Run python Script that reruns every 4 hours at system startup
What is the correct way to edit a crontab file?
Change web folders names with uppercase to lowercase or not? Zorn's lemma and maximal ideals Let's consider two statements: Zorn's lemma and theorem about existence of maximal ideals in commutative ring with $1$.  It's easy to prove that Zorn's lemma implies existence of maximal ideals.  I wonder if the converse proposition is true, i. e., does existence of maximal ideals implies Zorn's lemma? My approach was to construct a ring structure on an ordered set which satisfies Zorn's lemma condition such that existence of maximal ideal implies existence of maximal element in the set, but I didn't succeed. Okay so I remember almost nothing of this show. The only thing I know is that I used to watch this show on Cartoon Network and it had a hero who used a sword to fight monsters and a girl who used bow and arrows. Also, the monsters kind of looked like computer generated and there was a book and stones which were somewhat related to the story? I was wondering how light or any electromagnetic radiation can be "absorbed" if it consists of electric and magnetic fields. For example if there is a charge at point A, and the  light reaches point A, it will cause that charge to move depending on the value of the electric field. However, why wouldn't this electric field continue on and influence others particles everywhere else? For example, if you have a static charge creating an electric field, then if you put a charge at point A it will feel its effects and then if you bring a charge at another location B it will also feel its effects, namely it will not have been absorbed. Please shed some light on this. Thanks  edit: Does this mean the electron that absorbs this light moves in such a way that it creates an electric field of its own canceling the initial one? Or am I way off? External program from our Java program How can I write a program in Java that will execute another program? Also, the input of that program should be given from our program and the output of that program should be written into a file.  This is my small set of code to get its output:  Process p = Runtime.getRuntime().exec("C:\\j2sdk1.4.0\bin\\helloworld.java"); BufferedReader input =         new BufferedReader(new InputStreamReader(p.getInputStream())); while ((line = input.readLine()) != null)    System.out.println(line);  input.close();   This was my set of code but this throws an IOException. Undergraduate research field different from PhD field I am a Physics undergrad who is interested in pursuing a PhD in pure maths in the future (algebraic geometry/topology) but I am a bit unsure. My question is quite general, and I don't wish to provide more background for fear of bias in the answers.  My question: Is it is advisable/possible/unfavorable/favorable to apply for a PhD in a field, different from that in which you have done most of your undergraduate research?  My research (includes just reading and understanding papers, writing summaries until now, I havent published anything) mostly includes Quantum field theory, and gauge theory. Would the selection committee turn down an application to a pure maths field, if I have no research experience whatsoever?  I would also like to ask the question other way round. What if I concentrate my undergraduate research SOLELY on topics in pure maths such as algebraic geometry/topology and take other physics courses, would I be able to apply to a string theory PhD with a high chance of success?  Should I consider spending time on both of these (which is almost an impossible task), to improve my chances in both the areas or would research in one area, and grad level courses in the other suffice?  In brief: Should the PhD field you are applying to be the same as your undergraduate research area? How to create a bootable Ubuntu USB flash drive from terminal? I've read in several places that one motivation for category theory was to be able to give precise meaning to statements like, "finite dimensional vector spaces are canonically isomorphic to their double duals; they are isomorphic to their duals as well, but not canonically."  I've finally sat down to work through this, and -  Okay, yes, it is easy to see that the "canonical isomorphism" from $V$ to $V^{**}$ is a functor that has a natural isomorphism (in the sense of category theory) to the identity functor.  Also, I see that there is no way that the functor $V\mapsto V^*$ could have a natural isomorphism to the identity functor, because it is contravariant whereas the identity functor is covariant. My question amounts to:     Is contravariance the whole problem?   To elaborate:   I was initially disappointed by the realization that the definition of natural isomorphism doesn't apply to a pair of functors one of which is covariant and the other contravariant, because I was hoping that the lack of a canonical isomorphism $V\rightarrow V^*$ would feel more like a theorem as opposed to an artifact of the inapplicability of a definition.  Then I tried to create a definition of a natural transformation from a covariant functor $F:\mathscr{A}\rightarrow\mathscr{B}$ to a contravariant functor $G:\mathscr{A}\rightarrow\mathscr{B}$. It seems to me that this definition should be that all objects $A\in\mathscr{A}$ get a morphism $m_A:F(A)\rightarrow G(A)$ such that for all morphisms $f:A\rightarrow A'$ of $\mathscr{A}$, the following diagram (in $\mathscr{B}$) commutes:  $$\require{AMScd}\begin{CD} F(A) @&gt;m_A&gt;&gt; G(A)\\ @VF(f)VV @AAG(f)A\\ F(A') @&gt;&gt;m_{A'}&gt; G(A') \end{CD}$$  This is much more stringent a demand on the $m_A$ than the typical definition of a natural transformation.  Indeed, it is asking that $m_A=G(f)\circ m_{A'}\circ F(f)$, regardless of how $f$ or $A'$ may vary. Taking $\mathscr{A}=\mathscr{B}=\text{f.d.Vec}_k$, $F$ the identity functor and $G$ the dualizing functor, it is clear that this definition can never be satisfied unless $m_V$ is the zero map for all $V\in\text{f.d.Vec}_k$ (because take $f$ to be the zero map). In particular, it cannot be satisfied if $m_V$ is required to be an isomorphism.     