looking for a start in solidity

Question: I am totally new to the Solidity Programing. I was working on NodeJS, express and mongoDB. Few days ago I started working on blockchain. I have to write the API'S for contracts. We are using truffle and tronweb. Is there any simple project or tutorial available which gives me idea of how this whole thing works.
Comment: The question mentioned by Ismael is indeed a duplicate - consider also the official Solidity Intro, it's quite good: https://solidity.readthedocs.io/
Answer: May I suggest the udemy course: Ethereum and Solidity The Complete Developers Guide?
It's far from a complete developers guide but it was a nice introduction to developing smart contract enabled web applications.
The course is like 2 years old, it does not cover truffle and there are some quirks(none that couldn't be solved). Nevertheless the instructor was super pedagogic and i enjoyed the course!

looking for a start in solidity


New residential service installation 200 amp california

Question: I am running a 490' underground new service for a 200 amp, I was told to run one line of 4 aught, and two lines of 2 aught, aluminum wire. In a 3" sched. 40 pvc conduit, 18" inches deep in the trench.  Will this work for this type of install.
Comment: Is this a *service* (i.e. before the utility meter and main disconnect), or a *feeder* (i.e. after the utility meter and main disconnect)?
Comment: We need a few questions answered, is this for a residence (not a shop) what is the total load. For example a residence can allow a smaller wire size because of load diversity but to know for sure we would need to know the size type of heating / water heater, range etc, but with all electric and the size we can provide more information. Also is it possible to have the utility transformer located closer to the house in some cases this can save $ .
Comment: Yeah, why is this service or feeder so long?
Comment: It is from the utility pole at the street, to a pump house shop 490 ft.
Comment: @william -- I take it the pump house/shop building has a meter on it?
Answer: The trench is not 18" deep.  The trench is 22" deep.  The cover over top of the conduit is 18" per Code.
I would bump to Sched 80 conduit for the curves and entrances and exits, because it can stand some physical abuse.
You must lay the conduit in the ground, all complete, before pulling any wires into it. Assembling the conduit around the wires is illegal. It will be a very formidable pull.
Before committing to conduit, consider theft.  Any goober could chain your wires to a bumper hitch and drive away, dragging all 500' out of the conduit.  Whereas pulling it from 24" direct burial, the cable would snap.
Aluminum is the ONLY wire to consider for this. It is an excellent conductor, and in service and feeder wires, it performs just fine.  Honestly if you went to the distributor and asked for this in copper, it'd be fun to see that on hidden camera! Their eyes would light up with dollar signs, and then they'd need to special order it.
Make triple darn sure it's 490 feet. 500' is a standard wire spool size, and you really, really don't want to buy wire and find out it's 501'.  Count your conduit sticks carefully and measure three times.  On the upside, you get the whole-spool discount, so there's that.
Voltage drop is serious business. On a very long run like that, you lose a bit of voltage.  You can compensate by making the wires larger.  But we shouldn't do that lightly.  Let's go through it step by step.
The absolute minimum wire size which is mandatory
You are provisioning 200A service.  Thanks to 310.15(B)(7), we get a favorable "derate" on the wire size - we only need wire good for 83% of the 200A, or 166A.  In conduit, we get to size wires off the 75C column of the chart, so that brings us to 4/0 aluminum wire, at naturally 180 amps.
The "I was told" person has downsized your neutral. That's really decided by the provisioning of your particular loads, but I believe downsizing neutral to 2/0 is not unreasonable if  you have a lot of 240V loads.
So the minimum size, if we completely ignore voltage drop, is two 4/0 and one 2/0.  (as usual, the "I was told" person gets the facts wrong.)  Boy, I'm glad  you didn't buy the wire yet.
Now we need to think about voltage drop.
Look. I'll be the first person to roll my eyes and say "pshaw" when someone rants about big wire upsizing and 3%.  3% is not a rule, it's a wire salesman's dream.  And my rule of thumb is don't worry about it until you're at about 200'.  However, yeah. You need to worry about it.
So go to any handy-dandy voltage drop calculator, and first, change that 3% to 5% limit.  Or 6% if you're not overly fond of your tenants :)
Now, consider the current you'll actually be drawing on a regular basis.  For God's sake don't put 200A in there.  You're required to derate your loads to 125%, so the most you could ever plan on putting on a 200A service is 160A.  But realistically, you have to look at the actual nameplate numbers on the large appliances you expect to use simultaneously (e.g. NOT A/C and heater; and if you're running an electric oven, the house heater will be working less).
So you have to "play". And see what the chart tells you.
My usual calc is broken, but I tried it on another calc.  If you're running the service to the wall at continuous absolute limit (160 amps) and don't bump the wire and stay with 4/0, then your voltage drop is 5.05%.   If you bump wire one size to "250 kcmil" that cuts it down to 4.27%.  But again, that's at redline, so that better be uncommon.
Crunch with a somewhat more realistic 100A, and we get 3.16% drop, which nobody would have a problem with (except wire salesmen)... if we do 1 wire size bump to 250 kcmil, then you're at 2.67% drop.
Mind you, pulling 100A@240V, your electric bill will roll by at $3 per hour.
So I would do either

4/0, 4/0, 2/0, 4   -- or--
250kcmil, 250kcmil, 3/0, 2

with the last number being an aluminum ground wire.  If  you install receptacles at the meter point, or tap power in any way at the meter point (e.g. to an outbuilding), I would direct-bury the ground wire also, because you'll need it if you do those things.  If you stay in conduit, you could always add it later, though fishing an extra wire into such a very long run can be challenging.  Price it and decide.
Comment: Interesting..... have never thought about a goober pulling out wire..+


Is it secure to be using LDAP, or is LDAPS the only secure option?

Question: I am currently using an LDAP setup.  I am wondering if it is secure from Man-in-the-middle attacks or other vulnerabilities?
Answer: LDAP, by itself, is not secure against active or passive attackers:

Data travels "as is", without encryption, so it can be spied upon by passive attackers.
Active attackers can manipulate the stream and inject their own requests or modify the responses to yours.

At best, basic LDAP may rely on some authentication mechanism (through SASL) which is not trivially broken by an attacker: if the authentication is of the "show the password" type, then a passive eavesdropper can learn the password and then connect to the server with your identity; slightly more advance protocols, like CRAM-MD5, avoid that specific problem, but will still do nothing against attackers hijacking your connections or spying on your actual requests and responses.
So, really, if you value your security, then use SSL (i.e. "LDAPS"). In many respects, this is just like HTTP vs HTTPS.

As a side note, the Active Directory protocol from Microsoft, which builds up on LDAP, optionally offers a "sign & encrypt" feature, which appears to be some sort of cryptographic protocol embedded within LDAP (i.e. like LDAPS, but in reverse order), which might ensure enough security. I have not seen any decently detailed specification for that protocol, though, so I still recommend LDAPS in that case.
Answer: The LDAP protocol is by default not secure, but the protocol defines an operation to establish a TLS session over an existing LDAP one (the StartTLS extended operation).
Alternately, some authentication mechanisms (through SASL) allow establishing signing and encryption.
Most of the recent LDAP based directory servers support these modes, and often have configuration parameters to prevent unsecure communications.
LDAPS on the other hand is secure by default as long as proper ciphers are negotiated.
Answer: Create a certificate and log into your LDAP with your user certificate that you install in the admin entry.
Obviously the TLS session only raises the bar against packet sniffing, not the quality of authentication, unless a client certificate authentication takes place in TLS.
Plain text password would be the worst option, going up from there...SASL, etc.
If you are concerned with someone accessing your LDAP server from the Internet, and still want to allow access to "some" attributes, but not others, you can set up a proxy on 389 to filter requests going to the server. If you use the well known default ports for LDAP or LDAPS it makes it easier for users to find your services.
Also, consider putting in an SRV resource record entry into the DNS for your Internet connected LDAP server, so people will have the right address and port to hit your proxy server.


Is it okay if my unit tests contain 1-2 methods per class?

Question: When I'm testing my simple geometric library, I usually have one or two methods in every test class. All I check is whether objects do proper coordinates calculation or not. Is it okay (meaning the number of methods)?
Answer: Don't get hung up on the number of methods.  Just think about the ways in which your code could fail and add tests, preferably as separate methods, as you think of them.
The reason for separate methods is mostly to allow each test to be focused on just one aspect of your code and therefore keeping the test code short and simple to understand.
It's also acceptable to have a test method perform a number of Assertions, but once again these assertions should be checking a similar aspect of your code.
In some cases test code may need to be more complex eg: longer methods with calls to helper methods, but in general it is often possible to avoid this.
A good practical book that I've read on unit testing is Pragmatic Unit Testing in C# with nUnit.  It provides handy guidance on many questions such as this.
Comment: don't forget to think of all the ways your code should succeed as well as ways the code should fail. Tests should cover success ('happy path') as well as failure ('sad path').

Think of tests as specification instead of proof or evidence.
Answer: There is no inherent problem with that, nor does it indicate a test smell in itself.
In general, there are several different patterns for arranging test cases in classes. The book xUnit Test Patterns list several of them:

Testcase Class per Class
Testcase Class per Feature
Testcase Class per Fixture

Normally, the most common pattern is Testcase Class per Class, which means that you have a class contiaining test cases that all target the same System Under Test (SUT). That can sometimes lead to small test classes, but it's not a problem.
Answer: This is fine - it's not about having a million tests - it's about getting reasonable coverage. If you later find a bug, you can add a new test that exposes that bug before fixing it so your tests get better over time.
Comment: +1 - I find this is a good way to test, it's also good at finding and fixing edge cases which might go a bit crazy depending on certain setups.
Answer: My 2c/2p:
As others have said - it's not about the number of feature points tested, but it IS about isolation of each feature. So I would be wary of dependencies within each test - if you are dependent on one feature to produce an outcome/state/result which another feature uses in the same test then you have multiple possible points of failure.
Assuming you follow the Arrange/Act/Assert pattern, I certainly think it is OK to do multiple asserts on multiple objects after a single action.


Mapnik tiles have latitude distortion, projection issue?

