Should QA create a test plan for verifying bug fixes?

Question: I am new to QA and the single QA person on a team.  I have a pile of bug fixes that the Product Owner would like to release.  These fixes were coded by developers no longer with our company.  When it comes to writing test plans, is it typical to write a test plan for testing a bug fix?
Answer: It depends on your company's preferred methodologies. You should keep a record of the test cases you performed to verify the fix, but most likely won't need anything as formal as a test plan for that.
Depending on how the fixes are implemented, especially when testing old code, you may want to execute other test plans of the product where the bugs were found in order to ensure the bugfixes do not create any more problems.
Answer: I would suggest you add the new tests into your regression pack to ensure they are picked up in the future.
I assume you will run some form of regression prior to release, so between the two you have improved coverage and documentation going forward
Answer: How do you prevent the bug from happening again? Most of the times the answer to this question is a test-case.
Now where does this test-case go? Into an existing test-plan or -suite, preferable automated.

Fix the defect
Test the fix (manual, exploratory or automated)
Do a root-cause analyses
Create a test-case/step/plan to prevent the root-cause from occurring again
Execute all other tests
Release fix

So yes, you do need to test bug-fixes, certainly when reported by clients, since it is possible different users report conflicting bugs. This could lead to a pingpong state between two similar issues, where with each release the bug returns for the other client.
But no, you should not create a full test-plan for each bug-fix, just add a test-case to an existing plan. If you do not have a test-plan or -suite it time to start one :)
You could do a small risk analyses and wonder what the risks are that it breaks again. Based on that decide how much testing effort your team should invest.
Answer: It depends on your company internal policy and how your team is organized.
Typically, if you are the only QA in your team, it will be your responsibility to plan and execute fixed bug testing.
Regardless if the developers are still with your company, it is still your company's job to ensure your software meets customers' expectations.
Answer: If you have a bug for each fix, then I would use those bugs to document how they each fix was tested and whether the issue was corrected or not. A formal test plan seems like overkill to me.
Answer: I would suggest:

Ideal practice is, We write test cases across specifications
When we observed any bug, If its having step similar to any test case then fine [No need to update test plan]
But If bug is having different test steps than any test case, we should add those step in test plan and that bug would be a one test case
In our software field, team keep changing frequently[many if the times, not all the time & everywhere] So keep practice of lesser dependency
Regardless of role, Keep updating your team for bugs observed and testing results to be on same page
Answer: I wouldn't add bug verification to the master test plan since this will pile up un-necessary test cases, after all once you verify a fix you don't need to check this particular case anymore.
But do consider-

If applicable add the root cause of the bug to the plan
Add some verification tests to the nearest release, but also keep in mind branching and merging policies since bugs might reappear in the future


Dominant team members in a Scrum team

Question: What would you do in a situation where a team member tries to take responsibilities are not initially assigned to him but to the Scrum Master?
Comment: Should this be moved to http://pm.stackexchange.com/ ?
Comment: I don't agree with moving to pm.stackexchange.com because Scrum master is not PM and Scrum project should not have PM.
Comment: Seems like you're the only one on your team who is concerned. Challenge him to a duel.
Comment: I miss important context in this question. It could be that 'dominant' dev takes more responsibilities because he genuinely feel scrum master is under-performing and is trying to improve team performance, maybe he wants to feed his ego and superior skills, maybe he acts in a good will and is just not aware of potential destructive effect on team, maybe he is just jerk... or maybe scrum master himself is the problem.
Comment: I'm not sure how this really relates to Scrum, or programming for that matter. "Dominant teammember overshadows everyone on team".
Comment: In all reality if the team is producing working software in adherence to the Scrum process; let it be. Trying to give everyone equal footing is not the job of the ScrumMaster. Check out the [stages of group development](http://en.wikipedia.org/wiki/Forming-storming-norming-performing) and identify where this team exists. They may be stuck in the forming stage.
Answer: The Scrum team is self organised so there is room for somebody who is a little more dominant. Others should ask him for his ideas about tasks they are working on, but his dominance must be kept under control.
What you can do:

Motivate others to be independent but collaborative - this can be best achieved if you co-operate with their manager and HR who will set them some expectations, which will be evaluated on a regular basis.
During stand-ups, planning, and retrospectives; let other team members speak first. During reviews, let other team members present user stories which they implemented.
Let other choose tasks first so that the dominant developer cannot only choose tasks that he wants to do.
Involve pair programming for some tasks so that the dominant team member is collaborating with other developers.
Be Democratic - the opinion of one team member is not enough to make changes to the process - Scrum will only work if the team communicates clearly, or you will just be blocking each other.
If none of this helps you should speak with the dominant team member and explain the situation to him - but beware there is no team leader in Scrum for a reason. If you have support from management you could also threaten his job stability, but this should be the very last option.

If the dominant team member doesn't want to lose his dominance and passive team members don't want to be more active you will need support from their manager and HR. This could be a problem if the Scrum process is not encouraged by management.
Answer: Presumably, the reason for this question is because you feel that the team is somehow under-performing because of this dominant person.  Perhaps because the rest of the team aren't contributing 100% because, well, what's the point?
As a manager, if you are, it's your responsibility to make sure that all of your employees understand what their roles are.  Specifically, what's expected of them and how they are going to be evaluated.  As a team member in a Scrum team, each person is personally responsible for the success of the team.  So this dominating team member needs to know that they're failing in that responsibility, and will be evaluated accordingly.
Feedback is a key point. If there's a team meeting and this person dominates the discussion, forces his designs and approach on the rest of the team and pushes the rest of them into passive roles then he needs to be told, bluntly and privately, that he's failing to meet the requirements of the role. If you spot him sneakily highlighting only his own personal achievements, then he needs to be called on it, and made to understand that personal achievements are valued far, far less than helping the team to have a group achievement.
All of which is tough, but that's what managers and Scrum Masters are for.
There's one other possible way to go about this.  Make it a team problem.  Call them together and tell them they are under-achieving and here's why.  Ask them to come up with a solution.  You might be surprised.
Comment: Simply excellent answer.
Comment: I like your focus on the feedback.
Answer: Why not ask the alpha developer his opinion. It's entirely possible he has no idea of the effect he's having. Take him aside and tell him what you think. You could even couch the discussion of "hey you're our lead developer, but we need to bring these people up to your level, I need your help in working out how can we do it?". Turn his dominance into an asset. See if he can see ways of doing that.
If he's measured on how well he supports his team and brings them up to his level then he'll have more motivation to do just that.
You imply he's actually not working in the interest of the team (my guess), ie he's evil in some way, then yes, remove him. But a wise man once said to me: Don't assume ill will, when incompetance or simple ignorance are more likely.
Comment: That's an awesome way of re-framing the problem. Asking for his help completely changes the situation (and makes it a whole lot less scary).
Answer: Looks like he tries to be the Scrum Master.
Clarify your positions, and act accordingly.
The role of the Scrum Master is to enable team spirit. If you are unable to make this one and only person be a team player, remove him from the team.
Quick note: Your dominant developer is no more problematic than your passive developer.
Comment: While a nice line, simply removing someone from a team is easier said than done in most cases I would have thought Pierre.
Comment: I'm based in the UK, and having a dominant personality is not a reason to fire someone, you'd need a lot more than that!
Comment: @Pierre - it's a lot harder to fire people in the UK than say the US. But showing a pattern of bad behaviour and warnings, then yes of course someone can be fired. I'm not sure this particular situation would be easy to fire someone from in the UK. I could be wrong though.
Comment: @james: could you tell me why?
Comment: Well, having limited people to work on a project for one. Moving him to another team, the other team are inconvenienced and may resist. Knowledge retention on a project would be another (easy to say knowledge should be spread, but in reality it isn't). That's 3 very REAL reasons why it's easier said than done. Not that it can't be done of course. Of course maybe you mean fire him :-) ?
Comment: @james: Your first point is perfectly valid. Maybe it's not so easy to find a replacement. Because when I said removing, I was thinking about firing him. Being dependent on individuals is a real bad thing. Especially when he knows it, and plays with it. Which seems the case here :(
Comment: @james: I'm not familiar with UK laws, but I think anyone can be fired if the boss is not satisfied with his behavior, with a legal notice period. Unfortunately, I had to fire many people for those reasons.
Comment: @Pierre - Many european countries has strong laws "protecting" employees. Workers Unions are very influential. For instance, in Sweden we are only allowed to fire people for work redundancy reasons, and then it's a last-in first-out order. Often, the employer has to buy out the guy so it looks like he quit willingly.
Comment: @Martin Wickman: I guess hiring process is much more extreme in Sweden then?
Comment: @Pierre: Yes, it is often a very lengthy process considering you might be stuck with the person for a very long time.
Answer:
currently sprint planning is a role that rotates across the team

