Good resource for geometric optics
Where is a good place to learn classical optics for high school competitions?
Why is python ordering my dictionary like so? What can I do to manage my phone's internal storage? This is a  about avoiding outgoing mail being classified as spam.   Also related:                    I'm wondering how to prevent my emails from my site being marked as spam? I'm using sendmail.  I'm trying to send emails through my ruby-on-rails application. The mails are all written in swedish (if that does make a difference?). I don't know why they keep getting marked as spam.  Are there any things that I can do to minimize the risk? Do I need a transit visa for a technical stop in Australia? I'm travelling to New Zealand, and my itinerary mentions a technical stop in Australia.  Will I need to apply for an Australian transit visa for this? How to draw a picture by using tikz as backgroud of a character? I am writing some Chinese character and hope to draw simple pictures by tikz and use it as background of this chinese characters. In a short I'd like to get some results as this picture (ignore the lower right noise):    How could I do? Thanks in advance. As an example for least workable code:  \documentclass{article} \usepackage[CJKnumber]{xeCJK} \setmainfont{KaiTi}  \begin{document} \Huge 明月出天山 \end{document} Include Watir test cases in a Continuous Integration system What are the scope restrictions for setting shopt extglob. and other options? My non-interactive bash shell has extglob off. I would like to turn it on in the statement immediately before a command, but I have noticed that when shopt -s extglob is within an if .. then .. else block, it somehow does not register.   The following extglob-dependant command is not valid: syntax error near unexpected token '('.     Where can extglob be set, and why is there a restriction, at all? Does this apply to other options?  ... GNU bash 4.1.5     This works:    shopt -s extglob if true ;then     touch a.bcd; ls a.@(bcd) fi   This fails:    if true ;then     shopt -s extglob     touch a.bcd; ls a.@(bcd) fi ... line 17: syntax error near unexpected token `(' ... line 17: `touch a.bcd; ls a.@(bcd)' Precedence of user and group owner in file permissions How do I convert my PhD dissertation so that it can be published as a book? Is it possible to plot a graph of any shape? In school, I have learnt to plot simple graphs such as $y=x^2$ followed by $y=x^3$.  A grade or two later, I learnt to plot other interesting graphs such as $y=1/x$, $y=\ln x$, $y=e^x$. I have also recently learnt about trigonometric graphs and circle equations. In the internet, I have seen users posting graphs of different shapes like a heart-shaped graph and a Batman logo-shaped graph. I am sure there are numerous more graphs that I have yet to see. Seeing that the graphs can be shaped into shapes like the Batman logo and a heart brings me to my question: Is it possible to plot a graph of any shape regardless of its complexity? Perhaps, shaped into an outline of a person or a landmark? Why or why not? How did Luke know where to land on Dagobah? Ignoring invalid input features with Intersection tool in QGIS? In QGIS 2.18, when I used the Vector > Geoprocessing Tools > Intersection tool, I was able to check an option which said "Ignore NULL geometries [optional]".  Now I've downloaded QGIS 3.2 and that option no longer exists. But when I try to create the intersection between two layers I'm working on, I get a message saying that there is an invalid geometry somewhere and that I should either fix it it or "change the Processing setting to the 'Ignore invalid input features' option."   But that option doesn't exist! Or if it does exist, I just can't find it anywhere.  Does anyone know how/where to change this setting in QGIS 3.2? For some reasons my brush stroke suddenly looks like this:     As the picture showed, the edges are jagged and pixelated. It doesn't happen before and it's just one of the built-in inking brush.  I know there are a lot of similar questions here but please don't mark this as duplicate. I double checked all the thing the other answers suggested: It's brush tool, not pencil, and my mode of brush and layer is Normal, not Dissolve. It's pixelated even if I draw with mouse so it's not tablet driver issue. I disabled GPU render but nothing changed. How do I fix this? Maybe this is an easy question, maybe not.  I have a select box where I hardcode with width. Say 120px.    &lt;select style="width: 120px"&gt;   &lt;option&gt;REALLY LONG TEXT, REALLY LONG TEXT, REALLY LONG TEXT&lt;/option&gt;   &lt;option&gt;ABC&lt;/option&gt; &lt;/select&gt;     I want to be able to show the second option so that the user can see the full length of the text.  Like everything else.  This works fine in Firefox, but doesn't work with Internet Explorer6. I have Ubuntu budgie 17.04 (budgie-desktop 10.2.9). While pressing Alt+Tab to switch between the windows, the windows are switched randomly and I am not able to select which window to shift.  This feature was available in Ubuntu 16.04 but not in Ubuntu-budgie-17.04.    How to resolve this? GeoServer cutting off symbols close to tile edges? Proof of $f^{-1}(B_{1}\setminus B_{2}) = f^{-1}(B_{1})\setminus f^{-1}(B_{2})$ I want to prove the following equation:  $$ f^{-1}(B_{1}\setminus B_{2}) = f^{-1}(B_{1})\setminus f^{-1}(B_{2}) $$  Is this a valid proof? I am not sure, because at one point I am looking at $f(x) \in B_1$, but then $x \in f^{-1}(B_1)$ could be actually some different points.  $$\begin{align*} x \in f^{-1}(B_{1}\setminus B_{2}) &amp;\iff f(x) \in B_{1}\setminus B_{2} \\ &amp;\iff f(x) \in B_{1} \land f(x) \notin B_{2} \\ &amp;\iff x \in f^{-1}(B_{1}) \land x \notin f^{-1}(B_{2}) \\ &amp;\iff x \in f^{-1}(B_{1})\setminus f^{-1}(B_{2}) \end{align*}$$ Submitting a form by pressing enter without a submit button Well I am trying to submit a form by pressing enter but not displaying a submit button. I don't want to get into JavaScript if possible since I want everything to work on all browsers (the only JS way I know is with events).  Right now the form looks like this:  &lt;form name="loginBox" target="#here" method="post"&gt;     &lt;input name="username" type="text" /&gt;&lt;br /&gt;     &lt;input name="password" type="password" /&gt;     &lt;input type="submit" style="height: 0px; width: 0px; border: none; padding: 0px;" hidefocus="true" /&gt; &lt;/form&gt;   Which works pretty well. The submit button works when the user presses enter, and the button doesn't show in Firefox, IE, Safari, Opera and Chrome. However, I still don't like the solution since it is hard to know whether it will work on all platforms with all browsers.  Can anyone suggest a better method? Or is this about as good as it gets? Trying to rig a Map I'm trying to make a "simple" accordion map in Blender 2.79, for animated and still render purposes.  I will map graphics onto each face, and I don't want the faces to get stretched/distorted.  I do however, want to be able to create a hinge-type rig, to be able to fold open and close the map like an accordion without having to manually position each face edge (which would distort the faces anyways).  I looked into using an Armature, but the faces got distorted. I tried to use a Hinge constraint, but I if understand correctly, it would require each flap of the map to be a distinct element - I don't want that. I found solutions involving Weight painting, but I don't think I want any of the map's flaps to be anchored - also there is only one face per element and I don't know if Weight Painting is appropriate.    How can I rig this map properly to open it and close it without any distortion? $a =\prod_{i = 1}^{r} p_{i}^{ai}$ with $a_{i} > 0$ for each $i$ is the canonical representation of $a$...