Question: I'm trying to work out a kink in setting up mapnik-node.
My geos are in postgis, and I allllmost have them rendering properly, but my GIS experience is lacking so I can't seem to determine why this isn't working correctly.
Here's an example overlayed on a map. I'm getting a weird distortion on the latitude but not on the longitude. http://cl.ly/image/0K3i1x3n2L3v
I can provide a snippet of the mapnik code, if that helps. But hoping this is just a simple bug—some kind of projection issue that I'm not seeing.
My polys are stored in the db with srid=4326.
The bounding box I calculate for the tile x/y/z matches perfectly with what should work.
Answer: Figured it out.
I needed to add in:
map.aspect_fix_mode = mapnik.Map.ASPECT_RESPECT;

before:
map.extent = bbox;


Problems regarding meta titles

Question: We have problems regarding our meta titles on our Magento multi store.
Website: goo.gl/9W9hjQ
The multi store is running on one platform for both language.
We started the languages in sub-categories as following:
.com/en/
.com/da/
But we decided to change it for TLDs, so our 2 language is now running on following TLDs (3-4 months ago):
.co.uk
.dk
We have 301 redirected the old .com domain 1to1 to the .dk domain.
The meta titles is changed on the .dk domain as you can see in the browser. But Google is using the old meta titles from the .com domain.
Google is sometimes switching between the old and the new meta title in the SERP. The old meta titles is active 90% of time in the SERP. We are not able to see any problems with dmoz etc..
So is Google simply just confused, because of the .com redirect and will Google solve it over time?
Or is it a CMS/ Magento configuration?
Answer: Have you uploaded new sitemap to Google? Sometimes it helps.
Comment: Yes, and it have been in the air for 2-3 months. Link: http://goo.gl/g7nZKE
Comment: I see some pages, like http://www.meandmet.dk/temaer/print-my-dress  have NOINDEX robot tag.  Is it on purpose?

Problems regarding meta titles


Zero Volt reading when motorcycle is running?

Question: On a Suzuki gs 550 motorcycle battery reads 12.6 Volts DC when the motor is off. But when I start the motor, digital voltmeter reads zero. I have never seen this before. Any ideas?
Comment: Reads where, measured how, at what exact condition? When it is driving a load voltage drops
Comment: Are you sure you are reading the voltage across the battery terminals? It sounds like a measurement problem.
Comment: what @PlasmaHH said is right.Without knowing the exact way you are measuring we can't say for sure. Also, zero volts means a dead short which sounds kinda impossible in your case.
Answer:

You probably measured the voltage across a switch as shown in (a).

When the switch is open you get full battery voltage reading.
When the switch is closed the meter is shorted out so you get a zero reading.

Try measuring as shown in (b).


Where is the best place to put the export/upload function for a data table function?

Question: There's two Scenarios:
V1 Export appears above the top right but only shows to a specific tab, it is hidden if it is not available in other tabs.
V2 Export appears under the tab where it is only available to.
here's the screenshot:

Which one do you guys think is more appropriate and why?
Answer: Gestalt laws of proximity would suggest you go with version 2.
Since in your design, tabs delineate different data groupings (each with their own conditions for export), the closer the action is the object (the data set under the tab), the more related they appear.
From the Nielsen Norman Group: Closeness of Actions and Objects in GUI Design

One of the oldest principles of human-computer interaction is that things that are close together on the screen are seen as related.


Difference between rm and mp3 formats

Question: rm files are comparatively much smaller in size.
How do they compare quality-wise?
For a songs warehouse application, is it advisable to convert all mp3 to rm before archiving to save storage space?
Answer: I second Lunatiks answer, but suggest FLAC instead of WAV.
FLAC is a lossless compression method. Quote from http://flac.sourceforge.net/:

FLAC stands for Free Lossless Audio Codec, an audio format similar to MP3, but lossless, meaning that audio is compressed in FLAC without any loss in quality. This is similar to how Zip works, except with FLAC you will get much better compression because it is designed specifically for audio, and you can play back compressed FLAC files in your favorite player (or your car or home stereo, see supported devices) just like you would an MP3 file.

The reason why you'd want to go with a lossless method (FLAC or WAV) for storage in a warehouse is as follows:
Lossy methods like MP3 or RM are perfectly OK quality wise.
The problem shows up when you have to convert one lossless format to another lossless format.
If you do for example WAV -> MP3 -> RM you will end up with a file that has artifacts from MP3 encoding and RM encoding.
The proper way to store the files would be to store lossless and convert it to the appropriate format for your customers.
         OGG
          ^
          |
RM  <--  FLAC  -->  MP3
          |
          v
         WAV

Answer: Please, stay far far far far far away from Real Media files - they're very poorly supported by even such advanced encoding applications as mplayer.
I would stick with MP3s or at least re-encode them to a lower bitrate. You could alternatively re-encode them to .ogg, which is an open source format.
Answer: Unless audio quality is very much a secondary consideration, I would say you are best storing all audio in the highest quality, least compressed format you can; 16bit 44kHz WAV if at all possible.  This ensures you can encode in any format you wish to in future without losing further information during transcoding.
Oh, and Real Media?  Is this 1999?  Run, don't walk, to any other format.
Answer:
For losseless data storage use FLAC.
For loosy data storage use OGG.

Don't even get closer to RM... You would get problems.
Answer: Real Media is not bad at compression; but there are other codecs that better, and Real Media has patent and licensing issues, as well as being a poorly documented and supported protocol.
Xiph (the people behind Ogg Vorbis) have published a comparison between the leading codecs (rm, mp3, wma, aac, and ogg) - you can listen and compare each of the codecs.
If your data is already in mp3 (a lossy format) it would not be advisible to convert it, particularly to another lossy format.  You won't gain anything much in storage, and you will automatically lose quality.
Answer: I echo Artem Rusakovskii's and Lunatik's replies. Do not use rm, instead use ogg.

Difference between rm and mp3 formats


How to stop SimpleAnalytics from counting self views?

Question: I am using SimpleAnalytics for my website. But I do not want the views to be recorded when I myself visit the website. Is there any good solution available for this?
Comment: This isn't an answer to your question, but are you really concerned about your visits throwing off your analytics stats meaningfully? Analytics aren't super useful usually without a somewhat sizeable sample of users.
Answer: I'm the founder of Simple Analytics. We have an extension that you can use. You could also dynamically load the script when it's not you.


Why does antenna tuner in crystal radio use capacitor and an inductor?

Question: I know that antennas can be capacitive or inductive depending on the relation of antenna length to the \$\lambda\$. I know that crystal radios use both antenna and capacitor for tuning and I don't understand why because for me it would seem the antenna should only be capacitive and you should only need inductor for tuning. Why is there a capacitor in crystal radios and why is it used for tuning?
Comment: Just like a pendulum has two modes of energy storage ( movement, and height) to store energy, so does any other way to store energy, that is, to achieve a resonance. In electronics, the capacitor's electric field and the inductor's magnetic field are the two energy storage mechanisms **usually** used in radios. Your cellphone also uses quartz, with its own special two mechanisms, to precisely get the channels used. We can acid-etch the quartz more precisely to set a frequency, than we can build capacitors or inductors.
Answer: You are very astute. You can indeed construct a crystal set without a physical capacitor, relying instead on the capacitance of the antenna alone and tuning by varying the inductance. Of course, the value of inductance required will vary with the length of the antenna. Reckon on 10 pF per metre as a starting point. Perhaps a variable capacitor is popular because it can be arranged to dominate the antenna capacitance and make tuning less antenna length dependent. A wide range, reliable variable inductor can also present challenges. Go ahead and experiment.
Answer: It's a transmission-line thing. The electric current propagates down the antenna and, approximately, reflects from the antenna's end back toward the transmitter. Upon reflection, the current (but not the voltage) reverses.
By the time the reflected wave reaches the transmitter again—importantly, this does not happen instantaneously—the reflected wave (with current reversed) superimposes itself upon the signal the transmitter is generating at that moment.
Are you handy with your phasors? The impedance is the ratio of the sum of the voltages to the sum of the currents, phasor style. If the imaginary part of the ratio happens to be positive—and this depends on the wavelength compared to the antenna's length—then the antenna is inductive. Otherwise capacitive.
If you aren't sure that this makes sense, then here is a smaller idea to get you started: at the end of the antenna, the sum of the primary and reflected currents (but not voltages) must be zero because no current flows out the end of the antenna. This smaller idea does not explain the whole problem but it does serve to orient your mind so that, after you have slept on it, you might have a better frame to start to grasp the rest of the problem.
Answer: For the AM broadcast band, few long-wire antenna builders erect a full quarter-wavelength wire (about 75 m) and settle for a shorter wire. A shorter antenna benefits from added inductance to compensate for its capacitive reactance.
Once you include all the resistive components (don't forget ground losses), this inductor-compensated antenna has very low Q . To effectively select one station from another, an LC resonator should have loaded Q of at least 100. So you often see an antenna loosely coupled to a high-Q LC resonator.
Why is "C" varied rather than "L" for tuning?
Compare prices for each. A variable capacitor maintains high Q over its very large capacitance range. Variable inductors seem mechanically complex - some air-core types are bulky. Both tuning methods have been employed - there was a time when car radios used ferrite-slug-tuned inductors for tuning. These were not crystal radios.
Answer: The standard crystal set has an antenna that is electrically short so it is capacitive depending on antenna length and wire thickness, wire insulation, ground proximity etc. So precise Capacitance is install dependent. The ground resistance will generally be much higher than the radiation resistance because of non ideal ground conductivity.
The hobbyist is not likely to do 360 buried ground radials. Hence a loosely coupled antenna with a known high Q tuned circuit will give more predictable results. Remember that it was common in the 1930s for radio shops to make and sell crystal sets whilst just selling the more complex superhets. This LC tuned circuit can have a variable cap and a fixed inductor which is what you normally see. You can do a fixed cap and a variable coil. I have seen this as a child in 1969 where a tuning knob was attached to a slug tuned coil and it worked fine. For mobile stuff like car radios there is a vibration advantage to using permeability tuning but crystal sets are almost always stationary.


How should I reply if the potential supervisor ask me whether I contacted other potential supervisor?