Is this the right way to understand (categorically) why there is no natural isomorphism $V\rightarrow V^*$?   As an aside, are there any interesting cases of some kind of analog (the above definition or another) of natural transformations from covariant to contravariant functors?  Note: I have read a number of math.SE answers regarding why $V^*$ is not naturally isomorphic to $V$. None that I have found are addressed to what I'm asking here, which is about how categories make the question and answer precise. ( was closest.) Hence my question here. How to make a drive NTFS from EXT4? I want to know how to make a drive ( non-system ) from EXT4 to NTFS. Can anyone help me out? Android app should send edits to edit queue if necessary rather than rejecting them I edited a question, on Stack overflow and when submitting preview, got red bar telling me my account isn't allowed to make suggested edits.  On the Sandbox android question it works fine, but that is editing an answer to my question.   The message I receive is:     Account is not allowed to suggest edits   Are the rules for suggested edits different for Android app? How can one predict whether an individual infected with the SARS-CoV-2 virus will be asymptomatic? According to  , half of the   virus carriers show no symptoms.   What are the biological factors that determine whether an infected individual will be asymptomatic or not? Zooming in to a selection of vertices with the period key does not work as expected I am following through the exercises in BlenderBasics_4thEdition2011.pdf and have gotten to the section on texturing.  The book goes through the creation of a Lighthouse and now I am texturing it.  I want to have a closeup view of the roof of the lighthouse after partitioning it as the lesson describes on page 54.    In order to get a close up view I selected the roof and want to center the screen on the vertices of it so I pressed the Numpad . key.  When I do that the view zooms so that the roof is off the screen.    What am I am doing wrong?    I tried Ray's answer and was able to move the origin but pressing the Numpad . key still did not work.  The centering was still on the bottom of the lighthouse, as if the origin was still there. When can uncountable nouns be countable? Is there a method to split thousands of polylines by employing a tool like the "Editor" - "Split" method and using distance, in ArcGIS?      When I utilize the "split" - "distance" tool (fig1), it only splits one section leaving the remainder unsplit (fig2).    I have attempted the "COGO" tool - using "Proportion", and can utilize the "duplicate" button to assign the remainder of segments which seems like a viable option.  But I would like to find some way to automate this process.  Has anyone found a method to do this?  Or is Python (coding) the only way?  I am trying to split over 2000 lines into equal distance segments (fig 3) of ~600m. Each segment will have its angle measured, and will ultimately result in a rose diagram representing angular statistics.  The distance of each segment is important in my splitting regime (a long line will have a greater representation of a particular angle on my rose diagram than the neighboring shorter line segments).  I currently am using ArcGIS 10.2 with an advanced user license. When does SQLiteOpenHelper onCreate() / onUpgrade() run? How to show that $\lim \frac{1}{n} \sum_{i=1}^n \frac{1}{i}=0 $? Show that  $$\lim \frac{1}{n} \sum_{i=1}^n \frac{1}{i} =0 $$  I've proved that this sequence converges (it is bounded and decreasing). NOW, I need to find a sequence that is bigger than this one and goes to zero. Maybe something using geometric serie of 1/2  Thanks in advance! I installed the VMware bundle on my Ubuntu 11.04 successfully but when I open it it gives me this window     and I don't know the path to this C headers. Read it in the 90's. I remember one specific part where the inventor of the peeking mechanic has his Eureka moment, and has the presence of mind to give a thumbs-up/wave to the future, when they look back on his Eureka moment.  I also remember that they had computer screens that were like plastic cloth that they would just unfold on whatever surface was handy.  Sorry I don't have much more to go on.  Please and thank you! Using ArcMap 10.0, I am working on a map that shows many villages. Each village has a name attribute that I want to use for a label, but in some areas there are so many villages that not all can be labeled. I am using the Maplex label engine and have not checked the "Never Remove (allow overlap)" box.  I am quite happy that not all village names are displayed as this would be way too much. And it is ok that Maplex chooses which labels to display, as I have no way to know which village is more important than another one. This also means that I can't base the labeling on a field such as population or something else, even though that would be less random.  However, I would also like the village points that are not labeled to similarly not be displayed. I think it unlikely that someone would want to see the point without knowing the name of the village. If all points are shown, it is also difficult to tell which point a name label goes with in crowded areas. So instead of choosing just labels not to be shown, I also want the computer to chose features not to be shown.  To give you a better idea on what I mean:    Is there a way to make these features/points not draw if their label is not shown? Cleaning PCBs... using water? I found  where a technician with a lot of experience seems to clean his PCBs using water, and then he just puts it in the sun so it can dry. I was just amazed to see the circuit keeps working as intended, he says he keeps doing this all the time and several times, however I doubt this is normal.  Which leave me some questions:   What are the recommended methods to clean huge PCBs like the one in the video? What happens to charged capacitors when water shorts them out? Solder joints do not get any kind of corrosion? Is the antisolder mask waterproof?