Sprint planning should involve the entire team, not be handed to one person at a time. Unless there is a good reason for this I see this as a severe problem.
If what you are saying is true and objective then you have a major problem at your hands: people who do not participate and the "Dominator" who prevents a truly agile process. As the srum master it is up to you to take action. Maybe you would like to take your project manager into confidence about this situation.
Answer: Many teams strays from the core of agile, and it is your job to bring them back. You need to teach and re-embed agile values into the the team. In fact, you should constantly be teaching agile values. Hold out your vision of agile, make it clear and powerful. Show them your commitment to "agile done well".
To do this, walk them through the agile manifesto and Scrum values. Ask them what collaboration means for them and why it is important. Ask them about the role of trust in agile. This is a great time to talk about why there is no team lead role and no project manager role in Scrum and that it is the whole team's  responsibility to make great software, not the individual.
Plan an entire retrospective session around this. Get them to commit to some values and follow up during the next retrospective. Don't point fingers, use neutral methods.
Introduce methods that forces the other members to state their opinions safely. Something simple as fist-of-five is great for getting the silent voices in the team heard. It makes it painfully obvious that the team disagrees with the dominant guy. Planning Poker works well but the key is don't allowing any discussions before the cards are shown. Anything that helps getting the others heard without starting conflicts is helpful.
If that goes well, you're all set. Otherwise, talk to him about the problem. Use coaching and ask powerful questions that can help him realize the problem clearly. Try to get to the root cause why he has taken on the dominant role. Maybe he lacks trust in the team (why?) and maybe he feel he is responsible for success (why?). I suspect this role is not something he wants and quite possible he would like it to change. He may come around and realize it.
Answer: Perhaps the "dominant" dev is rewarded by his individual performance rather than team achievements?
In the past I have made sure that people that are very vocal and have clear ideas are also rewarded (via their objectives) by fostering others contributions.
In general, I think it is a bad idea to reward scrum members solidly on their individual contribution rather than on team achievements.
You could also try doing a 360 feedback round in your team for all team members, only if you think the other team members will be truthful in their comments.
Answer: Propose that he becomes the Scrum Master for the following sprint.
People willing to take on responsibilities is not a problem (as long as they don't want monopoly over them), it's precisely what we're looking to achieve with self-organization.
Teams with a rotating scrum master role aren't uncommon, by the way.
Answer: The sole task of the Scrum master is to make sure everybody plays by the Scrum book rules. If non-scrum masters would do that by themselves without interference of the Scrum master, that would just show you are in great (Scrum-)shape! The less the Scrum master has to do, the meaner and leaner your team is from a Scrum perspective. Eventually the role of Scrum master could/should be eliminated, he is there to get you started and teach you Scrum.
Again, the Scrum master does not participate in the development process. He should make sure all stakeholders communicate in time about the right things, he knows Scrum and provides guidance doing Scrum, he is not a product owner or project leader. If you experience something different, you may not be doing Scrum in the agile spirit. Of course, in smaller settings Scrum master could be a role one of the team members or stake holders plays on the side. Then it is just a cap one puts on when it is time for formalities. Do not confuse it with a
leadership role, it is a guidance role.
Answer: Embrace individualism and individual performance, but also try to empower passive individuals to become more participative.
My experience says that although they might seem similar at first sight, communism and agile are not the same. Agile doesn't aim for a classless society (team) but for software that works.
Try to make your alpha developer understand that he could help you develop others skills by asking and coaching, rather than answering always first, and achieving individual accomplishments. Surely your alpha developer is one that cares about good software, and that's something you cannot allow yourself to lose.

Dominant team members in a Scrum team


Is the "narayanam namaskrtya" benedictory initial stanza present in the critical editions of the Mahabharata?

Question: D.D. Kosambi writes that V.S. Suktankar's 1933 text-criticism rejected it.  What do different critical editions say about that?
Answer: Since you are asking specifically about critical editions, here's the BORI edition.
https://sanskritdocuments.org/mirrors/mahabharata/pdf/mbh-01.pdf
Page 13 of the file, the parva starts with the said verse.
Answer: It is absent from the Critical Edition of the Southern Recension of Mahābhārata edited by P. P. S. Sastri. As you can see at this link, the first numbered verse is a dhyānaśloka that begins with śuklāmbaradharaṃ viṣṇuṃ...

॥ श्रीः ॥
॥ महाभारतम् ॥
॥ध्यानश्लोकाः॥
शुक्लाम्बरधरं विष्णुं शशिवर्णं चतुर्भुजम् ।
प्रसन्नवदनं ध्यायेत् सर्वविघ्नोपशान्तये ॥ १
धर्मो विवर्धति युधिष्ठिरकीर्तनेन
पापं प्रणश्यति वृकोदरकीर्तनेन ।
शत्रुविनश्यति धनञ्जयकीर्तनेन
माद्रीसुतौ कथयतां न भवन्ति रोगाः॥ २

As for the BORI Critical Edition, although it does begin with nārāyaṇaṃ namaskṛtya..., the verse is not numbered (it's the 0th verse).

01001000a   nārāyaṇaṁ namaskr̥tya naraṁ caiva narottamam
01001000c   devīṁ sarasvatīṁ caiva tato jayam udīrayet
01001001A   lomaharṣaṇaputra ugraśravāḥ sūtaḥ paurāṇiko naimiṣāraṇye śaunakasya kulapater dvādaśavārṣike satre
01001002a   samāsīnān abhyagacchad brahmarṣīn saṁśitavratān
01001002c   vinayāvanato bhūtvā kadā cit sūtanandanaḥ

In his paper Epic Studies, V. S. Sukthankar explains the reason for including the opening stanza in the CE and yet not numbering it:

Type No. 2 (N ≠ S). I agree with R. [Dr. Ruben] that when there is a conflict between N and S and they stand opposite to each other with two divergent readings, no definite decision is, in general, possible as to which is the original; the versions should a priori be placed on an equal footing and treated with impartiality. Accordingly, in such cases, I have adopted as stop-gap the reading of N, placing a wavy line below it, to show that the reading is uncertain. R. mentions an exception to this procedure of mine, but the exception is only apparent. R. has failed to understand my motives in departing from my usual practice in the solitary instance of the benedictory stanza with which the epic begins (Nārāyaṇaṃ namaskṛtya etc.). I have printed it above the line not because it is found in K0 I or in K or even in N, but simply as a maṅgala, I am aware that the stanza is missing in S. There is, therefore, every probability that it did not belong to the archetype. I am also fully aware that the stanza is a characteristic mark of the works of the Bhāgavata sect and not peculiar to the Mahābhārata. There is, however, the fact that all our MSS. of the Ādiparvan begin with some benedictory stanza. These stanzas have probably usurped now the place of some simpler maṅgala with which the epic once began.
No orthodox Hindu work can begin without a maṅgala; and this edition of the Mahābhārata, critical though it be, is and remains a Hindu work which could not dispense with a maṅgala.
My recognition of the unoriginal character of the stanza is, however, clearly implied by the fact that I have omitted to give the stanza a number, differing in this respect conspicuously from the earlier editions, which treat it as the first stanza of the epic and an integral part of the text.
Comment: Ok. I got two more doubts here. 1. How he is sure that the verse is inspired from bhāgavatā sect and interpolated in mbh,why not otherwise? 2. He himself admits that the verse is present in few southern msnuscripts, how he completely ruled out the possibility of that verse being redacted? 1.He also says no hindu work dispense without a mangala shloka.2.Many northern manuscripts and even few southern do contain that shloka, these reasons are enough to not rule it out as an interpolation. Anyway you correctly answered my question to full satisfaction.His reasons not worth their salt(only for me)
Comment: "1. How he is sure that the verse is inspired from bhāgavatā sect and interpolated in mbh" - You can read more about it [here](https://archive.org/details/in.ernet.dli.2015.201383/page/n65/mode/2up). Apparently, Panini was aware of Vasudeva-Arjuna being worshipped but not Nara-Narayana. So this Nara-Narayana concept is a late develpment in Hinduism and a late addition to Mahabharata. Also, note that even if that verse was numbered in the BORI CE it doesn't mean it's a genuine verse. It simply means the critical editors did not find manuscript evidence to remove that verse. @Satya
Comment: @idol worshipper. 1.Nara-Narayana are same as Arjun-Krishna. Panini mentioning that only supports Nara-Narayana being worshipped,that is not later development.2.//critical editors did not find manuscript evidence to remove that verse// this also further supports the same point. These reasons are not valid even for those with Indologist mindset. Also,apologies to idol worshipper I somehow overlooked your invitation to chat,thanks for your concern.
Comment: Even bhāgavatās mention can also serve as a proof of it's authenticity. Why he didn't understand like this?
Comment: Additionally in south there is a own mahabharaha to each language translated and those are the one people follow mostly.
Comment: @idol worshipper. Please answer them.
Comment: Please use main chatroom for all discussions related to Hinduism. Here is the link: https://chat.stackexchange.com/rooms/15189/hinduism @Satya and Idolworshipper
Comment: @idol worshipper.Yes, direced towards sv. Apologies are for you.
Answer: In Gita Press edition of  Mahabharata, Adi Parva, 1st Chapter, 42nd verse is translated thus,
मन्वादि भारतम केचिदास्तीकीआदि तथा परे |
तथोपरिचाराद्यन्ये विप्र: सम्यगधीयते ||
कोई कोई इस ग्रन्थका आरम्भ 'नारायण नमस्कृत्यं ' से मानते हैं और कोई कोई आस्तिक पर्व से | दूसरे विद्वान ब्राह्मण उपरिचर वसुकी कथा से इसका विधिपूर्वक पाठ प्रारम्भ करते हैं |
Nārayaṇa Namașkṛtyam verse is indeed bona fide.


Do I lose my attack in Clash of clans war?

Question: If I start a battle in a war, but click "end attack' before I attack, will this still count as one of my attacks in the war?
Comment: @Mathias711: True, this is easily testable. However, when in a war, you  only get two attacks total. So, potentially wasting half of your available attacks "for science" isn't terribly fair to the rest of the people in your clan that you are playing along side. Personally, I would ask a question about it here before attempting it myself.
Comment: I'm not familiar to the game, but I think this is something you can try out yourself, and answer your own question!
Comment: @Ellesedil Okay. That is why I stated that I am not familiar to the game. I had no idea of that
Answer: No, as long as you don't place any units or spells you  can either skip or exit the attack screen completely.
Comment: I just went and tested it, he is correct
Comment: I'm not sure.... since this is war and you're being asked if you really want to attack
Comment: With the latest update, this is no longer true.
Answer: I think since the last update you can not end the battle without loosing. I just did it to myself. I started the battle and realised I wanted to check the range of the dudes cc.  I hit end and bam I got 0 out of 3 stars and now only have one attack left. I also watched the replay. 19 seconds of nothing. No troops or spells deployed.
Comment: Excellent point. This has indeed changed, but only for war attacks.
Answer: If you don't put a barb or anything else like a spell, you're good to go. Just make sure of the "Battle starts in: 20 secs" or anything. If it gets to zero, unless you wanna lose trophies like crazy, you gotta attack. Or BUSTED.
Comment: You can't lose trophies in clan war attacks.  Also, you won't lose trophies in non clan war attacks unless you place a unit or cast a spell, regardless of the timer.
Answer: Whether you are doing a regular attack or if you attack during a war, you can exit without losing your attack/trophies as the condition that you did not deploy any single troop on the map.
This is something I personally experienced in a war, where I suddenly realized that I only had half of my troops available. I exited and I was still able to attack later on.
Answer: I don't think so.
There are two labels for Ending the game:
<code>1: End Battle
2: Surrender
</code>
Of course, second one is not suggested.
Answer: Nope. You will not lose. I have tried this personally a few weeks ago. As long as the 30 seconds doesn't run out, or you place any troops or spells, you will be fine. If you do accidentally drop in a spell or troops, or the timer runs out, you should just go through with the attack.
Good luck with your war and have fun! :)
Answer: Yes you lose your attack, the people answering these questions don't know what they are talking about, today I lost a war raid because of this...