Question: I received a response from potential supervisor, in her response , she said ' I would, in principle be happy for you to apply to the university'. then she asked me whether I contacted with Professor xx (another potential supervisor), she said 'so I think your project will fit really well in their department! You are of course welcome to apply to more than one department, so keep me updated on what you decide to go ahead with.' How should I reply? Thank you!
Comment: Well, what do you want to do? This isn't a puzzle or test with a secret answer.
Comment: It reads like a 'suggestion' more than a serious 'question'. Did you contact the other potential supervisor? Do you think they'd be a good/better fit? I'd just do what she said -- consider your options and get back to her on what you decide to do.
Answer: With the truth: you either contacted the other potential supervisor or did not. In the latter case, you can still contact potential supervisor B.
It is completely understandable you want to find the best project/supervisor/university/etc. for yourself (I mean 'best' in the sense that it fits you best). As long as you are open and honest about it, you should be fine.
Answer: I already gave thumbs up ao Aolon's answer. Just wanted to emphasize.
A PhD supervisor will be a big part of your life for several years. And hopefully it will be a life-long relationship with many positive interactions. If you stay in academe it is very likely you will want to keep a good relationship with your supervisor. It is a good thing to be as open and honest with them as possible.
At least with respect to the subject of your academic work. You may or may not want to be telling them about your personal life. That's a very different subject.
If being honest on such a point causes them to reject you, you probably did not want to work with them anyway.


Get results from JMeter remote test in Java

Question: I´m trying to run JMeter tests directly from my Java code. The load should be produced via a JMeter server. In the code, I use a ClientJMeterEngine instance that connects to the JMeter daemon running on the server. This works so far, I can start and run my test. What I currently don´t understand is how to get the results from the test run after it has completed. My current code looks like this:
<code>// create ClientJMeterEngine instance
ClientJMeterEngine jmeter = new ClientJMeterEngine("myHost:4000");
// load test
File file = new File("C:/myTest.jmx");
// configure test
HashTree testPlanTree = SaveService.loadTree(file);
jmeter.configure(testPlanTree);
// run test
jmeter.runTest();
</code>
If I run the test using the StandardJMeterEngine object, I use a ResultCollector to get the results from my test runs. This works as expected. When I try to run the tests using the described server-based approach, I get the following exception on the machine where the server is running:
<code>2016/03/15 09:24:35 ERROR - jmeter.samplers.BatchSampleSender: testEnded(host) java.rmi.ConnectException: Connection refused to host: myHost; nested exception is:
    java.net.ConnectException: Connection refused
    at sun.rmi.transport.tcp.TCPEndpoint.newSocket(TCPEndpoint.java:631)
    at sun.rmi.transport.tcp.TCPChannel.createConnection(TCPChannel.java:228)
    at sun.rmi.transport.tcp.TCPChannel.newConnection(TCPChannel.java:214)
    at sun.rmi.server.UnicastRef.invoke(UnicastRef.java:142)
    at java.rmi.server.RemoteObjectInvocationHandler.invokeRemoteMethod(RemoteObjectInvocationHandler.java:238)
    at java.rmi.server.RemoteObjectInvocationHandler.invoke(RemoteObjectInvocationHandler.java:190)
    at com.sun.proxy.$Proxy1.processBatch(Unknown Source)
</code>
The server thus wants to call back to the client from which I start the test run. It fails, because I don´t have anything waiting for the remote call. I´ve worked my way through the JMeter source, but haven´t found an answer, what I need to do on my Java side to make this work.
EDIT
In the meantime I´ve made some progress. What I´ve found is the following:
The client thread must be kept in state running, so the ResultCollector on the client can receive the results from the server. I now do this by "busy waiting" in my client Java code.
The issue now is to know when the test execution on the server is finished, so I can also finish the client thread. The ResultCollector does have the capabilities to recognize the test is finished, but no public methods for this.
The solution would be to extend the ResultCollector and override the testEnded method: This method sets a public boolean variable "testFinished" to true.
Though this feels a little bit "hacky". Does anybody have a better solution here?
Comment: Well it's invoking something, on some remote stub, that it got from somewhere. I have no idea about JMeter remoting but you clearly did export a remote object and provide its stub, and subsequently unexported it. Or JMeter did.
Answer: I had the same problem and tried your solution with overriding testEnded() method of ResultCollector class. My test was endlessly waiting for response from remote JMeter slave server.
The actual problem is that remote server - Slave - succeed in replying of requests from Master at early stage. However, the connection dropped at some point and the test finished without receiving any report from a Slave. Slave is unable to start a new TCP connection, and can't send results when it is ready with his testing report as its hostname (IP) hasn't been set/sent to Master correctly.
Therefore, the solution is to start a Slave with the correct parameter in command line or in the jmeter.properties.
I started my slave like this:
c:\JMeter_new\apache-jmeter-3.1\bin>jmeter-server -Djava.rmi.server.hostname=IP
where IP is of the Slave
PS If you run tests in GUI, run Master with the parameter as well:
D:\JMeter_new\apache-jmeter-3.1\bin>jmeterw -Djava.rmi.server.hostname=IP_of_master


How do I make something summon when I use a fishing rod?

Question: I was trying to use the execute command but I was never able to find something where I could trigger it at the use of the fishing rod. I'm trying to make my own weapon using commands and I want to make a line of particles that do damage and deal knockback I thought a bow or fishing rod was a good place to start but I can't find any command that works.
Answer: You can execute a command whenever a fishing_bobber (end of fishing line, only spawns when you use your fishing rod) is spawned. An example:
/execute at @e[type=minecraft:fishing_bobber] run particle minecraft:happy_villager
If you put that command in a repeating command block it will show the happy villager particle along the path the fishing bobber takes.
Comment: This is exactly what I needed tysm.
Answer: You have to wait until there is a bubble trail going to your bobber. After that, the bubbles will hit the bobber and it will go underwater. When that happens right click and if that doesn't work left click.
Comment: Please read the question properly. This isn't just "how do I fish" but "how do I use commands to make something special happen when I fish"


How much do R2-D2 and BB-8 weigh?

Question: The question is self-explanatory:  How much do the astromech droids R2-D2 and BB-8 weigh, in-universe?
Comment: I was thinking of this myself rewatching ESB. Threepio and Artoo fall from a short height from a hovercraft into the sand, yet manage to get almost completely buried. That's some impact!
Comment: @CreationEdge that was in RotJ :)
Comment: @RedCaio Geez, and I almost typed Ep XI instead. I don't know why I mixed up the titles, I hadn't even started drinking yet!
Comment: @CreationEdge - as we see in Ep7, there is quicksand on sand planets in TGFFA. Could have possibly been that? (ok, ok, I know it was Lucas doing his Kurosawa thing. But I'm allowed to my own fan theories, ain't i?)
Comment: @DVK I think comedy relief gets a free pass. Comedy is the power that bends the universe to its will.
Comment: @CreationEdge - Mark Twain is a Sith Lord?
Answer: R2-D2:

Wookieepedia says (citing "Star Wars Rebels: Head to Head") 32 kg.
Apparently the same number was in EU/Legends, in "Star Wars: Head-to-Head Tag Teams, pg 50" as per the same page.

BB-8

couldn't find in-universe evidence in the film, novelization, or Visual Dictionary page.
Sphero's out of universe popular toy is ~200 g (at the height of 11cm, compared to real one's Databank height of 0.67m). If we proportionally size the weight up by 6^3=216 times, we get ~42kg.
Which probably isn't accurate, since I assume TGFFA has much better/lighter materials than Sphero (since R2-D2 could fly and BB-8 has liquid metal cables that hold its weight and pull him up).
Doing very back of the envelope guessing it looks like his body is ~20-30% of R2-D2 by volume. (they fit in same size X-Wing astromech socket.

R2-D2's body seems to be exactly 1.5 times BB-8 body, since his body+head fits exactly 2 BB-8 bodies and his head is an exact hemisphere of the same radius. That gives us 9.42+4.19/2=11.5 for R2-D2 volume given radius of 1 (for simplicity of comparison), excluding appendages. BB-8 is 4.19 for the body and roughly 0.26 for head (half a sphere size of half the body), totalling 4.45.

Assuming he's not made of some advanced ultralight composites that severely affected his density by comparison the above gives us ~12.4kg. It may be less because of more advanced materials, or because I didn't count the volume of R2-D2's legs; or it can be less because I assume a spherical droid's internal space has less empty lacunae due to better space packing. Without better canon info, I'll call those factors a wash for hand-waved calculation purposes and settle on 12kg.
Comment: Before it falls on him he attempts to lift BB-8 down into the hold. He makes a sort of struggling gasp and falls over being crushed by BB-8. 12kgs is really not a lot at all specially for a storm trooper who has probably had some form of fitness training :P
Comment: Finn needs to hit the gym if he struggles with 12kgs
Comment: @CandiedMango - huh? when was he carrying BB-8? I know it FELL on him... but I wouldn't be happy if 12kg fell on me either :)
Comment: @CandiedMango - I must have mis-seen it. I was under firm impression that Rey dropped BB-8 on him from above. Can't wait for DVD!
Comment: Hmm I seem to remember him rolling off the edge into an expecting finn. Knowing modern times we'll only have to wait a few months!
Comment: @CandiedMango  As a trained stormtrooper, Finn is used to wearing protective armour!
Comment: @MrLister - clearly not, **since he takes off the bloody helmet the moment he can, contrary to regulations** :)
Comment: According to TFA: Head-to-Head, BB-8 weighs 18Kg

How much do R2-D2 and BB-8 weigh?


Travelling with US H1B visa on old cancelled indian passport

Question: I renewed my Indian passport before its expiry. My old passport is marked cancelled. I have a valid H1b visa till Jan 2019 on my old cancelled passport, do I need a new H1B stamp on new passport or can I use valid visa in my old cancelled passport?
Answer: The unexpired visa in your expired passport remains valid, and you can continue to use your existing visa to enter the US, without getting a new visa, as long as you use it with a new passport of the same type and issued by the same country as the old passport.
FAQ from the Department of State:

My old passport has already expired. My visa to travel to the United
  States is still valid but in my expired passport. Do I need to apply
  for a new visa with my new passport?
No. If your visa is still valid you can travel to the United States
  with your two passports, as long as the visa is valid, not damaged,
  and is the appropriate type of visa required for your principal
  purpose of travel. (Example: tourist visa, when your principal purpose
  of travel is tourism). Both passports (the valid and the expired one
  with the visa) should be from the same country and type (Example: both
  Uruguayan regular passports, both official passports, etc.). When you
  arrive at the U.S. port-of-entry (POE, generally an airport or land
  border) the Customs and Border Protection Immigration Officer will
  check your visa in the old passport and if s/he decides to admit you
  into the United States they will stamp your new passport with an
  admission stamp along with the annotation "VIOPP" (visa in other
  passport). Do not try to remove the visa from your old passport and
  stick it into the new valid passport. If you do so, your visa will no
  longer be valid.


