How can I get my cat to stop chasing my other cat?

Question: I have three cats. All neutered and spayed. A tuxedo named Pip, a tabby named Kitty, and a tortie named Faith. Pip isn't really all that important in this question, I'll give more info about him later. Faith is a grumpy cat and she is pretty aggressive, but it's more out of fear. She didn't trust me for most of my life but now she won't stop kneading my face in bed and keeping me up all night because she wants me to pet her. She's about 7-9 years, we aren't really sure. Now enter Kitty. He's a li'l devil. We got him in about September or August of 2015, he's, I think, 3 years. We thought he was a kitten when we first found him because he was so small and skinny. Now he's small and fat. He's very territorial, constantly rub-rub-rubbing on every little thing to get his scent there and claim it. Pip, our sweet li'l innocent angel that wouldn't hurt a fly. Well, he would, but he's a cat, what do you expect? He used to be more aggressive until my brother traumatized him (more on that in another question) and now he's very skittish.
This all sounds nice. You have 3 happy cats, what could possibly go wrong?
... A lot. Kitty's favorite activity is chasing Faith and battling with her. He's gotten into quite a few 'duels' with Pip (all of which Pip won) and so I think he's trying to show his dominance. He failed with Pip but Faith is weak and old so she's the perfect target for our little bully. I've not been sure on if I should ask this or not but the final straw was when I heard yowling and hissing from outside today. (Note: whenever Faith comes in Kitty tries to chase her, so we have to hold him down so he won't chase her.) and Kitty was chasing Faith up the stairs to our deck and Faith was backed into a corner. I opened the door, shook Kitty a few times and flicked him on the nose as punishment.
Things get worse when Faith fuels the fire. Whenever she sees Kitty she hisses and tries to scratch at him. Things get even worse when Pip joins in though. Pip doesn't know what he's doing, he chases her very rarely, and I think he only does it because Kitty does it. (Kitty is then ultimate bad role model.)
I'm sure one of you will suggest to have them 'make friends' or something but that definitely wouldn't work. They'd murder each other, and if they didn't, Faith would collapse under the stress.
Maybe one of you will suggest "Maybe give one of them up." However, I could not give up any of my kitties even if I wanted to. Faith, although she'd be freed from the stress of Pip and Kitty, will be separated from me. She gets very attached very easily and it would definitely not be a good idea. My sister (her original owner) left her in September to go to college and Faith got attached to me, somebody she already had a slight bond with. In a new house, she wouldn't recognize anyone, and I think it would be even worse than the stress with Pip and Kitty she is facing now.
Suggestions, please? I am so tired. I've tried everything, clapping, flicking Kitty/Faith (when she starts the fight) on the forehead/nose, nothing works.
Comment: Only solutions I know are either to let the cats settle it their way, separate them, or separate them and try re-introfucing them in a much more structured and controlled manner -- which may not work
Comment: There are a lot of previously answered questions about fighting cats. Do some searches here if you need more information than the answers below.
Comment: Are all of them fixed/altered @booty tornado ?
Answer: You need to establish a pecking order, one where you are at the top. Your cats need to abide by your rules, and that includes not picking on each other. Even if they cannot stand each other, they need to respect your wishes and not start fights.
1. No picking on each other.
You need to consistently enforce this rule in all directions. Cats don't respond to your intention of teaching them (dogs do), but cats do respond to their environment and situation.
A cat might never stop doing something because you tell it to, but you can get it to stop doing something because it causes you to retaliate

Disclaimer:
  A retaliation by you is supposed to be negative feedback and an inconvenience to the cat. It should never cross ethical boundaries. No violence, fear, or harsh punishments.