Which filter do I need to perform NDVI measurements?

Question: I have Nikon D70s DSLR camera and i would like to modify it for NDVI (Normalized Difference Vegetation Index) measurment. I am looking for the filter that can pass effectively RED and NIR band but block others. From the modified D70s (after installing new filter), I should be able to get RED and NIR as a separate bands of a image.  I would like to get suggestions for the filter for my purpose. Thanks in advance.
Answer: I was in Valencia for the AgEng/CIGR 2012 conference where I saw a presentation about adapting a DSLR to do exactly this. The paper contains the details how to get the best possible Red/NIR colour space for NDVI and shows application of the results. You need to remove the ir cut filter and replace with a ~600nm long pass filter.
http://cigr.ageng2012.org/images/fotosg/tabla_137_C1122.pdf
Then you do linear combinations of the the 3 colour bands. Their work base the calculations on actual measured spectral response curves, which you might not have the gear for. The result depends on the white balance but I have tried this technique without actually doing all this on a Basler ACA640-90GC camera. I got good result incorporating it into the bayer demosaiqueing, separating the NIR channel into the blue colour and red into the red colour and nullifying the green. The hard part is balancing the intensity balance of the two bands, since the response to NIR in the cmos is about 25%. I also found that the result is very sensitive to the bayer conversion algorithm. I found that Basler's SDK actually use nearest neighbor, which makes very bad NDVI images.
Example image - before and after converting to NDVI index:

And an example when using the Basler bayer conversion:
Answer: NASA tells me that to capture NIR, you'll need a filter cutting at about 700nm, such as Hoya R70, Marumi 700HB or B+W 092 (that last one is actually 695nm). The diameter should be chosen the same as thread diameter of lens you're going to use (if there are several lenses, either get all different sizes, or the one with biggest diameter and step-up rings for smaller diameter lenses).

I should be able to get RED and NIR as a separate bands of a image

I'm not quite sure what was meant by this, but if you meant you'd like to capture NIR in one color channel and visible light in another, then unfortunately there's no filter that could do exactly that. It would require you to replace the Bayer filter on sensor, i.e. the microfilters on each sensel.
However, my experiences with a Olympus C-2020Z (known for its hot mirror doing a lousy job when blocking IR) with a Hoya R72 and tells me that IR affects all color channels. So when letting in red+IR, you'd have IR in blue/green channels and red visible light+IR in red channel. By subtracting average of other channels from red (perhaps multiplied by some factor found experimentally through testing with IR only), you'd get approximate value for visible light only.
Comment: Thanks Imre. I found that lifepixel.com has super color filter which can pass red and NIR band effectively. But i am not sure about the technique to spearate the bands. As per my basic knowledge i learned that Bayer filters are also sensitive to infrared band of light so, if i remove the internal hot mirror filter and replace this with the above mentioned filter will it be able to acquire a 3 band image just like before the hot mirror filter is in place. Anyone please if you know any idea about separating the channels in such situation would be highly appriciated for your suggesstions. Thanks.
Comment: I [have used](http://nagi.ee/photos/a4imrel/sets/265913/) the Olympus C-2020Z (known for its hot mirror doing a lousy job when blocking IR) with a Hoya R72 and my experience is that IR affects all color channels. So when letting in red+IR, you'd have IR in blue/green channels and red visible light+IR in red channel. By subtracting average of other channels from red (perhaps multiplied by some factor found experimentally through testing with IR only), you'd get approximate value for visible light only.
Comment: Thanks . May i get a bit elaboration about your work. Have you published any papers so that i can take it as a reference. thanks
Comment: No papers... Just the curiosity for IR photography has shown me that the resulting images tend to be red-grayish or gray depending on chosen white balance (meaning the IR light affects all color channels, but perhaps red stronger than others).
Answer: Five years on this topic remains of research interest, particularly regarding drone carried cameras, so another answer is probably of some value.  The AgEng2012 site linked above has been removed but this paper is the best I've come across:

Rabatel, G.; Gorretta, N.; and Sylvain, L.  Getting simultaneous red and near-infrared band data from a single digital camera for plant monitoring applications: Theoretical and practical study.  Biosystems Engineering 117(1):2–14, January 2014. DOI 10.1016/j.biosystemseng.2013.06.008.

Methodology and findings are similar to what Michael mentions: converted camera, ~600nm red filter, sensitivity to demosaicing, linear band calculations, and improved spatial resolution compared to NDVI specific cameras at the expense of some SnR limitations.
Answer: Newport.com, thorlabs.com, or edmundsoptics all sell longpass filters that can be used for you app.
It's been my experience that the NIR bandpass of the green and blue Bayer filters are almost the same ( less than 10 digital numbers difference at up to 90% of the sensors dynamic range). The red channel NIR bandpass is slightly smaller than the blue and green. But it's going to depend on the camera manufacturer.
You might also want to work with raw data images as the built in demosaicing process can create artifacts. NN demoaicing might not look the prettiest but it retains the trues spectral signal of the scene.


How does the Dyson air multiplier fan create negative pressure?

Question: I'm curious as to how the Dyson air multiplier fan works. So far I understand that it creates negative air pressure that causes air behind and around the device to be sucked into the air stream and blown out. My question is how is the negative air pressure created? I haven't found an explanation for that yet.
Comment: The negative pressure is to be understood with respect to atmospheric pressure. As people pumping up water from very low sources know very well, there is a limit on how "negative" a pressure can be. Planes fly with that kind of negative pressure created above the wings.
Comment: Related: https://en.wikipedia.org/wiki/Injector#Well_pumps
Answer: There seems to be a lot of B.S. in the ads for this product, making it seem much more complicated than it really is.
It seems that the fan contains an ordinary fan in its base, and squirts out high speed jets of air from around the big ring. These jets of air push on the ambient air.  This slows down the individual jets but pushes along a greater volume of air.
This link seems to have a more reasonable explanation than many other sites, which tend to simply reproduce press-release hyperbole:
http://www.pocket-lint.com/news/27887/how-dysons-air-multiplier-works
Comment: it's the same principle as any fan (at least any i know of) works: it propels the air in one direction, and because there are then less air molecules behind (because they moved towards you), the pressure is lower than the average room air pressure, which causes the air molecules behind to move towards the fan to equalize that pressure gradient.
Comment: Thanks for your reply. Still a couple things I don't understand. First, why does friction in the air create entrainment? Second, how does "air that gets pushed away from the ring towards your beautiful face creates an area of low pressure"? I don't understand how that area of low pressure cone about. Thanks
Answer: For your first question, think of the air kind of like sand.  If you just slide you hand across the surface, some grains beneath the surface will move due to friction between the sand grains transferring the energy of your hand to other sand grains.  The same thing happens in the air inside the ring, as air shoots out of the slit surrounding the ring it collides with the air particles already inside the ring and transfers some energy to them, thus pushing them out of the front of the fan.
Second, the low pressure area.  As the air inside the ring is getting pushed out of the center of the ring by the air blowing out of the slit, then there will be a void, for lack of a better term, in the center of the ring. Think of this void as the low pressure zone, as the void grows in size, the air at the back of the fan wants to flow into the void and there fore gains momentum towards the front of the fan and it gets accelerated from the friction as it flows past the slit in the ring.
Answer: air moving along the inner surface of the fan naturally creates a lower pressure inside... bernoulli's principle.
Comment: 3 down-votes and no comments..? Too bad of these *guys*. I'd say that, It's better to comment and start *arguing* rather than posting an answer & getting down-voted..!


Shimano Rear Derailleur Max Capacity

Question: I need to get a new rear derailleur. My chain rings are 53/39 and my cogs are 28/11 so if I am not mistaken I need a total capacity of (53-39) + (28-11) = 31. I need a 11 speed compatible rear derailleur. I was thinking about getting a Shimano 105 5800. On their website they say that the capacity of the SS is 32. Should I go for it or get the GS since the capacity is 37?
Answer: There is also a spec for the maximum size of a rear cog.  The 5800SS allows a 23 to 28 tooth largest cog, so you are OK.  The GS allows a range of 28 to 32 teeth for the largest cog.  I would suggest you decide based on which direction you are more likely to want to change-if you go to an 11/32 cassette you need the GS, while if you go to tighter gearing you need the SS.  These are the specified numbers and you can often make it work over a little more range.
Comment: The GS is more useful since it also handles a 11-25 cassette without any problem. So there's no need to swap the RD if you want to use the smaller cassette and it's just a tiny bit heavier.
Comment: @Carel:  I looked up the spec on the Shimano page.  It says the minimum large cog for the GS is 28.  I suspect it would work for the 25 as well.  The idler pulley will be farther from the cassette than intended.
Comment: @ Ross Millikan: I use the combination on my bike, it works very well with 11-32, 11-28 and 11-25.