What is really instantaneous?

Question: How can a body travel at an instant and what does instantaneous speed tells us?
What really is meant by speed of an object at an instant if an object does not travel at an instant? I would like a mathematical explanation.
Comment: Zeno's paradox? Mathematics of limits? Derivatives?
Comment: An interesting side note. Multiple rapid observations seems to "pause" time. https://en.m.wikipedia.org/wiki/Quantum_Zeno_effect
Answer: Instantaneous speed (and indeed many other instantaneous concepts) is a bit of a formalism, but in layman's terms, the instantaneous speed is the ratio of distance covered to time, taken over a very small time interval. Formally, we say that the time over which the distance is measured is actually infinitesimally small, so we can represent instantaneous speed as the following limit:
$$v=\lim\limits_{\Delta t\to 0}\frac{\Delta x}{\Delta t}$$
Answer: It is an experimental fact that an object that is at rest at a particular instant behaves differently than an object that, at that instant, is in the same position but in motion, would behave. It is up to us to fashion our physical theories such that they agrees with this observed behavior of the world.
Experience forces us to conclude that knowing where every particle in the object is at some particular instant is not a complete description of the state of that object. There is something more than position that can be different, something that is responsible for the object sometimes being found at slightly different positions in the following instants.
In classical mechanics we don't even attempt to understand what the "something more" is -- but we do give it a name: momentum. It is common to learn in school that momentum is an abstract quantity that you can compute once you know the mass and the motion of an object, but it is just as fruitful from a physical point of view to imagine that it is something that really exists: In order to fully specify the state of an object, you need to know not only where it is but also the quantity of momentum it "contains". If you know both of these, you can make a pretty good guess at what its position in the next instant will be.
Momentum (together with mass) tell us how the position of an object changes over time. We also have (experimental) rules for how the momentum of an object changes over time -- these are the laws of mechanics.
In particular, it turns out that momentum is conserved, so it does make good sense to think of it (at least some of the time, informally) as a "substance" that is inherent to an object, rather than just a helper quantity for calculations. The transport of momentum from one object to another is known as force.
Quantum mechanics has its own mathematical description of what momentum "really is", in terms of quantum amplitudes. This is not particularly accessible and is best ignored as long as you're on a stage where you can ask this question. It is a matter of some contention whether the words "really is" even apply to any quantum-mechanical description at all.`
Comment: I was thinking to vote to migrate this question to math.SE. What do you think? should it be migrated?
Comment: @AccidentalFourierTransform: That depends on assumptions about what the OP is really asking. The _conventional answer_ in terms of kinematics would certainly be at home at Math.SE. But for the purposes of this answer I'm assuming that the OP already knows (or can learn) that and is interested in how the derivatives (which assume that we know the motion of a body over many instants) connect to physical reality in a _single_ instant. And that seems to be to be essentially physics.
Answer: Instantaneous (linear) speed isn't that helpful. In planar rigid body mechanics instantaneous rotational speed and the point about which the body is rotating is extremely helpful as it describes the velocity of the body at every other point (ie. describes the motion completely).
For example see my answer in Rotation of Slipping Ladder where the motion of the ladder is described in terms of the instant center of rotation (Point S below).

The helpful insight extends to the fact the once the linear velocities of the body are described everywhere, any reaction forces applied have to be perpendicular to the motion (reactions do no work). So the direction of the reaction forces is always pointing towards the instant center of rotation.
Note that the instant center is not fixed in space. At every instant it can be located at a completely different point. There is no concept of speed of the instant center. It is just a point in space valid only for one instant in time.

What is really instantaneous?


Automate creating an image pyramid store with Geoserver?

Question: Geoserver has a REST API to automate Geoserver commands, however I do not see any examples of how to automatically create an Image Pyramid store and associated layer. Is that possible? Can anyone provide me sample code?
Answer: You can create pyramid with gdaladdo, you not create pyramid with geoserver. Here is documentation of it
https://gdal.org/gdaladdo.html


Can I recover my 4TB SATA HDD accidentally re-initialized with MBR?

Question: I changed my 4TB SATA HDD to a different 3.5 inch USB enclosure.
When plugging the new enclosure's USB cable into my Windows 7 x64 PC, in the Disk Management MMC, it asked to be initialized with MBR, and I consented. I now have two 1.8TB drives without data.
Is there a way to go back to a GPT partition table and recover the data that was originalky on the drive?
Comment: Look into how to restore the partition data if you cannot do that then the original data lost.  In the future you know that MBR disks can not support 4TB Disks
Answer: testdisk, a popular open-source tool to search for and salvage partition metadata, may be able to help you here. While other tools (such as Recuva or photorec, the latter being from the same company that makes testdisk) will search your partitions for traces/remnants of your files and attempt to rescue them, testdisk will search for lost/overwritten information about where your partitions were originally located and what their structure was like (filesystem, start sector, length, etc).
If you're lucky, testdisk might be able to simply virtually undo all changes you made by either rebuilding or recovering the GPT structure.
If you've written data to the disk (other than simply creating a new, incorrect MBR), your odds of recovering the data are considerably reduced.
Answer: It's possible that the GPT backup data, and maybe even the primary GPT data, remain intact. If so, my GPT fdisk (gdisk) utility should be able to recover the original partitions. See the gdisk documentation's page on data recovery for some basic information and advice.
Two important caveats:

Switching external enclosures can change the way the disk looks to the computer. In particular, some enclosures translate from 512-byte logical sectors to 4096-byte logical sectors. If your first enclosure did such a translation but the new one doesn't (or vice-versa), or if either enclosure does something else unusual, you'll need to put the physical disk back in its original enclosure before this will work.
If Windows "formatted" the new partitions, chances are critical data structures from the original filesystems will be overwritten. This may make it difficult to recover anything from the disk. Running CHKDSK (or its GUI equivalent) may help, or you may need to run a file-scanning tool on the disk. The only such tool I know anything about is PhotoRec, but there may be Windows-specific tools that will do a better job on NTFS volumes.

Note that partitions are just chunks of the disk, as defined in a partition table. Filesystems are more complex data structures that are typically stored within partitions. Although many tools manipulate both together, they're different data structures, and understanding this distinction can be critical for data recovery. In particular, gdisk is a partitioning tool that doesn't touch filesystem data, so gdisk may be able to help with the partitioning issue, but if there's filesystem damage gdisk can't help with that.
Answer: Some files or file directories are likely to be lost, but you should be able to recover most of the file data this way:
If you have another machine with an OS that supports GPT, first install an unerase utility.
Then attach the drive, delete the MBR partitions so the drive is completely unallocated.
Then create the single GPT partition but opt for quick formatting.
This will create empty file tables, but not delete the file data.
Then run the unerase utility. This will find both valid files and old delete fragments. Hopefully there will be enough structure left to recover anything critical.


Data Import Wizard not working

Question: When I try to use the Data Import Wizard (which I have used many times) I am getting the below error.  I do not have a field named status in my import file.  Is anyone else experiencing this problem?  I successfully used the import wizard a week ago.  This appears to be a new problem.  I have logged out and back in & the problem still exists.  I am using a System Administrator account.
Comment: what object are you trying to upload?
Comment: @cropredy  The data import error was occurring on any object that I tried to import into.
Answer: I determined that the problem was caused by the "Salesforce Community Page Optimizer" Chrome extension.  Once I disabled the extension, the data import wizard worked fine.

Data Import Wizard not working


Before proposing reviewers, should I notify them?

Question: Some journals require the author to propose potential reviewers. Should I just propose reviewers and let the editor contact them? Or should I first contact the potential reviewers myself and ask if they agree?
Comment: You can leave contacting the reviewers to the editor. The editor, in any case, may ignore your recommendations for potential (or blocked) reviewers.
Answer: I never asked them and never heard of anybody doing so. It's the journal who is responsible to organise peer review, not you, and they have to and will humbly ask¹ potential reviewers anyway. By asking potential reviewers yourself, you cause the following problems:

Not every reviewer you suggest will be chosen by the editor. For a reviewer who will not be chosen to review, deciding upon your request will be a waste of time.
Even a reviewer who is eventually chosen will waste a little time by responding to the same request twice.
If a potential reviewer declines your request and you do not suggest them, the editor may still choose them on their own (most editors to whom I have talked make a point of choosing at least one reviewer who was not suggested). In that case, the potential reviewer may think that you ignored their wish.
Asking reviewers directly is a first step to breaching the independence of peer review as it opens up the path for manipulating peer review: "Sure I will review your paper [favourably] – provided that you will do the same thing for my next paper." Some oppose even the mere option to suggest peer reviewers for this reason.
If you know only a limited list of potential reviewers for your paper, removing those who deny your request may make the list smaller than what is requested by the journal. If those reviewers only refuse when asked by the journal, you still did your duty of suggesting sufficiently many potential reviewers (and this may not even be a problem, if the journal requests a list that is longer than the number of actual reviewers).
You waste some of your time by asking the potential reviewers and waiting for their response may delay the actual submission.
If you show your paper to your potential reviewers privately, they have an advantage in the rare case that they try to steal your ideas or commit other dishonesties. By contrast, peer reviewers are often bound to terms of non-disclosure and similar and also the journal serves as an independent register of who had knowledge of the manuscript. This point does of course not apply, if you trust all potential reviewers or published a preprint of your paper anyway.

