for physics students to learn principles of classical and quantum mechanics. The harmonic oscillator potential can be used as a model to approximate many physical phenomena quite well.   The first point is sort of meaningless though, I think the real reason is my second point. I'm looking for some materials to read about the different applications of the HO in different areas of physics. Find the value of : $\cos x \cos 2x...\cos 999x$ given that $x=\frac {2\pi}{1999}$ What happened to the voting buttons? Starting from yesterday, the up-/downvote buttons seem to have moved from the left of a post towards above (in the middle) of the bodies of all questions and answers. Here is a screenprint of a sample (containing such buttons for both a question and an answer to it):    This happens on an iOS device (running 9.3.5), using various browsers and this in any SE site. No idea so far how it looks like in the most recent iOS version. Same for the (green) checkbox below those voting buttons (to mark an answer as accepted). Is this intended to be like this (IMO it looks ugly ... and a huge waste of available space), or is it rather some sort of CSS related issue that needs to be fixed? PS: Looking at the same page around the same time on a Win PC (using any of the common browsers) seems to work fine. Difference between "the very first" and "first" I have the sentence:     Who wrote the very first dictionary ever?   Is it any different from     Who wrote the first dictionary ever?   I don't get how something could be more first. Email to salesforce error While sending an attachment as an email to be attached to the salesforce record.Salesforce is giving the below given error.  Generating server: mx1.madison.gov  6-2j26dncatawfg1112dv2uwxupq9hzfgmjrrlrn2rx48cxm93nt.8-dea8eae.c.le.salesforce.com mx1-asg.mta.salesforce.com # #SMTP#  Is this salesforce problem or it is the Internal company server problem as I could see that error mentions company server name also.But to me doesn't look like company server error. Epsilon delta proof for $\lim_\limits{x\to a} \sqrt{x} = \sqrt{a}$ where $a>0$ $f(x) = \sqrt{x}$. For right limit, $x-a &gt; 0$. Let $x-a &lt; \delta$ then $(\sqrt{x}-\sqrt{a}) \lt \frac{\delta}{\sqrt{x}+\sqrt{a}}$. Then chose we must choose $\epsilon $ so that $\epsilon &gt; \frac{\delta}{\sqrt{x}+\sqrt{a}}$. Is this correct? How would I conclude the proof?  Edit - Follow up  From answers, I try a proof for the same limit but this time Left side:  For left side limit, $x-a &lt; 0$ so $a-x &gt; 0$. Let $a-x &lt; \delta$ then $\sqrt{a}-\sqrt{x} \lt \frac{\delta}{\sqrt{a}+\sqrt{x}} \lt \frac{\delta}{\sqrt{a}}$.   So let me begin the proof by saying to choose $\delta = \epsilon \sqrt a$ then $a-x \lt \delta $ so $(\sqrt a - \sqrt x )(\sqrt a + \sqrt x) \lt \epsilon \sqrt a$ so $\sqrt a - \sqrt x \lt \epsilon$ When I choose the Connection Type in VPN Connections, I have only PPTP and I can't find OpenVPN, but I already installed it via terminal with the command sudo apt-get install openvpn easy-rsa  Why I can't find it? Adding values to a picklist using apex How to initialize private static members in C++? What is the best way to initialize a private, static data member in C++?  I tried this in my header file, but it gives me weird linker errors:  class foo {     private:         static int i; };  int foo::i = 0;   I'm guessing this is because I can't initialize a private member from outside the class.  So what's the best way to do this? GR. Einstein's 1911 Paper: On the Influence of Gravitation on the Propagation of Light Sums in math mode: how to display index under the sigma sign? I have a fraction with sums above and under the line. How can I convince LaTeX to write the indices of the sums under the sigma instead of next to it?  \begin{displaymath}   \frac{\sum_{s \in S} s^2}{\sum_{p \in P} p^2} \end{displaymath} I seem to be having issues when I try and save the output file as a .png, or .jpeg for that matter. I've set up my user preferences, File, Render Output to my desktop as well as render output in the editor window. What am I missing here? I'm obviously very new to this program and your help would be appreciated. Thanks for your time Suppose we have a sample of size n= 9, where the sum equals 27, and the sum of the squaredvalues equals 113. Sample mean is 27/9=3 and mean of squared values is 113/9 = 12.56 Can we then say that the sample variance $S^2 = E(\overline X^2) - E(\overline X)^2 = 12.55 - 9 = 3.56$ ? I have a select control, and in a javascript variable I have a text string.  Using jQuery I want to set the selected element of the select control to be the item with the text description I have (as opposed to the value, which I don't have).  I know setting it by value is pretty trivial. e.g.  $("#my-select").val(myVal);   But I'm a bit stumped on doing it via the text description. I guess there must be a way of getting the value out from the text description, but my brain is too Friday afternoon-ed to be able to work it out. Posting and answering questions you have already found the answer to I tried to follow the new Blender 2.8 tutorial from Blender Guru.   When entering "Sculpting" with the icing there were suddenly alot of holes in it. Why is this? Batch/Multi rename only files with a specific number range I have a situation where I have the following:  file01.ext . . . file99.ext file100.ext . . .   And I want to (using rename ) insert the '0' for the files 01-99 (so that they become 000-099) but not modify files larger than 100. Which expression should I use and what is a good source on learning regular expressions such as these?   On Windows I would have used Total Commander's multi rename function which has a very simple renaming syntax. Are there some good alternatives for that tool (multi-rename) for Ubuntu? Summation of series $\sum_{n=1}^\infty \frac{n^a}{b^n}$? How to find out the reason, why my PC can't run Windows 11? Windows 11 requires this features:  Processor 1 GHz or faster with 2 or more cores on a compatible 64-bit processor or System on a Chip (SoC) Memory 4 GB RAM Storage 64 GB or larger storage device System firmware UEFI, Secure Boot capable TPM Trusted Platform Module (TPM) version 2.0 Graphics card DirectX 12 compatible graphics / WDDM 2.x Display &gt;9” with HD Resolution (720p) Internet connection Microsoft account and internet connectivity required for setup for Windows 11 Home  I have a 15-ay108nh laptop. As far as I know, it supports all the required features. It has 8GB ram, 256GB storage, UEFI support, TPM 2.0 ( I enabled it in the BIOS ), DirectX 12 compatible graphics / WDDM 2.x, and 1080p screen. I installed the , but for some reason, I get this result:  How is it possible to find out exactly, why it fails? I have installed Ubuntu on my D: partition using Wubi. Is there any way in which I can remove my Windows from C: drive and move my existing Ubuntu installation to C: drive and make it a regular install.