Shimano Rear Derailleur Max Capacity


Difference between "kernel" and "filter" in CNN

Question: What is the difference between the terms "kernel" and "filter" in the context of convolutional neural networks?
Comment: They are the same! Filter or kernel is simple group of weights shared all over the input space.
Comment: is a filter the weight in a cnn? i see always weights in decritptions of neuronal networks, but i was wondering where the weights of a cnn are:
Answer: In the context of convolutional neural networks, kernel = filter = feature detector.

Here is a great illustration from Stanford's deep learning tutorial (also nicely explained by Denny Britz).

The filter is the yellow sliding window, and its value is:
\begin{bmatrix}
    1      & 0 & 1  \\
    0      & 1 & 0  \\
    1      & 0 & 1
\end{bmatrix}
Comment: @PratikKhadloya no, those names are given subjectively by researchers based on what the filter appears to be detecting.
Comment: Is there a specific name for the filter? Like a "Blur" or an "Edge detect"
Comment: @Frank Dernoncourt according to this link https://towardsdatascience.com/intuitively-understanding-convolutions-for-deep-learning-1f6f42faee1 they are different when we have a couple of channels. actually when there are multiple channel Each filter actually happens to be a collection of kernels, with there being one kernel for every single input channel to the layer, and each kernel being unique. I would be happy to have your idea
Answer: How about we use the term "kernel" for a 2D array of weights, and the term "filter" for the 3D structure of multiple kernels stacked together? The dimension of a filter is $k \times k \times C$ (assuming square kernels). Each one of the $C$ kernels that compose a filter will be convolved with one of the $C$ channels of the input (input dimensions $H_{in} \times H_{in} \times C$, for example a $32 \times 32$ RGB image). It makes sense to use a different word to describe a 2D array of weights and a different for the 3D structure of the weights, since the multiplication happens between 2D arrays and then the results are summed to calculate the 3D operation.
Currently there is a problem with the nomenclature in this field. There are many terms describing the same thing and even terms used interchangeably for different concepts! Take as an example the terminology used to describe the output of a convolution layer: feature maps, channels, activations, tensors, planes, etc...
Based on wikipedia, "In image processing, a kernel, is a small matrix".
Based on wikipedia, "A matrix is a rectangular array arranged in rows and columns".
If a kernel is a rectangular array, then it cannot be the 3D structure of the weights, which in general is of $k_1 \times k_2 \times C$ dimensions.
Well, I cant argue that this is the best terminology, but it is better than just use the terms "kernel" and "filter" interchangeably. Moreover, we do need a word to describe the concept of the distinct 2D arrays that form a filter.
Comment: You are the MVP. This answer made my day
Answer: Filter consists of kernels. This means, in 2D convolutional neural network, filter is 3D. Check this gif from CS231n Convolutional Neural Networks for Visual Recognition:

Those three 3x3 kernels in second column of this gif form a filter. So as in the third column. The number of filters always equal to the number of feature maps in next layer. While the number of kernels in each filter will always equal to the number of feature maps in this layer.
Comment: please add the source of the image you're using (Stanford CS 231): http://cs231n.github.io/convolutional-networks/
Comment: @Antoine Thanks for reminding
Comment: no prob, welcome to the site!
Answer: A feature map is the same as a filter or "kernel" in this particular context.
The weights of the filter determine what specific features are detected.
So for example, Franck has provided a great visual.  Notice that his filter/feature-detector has x1 along the diagonal elements and x0 along all the other elements.  This kernel weighting would thus detect pixels in the image that have a value of 1 along the image's diagonals.
Observe that the resulting convolved feature shows values of 4 wherever the image has a "1" along the diagonal values of the 3x3 filter (thus detecting the filter in that specific 3x3 section of the image), and lower values of 2 in the areas of the image where that filter didn't match as strongly.
Answer: The existing answers are excellent and comprehensively answer the question. Just want to add that filters in Convolutional networks are shared across the entire image (i.e., the input is convolved with the filter, as visualized in Franck's answer). The receptive field of a particular neuron are all input units that affect the neuron in question. The receptive field of a neuron in a Convolutional network is generally smaller than the receptive field of a neuron in a Dense network courtesy of shared filters(also called parameter sharing).
Parameter sharing confers a certain benefit on CNNs, namely a property termed equivariance to translation. This is to say that if the input is perturbed or translated, the output is also modified in the same manner. Ian Goodfellow provides a great example in the Deep Learning Book regarding how practitioners can capitalize on equivariance in CNNs:

When processing time-series data, this means that convolution produces a sort of timeline that shows when different features appear in the input.If we move an event later in time in the input, the exact same representation of it will appear in the output, just later. Similarly with images, convolution creates a 2-D map of where certain features appear in the input. If we move the object in the input, its representation will move the same amount in the output. This is useful for when we know that some function of a small number of neighboring pixels is useful when applied to multiple input locations. For example, when processing images, it is useful to detect edges in the first layer of a convolutional network. The same edges appear more or less everywhere in the image, so it is practical to share parameters across the entire image.
Answer: To be straightforward:
A filter is a collection of kernels, although we use filter and kernel interchangeably.
Example:
Let's say you want to apply P 3x3xN filter to a K x K x N input with stride =1 and pad = 0. So each of the 3 x 3 matrix in 3 x 3 x N filter is a kernel. And your output will be K-2 x K-2 x P .


D7500 exposure changes when in "M" mode and zooming video

Question: Using a 16-85 3.5-5.6 lens.  Taking a head and shoulders video at 1/60 and f5.6 in mode "M".  This is the exposure I want to lock in.  When I zoom out and include some darker areas in the frame, the aperture will change to f4 and f3.5 thereby overexposing my subject.  How can I lock my exposure?  (Even in aperture priority mode it will do the same thing.)
Comment: Does this problem happen only for video?  I don't know for Nikon, but for stills on some systems, if you change the aperture while the lens is fully zoomed in (up to F8, then back down to F5.6), it will keep the same setting when you zoom out.
Comment: I'm just going to drop some 'buzzwords' to look into, because I don't know enough about it to make it into an answer. T-Stops, aperture ramping, focus ramping. (You could check out parfocal lenses while you're at it). These are what makes the difference between a $1,000 stills lens & a $40,000 cinema lens. Cinematographers need finest control over exposure, stills don't.
Answer: This happens because you are using a variable max aperture lens. The aperture iris is not actually opening or closing as you zoom out – its actual opening size is staying constant. It's just that the entrance pupil, the apparent size of the aperture opening when viewed through the front of the lens, is changing size due to the movements of the lens element groups as you zoom.
It is for this reason, amongst others, that variable max aperture lenses are not the best choices for video. The only way you can lock your desired exposure is to shoot in aperture priority. If you shoot video in manual mode, you cannot avoid the exposure changing as you zoom.
Comment: @Tetsujin ah, okay, thanks for the article. I can see where you're coming from. For a lens that is supposedly controlled for constant aperture, then a reduction in exposure due to focus breathing is focus ramping, which is the T-stop issue you're talking about. However, the 16–85mm lens OP is talking about is **not** controlled for constant aperture throughout its zoom range. The spec'd "3.5–5.6" is not a measured T-stop exposure change—it is actually observed f/D (focal length over entrance pupil diameter) (at least to the nearest standard f-number)
Comment: @Tetsujin how is it T-stop? The camera isn't aware of the lens's transmission characteristics. It only knows and controls the lens's aperture linkage, the position of which correlates to reported f-number.
Comment: Seems like this is a video specific issue... For stills, the camera should be able to retain F5.6 through the zoom range because the camera only stops the lens down right before the photo is taken.
Comment: @xiota you're pretty much right. I'd say it's _mostly_ a video-specific issue. It also comes into play when in live view, most Nikon DSLR bodies can't change the aperture. So if composing in live view while zooming, the camera will _appear_ to be changing the aperture on variable aperture lenses. Of course, after pressing the shutter button, the camera will come out of live view, reset the aperture linkage mechanism, close down the aperture, and capture the image. As you note.
Comment: @xiota, having said that, it might be 100% a video-only issue. It appears that the more recent "prosumer" Nikon bodies can change the aperture in live view.
Comment: I'm pretty sure it's a t-stop vs f-stop issue… but I don't know where to go to confirm it.
Comment: @scottbb - It's honestly well above my paygrade, but I found a post from an engineer at Arri who was surprised by it & did some investigation - https://www.provideocoalition.com/focus-ramping-when-t-stops-go-bad/


Why Quantum Mechanics states that nature is probabilistic?

Question: I am new here and I am studying the philosophical implications of Quantum Mechanics. I read somewhere that QM and determinism are mutually exclusive and that QM involve a number of philosophers to think that nature is probabilistic. Is this true? And what are the theories or physical laws (in QM)that state that nature is probabilistic? In other words, where is the modal aspect in QM?
Comment: QM, as a mathematical formalism, and determinism are not mutually exclusive, the *standard (Copenhagen) interpretation* of QM (and many others) and determinism are mutually exclusive. What suggests indeterminism is that, unlike classical physics, quantum mechanics only predicts distributions of outcomes. There are deterministic interpretations of QM, e.g. Bohmian mechanics, although they are, arguably, "artificial" because the deterministic hidden variables are undetectable. Many consider even the Many Worlds interpretation "deterministic", although it is a quaint kind of determinism
Comment: Mathematical formalism (with some minimal interpretation relating numbers to meter readings) expresses nothing by itself, so it can not exclude anything philosophical. Abstract generalizations, like realism, determinism, materialism (or lack thereof) require postulates way beyond the formalism. Some weaker relations can be argued, like "suggests" indeterminism, or makes it "more plausible", the way classical mechanics made determinism more plausible (because it made unique predictions for any initial values), see [QM vs determinism thread](https://philosophy.stackexchange.com/q/1210/9148).
Comment: Thanks @Conifold. I am ignorant on the theme and I just started with it. For this reason, Could you clarify me why QM as mathematical formalism and determinism are not mutually exclusive? Are you saying that the mathematical equations and formulas of QM express a certain kind of determinism?Thanks for the help.
Comment: Moreover, are the mathematical formalism of QM and determinism never mutually exclusive? @Conifold
Answer: The basic formula of QM in its most simple form is Schrödinger's equation. That's a linear differential equation like other differential equations from classical Newtonian mechanics or Maxwell's electrodynamics.
The distinctiveness of the Schrödinger equation: The equation describes in a deterministic way the time development of a probability.
According to the Copenhagen interpretation QM is a complete theory. Hence in QM we cannot do better than deriving a probability for the outcome of our experiments. And there is no other theory which derives more precise predictions than QM - also Bohm's theory does not.
I consider a good introduction to the Copenhagen interpretation - without any mathematical formalism - the book
"Werner Heisenberg: Physics and Philosophy".
Heisenberg is one of the founders of QM.