¹ Except perhaps for some journals with a questionably demanding demeanor.
Answer: You should never ask someone as an author whether they would be willing to serve as a referee for your paper.  In addition to Wrzlprmft's issues:

It looks suspicious, like you may be filtering your suggested referees based on how enthusiastic they seem to you.  This could happen even if it's honestly not your goal.  For example, suppose you ask A, who replies "Wow, I can't wait to read it!", and B, who replies "I'm really busy but I could do it if necessary."  If you suggest only A to the editor, are you biasing the process in favor of your paper, or just being respectful of B's time?  There's no clear way to draw the line.
You're putting potential reviewers in an awkward position by making them tell you whether they could do it.  For example, they may feel more comfortable telling an editor that they are too busy or feel the paper is outside their area of expertise, rather than telling you.  They may also worry that giving you this information decreases their anonymity.
Answer: I would say that any contact between referees and authors should be minimized. Just mention any potential referees when submitting.
Secondly, my personal experience is, is that referees are chosen from the existing journal's database, regardless whether and how many potential referees are provided. This, on the basis of the types and quality of the referee's reports I received in a number of cases; for example quite poor and obviously not one of my suggested referees, or their response was way too knowledgeable about certain topics to meet the knowledge on those topics of my suggested referees.
And to add to that: I've talked to an editor-in-chief who gave a crash-course in article writing and he said that he would rather not use the potential referees as they are likely related to the manuscript author(s) in one way or another, professionally or otherwise. He told us that he, therefore, never provided any potential referees with his submissions, unless he had to.
Answer: Managing the review of the paper, including the contact with reviewers, is the task of the editor in charge. The process should remain as blind a possible. Knowing too much may bias the review.
However I have witnessed some outlier cases:

a researcher I know had trouble with a paper, for which the editor add difficulties finding reviewers, asked me (and somebody else he knew) as a service to review it. I would probably have refused if it were send by the editor only
acting as both an author and a guest editor for a special issue or a book.
Answer: You definitely should contact with reviewers, talk to them and then redirect to the editor.
Comment: The motivations (time savings) you expose in your comments are important to understand your answer. I would suggest to edit your answer accordingly.
Comment: Welcome to Academia stack exchange! This answer is very brief, and doesn't really provide a rationale as to why someone would want to follow this advice. Could you expand your answer to explain your reasoning?
Comment: I strongly disagree with this approach. For reasons, see the other answers.
Comment: I had the same dilemma, i talked with reviewers, asked them if they agree, and then they had a conversation with editor, it saved editor`s time. If you disagree, its your choice)
Comment: @Bertram if you're willing to come back and improve/add-to your answer in response to comments, others will be much more willing to upvote you.


How do you teleport a player to spawn when they win a 1v1

Question: On a kitpvp server I manage, players can duel. When they duel each other, they get teleported to a room. I'm having trouble teleporting the player back to spawn after they win. I can do it manually, but I'd rather it be automatic.
Comment: As someone who is unfamiliar with KitPvP duels, does the losing player die when defeated?
Comment: Hi Sen, what have you tried so far?
Comment: Well i have tried beds thats about it
Answer: I did a similar thing back in 1.12 for a small server. The way I did it was to have a command block assign each player to a different team when they start the game (using scoreboards). Then I would have 2 command blocks checking for at least 1 player on those two teams to be in a certain radius. Using gates, once any of the command blocks doesn't detect a player in a team, it runs another command block that teleports the remaining player/players back to a specific location. The players are then removed from the teams. Due to my lack of knowledge of 1.13 commands, I am unable to provide the commands that you could use for this.
I wish you luck


Meaning of "Officious" to include exploiting a minor office?

Question: Years ago (like in the 1990s), I looked up the word in the OED and saw only meanings like offering unsolicited advice. But I think many people use it to describe people who take their small amount of power seriously. Barney Fife might be an example, maybe there are better ones. My question is, is this Barney-Fifish meaning now accepted and if so, is it pretty new usage?
Answer: tl;dr: you're right.

in the 1990s), I looked up the word in the OED and saw

In the 1990s you would have been looking at the second edition of the OED, finalised in 1989. In OED2, officious is given as(£):

†1. Doing or ready to do kind offices; eager to serve or please;
  attentive, obliging, kind. Obs.

†b. officious lie (L. mendacium officiosum, F. mensonge officieux): a
    lie told as an act of kindness to further another's interests. So
    officious falsity. Obs.

†2. Dutiful; active or zealous in doing one's
  duty. Obs.

b. Of a thing: Performing its office or function, serving its purpose,
    efficacious. rare.

3 Unduly forward in proffering services or taking business upon oneself;
  doing, or prone to do, more than is asked or required;
  interfering with what is not one's concern; pragmatical, meddlesome.
†4. Pertaining to an office or business, official; hence, formal. Obs.

Diplomacy. As opposed to official: Having an extraneous relation to official matters or duties; having the character of a friendly
  communication, or informal action, on the part of a government or its
  official representatives. (= F. officieux (Littré), It. uffizioso.)

Noting that † means obsolete, we see that the only non-rare still-current general meaning is 3, which accords with your memory.
By contrast, the current OED (OED3, from March 2004) has (£) - my emphasis:

†1.    a. Of persons or their actions, etc.: active or zealous in the
  exercise of an office; dutiful. Obs.    b. Of things: performing the
  proper office or function; serving the required purpose; efficacious.
  Obs.
2 Doing or ready to do kind offices; eager to serve, help, or please; attentive, obliging, kind. Now rare, exc. as passing into
  sense 3. Cf. officious lie n., officious falsity n. at Special uses.
3 Unduly forward in offering one's services, or in taking business upon oneself; doing, or prone to do, more than is asked or required;
  interfering, intrusive. In later use esp.: inclined to assert
  authority in a self-important or pompous way, esp. with regard to
  petty or trivial matters. (Now the usual sense.)
†4. Relating or
  belonging to an office or business; official, formal. Obs.
5 In diplomats' use (as distinguished from official): not directly bearing on official business; having the character of a friendly
  communication or informal action on the part of a government or its
  official representatives; informal, unofficial. Now rare.

So we see that that same sense 3 has evolved into what is now (I agree) the usual sense. And it is this 'usual sense' that is given prime billing at oxforddictionaries.com, which is fair enough. The quotations for sense 3 show this evolution starting well before 1989, so it's perhaps surprising that OED2 doesn't include this nuance at all.

oed.com pages are subscriber-only, but UK library membership often grants subscriber access
Comment: Thanks very much -- my memory was correct. I don't know if anyone using officious nowadays except in the "petty tyrant" sense at all and if I understand your response entirely, this evolved fairly recently.
Answer:
"people who take their small amount of power seriously"

This sense of the word you refer to is exactly how Oxford currently defines it, and it does not seem to be a 'new' meaning of the word:

officious [adjective]

Assertive of authority in a domineering way, especially with regard to trivial matters.

The commentary on origin and changing meanings is notable:

Origin: Late 15th century: from Latin officiosus 'obliging', from officium (see office). The original sense was 'performing its function, efficacious', whence 'ready to help or please' (mid 16th century), later becoming depreciatory (late 16th century).

The other meaning is

1.1 Intrusively enthusiastic in offering help or advice; interfering.

Source: https://en.oxforddictionaries.com/definition/officious
Comment: The first definition I am sure was not even listed in the OED in 1990 or so. Although the origin is shown, it is not clear at what point "my" definition became first unless you mean "becoming deprecatory" but that might include offering unsolicited help rather than (1.). I would actually be interested in how the OED showed it in 1990 to see if I am even remembering it rightly: I did not have an OED at home, and so I looked at the library and remember chatting about this with the librarian.
Comment: It's very unlikely to be an entirely 'new' meaning but it is very possible @Jeff that one traditional meaning became more widely used than the others over these 27 odd years. Unfortunately I do not have access to the full version of the OED so I hope some other member will clarify that point about the 1990 listing.


What are the key differentiating factors between the flight control systems employed in unmanned drones and those utilized in piloted aerial vehicles?

Question: "I've come across open-source projects like ArduPilot that are commonly used with microcontrollers like Pixhawk for unmanned aircraft. However, I'm unsure whether these systems can be adapted for use in manned aircraft. My knowledge of control algorithms in manned aviation is limited to basic concepts like PID controllers. I would greatly appreciate it if someone could kindly provide insights into the control algorithms employed in manned aircraft and share guidance on developing communication protocols for such systems. Your expertise in this area would be immensely helpful. Thank you.
Comment: Simple aircraft use pushrods and cables for their flight controls.

Manned aircraft using fly-by-wire controls do so with extremely sophisticated software that include carefully verified algorithms and extensive backups -- not at all the stuff of "limited knowledge of basic concepts."

You're going to need a lot more knowledge than can be gained on an internet forum before you're ready to "develop communications protocols for such {manned} systems." There are discussions on Av.SE of fly-by-wire designs and certification requirements, but this question as it stands is far too broad.
Comment: There's not a single computer on a transport aircraft. Navigation is not the same than autothrotle or engine control (FADEC), and control surfaces have their own controllers which use different "laws" in case of failure. Some of these computers run on real-time OSes. So which function do you want to compare to Arduino application? Related: [Airbus control architecture - where does the actual autopilot live?](https://aviation.stackexchange.com/q/46989/3201)
Comment: The design configuration of an octocopter is supported by Ardupilot. There's also a single-seat manned aircraft called the Jetson One, which has a design that resembles an octocopter. As a result, the control surfaces on both of these aircraft may appear similar. Then, why not use open-source software, given that the configuration is supported by these software packages?
Answer: The primary difference is liability, which drives a chain of other items.
There is an expectation of the various parties involved in approving, licensing and insuring aircraft that there is a clearly designated chain of responsibility to the various decisions and actions should something turn out to be wrong.
So it would be perfectly feasible to take an open source autopilot and slap it into a full sized aircraft registered for experimental use, and quite possibly it would not immediately crash*. However if you want to operate outside the experimental limitations (actually have a useful aircraft) you will need to convince various parties that your autopilot is safe. And saying 'I downloaded software built by strangers from the internet' will not go very far.
The process from here would be basically a line by line effort documenting and explaining how the software worked and makes decisions, and how those decisions maximize safety. This is probably easier than building an autopilot from scratch but still a multi man year project. At the end of it you would be taking liability for any unfortunate outcomes of the system working as designed, which possibly should give you pause if you have had much involvement in crowd sourced software.

Some reasons why a model aircraft autopilot might sorta work but run into trouble entrusted with a full sized aircraft without re-design include:

Different aerodynamic behavior due to scale so various
constants will be wrong.

Square cube law will mean that controls will respond slower/have more
inertia than at model scale leading to induced oscillation.

