Call to a member function prepare() on a non-object
Call to a member function on a non-object
Are two (or N) resistors in series more precise than one big resistor? Let's say I have one 2 k&ohm; resistor with 5% tolerance. If I replace it with two 1 k&ohm; resistors with 5% tolerance, will resulting tolerance go up, down, or remain unchanged?  I'm bad with probabilities, and I'm not sure what exactly tolerance says about resistance and its distribution.  I am aware that in the 'worst case' it will be the same; I'm more interested in what will happen on average. Will the chance of getting a more precise value increase if I use a series of resistors (because deviations will cancel each other out)?  On 'intuitive level' I think that it will, but I have no idea how to do the math with probabilities and find out if I'm actually right. I want to add the Apache user (www-data) to the audio group. I've read the man page for useradd, but I'm not having any luck. I'm running xubuntu 11.10. Here's what I'm doing:  $ sudo useradd -G audio www-data useradd: user 'www-data' already exists   If I leave out the -G option, bash, prints the help info for useradd:  $ sudo useradd  audio www-data Usage: useradd [options] LOGIN Options: -b, --base-dir BASE_DIR       base directory for the home directory...   It's not clear to me from the man page what options I should use to make this work. Proof of Convergence: Babylonian Method $x_{n+1}=\frac{1}{2}(x_n + \frac{a}{x_n})$ Having some trouble proving this:     If $f:\mathbb{R}\rightarrow \mathbb{R}$ is differentiable and $\lim_{x\to \infty } f^\prime(x)=0$ show: $\lim _{x\to \infty } (f(x+1)-f(x))=0$   Attempt: from $\int \lim _{x\rightarrow \infty } f^\prime(x) \mathrm{d}x=0$ we can say that $f^\prime(x)=const$ would we then have to show that  $\lim _{x\to \infty } (f(x+1))$ is also a constant? From there if we show that both are equal then the difference is zero. I am on the right track?  Thank you for the help. Do the druid's uses of the Wild Shape feature recharge while still in Wild Shape?  Say a 10th-level Circle of the Moon Druid spends two uses of Wild Shape to transform into an Air Elemental, whose form she can stay in for the next 5 hours. After 2 hours, she takes a short rest, but does not drop the Wild Shape. She has another couple of hours to keep the Air Elemental form.  My main concern: Does she get her two uses of Wild Shape back with that short rest? Or, does she have to drop the Air Elemental form first before taking the short rest to recharge Wild Shape? What value to use to convert ADC value to voltage? While working on my project, I got to a point where I don't know the answer. I have a 10-bit ADC (the one incorporated in the ATMEGA328P) and it returns values up to 1023 (210 - 1).   The official documentation states that the equation to convert the values from the ADC to the corresponding voltage is:   $$ V = \frac{V_{in} \cdot ADC}{2^{10}} $$  But, if the values go up to 1023, then wouldn't it be better to replace the 210 by 210&nbsp;-&nbsp;1?  Thanks. Can we raise the bar for reputation for late answers to bypass the review queue? I would like to raise the bar for late answers to enter the Late Answers Review Queue to 50 rep, which is the threshold after which users gain the ability to comment. Here are some answers that were posted by users with between 10 to 50 rep (yes, there's a little selection bias to be sure. I suspect most/all of these will be 10k links only by the time you're reading this. Note, all of these are from the last 5 hours.  These are all from Stack Overflow, where I have the most experience, but since Late Answers have a much higher probability to be bad across all stacks, I think this applies generally. Also, I have bolded all the users references to the inability to comment, that were not in their original answers:    (23 rep)     Solution provide by @brandonyang works for me too! (I can't add comments in a response 'cause my low reputation, sorry).   (47 rep)     I tried your answer but it doesn't work in office 365 outlook online. Do you have any suggestions?   (26 rep)     My question is somehow related to the same issue. I am fairly new to C# and have been doing JAVA for quite a long time. When I am coding in C# with .Net framework 4.5.1 using visual studio 2013, I get errors for using certain libraries but I donot know which dll should I reference in my project How do C# people figure that out? For example I wanted to code to draw an ellipse and needed system.Windows.Media and System.Windows.Controls both of which were giving me errors, after googling for quite sometime I got to know that we need to reference PresentationFramework.dll, is this the only way to figure it out?      PS >> I am unable to post this as a comment hence putting it as answer.      Thanks.   (19 rep)     i like  video. It explains the differences between int and Integer and cautions of using Integer   (21 rep)     I have a question, i am actually trying to copy the database from Prod server to test server ( SQL server 2008R2). I am wondering would it copy the users logins, roles and permission as well ?      I know I suggested 50 rep above, but while I was compiling that, I . Perhaps it should be 100 rep, not 50. I am less confident about this, but I thought I'd add it as an addendum for discussion.     The answer posted  is the simplest solution that you are looking for to solve your problem, so take a look ;) I recently switched from Windows to Mac. Steam says I have fewer Mac specific games than total games and it should be an equal amount unless you can only download, install, and play once from your account. I thought I bought it so that it would work across all operating systems. If this is true, is there a work around or a discount for re-purchasing for the other OSes? What kind of components are black blobs on a PCB? In low-cost mass-produced items I often run into black blobs of what looks like resin applied directly on top of something on the PCB. What are these things exactly? I suspect this is some kind of custom IC that is layed out directly on the PCB to save on the plastic  housing/connector pins. Is this correct? If so, what is this technique called?    This is a photograph of the inside of a cheap digital multimeter. The black blob is the only non-basic piece of circuitry present, along with an op-amp (top) and a single bipolar junction transistor. Code block is not properly formatted when placed immediately after a list item Consider the following piece of Markdown code:  This is some regular text.      &gt;&gt;&gt; def factorial(n):     ...     return 1 if n &lt; 2 else n * factorial(n - 1)     ...   * This is a list item.      &gt;&gt;&gt; def factorial(n):     ...     return 1 if n &lt; 2 else n * factorial(n - 1)     ...    Notice that the second code block is preceded by a list item.   Why is the factorial code not properly formatted under these circumstances? To demonstrate the problem, I'll insert the Markdown code show above:    This is some regular text.  &gt;&gt;&gt; def factorial(n): ...     return 1 if n &lt; 2 else n * factorial(n - 1) ...     This is a list item                 def factorial(n):       ...     return 1 if n &lt; 2 else n * factorial(n - 1)       ...               This applies to numbered as well as unnumbered lists. Proximal Operator Fixed Point Property for Matrices $\newcommand{\prox}{\operatorname{prox}}$ $\newcommand{\argmin}{\operatorname{argmin}}$ $\newcommand{\dom}{\operatorname{dom}}$  Recall again that the proximal operator for vectors $\prox_{f}: R^n \rightarrow R^n$ of $f$ is defined as:  $\prox_f(v) := \argmin_{x} \left(f(x) +(1/2)\|x-v\|_2^2 \right) $,  where $f: R^n \rightarrow R \cup \infty$ is a closed proper convex function and $\|\cdot \|_2$ is the Euclidean norm. $\prox$ is strongly convex and not everywhere infinite, so there it will have a unique minimizer for every $v \in R^n$  The crucial property of proximal operator is that $x^*$ minimizes $f(x)$ iff $x^* = \prox_f(x^*)$, i.e. $x^*$ is a fixed point of $\prox_f$.  Let us consider the proof of this property. The first part, namely, that if $x^*$ minimizes $f$ then $\prox_f(x^*)=x^*$, is trivial:  $f(x) +(1/2)\|x-x^*\|_2^2 \le f(x^*) = f(x^*) +(1/2)\|x^*-x^*\|_2^2$  The second part uses the notion of subdifferential. In the proof authors say that $\tilde{x}$ minimizes  $f(x) +(1/2)\|x-v\|_2^2$  i.e. $\tilde{x} = \prox_f(v)$ iff $0 \in \partial f(\tilde{x}) + (\tilde{x}-v)$,   where the sum is of a set and a point. Recall the definition of the subdifferential:  $\partial f(x) = \{ y$ |  $ f(z)\le f(x) + y^T(z-x)$ $\forall z \in \dom{f}\}$  Take $\tilde{x}=v=x^*$, it follows that $0 \in \partial f(x^*)$ so $x^*$ minimizes $f$.  