In any form of ball game, is there a term for when a ball gets caught in the wind and changes the play or affects how the play ends?

Question: So far I haven't had any luck trying to find an answer to this question, but I am not all that familiar with sports as a whole - only baseball, and Fly Rule doesn't seem to apply. A term from -any- outdoor ball sport would be acceptable,
Answer: As far as I know, there are no rules regarding the situation you described. In many sports, wind plays a large factor in how the ball travels through the air (American Football, Soccer (Football), Baseball, Golf, etc).
In Track and Field, there are rules that stipulate that the wind speed has to be under a certain amount for a record to be counted (see wind assistance).


Clubs/meetups: in scope?

Question: Are threads like this one about clubs/meetups on topic for the site?  It's so far off from my concept of a "retrocomputing" SE site that I've got no idea what to think about it, but it's clearly not totally unrelated.
Answer: I'd say it's off-topic.
On the one hand, such meetups are of interest to retro enthusiasts (so as you say, it's not totally unrelated).
On the other hand though, answers to this question would have a limited geographical scope, and probably a short half-life — some meetups are long-lasting, but many aren't, and the information on them tends to move around so even external links might not be useful very long. An answer which is valid now might well be useless or misleading in a couple of years' time... Both these factors mean this seems like a poor fit for a StackExchange site.
Answer: This kind of question doesn't seem to fit SE very well.  Its not off-topic exactly, but its a bad question.  Thinking about what kinds of answers we would get to questions like this and I think they would be short, every answer would be equally valid, and would be mostly useless without links.  None of that sounds like a good QA idea.
Comment: Totally agree.  We should probably also consider the same policy toward inquiries like "What's a the best place to buy a vintage XXXXX brand computer?"
Comment: @Aaron I agree, a question like that is way too vague to be useful.  A specific question about how to buy something would be ok though.  For example something along the lines of "How can I import an MSX from Japan?"
Answer: Questions about meeting have cropped up on other site metas in the past.  Certainly on Genealogy and more recently on Motor Vehicles.
The consensus seems to be that suggestions of a meeting should be made in the chat room.
That would seem to be appropriate for this site too.
Comment: The original question wasn't about having a meeting(s) for Retrocomputing SE members, but rather it was asking for a list of local computing clubs / meetings in the user's area.
Comment: I'd argue it does answer the question asked: Users asking such questions should be offered a comment that the question would better be asked on a chat room and the question closed.
Answer: I'd argue it's conditionally off-topic.
I suppose if you can find a way to have a specific question of general interest and use on the topic of clubs/meetups, that could be quite topical.  For example, a question about the history of the Home-Brew Computer Club where both Microsoft BASIC and the Apple 1 were famously introduced, that's should be topical.
A question such as "Are there any 8 bit Commodore clubs/user groups/meetups in the Portland, Oregon metro area?" would not be.  I could answer such a question in the affirmative--but I see two reasons why that is a poor question.  First, the question and its answer are not useful to people in Tampa (or anywhere else).  Secondly, it's a request for information that may change in the future.  Currently there is a Portland CUG, whose current meetings are held at this location on these dates, at these times.  Any of that information may change in the future, causing a well-accepted answer to become completely wrong.
We have questions of selective interest now, but there are a lot of people who share those selective interests.  I don't have an Amstrad CPC and am therefore not hugely interested in questions and answers about it.  But even if the Amstrad users are a small segment of our userbase, it is not exclusive the way questions of gatherings based on geography are.
Answer: This might be a good candidate for a community wiki question. In a round about way I think it'd be of benefit to the site.
Comment: Everything I've read on the use of community wikis suggest that using them to get around bad or borderline questions is poor form.  Making it a community wiki would make it easier for any member to update the answer, but it would still need to be constantly updated to not fall out of date.  This kind of question really does seem like its less suited to the QA format and something more suited to the various forums out there, or a well worded search.
Comment: Yeah, that's a fair call.

Clubs/meetups: in scope?


Philosophy - If both the Rebound Universe and Block Universe theories are true would that mean Nietzsche's Eternal Return theory true?

Question: If the Block Universe is true, and if the Rebound Universe is also true where the universe goes from Big Crunch to Big bang then doesn't that mean that the Block Universe is shaped like a circle where history repeats the exact same way and so do we - therefore wouldn't that verify Nietzsche's Eternal Return theory to be true?
Comment: Possible duplicate of [Philosophy - Does the block universe theory of time mean that life will repeat after death?](https://philosophy.stackexchange.com/questions/54024/philosophy-does-the-block-universe-theory-of-time-mean-that-life-will-repeat-a)
Comment: Please specify your train of thought. Would a built block of time traversed in a direction reverse to its build-direction and ( after the next Crunch) in build-direction be a rewind /replay of the first happenings? Is that it? How would that differ from an eternalist view? Why is the block theory of time needed here? Also, do you envison spacetime after the Crunch to be a mirror image if before? Why? Etc.
Comment: I edited my question
Comment: If x and y theory are true does that not verify z theory? First if all, theories are not true. They may fit the available data best, but what makes them (good) theories is being falsifiable. Second, your edit was miniscule. Please talk at length about your understanding of the block theory and the reboind theory, and how that would give credence to nietzsches  theory (is that one even falsifiable?)
Comment: I'm so sorry to read that you're experiencing such a deep mental health crisis.  It's not clear to me whether you've sought counseling for help.  If you haven't done that already, I encourage you to do so. I hope things turn around for you soon.
Answer: Nietzsche's eternal return is a thought experiment for his existentialist ethics, not a metaphysical hypothesis.  Roughly, the idea is that, as I reflect on the decisions that I've made in life, I imagine that my life will endlessly repeat in exactly the same way.  If this thought makes me happy or excited, because I get to endlessly relieve my great triumphs and accomplishments, then I've lived well.
Nietzsche doesn't care whether or not the eternal return is actually true.  It's a heuristic to evaluate the quality of our lives.
Now, in terms of physics, general relativity does allow the existence of "closed timelike curves."  The basic idea is similar to science fiction stories (movies, TV shows) that feature "time loops" as a plot device.  However, in those stories, the protagonist has some way to carry information from one loop to the next, usually deja vu or a vague memory.  That wouldn't be the case with actual closed timelike curves.  Closed timelike curves aren't like a dog chasing her tail, spinning around and around.  They're more like a statue of a snake with her tail in her mouth.  There aren't actually iterations or loops of the closed timelike curve; it's just that spacetime is structured in a closed circle.
Closed timelike curves are possible in general relativity, but AFAIK they require very specific arrangements of matter, and have never been observed in the real world.  (IIRC the original example to show they were possible had a universe full of motes of dust spinning in a particular pattern.)  So there's no reason to think that we are living in a closed timelike curve.
Comment: And will living in a closed timelike curve never happen for us?
Comment: Again, closed timelike curves are possible in general relativity, but there's no reason to think we're living in one.  And, insofar as they require very specific arrangements of matter, there's good reason to think that we're *not* living in one.


How to update all hyperlinks which are in the content

Question: Hello I am using book_copy module for creating a new version of the book so i have copied a book which is having more than 1000 pages with the book copy module.
Now the new book copy has all working links but for the new book i needed the new urls.
like for previous book it was content/2012 now i needed content/2013 which i have done with pathauto but problem is that while copying a book it takes url for pages as content/2013 but in the page there are hyperlinks which are not updated and pointing to old book pages.
 So let me know if any module for fixing this issue o/w i need to go on each link and update it which is really a difficult task.
Answer: Export the database, run a find and replace over the dumped sql and re-import it.
e.g.:
<code>#1 mysqldump ... > output.sql
#2 copy output.sql to output_mangled.sql as a backup
#3 perl -pi -e 's/content\/2012/content\/2013/' output.sql
#4 cat output.sql | mysql ...
</code>
Replacing ... with authentication and database specification parameters.
To be safer, make the regular expression in the replacement step more specific.
Comment: That is _very_ risky...
Comment: OK. i) keep copy of output.sql ii) limit tables to field_* iii) try to match your sql as carefully as possible (check for href around) iv) test thoroughly. Since you have a backup of the database it can't be destructive
Comment: Sorry, a string substitution on an SQL dump for varchar/text data is just a bad idea. Reading every piece of content in the site (not to mention variables, titles, and a plethora of other text data in a Drupal site) to make sure the replacement hasn't messed anything up is just untenable. The correct way to handle this would be a custom filter
Comment: ok but i needed to modify all hyperlinks at one go.so how is it possible.
Comment: If you don't want to play with the sql you could export the content, then find-and-replace then reimport. Feeds export/import, migrate module or one of the other data migration tools.


How do I add a picture to a Stack Exchange post?