Square cube law also means structure safety and power margins are
lower, so less aggressive control actions required to avoid mid air
destruction.

Unmanned vehicles detecting failure should find a safe place to crash
ASAP to protect those on the ground. A manned aircraft probably
should not still have 'crash now' code in it or at least very
carefully tuned.

Handling disputes between humans and autopilot for control.

To achieve higher reliability redundancy is involved which means programming systems to read from duplicate sensors, and have multiple controllers with some sort of arbitration managing which actually pilots the vehicle. Dealing with noisy/stale/lying data in aircraft is a still a hard problem.
Comment: *And saying 'I downloaded software built by strangers from the internet' will not go very far*


How to get back to original spawn when in creative and spawn resets?

Question: I made a really good castle in creative mode but had not slept in a bed on superflat mode. Someone killed me and that somehow reset my spawn more than once. I did tests in a new world of the same kind of circumstances and found that when killed and not having slept in a bed you respawn in a totally different area. If anyone knows how to get back to my original spawn point I would really appreciate the help.
Answer: Use Minutor, or some other mapping program to work out the coordinates of your epic castle (in Minutor, the coordinates of the block under the cursor are shown in the status bar at the bottom). Then in-game, press F3 to see your coordinates and walk to the coordinates you found.
Then sleep in a bed and reset your spawn!!
Comment: The compass works too to get the the original spawn without installing any tools.
Comment: @fredley Is there any way to simply teleport back to the original spawn (instead of walking perhaps thousands of meters?)
Comment: @AndersonGreen Yes, you can use the `/tp` command and teleport directly to the coordinates you found, e.g. `/tp 345 64 923456`
Comment: How does the compass help get back to spawn? All it does is show you the direction you are facing.
Answer: If you built it near your original spawn point, a compass will point you there. You can give yourself one in Creative Mode.
Answer: Honestly, all I did was follow a compass. I too, was stuck and I had absolutely no idea how to find my whole city I built. All I did was follow a compass and I was back at my city. I went pretty far from my city and it took a while to get back so don't get impatient. It might take a while to get back to you house or whatever it is you built, but you will get there.
Answer: The compass points to your initial spawn point (world spawn point). Just point the compass so the red line appears straight ahead as if north and then keep following until you are back to your starting point. If you don't remember what it looks like you will know when the compass keeps pointing back behind you as you try to continue straight ahead.


Escape velocity when Sun is present

Question: When we calculate escape velocity for an object from earth, what we do is, we conserve energy. So if $v$ is the escape velocity, we write
$$-\frac{GM_em}{R_e}+\frac{1}{2}mv^2=0$$ since we are assuming that the object escapes earth's gravitational field at infinity and the velocity just becomes $0$ at infinity. But there are other celestial objects in the solar system. So,for example,when the sun is considered,won't the escape velocity of earth change as well? Since then the energy conservation will be as follows:
$$-\frac{GM_sm}{r}-\frac{GM_em}{R_e}+\frac{1}{2}mv^2=0,$$ where $r$ is the radius of earth's orbit around sun. Here, the object will again escape the earth only at infinity and the velocity also just becomes $0$. Here we can clearly see that escape velocity of earth is changing as other celestial objects are taken into consideration. So why do we still say escape velocity of earth is $11.2$ km/s whereas this value was derived assuming that no other celestial objects other than earth is being considered. Isn't it wrong? I am really confused.
Comment: This may help: https://xkcd.com/681/ or it might add to the confusion. ;)
Comment: If you include the Sun's gravitational potential in your energy calculation you also need to (somehow) include the kinetic energy due to the Earth's speed relative to the Sun.
Comment: See https://space.stackexchange.com/q/3612/
Answer: It depends how you calculate the escape speed. 11 km/s is not the speed required to escape to infinity, for the reasons you describe, but it is (approximately) the speed to escape being gravitationally bound to the Earth. Thus launching something at 12 km/s from the Earth means it won't be coming back, but it will still be orbiting the Sun within the Solar System.
We can make the usual escape speed approximation if the other celestial bodies are far enough away that the additional gravitational potential due to them can be considered unchanging on either side of the Earth. In this case, the gravitational potential of the Sun is just an additive background perturbation that applies equally wherever the body is in the system.
If this approximation applies, then the sum of the kinetic energy and change in gravitational potential energy being zero tells us that the object can escape the gravitational potential of the Earth, but not necessarily from the background potential of the Sun in which the Earth is embedded.
Of course, the gravitational potential of the Sun is not constant either side of the Earth, it varies with distance from the Sun. Leaving aside the complication of angular momentum, if you imagine the potential wells of the Sun and the Earth embedded within that; you can probably see that it would be slightly easier (than the simple calculation of escape speed) to escape from the Earth towards the Sun than away from it.
Comment: I don't actually understand what's wrong with the last equation I made. I mean I don't see any thing wrong in conserving the energy though I must be wrong. Could you please correct the final equation in that case?
Comment: @a_i_r There's nothing wrong with it of you want to work out the escape speed required to get out of the Solar System.
Comment: @a_i_r But if you do, don;t forget that the Earth already has a velocity with respect to the Sun, which should be added (as a vector) to your launch velocity, prior to calculating the kinetic energy in that case.
Comment: Thank you,so in this case,if the launching speed is $v$ for getting out of the solar system with $v_e$ being earth's velocity wrt sun,then the desired equation is : $-\frac{GM_sm}{r}-\frac{GM_em}{R}+\frac{1}{2}m(v_e+v)^2=0$. Am I right?
Comment: I think that, because of the way tidal forces work, while it is _very_ slightly easier to escape from the Earth toward the Sun than away from the Sun, the latter is still slightly easier that it would be to escape from the Earth if there was no Sun; but escaping from the Earth if there was no Sun would be slightly easier that escaping from the Earth in a direction perpendicular to the direction to the Sun.
Comment: @a_i_r the velocity of the Earth and the launch velocity would need to be combined as vectors.  So you would get a combined speed from your formula which would equal the magnitude of the vector sum of $\vec{v}_e$ and $\vec{v}$. From that you can get a maximum and minimum escape speed. The minimum launch speed from the Earth to escape the Solar System is only about 13 km/s I believe, because the Earth's velocity around the Sun is already 30 km/s. The maximum would be more like 72 km/s if you fire the object opposite to the Earth's velocity.
Answer: The escape velocity of 11.2 km indeed does not include the effect of other bodies, such as the Moon, the Sun, the planets, but also the Milky Way etc. It also does not include the rotation of Earth around its axis nor the Sun, nor the rotation of the solar system around the galaxy.
The precise escape, or rather required initial, velocity depends on the latitude, the orbit, say around the moon or a planet, that you are aiming for and how much propulsion you are bringing on your rocket.
To save fuel you could decide to launch your rocket to the moon from the equator at new moon.
Answer:
to obtain the escape velocity of a mass $~m~$ unter the gravitation forces of the sun and the earth , we first transformed , the velocities to   the center of mass velocity and the relative velocities . with
$$ \dot r_{{C}}=0={\frac {M_{{S}}\dot r_{{S}}+M_{{E}}\dot r_{{E}}+m\dot r_{{m}}}{M_{{S}}+M_{{E}
}+m}}
\\\dot r_{ES}=\dot r_E-\dot r_S\\
\dot r_{mS}=\dot r_m-\dot r_S$$
solve those 3 equations for $~\dot r_S=v_S~,\dot r_E=v_E~,\dot r_m=v_m~$
$$v_S=-{\frac {M_{{E}}v_{{{\it ES}}}}{M_{{S}}+M_{{E}}+m}}-{\frac {mv_{{{\it
mS}}}}{M_{{S}}+M_{{E}}+m}}
$$
$$v_E={\frac { \left( M_{{S}}+m \right) v_{{{\it ES}}}}{M_{{S}}+M_{{E}}+m}}-
{\frac {mv_{{{\it mS}}}}{M_{{S}}+M_{{E}}+m}}$$
$$v_m=-{\frac {M_{{E}}v_{{{\it ES}}}}{M_{{S}}+M_{{E}}+m}}+{\frac { \left( M_
{{S}}+M_{{E}} \right) v_{{{\it mS}}}}{M_{{S}}+M_{{E}}+m}}
$$
with $~r_{mS}\approx r_{ES}=r\quad\Rightarrow v_{mS}\approx v_{ES} =v$
thus the   energy is
$$ \frac 12 \frac {v^2\,M_S\,(M_E+m)}{M_S+M_E+m}-\frac {G\,M_S\,M_E}{r}-\frac{G\,M_s\,m}{r}-\frac{G\,M_E\,m}{r_E}=0$$
form here  the escape velocity
$$v^2\approx 2\,\frac{G\,(M_S+M_E+m)}{r}=  2\,\frac{G\,(M_S+M_E)}{r}  $$

$$M_E= 6.370\,10^6 ~,M_S=1.989\,10^{30}~,r=1.496\,10^{11}~,G=6.67430\,10^{-11}$$
thus the escape velocity $~v=42.12~$[km/s]

$~r_C~$ center of mass position
$~r_S~$ sun position vector inertial system
$~r_E~$ earth position vector inertial system
$~r_m~$ mass position vector inertial system
$~M_S~$ sun mass
$~M_E~$ earth mass
$~r~$ distance earth sun
Answer: Escape from the Earth is a specific, solvable problem. The general, multibody,  problem is much messier. Gravity assist trajectories allow a spacecraft to, in principle, escape the Solar System, the Galaxy, and even the local supercluster with little change in velocity beyond that needed to reach a Lagrange point. From Earth's surface, that's a bit less than what we call "escape velocity". From there, a spacecraft can escape into solar orbit and subsequently gain velocity from encounters with planets. This can lead to its ejection into interstellar space, where encounters with stars can continue the process.
So, regardless of where you want to go, "escape velocity" in its common meaning suffices. However, in general, low initial velocity trajectories take a long time. Investigators seeking results in their lifetimes may want to choose trajectories with higher initial velocities for their spacecraft.

Escape velocity when Sun is present


How do you confirm payment via a JSON RPC call?