The rule is clear, no picking on each other. If you can't interact nicely, then don't interact. All you need to do now is consistently enforce it.
2. Responding to transgressions.
A real world example might help here. We have three cats: Cleo, Misha and Alfie. Cleo and Misha are sisters (about 15 months old), Alfie is a brand new addition (about 6 months old). Alfie always wants to be first, everywhere, every time.
Treats? Alfie is front and center. Food? Alfie rushes to the first bowl and quickly jumps to the next bowl when his is finished. I'm giving love to a cat? Alfie will come and ask for attention.
I may be biased because I like the little bastard, but imo Alfie does not mean to undercut the girls, he's just eager for everything. He is always highly suggestible and easily distracted by anything that catches his attention. Literally anything distracts him, both in cases where it is a benefit or a drawback. He's not gaming the system, he's just incessantly eager.
However, and this is key to how to respond to transgressions, Alfie's intent does not matter. He gets punished for breaking the rules, not for why he broke the rules. The rules are non-negotiable, and need to be enforced as such.
He's allowed to be highly energetic and eager. Cleo and Misha are a bit more relaxed, so it's natural for Alfie to generally be first. I don't punish him for being first if the others are slower than him. However, whenever he crosses the line and actually cuts in between me and another cat, he does get punished.
I (softly) put my hand on his side and then give him a strong push. At the very least, he'll slide to the side. At worst, he'll fall on his side. It doesn't hurt him, but it inconveniences him to be pushed away and have to walk towards me again.
If he rushes at a treat that was clearly placed in front of another cat (I tend to also say the name of the cat whose treat it is, for further clarity) I will do everything in my power to ensure that the treat is eaten by the cat I gave it to. I will take it from Alfie's mouth if I have to (luckily, he doesn't put up a fight).
The core of my feedback is always consistent: No cat gets to bend a rule that I have made. If I give a treat to Misha, no one gets to take it from her.
This also extends to unjust behavior that I see happen. If a cat drives another cat from their sleeping spot in order to claim it for themselves, I will actively remove that cat and make sure their crime did not pay. This includes dunking water on the spot (after the cat has been removed) to make it an inhospitable sleeping spot.

Interestingly, before Alfie joined our family, Cleo was similarly eager. It was always "me first!" with her, and we needed to push her aside to actually be able to interact with Misha. However, Alfie is much more energetic (and socially oblivious) than Cleo, and now Cleo is basically being treated the way she (obliviously) treated Misha. Since Alfie, Cleo no longer cuts in front of Misha, not even unintentionally.
This sort of proves my point: cats learn to not undercut others (unintentionally or intentionally), when they themselves become the victim of being undercut. When I see a cat undercut another cat, I will interfere in a way that the offender becomes a victim to their own crime.

3. Patience and consistency.
Most people who see me interact with my cats make a comment about how much effort it must take to enforce rules that get broken constantly, since cats are not really trainable (compared to dogs).
But I disagree with that. Cats are slow learners (compared to dogs) because they do not observe a human's intent to teach, but that doesn't mean that cats are unable to learn something.
Consistency is the most important factor here. If you always apply the same rule, and apply the same punishment, then the cat will eventually see the pattern and learn to avoid it.
Cats already learn this way. There's a rope hanging off of a bucket. The bucket is standing on a table, filled with water. Cleo jumps and tries to catch the rope. The inevitable happens: she catches the rope and throws a bucket of water on herself. Cleo made the mistake a second time, but has not played with the bucket since then. Cause meets inevitable effect. Cleo saw the pattern (play with the bucket => get drenched) and actively avoided it in the future (don't want to get drenched => should not play with bucket).
The key part here is inevitable. It's not just a random occurrence, the effect consistently happens. I try to artificially recreate that same environment of cause and effect. Alfie takes food from another cat, and gets pushed aside (roughly but without hurting him). The more aggressively he tries it, the more aggressively he gets pushed back.
Misha sometimes walks away from treats. When she does, he's allowed to take it once she turns her back, unless he has already tried to steal it.
It has taken about 2-3 weeks, but Alfie understands that now. He will eagerly look at the treat and Misha, but he will generally not try to steal it anymore (exceptions made on days where he's much too playful to be thinking straight, he's still a kitten after all).

4. Focusing on your situation.
My situation has been a bit more relaxed. It's mostly about a social fax-pas. Your situation is more severe, your cats are antagonizing each other. You can apply the same principles as above, but you'll have to tailor it to the situation.
The rule: No cat shall be aggressive or unkind towards another cat. No cat is above this rule. As the pets' owner, you will not accept any disregard for this rule (no matter which cat is breaking the rule).