Question: I'm trying to make people see what I did. Can someone help me.
Comment: Could you please give more Information about your problem ? Where do you have your problem ? (ProfilePicture, Question with Pictures, Pictures in Answers etc.)
Answer: You can add an image using the following markup:
![Image description][1]
Then at the bottom of the page, you need to add a link for the image
[1]: https://i.stack.imgur.com/IUbiN.png
The system supports uploading images from your PC as well as links to images.  Just click the icon that looks like a picture (6th option from the left, between {} and numbered list) and follow the instructions.  These post tools are located above the box you type in.  For more help on them, see here
If you don't want to list links at the bottom, you can keep it all in one place like this:
![Image Description](https://i.stack.imgur.com/IUbiN.png)
Comment: to add, you can also do `[![Image description][1]][1]` on large images which get re-sized, this way they also become links that when licked on will open up a page where you can view the full sized image
Answer: In addition to the above method, you can let your answer add the image automatically by clicking the highlighted button pictured below:
Comment: And please change the default description from *'enter image description here*'. People using screen-readers rely on this text to describe the image that is displayed on the screen.


Can Euchromatin convert into Heterochromatin?

Question: I know that Heterochromatin can convert into Euchromatin but is the reverse possible? If yes, then How?
Answer: I would suggest in the future doing a small amount of research before asking. For example, the wikipedia page for euchromatin says this:

Euchromatin participates in the active transcription of DNA to mRNA products. The unfolded structure allows gene regulatory proteins and RNA polymerase complexes to bind to the DNA sequence, which can then initiate the transcription process. Not all euchromatin is necessarily transcribed, but in general that which is not is transformed into heterochromatin to protect the genes while they are not in use. There is therefore a direct link to how actively productive a cell is and the amount of euchromatin that can be found in its nucleus.
It is thought that the cell uses transformation from euchromatin into heterochromatin as a method of controlling gene expression and replication, since such processes behave differently on densely compacted chromatin, known as the 'accessibility hypothesis'. One example of constitutive euchromatin that is 'always turned on' is housekeeping genes, which code for the proteins needed for basic functions of cell survival.

Facultative heterochromatin is chromatin which converts back and forth depending on circumstances. This occurs through (de)"condensation" or (de)"compaction" of chromatin:

Among the molecular components that appear to regulate the spreading of heterochromatin are the Polycomb-group proteins and non-coding genes such as Xist. The mechanism for such spreading is still a matter of controversy.[19] The polycomb repressive complexes PRC1 and PRC2 regulate chromatin compaction and gene expression and have a fundamental role in developmental processes.

I would suggest reading those pages a little bit and coming up with a more specific question if they don't answer it.
Comment: I am truly sorry and thanks for the advice and help, will see it in the future and I actually read the Wikipedia article and some other websites and my mistake, i didn't read it well and got them all mixed up and confused.  Thanks a lot for help.
Comment: @Cerebralcortex No worries. Like I said, in the future. I know that it is not always obvious how to follow the rules. Just quoting from the sources you have explored is always a good idea, even when it's trivial, as it helps us out at least that far.
Comment: I understood your point. Thanks a lot.


How can flow in a centrifugal compressor at fixed conditions vary?

Question: Consider a centrifugal compressor. The compressor map usually plots the adiabatic head vs volumetric inlet flow for rated conditions of pressure, molecular weight and temperature at a given speed. But when pressure, temperature, molecular weight and speed are fixed, how can the inlet flow be made to vary ? Is it accomplished through the use of inlet vanes ?
Comment: What do you mean by speed?  RPMs of the turbine?
Comment: Yes, rotation speed.
Answer: The generic equation for mass flow through a cross-section of area $A$, traveling at velocity $V$ is
$$
\dot m = \rho \, V \, A
$$
where $\rho$ is the density of the fluid and $\dot m$ is the mass flow rate.  If the molecular weight, temperature, and pressure of the fluid are fixed, then density is fixed too.  That leaves $V$ and $A$ to play with.  Since $V$ will be largely determined by the compressor rotation speed, all you have left is to change the cross-sectional area of the inlet.
Calculating the area for a particular flow rate is a bit more difficult in practice however, since the fluid velocity will want to increase as the area is increased.  Also, if the compressor is going to pump liquid, then there isn't much that can be done to alter the flow rate without incurring cavitation in the pump.
There is another alternative; you can feed back some of the outlet from the compressor to the inlet, so that the net outlet of the pump is reduced.  You could control the fraction of feed back (a.k.a. reflux) with a valve.
Comment: So if I follow you, the only way to increase the flowrate under the conditions described is through modification of the geometry of the inlet ?
Comment: Yeah, or if you can, modify the pump internals in some way to make it move less fluid on each rotation.  If it's a piston pump, shorten the stroke, if it's a turbine, change the blade size and/or angle.
Answer: In general every compressor (axial and radial) will deliver a certain flow rate depending on the back-pressure down stream of the compressor. So by changing the pressure difference over the compressor: for example closing a throttle upstream or downstream of the compressor the flow rate will change accordingly. This can also be achieved by changing the geometry of the compressor. One possibility are the mentioned variable inlet guide vanes.
Below is a compressor map taken from Wikipedia. As you can see (away from the surge line) every mass flow rate is connected to a pressure ratio. At constant rotor speed, the flow rate changes according to the pressure ratio which is needed.


How to switch between full and half-width input for Windows 10 Japanese IME?

Question: How to switch between full and half-width input for Windows 10 Japanese IME?
Answer: Not sure if this will work since I am not on Windows 10, but clicking on the icon in the taskbar that indicates the mode of typing (katakana, hiragana, romaji etc.) I can select halfwidth from the menu that appears after that.
My options are:

Hiragana
full width katakana
full width romaji
half width katakana
half width romaji

Hopefully this helps
Answer: I can't check right now but IIRC it was Shift-CapsLock. You can also just type the word in any kana and press F7 to make it full width katakana or F8 for half width.


What is the best water to use for yeast being used in bread making?

Question: I'm just starting back on the road to bread making, and I have been wondering about the water that I am using, and how it will affect the yeast.  Do I just use (1) the stuff that flows from my pressure water system, (2) the water from my drinking water filtered system or (3) go out and buy bottled water? For #1 I just open the tap - the water that comes out of it is de-salinated using RO.  #2 is the same water but put through two filters, I use this for drinking.  #3 the bottled stuff, but if I go down this route, what type - filtered or purified (is there a difference- see below)? How will my yeast react?  So, had a mooch around google and found all sorts of articles (below), but am still wondering.  What is the best water to use for yeast being used in bread making?
This is a fascinating article
A bit more info here
These guys seem to know a thing or two
Comment: Though the third link initially talks about sourdough starter - it is a fascinating discussion about different water types, thus the inclusion in my references (hence the 'these guys seem to know a thing or two').  I am 'just' making bread.
Comment: Are you making sourdough starter, as in the third link, or just making bread?
Answer: There's no single right or wrong answer to the question as it depends on the mineral content (ie hardness), acidity, and chlorine content of your water. Yeast and gluten development is affected by these factors, see this good summary from the King Arthur Flour site:

The degree of hardness is an indication of the amount of calcium and
  magnesium ions in water, expressed in parts per million (ppm). Soft
  water has less than 50 ppm, while hard water has over 200 ppm.
  Generally, water of medium hardness, with about 100 to 150 ppm of
  minerals, is best suited to bread baking. The minerals in water
  provide food for the yeast, and therefore can benefit fermentation.
  However, if the water is excessively hard, there will be a tightening
  effect on the gluten, as well as a decrease in the fermentation rate
  (the minerals make water absorption more difficult for the proteins in
  the flour). On the other hand, if water is excessively soft, the lack
  of minerals will result in a dough that is sticky and slack.

Acidity has an effect as well, too acid or too basic will impact your yeast, neutral or very slightly acidic is best.
Water with a high chlorine content is not ideal as it may impact the flavor of the bread, and can impact the development of cultured starters like sourdoughs. Chlorine is not an issue though as simply leaving water in an open container for a day or two will let it naturally disperse.
So, your bread will be best if there's some minerals in the water to feed the yeast and aid gluten development, it will also be best if the water is very slightly acidic. In many places water that comes out of the tap is perfect, and in most places it will work fine. Purified water is not good because it doesn't have enough minerals in it. Filtered water from a RO system may be fine, whether you want to go further really depends on how much residual mineral content there is in it. I live in a hard water area, about 250ppm, and I use a brita filter for my bread making and cooking and as long as the filter is not used up it seems to work, although I don't know what the actual mineral content after filtering is. If I really wanted to know I would get a testing kit, they do not cost much and are simple to use. I would suggest you get test kits for hardness and acidity, you probably don't need one unless you know you are adding it in your RO system.
Another good article I've seen on the subject is here.
EDIT: chloramines
Chloramines are sometimes added to water instead of chlorine because it does not evaporate easily and therefore much less can be used. The EPA's guidelines is a maximum of 4ppm, however beer brewers state that yeast eating chloramines can produce off flavors in the end product even in small concentrations. I don't know how that translates into bread baking though. Chloramine can be boiled off, but that takes a lot of energy, or you can use Campden tablets to remove it. A charcoal filter like a brita can also remove some, how much I do not know. All I can say is that I've never detected any off flavors in bread when I've used filtered water.
Comment: Chloramines however will have to be boiled off, those don't disperse by standing like chlorine does.
Comment: Just off shopping, so will think about everything above, many thanks for it all.  I hadn't really appreciated all of this before, but now it really does start to make sense.  Thanks for the link as well.
Comment: Back from the shops - proper work stuff all completed, so have time to concentrate on things here.
Comment: See my edit @thrig
Answer: I use tap water (chlorinated) without incident, but if convenient (I won't boil vegetables/potatoes/pasta just to make bread) I will reuse (cooled to 95F/35C or so) cooking water if it's handy (should have some extra "stuff" in it by then - potato water in particular is a classic) or use cooled water that has been in the kettle (so chlorine should be boiled off.)
But I have no problem with using the tap-water directly. I also use milk, eggs, yogurt, nut or bean milks, beer, etc. Hmmm; I don't know that I've tried bread using coffee as the liquid - a project for the weekend, perhaps.
All work, some with more obvious effects on the results than others.
Comment: The coffee idea has me thinking - yes perhaps a project for the weekend, first have to hone my baking skills though.
Answer: Simply use what you have at hand. All other being equal, I would resort to the same water I drink.
Bread has been baked for centuries before we started purifying our water. If you make your own sourdough (like historical bakers), the main thing is to be consistent. Always use the same water, and then you will get a stable culture. If you change the mineral content, you may change the species equilibrium in the sourdough. But it does not matter which potable water you use, as long as it is the same. OK, if you happen to have a culture whose taste you dislike, starting a new one with different water may be a good thing to try, but that is rare and not very predictable.
If you are baking with commercial yeast, it won't have a noticeable difference. Commercial yeasts are hardy little beasts, designed to thrive in countless scenarios, misused by amateurs, thrown into choking-heavy doughs, etc. Sure some conditions are better for them than others, but if there is any noticeable difference, it will probably be that the yeast with the suboptimal water takes a few more minutes to rise. And these fluctuations in rising time are small when compared to fluctuations caused by other factors, so you have to rise your dough until ready anyway.
Only if you have an unusually impure water, and/or notice weird smells in your bread (and know they are not caused by some of the more common reasons like thiosulfates or ammonia from wrong rising speeds) is it worth changing your water. Maybe also if you are a high-volume artisan baker and have all other variables of the process perfectly controlled and are now finetuning the last details. But for the normal home baker, tweaking the water is unlikely to give you any returns on the invested time.
Answer: I think rumtscho, above, said it well, as to your question on water. However, it is recommended that if you have tap chlorinated water to let it set out on the counter for overnight, to let the chlorine evaporate.
When I thought I had 'killed' my starter, I looked back to see what things I had changed from one day to the next, and realized one item that I did differently was, in a moment of haste, I used my well water. I had also used a metal spoon, and I think I had reduced it before it was strong enough to survive that.
I have not yet totally gone off of bottled water, but am now adding 1/2 well water, after letting it set on the counter to warm up to room temperature, as our well water is very, very cold. I guess I'm doing as rumptscho suggested: bakers only recently have had 'purified' water, and I am introducing my well water a bit slowly, as it is quite hard water.
Comment: Um, now that is interesting - I don't have any well water, but the more I learn, the better things will be.