Question: I know you can use e.g. monero-wallet.cli to confirm payment via a payment ID, but how do you do this with a JSON call to the daemon?
Answer: There are get_payments and get_bulk_payments JSON RPCs you can send to a running monero-wallet-rpc daemon to get a list of incoming payments using some given payment IDs.
There are some examples at https://getmonero.org/resources/developer-guides/wallet-rpc.html

curl -X POST http://127.0.0.1:18082/json_rpc
     -d '{"jsonrpc":"2.0",
          "id":"0",
          "method":"get_payments",
          "params":{"payment_id":"4279257e0a20608e25dba8744949c9e1caff4fcdafc7d5362ecf14225f3d9030"}}'
     -H 'Content-Type: application/json'

{ "id": "0",
  "jsonrpc": "2.0",
  "result": { "payments": [{ "amount": 10350000000000,
                             "block_height": 994327,
                             "payment_id": "4279257e0a20608e25dba8744949c9e1caff4fcdafc7d5362ecf14225f3d9030",
                             "tx_hash": "c391089f5b1b02067acc15294e3629a463412af1f1ed0f354113dd4467e4f6c1",
                             "unlock_time": 0 }] } }


Is the set of undecidable problems decidable?

Question: I would like to know if the set of undecidable problems (within ZFC or other standard system of axioms) is decidable (in the same sense of decidable). Thanks in advance, and I apologize if the question is too basic.
Comment:  This set in non-computable. Why? Let us suppose that the set of independent statements is computable. Then, so is its complement, i.e., the set of formulas $\varphi$ such that either $ZFC \vdash \varphi$ or $ZFC \vdash \neg \varphi$. Using this it easily follows that the set of consequences of ZFC is decidable, which is well-known to be false.
Comment: Perhaps it is worth clarifying that the term "undecidable" refers to "independece in ZFC", while the term "decidable" refers to "computable (also called recursive)"
Comment: I think that this was addressed somewhere on http://math.stackexchange.com however, I cannot find it.
Answer: No, in fact the situation is even worse than that. The set $T$ of all (Gödel codes for) sentences that are provable from ZFC is computably enumerable; the set $F$ of all sentences that refutable from ZFC is also computably enumerable. These two sets $T$ and $F$ form an inseparable pair: $T \cap F = \varnothing$ but there is no computable set $C$ such that $T \subseteq C$ and $F \cap C = \varnothing$.
In other words, there is no finite algorithm that can reliably tell whether a sentence is "not provable" or "not refutable." This is much weaker than asking for an algorithm to tell us whether a sentence is independent or not (in which case we could determine whether it is provable or refutable by waiting until it enters $T$ or $F$).
This remains true if ZFC is replaced by PA and some still weaker theories.
Answer: For every consistent recursively axiomatizable theory $T$ (one of which $ZFC$ is widely believed to be) there exists an integer number $K$ such that the following Diophantine equation (where all letters except $K$ are variables) has no solutions over non-negative integers, but this fact cannot be proved in $T$:

\begin{align}&(elg^2 + \alpha - bq^2)^2 + (q - b^{5^{60}})^2 + (\lambda + q^4 - 1 - \lambda b^5)^2 + \\
&(\theta + 2z - b^5)^2 + (u + t \theta - l)^2 + (y + m \theta - e)^2 + (n - q^{16})^2 + \\
&((g + eq^3 + lq^5 + (2(e - z \lambda)(1 + g)^4 + \lambda b^5 + \lambda b^5 q^4)q^4)(n^2 - n) + \\
&(q^3 - bl + l + \theta \lambda q^3 + (b^5-2)q^5)(n^2 - 1) - r)^2 + \\
&(p - 2w s^2 r^2 n^2)^2 + (p^2 k^2 - k^2 + 1 - \tau^2)^2 + \\
&(4(c - ksn^2)^2 + \eta - k^2)^2 + (r + 1 + hp - h - k)^2 + \\
&(a - (wn^2 + 1)rsn^2)^2 + (2r + 1 + \phi - c)^2 + \\
&(bw + ca - 2c + 4\alpha \gamma - 5\gamma - d)^2 + \\
&((a^2 - 1)c^2 + 1 - d^2)^2 + ((a^2 - 1)i^2c^4 + 1 - f^2)^2 + \\
&(((a + f^2(d^2 - a))^2 - 1) (2r + 1 + jc)^2 + 1 - (d + of)^2)^2 + \\
&(((z+u+y)^2+u)^2 + y-K)^2 = 0.
\end{align}

The set of numbers with this property is not recursively enumerable (let alone decidable). But curiously enough, one can effectively construct an infinite sequence of such numbers from the axioms of $T$.
The equation is derived from Universal Diophantine Equation by James P. Jones.
Comment: Interesting example. This is a concrete case for a set $S$ (the set of $K$'s for which your equation has no solutions over non-negative integers) that is not recursively enumerable but nevertheless is explicitly definable in all (sufficiently strong) recursive  theories. Being non recursively enumerable dictates $\exists k\in S$ that can't be proved to be in $S$ in the theory $T$, because otherwise enumeration of all such proofs is an effective way to generate $S$, contradicting $S$ being non recursively enumerable.
Comment: I guess a succinct summarization of this situation is this: **In a non recursively enumerable set there hides an Achilles heels for every recursive theory.** One set catches them all - once you find it, you're done. But hard work lies in identifying such a set and proving that it's indeed non recursively enumerable.
Comment: @Eric If I understand your metaphor correctly, you are saying there is essentially only one non-recursively enumerable set (all others can be obtained from it using recursive procedures). This is not the case. There is a very complicated and fascinating hierarchy of different kinds of non-recursively enumerable sets: https://en.wikipedia.org/wiki/Turing_degree
Answer: I believe that one can expand on boumol's answer, as follows.
The spirit of the OP's question attempts to regain Eden after the Turing-Godel expulsion.
One might attempt to repair the OP's attempt in a more general way by adjusting the mathematician's aim from settling questions (with a proof of either yes or no) to resolving questions (say, with proof of yes, or no, or independence (relative to consisistency), independence of independence, etc).  One can even allow, in many different ways, stronger and stronger reasonable axioms (iterated consistency or reflection principles) as one pursues all these statements of iterated independence.
Trouble always ensues because a programmer can always dovetail a countable number of uniformly specified computer searches, making the program terminate if any one of those searches turns out successful.
Then having every formula in the theory fall into one class or another again makes
the set of consequences of ZFC decidable, contradiction.


Managed property not showing office 365

Question: I have a managed property in office 365 which doesn't show in one site collection but does in all the others.
Any ideas?
Answer:
You have to perform a full crawl of the content source or sources that
  contain this new managed property to include it in the search index.
  If the new managed property is in a SharePoint library or list, you
  have to reindex that library or list.
  https://technet.microsoft.com/en-us/library/jj219667.aspx#proc7

As you are in the Office 365, i would do following.
Make sure my site collection crawled after the Managed Property created.The full reindex of the list or library will be performed during the next scheduled crawl.
You can also Contact the Office 365 Support and check if they can behind the scene.

Managed property not showing office 365


How can I set up a click and collect type of shipping option where a customer collects their order from us

Question: I want to offer people shopping on my Xpresso Store the option of collecting their order from our HQ. I call this 'Click and Collect'. So basically they select this option when checking out and no shipping costs are charged, instead they simply come and pick them up. Thank you
Comment: Could you please clarify what you're asking? Do you want someone to do this for you? This isn't a forum for hiring developers. http://expressionengine.stackexchange.com/help/on-topic
Answer: it would be preferable if you look at the Documentation before asking. I don't know this extension, but on this page:
https://www.exp-resso.com/docs/settings_shipping_methods.html
it seems clear that you can just create a shipping method with no shipping cost and title it "Click and Collect".
Comment: I did look at this same page. I have created a shipping method with no costs and called it 'Click and Collect' but as it stands when someone puts something in their cart and goes to check out it does not offer any shipping options. It just applies the standard shipping that is already set up. It does not offer any other options. That page and all documentation I have been able to find offer no information about how to configure it to allow selection of other shipping methods. Thank you
Comment: If you're not a developer, I think you need to hire one. We need code examples from your templates to even approach helping you; hire a developer or hope someone here is willing away to give away their very valuable time.
Comment: sorry for any confusion. *I want to be able to set it up myself not hire someone to do so. I just don't know how to set it up and hoped someone here could help me figure it out. Thank you
Comment: Please edit your original question, and delete this answer, since it's not an answer, just an addendum to your question.


Can we use electrolytic capacitors on IC power rails?

Question: Can we use bulk electrolytic capacitors on the power rails of an IC? I know that we will use bulk capacitors in switching regulators at input. Can anyone explain why or why not and how to find ripple current of an IC? Is there any formula?
Comment: Data sheets on ICs normally give some indication of current.
Answer: It all depends on the necessary power supply impedance. The IC maker either suggests a maximum power supply impedance over a certain frequency range in their datasheet or they give you some guideline as to what the required ripple current and voltage tolerances are, which allow you to calculate the necessary supply impedance.
After you know the \$Z_{max}(f)\$, you can decide which type of capacitor is most appropriate for your bulk caps.
An aluminum electrolytic capacitor (the standard type with liquid electrolytes) will have an intermediate ESR/capacitance ratio. Solid tantalum capacitors have a rather high ESR/capacitance ratio. Ceramics and aluminum polymer capacitors have a rather low ESR/capacitance ratio.
Now when do you need high or low ESR/capacitance ratio? You need low ESR if you need to achieve very low values of supply impedance, e.g. \$m\Omega\$ and below. High ESR/capacitance ratio capacitors are good if you have a sluggish voltage regulator and need to cover rather low frequencies with the bulk capacitors (e.g. below 1 kHz) but don't need a very low impedance. In this case it is cheaper to provide the large capacitance with e.g. aluminum electrolytic capacitors instead of ceramics. High ESR capacitors are also a simple way to work as a snubber with its ESR in the ohms range. That means that it can be wise to provide a single tantalum or aluminum capacitor even if you provide the decoupling capacitance in the form of ceramics.