Question 1:  Consider proximal operator defined for matrices now:  $\prox_f(Y) := \argmin_{X} \left(f(X) +(1/2)\|X-Y\|^2 \right)$,  where $X$ is some real $m$ by $n$ matrix. What norm function should be taken in the definition of the $\prox$ in this case? Frobenius? Or spectral norm (induced 2nd norm) which is the largest eigenvalue (is spectral norm even differentiable?)? Or something else?  Question 2:   Can the norm for the definition of $\prox$ in case of matrices be chosen in different ways? What conditions does the proof above impose on the norm function? Will Stack Overflow's engine be (someday, or ever) open sourced? Empty drives show 2 GB used, Explorer shows nothing I have 4 drives in Windows: C:, D:, E: and F:. Except C:, all of them are empty.    Suddenly it shows up to 2 GB space used – even when there are no files.     [  When I format the drives, it's fine for a while until it shows used space again.  I use Windows 8.1 Pro 64 Bit. How to explain in layman’s terms the significance of the difference of qubits of the D-Wave and IBM QX? So, @AndrewO mentioned recently that he has had 'encounters' with people wondering why D-Wave has a lot more qubits than IBM. Of course, this comparison is faulty, since the IBM and D-Wave's machine may both exploit quantum effects to a certain degree, IBM's machine matches the thing the TCS people call a 'Quantum computer' a bit more than D-Waves's alleged quantum annealer.   How do you explain to a novice why IBM is still reaching important milesstones, even though the D-Wave has a lot more 'qubits'. I understand that an easy answer is, 'well, you're comparing apples and pears', but that is merely relying on your possible authority and simply doesn't explain anything!   How can you explain that those devices are different, how can you dispell the myth that the number of qubits is not the only metric to judge quantum devices? (preferably to a layman, but assuming basic (undergrad?) physics knowledge is ok, if needed) Is the electromagnetic spectrum discrete? I'm just starting to learn physics and I have a question (that is probably stupid.)  I learned that energy levels that the bound electron can have are discrete. I also learned that when an electron transitions from one level to another, a photon with a specific wavelength (energy) is created and released into the wild.  My question is this: are there holes in the electromagnetic spectrum, values of frequency that the created photon can't possibly have? MySQL Access denied for user 'root'@'localhost' Word with the same meaning as "good moments" or "pleasant memories"? Is there a single word with the same meaning as "good moments" or "pleasant memories"? I'm looking for a word which describes good thing(s) that happened to someone. The only word I can think of is "joy".  Some examples might be:   Wayne has many pleasant memories of the days spent with the family. She could remember many good moments from the last weekend. Live CD asks for a username and password E:Error, pkgProblemResolver::Resolve generated breaks, this may be caused by held packages I am not sure what caused this error, but here is what the whole error says, and also this is sitting as a notification, and preventing me from updating any software using Update Manager - Please provide some assistance or tell me how to figure out what to do to fix it.  Could not calculate the upgrade  An unresolvable problem occurred while calculating the upgrade.  Please report this bug against the 'update-manager' package and include the following error message: 'E:Error, pkgProblemResolver::Resolve generated breaks, this may be caused by held packages.' Intuitive explanation of entropy I have bumped many times into entropy, but it has never been clear for me why we use this formula:  If $X$ is random variable then its entropy is:  $$H(X) = -\displaystyle\sum_{x} p(x)\log p(x).$$  Why are we using this formula? Where did this formula come from? I'm looking for the intuition. Is it because this function just happens to have some good analytical and practical properties? Is it just because it works? Where did Shannon get this from? Did he sit under a tree and entropy fell to his head like the apple did for Newton? How do you interpret this quantity in the real physical world?