How and where to get you research work assessed for PhD?

Question: I completed a masters in Chemical Engineering long ago. I have since done some research which I think suffices to achieve a PhD. Can it be done (and how)?
Comment: The short answer would be "most likely not". What you _could_ do in principle, I suppose, is to approach a relevant professor and present your material, and ask them directly. But if you don't already have international publications of your work and fulfill other requirements from the PhD curriculum that the university may have, your chances will be very slim.
Comment: What research have you done? Why do you think it is enough for a PhD? What do you already understand about PhDs? Do the answers to the last two questions give evidence to suggest that you've done enough?
Comment: Generally, you enroll in a Ph.D. program and do your research then.  They do not award Ph.D. for previously-done work.  Actually, there may be some shady institutions that award degrees (not only Ph.D.) in return for your money, claiming to justify it by saying the degree is awarded for your "life experience".
Answer:
How and where to get you research work assessed for PhD?

The first person to evaluate if the work is sufficient to warrant submitting it for official PhD evaluation is the PhD supervisor. Therefore you need to find a supervisor first.

I have since done some research which I think suffices to achieve a PhD. Can it be done (and how)?

These things vary a lot by country and by institution, but it's common nowadays that the requirements for passing a PhD involve:

validating some classes (in particular about general scientific methods, ethics, etc.)
having published a few papers in peer-reviewed journals or conferences

The question doesn't specify whether the work has been published/peer-reviewed or not? If not, it's a bit risky to assume that it is sufficient for a PhD.
Answer: Your best bet will always be to contact the administrative department at a university (or universities). Which ones you decide is up to you; whether you want to try a smaller local university or a more reputable one. The point is, they will be able to give you a real, factual answer, which is more likely to be correct compared to un- or semi-qualified comments from people on forums. As the advice you receive from the admin will also be what you will need to follow, it also carries more 'weight' than comments here.
That said, it can depend on the institution. In Australia (Specifically the University of Southern Queensland) there is a 'Master's of Engineering Science'. This is intended for predominately international students who have completed a Bachelor's elsewhere, and allows an internationally recognised Bachelor's equivalent. It is still 2 years of full time, or 4 years of part time work, although the work is more project related and less course-work. I know of no PhD equivalent to this Master's.
It has become more common in recent years (at least in Australia) for a PhD by publication. This requires 3 years of enrollment (as per a typical PhD.) with a number (typically three) publications produced during this enrollment. These publications must be fully peer reviewed and the journal of sufficient ranking. Supervisor's opinions of PhD by publications differ (mine dislikes them) but one theory is that since the work is already peer reviewed, it means any examiner's comments on that work carry less weight if they dispute the results. If you choose to do a PhD by publication, it's thus advisable to find a supervisor who agrees with the system.
So unfortunately, as far as I'm aware, there's no way out of doing further research. No reputable university is going to 'give' you a PhD based on previous work. In all likelihood, your work may be innovative, but will have lacked the level of detail required to be PhD-worthy (a preliminary appointment with a potential supervisor or two will quickly answer this question). On the plus side, the fact that you've demonstrated innovative work in the past likely means you're capable of doing a PhD; the university enrolment process will definitely take this into consideration. Try and find a supervisor with a passion for the field you've demonstrated innovation in. They'll likely be interested in letting you continue your current work, and will advise on how to bring it up to a recognised PhD depth of research. It's likely to still be 3 years of study, but this way you'll have a big head start with a topic you're already very familiar with; those 3 years will be spent gaining deeper insight, which is what the examiners at the end of 3 years will look for.
It may be an aside to your original question, but if you're considering options then look into a Doctor of Professional Engineering (D.Eng) or equivalent for your field. This is typically industry-based (you work for a company doing innovative work for them, while reporting to the university.) and is typically 6 years of part time study. Something like this may be of benefit to you, although once again, contacting a university administration, presenting them with your qualifications, and asking for a list of supervisors you could organise a consultation with is going to be your best bet.
Information on the USQ D.Eng can be found here. Directly quoting from that page:

Have a passion for engineering and want to further your knowledge? USQ's Doctor of Professional Engineering is designed to expand your knowledge in technical investigation and innovative design and analysis, as well as take your research skills to a higher level.
  Your studies will include 8 units of discipline-based coursework and research methodology studies, before embarking on 16 units of independent research in a specialised field of your choice, contributing to our future world.


Is there a summary of Russell's Principia Mathematica?

Question: Perhaps better, is there an accessible version of the Principia? I am looking for a summary that would summarize and clarify Russell's reasoning behind his famous conclusion that 1 + 1 = 2.
Comment: Have you tried the wikipedia article? Or the SEP article? And by "reasoning behind", do you mean you want to know why Russell thinks that 1+1=2 or do you want an explanation of what axioms/definitions/rules of deductions are used in PM? Because the conclusion that 1+1=2 might be famous, but it isn't due to Russell (or Whitehead), or any particular person of history for that matter.
Comment: Here's something that might help but mainly to understand *54.43. I have only skimmed it: https://blog.plover.com/math/PM.html The three volumes of the Principia Mathematica are on the Internet archive: https://archive.org
Comment: @Not_Here  Well, frankly and probably unrealistically, I was hoping for something that would spoon feed the proof to me. Thanks to you and Frank Hubeny for the citations.
Comment: Since you seem to be asking for a logic/philosophy reason, are you sure you want Russell's reasoning in particular? As Mauro said, other proofs are much simpler, and the system used in PM is not really used by others.
Answer: For an introductory exposition, you can see : Richard Zach, Principia Mathematica and the Development of Logic (2010).
A more detailed exposition is into : Ivor Grattan-Guinness, The Search for Mathematical Roots, 1870-1940, Princeton UP (2000), Ch.7.
The issue is that the "standrad" proof of 1+1=2 from Peano's axioms is quite simple: it needs very few lines starting with the definition of 1 and 2 and the axiom for +.
In PM, instead, the Peano axioms are themselves derived from more basic principles and definitions, and this is much more longer.
As you can see from the reprint of the first chapters : Alfred North Whitehead & Bertrand Russell, Principia Mathematica to #56, Cambridge UP (2nd ed 1927), the definition of 1 is at page 345 : Def 52.01.
The definition per se is quite simple :

1 = the class of all classes α such that α = { x } for some x (Def).

{ x } is a singleton, i.e. a class with the only element x. Thus 1 is defined as the class of all classes with exactly one member.
The first result proved from it is :

52.1 a class α ∈ 1 iff α = { x } for some x.

As you can see, the level of details is very high.
Then we have (page 358) the definitions of 0 and 2; from them several further results are proved :

54.101 a class α ∈ 2 iff there are x,y such that x≠y and α = { x } ∪ { y }.
54.102 a class α ∈ 0 iff α is the empty class.

Finally (page 360) we arrive at :

54.43 if classes α, β ∈ 1, then α ∩ β is empty iff α ∪ β ∈ 2.

From this proposition it will follow, when arithmetical addition has been
    defined [emphasis added], that 1 + 1 = 2.
Comment: Thanks for the summary. PM is extremely hard to read, even for a mathematician. I don't understand 52.1: isn't it true by definition 52.01?
Comment: @Taladris yes, it is an immediate consequence of the def.