Transferring files to a USB attached drive

Question: I did a quick search and found someone who had a similar issue to this but was ahead of me so I'm not sure how to set this up?
I have my pi connected to my router with a 4 bay nas drive connected to the pi via USB. When I turn on the pi and type startx I have to add the drives manually each time, which is fine if I keep the pi running 24/7. I have installed netatalk and the home directory shows up on my shared network, but I can't seem to access the media folder. I know I'm supposed to type:
sudo nano /etc/netatalk/AppleVolumes.default
and edit the drive I want to mount but I'm not sure what aprt to edit and when I do change a line down at the bottom which relates to home directory I can no longer connect to the pi?
Any help is appreciated.
Comment: Your question is unclear. Are you trying to connect to or from the Pi? Tell us WHAT YOU DO to " add the drives". `AppleVolumes.default` controls which directories the Pi exposes.
Comment: I apologize. I am trying to connect from a Mac to the Pi with the drives attached to it. I am trying to transfer files from my Mac to the usb drive rather than the pi itself.  When I "add them manually" what I mean is File Manager pops up and asks me what to do with these drives it has found connected to the Pi. I then click ok to make them attach I assume.  The media folder is where my external drives are located.  Hopefully this is a bit clearer.
Answer: You are asking 2 questions:-
1. How to automount network drives
2. How to connect to a mounted drive on th Pi using netatalk.
A suggestion for the 2nd is to add lines at the end of /etc/netatalk/AppleVolumes.default. Mine contains the line /etc           "$hRoot" which lets Appletalk mount the directory /etc as "$hRoot" where the $h is replaced by the Pi Hostname (default raspberrypi). You could use /media/MountPoint for each drive or just /media for all mounted volumes.
# The line below sets some DEFAULT, starting with Netatalk 2.1.

:DEFAULT: options:upriv,usedots

# By default all users have access to their home directories.
~/          $hHome

:PIROOT: options:upriv,usedots,ro
/etc            "$hRoot"
# End of File

I cannot give a definitive answer for the 1st, as I don't do this, but you normally would add an entry to /etc/fstab. If you run mount after you have manually mounted a drive this will give an idea of the settings. Post a new question about this if you need further help.
Comment: Thank you for the info. I will try this out. I am not so concerned over #1. It was more to do with the second question.


I am trying to exploit port 7000/tcp afs3-fileserver

Question: I have been trying to exploit a cheap smart tv box that I have bought a while ago and after my nmap scan I found that port 7000/tcp was open but researching about the port gave back not much information I even searched searchsploit and vulners for any information or vulnerabilities within the port but I couldn't find anything useful. (I am new to pentesting I would use some more knowledge)
Comment: Ports don't have vulnerabilities. The things running on the port have vulnerabilities. Focus on that, not the port.
Comment: The first step is to figure out what the device uses the various ports for. Are you able to grab a firmware image for it? Are you able to get a shell? Fuzzing may also be relevant to try.
Comment: Seconding the 'Get a firmware' image recommendation on this.  Being able to download it, and analyze what's actually running on it with binwalk/qemu will be a huge help.  You may want to check out archive.org for results from devttys0.com  That was a wonderful resource for reverse engineering firmware.
Answer: Is it a Roku box? They have port 7000/tcp open. If so, I think it's a web service. You may want to try this: https://github.com/RoseSecurity/Abusing-Roku-APIs
Comment: Welcome to the community, but the specified link is not for research per se but rather to cause pure chaos/DoS...


Cobordism/bordism group based on orbifolds with corners

Question: We define a geometric homology group of a topological space $X$ as follows: the chain complex $C_{\bullet}$is freely generated by the maps $f$ from a compact oriented orbifold with corners $P$ to $X$, graded by the dimension of $P$, the boundary map from $C_n$ to $C_{n-1}$ is defined to be the restriction of $f$ to the (oriented)sum of its codimension $\ge$ 1 faces. If $X$ is itself a compact oriented orbifold with corners, is it true that the homology $H_{\bullet}(C, Q)$ over $Q$ is isomorphic to the rational singular homology group of $X$?
Comment: Why does the boundary square to zero if one allows corners? You probably want to add compactness somewhere as well, otherwise every orbifold bords
Comment: Does "oriented" condition avoid the issue you mentioned?Why does not it square zero, can you give an example?
Comment: The orbifold with corners is well formed,similar to the simplexes. Any point $x$ lies in exactly $n$ faces, where $n$ is the codimension of $x$, where "codimension"  is the number of zero coordinates in any chart containing $x$
Comment: The problem i was thinking of is that the boundary of a manifold with corners is not a manifold with corners. This is already not true for the model of the positive quadrant in $R^n$. Or think of a space homeomorphic to a disc, but with one corner. I don't see how orbifolds solve this.
Comment: Also interesting to note is that a point is nullbordant if one admits orbifolds.
Answer: This seems closely related to Baas-Sullivan theory of bordism with singularities.  Look up Nils Baas's 1973 Math. Scand. paper on MathSciNet, and then look at the various papers that cite this (including very recent ones).


Ignore an invoice issued by an internet registration

Question: I have registered for a training which is going to be organized in Germany. The registration was done by a form in which I have mentioned my personal information, my address, and the company I work for. The problem is that I have submitted the form without reading the conditions where some cancellation conditions were mentioned. The manager sent me an expensive invoice and he asked me to pay it even if I am not going to attend the training. All want to know is what will happen if I don't pay the invoice and just ignore their emails. I haven't signed any document, all I did is I answered some of their email telling them I am sorry for not replying to your emails and I am not able to attend this training.
Answer: It will depend on the exact language on the registration form that you submitted, but it may well constitute a contract. A physical, pen and ink, signature is not required to form a contract, and online document can do so, provided that there was agreement.
If the form created a contract, you are probably bound by the terms, subject to any limits in law. If cancellation is not allowed, or allowed only under circumstances that do not apply, you may be liable for payment.
If you just ignore this invoice, the organizers could sue you, with a plausible chance of success.
You would be wise to clearly tell the organizers right away that you wish to cancel any reservation, and that you never agreed to the price they wish to charge.  That should put you in the best possible position to dispute any charge.
The organizers might not choose to sue you, particularly if you have given them reasonable notice of your intent to cancel. But you cannot be sure of that.
There may be laws in Germany or France which require the organizers to give you a chance to cancel. I do not know if such laws exist, and if they do, what their provisions are. They might well be limited in time.
You could claim that since you never agreed to the price, no actual contract existed. The sooner you indicated a desire to cancel after learning the price, the better this argument will be. You may need to consult a lawyer on the best way to make such an argument.
Comment: Not agreeing to a price does not invalidate a contract in most circumstances- in the absence of such agreement there is an implied term that a reasonable price will be paid.
Comment: @Dale M That is quite true, but a failure to agree on a price **may** mean that there was no "meeting of minds", no actual agreement, which is essential for a valid contract.
Answer: Ignoring this invoice is a very bad idea. But it is a good idea to check if there is a valid contract and if you can get out of it. If you "signed up" to the training online, you may benefit from consumer protection regulations on internet sales.
You are in France, the company is in Germany, right? Check which law applies. Either way, it would implement the relevant EU regulations.
It might be a good idea to talk to a real lawyer, not some guys on the internet.


Troubleshooting Inconsistent Routing Issues

Question: I'm porting an application over from mvc4 to mvc6. Running under windows with iis platform host. rc1-update1.
So far everything is good in windows / iis, but once deployed to linux + mono 4.22, I get lots of weird routing issues including stating its not finding the routes for an action (but everything else in that controller is fine) and sometimes throwing AmbiguousActionException where it seems the candidates are completely off.
I've got this in startup
<code>app.UseMvc(routes =>
        {
            routes.MapRoute(
                name: "default",
                template: "{controller=Home}/{action=Index}/{id?}");
        });
</code>
And have marked up actions like such
<code>[HttpGet, Route("/Users/Find/{id?}")
public ActionResult Find(int id){
</code>
I'm finding frequent cases where Url.Action just sticks random id's in (even passing null or empty parameters for routing dictionary object).
so the question...
What can I do to better troubleshoot the issues... there are definite differences in the linux and windows environments... whether thats windows vs linux, or dev vs prod, or iis vs kestrel. Is there a routing middleware that helps troubleshoot the problem.
I feel like there is a bug here somewhere but don't even know what kind of information else I can specify, or use to do some more legwork. The places where the routing breaks is intermittent and part of a much bigger solution, so not a simple repro.
Answer: So after fighting oddities here for days on end, I upgraded to mono 4.3.2 on a whim, and things miraculously just started working as expected. Couldnt tell you why, but apparently something in the other versions I tried (4.0.5 and 4.2.2-sr1 or whatever) had issues with parsing urls or something?


Bad battery? Bad alternator? Bad starter? 2004 honda civic?

Question: Today went to start my 2004 honda civic and when I put the key in the ignition I got nothing no power no sound no crank nothing..... I just replaced the battery with a good die hard battery less than 3 months ago.... But when I jumped the car it started right up and off I went....the car ran great as it usually does got to my destination did my shopping car wouldnt start this time I got multiple/rapid  clicking...... What in tarnations???
Comment: @Sixtyfive, easier to measure voltages first, which doesn't require disconnecting stuff. See [Paulster2's answer](https://mechanics.stackexchange.com/a/89678).
Comment: Welcome to Motor Vehicle Maintenance & Repair!
Comment: If you have a multimeter that has an ammeter function, you could measure the power draw directly (unplug "+" pole of the battery and attach multimeter between battery "+" and car "+" ... assuming chassis is "-") to exclude something sucking it empty. Second thing, charge it up and let it stand disconnected for a bit, then see if it held its charge. At this point the only part left would be the alternator.
Comment: Yeah, I'd seen his answer and agree. Comment was more intended as another way. Sorry if it caused confusion.
Answer: It sounds as though the battery is dead because the alternator isn't putting out the juice needed to recharge it. You should have the voltage checked at the battery with the engine running. It should be between 13.5 to 14.1 vdc. If it is below that, let it run and check to see if it is discharging (ie: the voltage is getting lower). If it is, the alternator is probably bad. You should take it to a auto parts store to have it tested.
Comment: Conversely, if the voltage is within that good range and holding steady (with the engine running), it would be another part of the system: the battery, the starter, the connections, or something draining the battery when the car's off. A multimeter or other voltage tester can tell you more about that, too: a good battery should be around 12.3 to 12.6 volts (assuming not AGM) when fully charged, with the car not running. Finally, some places (like the retailer I work at) can test the battery for you if you bring it to them loose. But work step by step. Answer Paulster2's question first.