Is there a summary of Russell’s Principia Mathematica?


tricopter simulation to test control algorithms

Question: I am looking to write and test my own control algorithms for tricopter flight. I am looking for a simulator that can simulate a tricopter but at the level of receiving simulated PWM and returning simulated gyro, compass and other sensor readings. Ideally it would also have graphics for visualization (need not be fancy). Ultimately, I want to port this to a real tricopter but at the moment I would just like to simulate it. Any suggestions for free simulators that are low level as I described?
Comment: Matlab/Octave with your own physical model of the aircraft?
Answer: I have developed a framework that compiles both as an AVR program to run directly on Multiwii board and also having the option to test the algorithms as a desktop 3D app. The 3d app is based on the irrlicht game engine.
It can be found here: https://github.com/mkschreder/bettercopter


Field service lightning crashing on Android constantly

Question: We have just implemented FSL in our business and all installers using FSL app on Android phones are reporting multiple crashes without exception. Has anyone else experienced this issue? Is there a work around? Is sales force working on it at all?
Answer: Looks like there is a know issue which hasn't yet fixed, but it only states For Technicians who are using Salesforce Field Service Lightning Android Mobile app and trying to make a post with a file attachment other than image or video, the app crashes.
The issue occurs with file types such as pdf/doc/csv/xml/txt/rtf etc. except for image (png, jpg, jpeg) and video (.mp4) files.
Reference: Salesforce Field Service Lightning Android Mobile - App crashes when trying to make a post with a file attachment other than image or video.

Workaround: Use the FSL Android app to attach only image or video files till the issue gets fixed, and use the Salesforce mobile app or
  browser based Full site to attach any other file types.

If your issue is not related to attachments and still experiencing this issue I would strongly suggest to raise a case with support.


Avocado tree leaf covering by brown dots

Question: I have a avocado tree in the backyard. The leaves are getting a lot of brown dots. Is it getting infected?thanks a lot!!
Answer: Sarah, your avocado has injury from a pest called the "persea mite". If you do an image search on this term you will find a number of images that show the same kind of damage. This article by the California Avocado Growers contains some interesting details about treatment and also a list of other plants which can act as hosts to the same mite. If you have these plants locally then they can jump to your avocado when the time is right.
Your situation is pretty far advanced, so read this article from University of California which suggests an insecticidal oil application.
Comment: Thank you so much Colin!


Installing Epson printer/ scanner

Question: How can I install my Epson ME Office 535 All-in-one printer/scanner in Ubuntu 13.04?
.
Answer: First, press Super KEY and search for Printers.

Then, click Add/Plus Icon

You will see this screen with your connected printer. Select your printer and click next. It will than ask you for a name and ETC ETC. Fill out. It will search the drivers for your printer. You should first search the internet and then from database.
Search in list EPSON
and then select model and apply

Installing Epson printer/ scanner


What should I do if I don't know the answer to a question posed by a student?

Question: If I am the TA for a class, what should I do if a student asks me a question which I can't answer? While "tell the truth and say you don't know" is one approach, are there other options?
Answer: I agree that honesty is the best policy, and it's too bad if you're in a situation in which you feel worried about admitting you don't know the answer.  You shouldn't try to bluff, by pretending you know but don't have time to explain or by giving an intentionally vague answer.  However, there are ways of handling it more smoothly than just saying "I don't know" and leaving it at that.  Depending on the circumstances, you can say "That's a really interesting question.  I haven't thought about it, so I'll have to look into it, but let's talk about it in office hours."  (Or you can promise to return to the topic in the next class meeting if it's really relevant to the course and everyone in the class will want to know the answer.)  Or "These issues can be complicated.  I don't know the details off the top of my head, but the place I'd look them up is Reference Work X.  I'd be happy to show you where to find it after class."  Or "That's a good question, but it's somewhat beyond the scope of this class.  I'd be happy to investigate it with you outside of class."
The key is to respect the student's desire to learn.  If you avoid the question or give an answer you know is inadequate, then you're being deliberately unhelpful.  If you just give up and admit defeat, then at least you're being honest, but the student still isn't finding out what he/she wanted to know.  If you respond by pointing the student on the road to an answer, even if you can't supply it off the top of your head, then you've done everything that can be expected of you.
Comment: And, even if you suspect that the student might be just hassling you, nevertheless treat every question as genuine and sincere, and respond as sketched by AnonymousMathematician. That is, the conscientious (and "innocent") response is also the best defense against suspected hecklers.
Comment: @math137: If you are repeatedly unable to answer elementary questions, then yes, the students would (and should) become alarmed.  On the other hand, the solution to that problem is to learn more, not to try to hide your ignorance from the students.  But it's easy for students to ask arbitrarily obscure or difficult questions, and curious students will sometimes do so; there's no shame in not having an immediate answer.  Naive students may believe their instructor should know everything, but it's valuable for them to learn what sorts of things an expert does or doesn't know offhand.
Comment: @IlmariKaronen I have done that and sometimes was not able to find the answer. No biggie -- just abort after 5/10/... minutes, say "This seems to be harder than I though, I'll get back to you! But now we need to continue with the other material" and all is good. I think that students can learn more from watching how you approach a problem you don't know already know the answer to than they can from watching you present a polished solution.
Comment: +1. In some cases, "I don't know, but let's find out!" may also be an option. Of course, you should only try that if you're confident that you *can* work out the answer reasonably quickly; spending too much time working on a single question not only cuts into your available time, but can also easily bore the students.
Comment: +1. Or - *gasp!* - you might even ask whether some other student has an idea (assuming that this happens in a classroom setting)!
Comment: I believe directly, and if that happened more than one time, admitting that you don't know the answer may cause you to lose students trust on you.
Comment: Also I try to deal with it with some humor.  "Well congratulations!  You have stumped the professor!  I do not know the answer but..." and then either "let's figure it out together" or "I'll have to get back to you" with follow-up.
Answer: I'm a professor, so I am expected to know the answers, but sometimes I don't. This often involves some minor detail in a programming language.
So I usually say, "That's a good question. I don't want to give a wrong answer, so let me think about that and get back to you." We use a course management system which includes a discussion board, so I will usually then add, "I don't want to forget, so post that on the forum. That way everyone will see the answer." Then in the posted answer I try to explain how I found the information. I find this works well.
Answer: It's happening a lot so in this situation  I prefer to not saying I don't know the answer" but I will say good question and let us think about it and we will discuss later
Comment: And the students will know right in that moment that you doesn't know the answer. I never respected any teacher who couldn't admit that he doesn't know/remember something.
Comment: Agreed.  I have never really liked professors who tried to be infallible.....
Answer:
I don't know the answer to your question at the moment, let us all try to find a
  solution together.

In that way, you are communicating the fact that there are always new ways to look at things and presented for the first time, it is difficult to answer.
Then, you might well be in the same shoes as the questioner and other students and one logical way is to sit and solve it together. You could invite the whole class if you wish; a cooperative effort. The main idea is to try to find a way to tackle the problem before it dies away.
Answer: You should say that you don't know the answer or did not prepare for answering it (especially if it is a question that is out of topic). Then you can either search for the answer with the student if you have time and it is appropriate. Or if you cannot at that moment, you can say that you will look for it and give the student some explanation by e-mail or next time. This is something that happens even to professors sometimes.
Comment: An alternative is to ask whether any of the students want to answer it. Or even whether they would be willing to research it and provide an answer in the next class. You needn't assume that all of the answers need to come from you. Students need to learn, not to "be told".
Answer: If that is course related Just tell them  "we will get there", don't hurry. Go home find out the answer and give them answer the next day or so. :)
Comment: That is an attempt to fool the students. They are not stupid, and will probably see through it (specially if you do this often). I thin there is more value in teaching that it is OK not to know everything, but give the tools to figure out, as Anonymous Mathematician wrote in his answer.
Comment: Best action is to say I do not know the answer. Then research it. That's it.


How to get more concrete, specific, tangible feedback?

Question: I am receiving training for an in person sales position. The trainers feedback was that I appeared stiff. I asked him what I could do differently to appear less stiff and he just said "It looks like you have an invisible force field between you and the person you're talking to". This doesn't help me at all. How can I communicate I'm looking for specific, actionable items? Such as I should stand closer when talking to someone?
Comment: You've already communicated this. You can try again like "I've thought about your feedback that I appear stiff and I still don't know how I should approach improving this. Do you have some specific recommendations?" but a good trainer would have already made some suggestions.
Answer: When you've given feedback you don't understand try to get the person to give you more information. When the trainer tells you:

It looks like you have an invisible force field between you and the
  person you're talking to.

Respond with:

That's good to know. Thank you for telling me. Please tell me more. What would you do differently?

If I had to guess what the trainer meant by:

It looks like you have an invisible force field between you and the
  person you're talking to.

I would also guess that you keep too much distance between you and the other person. You may also need to be more animated in your communication (e.g. use your hand or objects to illustrate your point) and relaxed body language (e.g. https://blog.udemy.com/positive-body-language/ for working in the United States).
Comment: The article you linked to is very good. Funny enough point 1 has "Keep your back straight but not stiff and let those shoulders relax" I still find this hard to know what changes to make. I guess for me having a stiff something is a physical thing, not something people can see. For example just by looking at someone how would you know their back feels stiff?
Comment: @Bertelem I think the idea is that people are social creatures and when we have a conversation, we look at non-auditory cues from the person we're having a conversation with. You can have your back straight, but your shoulders and neck stiff e.g. a soldier standing at attention. However that stance is too "formal" and "rigid" for a typical sales conversation. There's this popular TED talk on body language too: https://www.ted.com/talks/amy_cuddy_your_body_language_shapes_who_you_are?language=en.
Answer: You could ask him to record a short training session and then give you feedback based on the recording. If anyone is concerned about data privacy, record the session on your personal device and show everyone involved that you delete it after the feedback is given.
Why the recording works:
During the interaction with a customer, you have to concentrate on your role and product knowledge, on the questions and problems of the customer, and on your own guestures and postures. That is too much to be aware of your body language all the time.
The recording gives you the chance to see yourself from a (literally) different perspective. You get to see guestures and postures that seemed so unimportant during the interaction that you already forgot them.
If the feedback is too vague for you to understand, you can ask the trainer to point out specific details in the video.