Kitty's favorite activity is chasing Faith and battling with her. ... Faith is weak and old so she's the perfect target for our little bully.

The moment he acts out, you respond. This can be done by name calling (neutral, then angrily), throwing soft things (e.g. pillows) or physically intervening. The important part is that Kitty is guilty, and must be appropriately punished. I'll address possible punishments further in the answer, but the general idea here is that you need to tailor a punishment that fits Kitty's crime. Crime does not pay, and it is your job to deliver that message to Kitty's doorstep.

He's gotten into quite a few 'duels' with Pip (all of which Pip won) and so i think he's trying to show his dominance. He failed with Pip.

If you see it happen, you need to intervene. Regardless of who started it, if both are actively engaging each other in the fight, they are both guilty. Note that you should observe what is going on. If one of them is attacking, and the other is fiercely defending (but doing nothing more than defending incoming attacks), then only the aggressor is guilty. But if both of them are aggressive (even if one of them started off simply defending themselves), both are actively breaking your rule and need to be punished for it.
If Kitty is the aggressor, and Pip consistently wins before you can intervene, you do not need to intervene after the fact (as long as no cat gets hurt in the process). Kitty has already experienced the effect from what he caused: Pip dominated him. Kitty must recognize the pattern and decide: do I want to keep losing this? Or do I want to avoid poking the bear?
However, if the aggressor ends up winning, then you do need to intervene after the fact and make sure that the aggressor does not gain anything from their behavior.

I've not been sure on if i should ask this or not but the final straw was when i heard yowling and hissing from outside today.

In order to retain consistency, you should yell at the hissing cat before you even know who it is. That proves the point that you're not upset with a specific cat, but rather whoever is being unkind.

whenever Faith comes in Kitty tries to chase her so we have to hold him down so he won't chase her.

Holding him down is a good idea. However, maybe a pedantic niggle, but I'd make sure to only hold him down after he has shown to make a move. If you immediately hold him down before he has even misbehaved, then there's no lesson for him to learn. However, if he only gets held down after misbehaving, then he can learn the pattern (run after Faith => get held) and learn to avoid it (don't want to get held => shouldn't run after Faith)
Don't be afraid to hold a pillow and use it as a projectile to block him when he makes a move for her. If he's being that aggressive that you can't easily stop him, then you're morally allowed to respond in equal measure by throwing a pillow (I would aim it in front of him rather than on him).
Even if the pillow doesn't stop him, it still serves a purpose as a signal that you are telling him off. It also has the added benefit that you holding a pillow will eventually become a reminder of what will happen if he misbehaves.

Things get worse when Faith fuels the fire. Whenever she sees Kitty she hisses and tries to scratch at him.

Faith's reaction is understandable. But just like how I said that Alfie's lack of ill intent is irrelevant; so is Faith's justification for hissing. Hissing is unkind, and unkindness does not get tolerated.
You may think that this is unfair on Faith. But keep in mind that you're also teaching Kitty the same lesson at the same time. He sees that other cats get punished for the same misbehavior. This causes him to realize that getting told off isn't something that's unique to Kitty himself.
Again, you are showcasing a pattern (cat acts unkindly => cat gets told off) that the cats will eventually learn to avoid (cat doesn't want to get told off => cat shouldn't act unkindly).

Things get even worse when Pip joins in though. Pip doesn't know what he's doing, he chases her very rarely and i think he only does it because Kitty does it. (Kitty is then ultimate bad role model.)

I think the pattern is starting to be clear. Pip is guilty. Pip gets punished. Kitty is not to blame for being a bad role model. Pip is to blame for following Kitty's example, especially if he has also seen Kitty getting told off for this misbehavior (and should therefore know better).

I've tried everything, clapping, flicking Kitty/Faith (when she starts the fight) on the forehead/nose, nothing works.

I think I'm coming across as a general hard-ass. Keeping that in mind, I'm not a fan of physical punishment such as flicking on the nose. As I see it, only a human is able to flick a cat on the nose, and thus it leaves the possibility for the cat to blame humans for being flicked on the nose.
I push Alfie, because he pushes the other cats. When he bites them (more than just horseplay), I slightly pinch him when I grab him. When he chases them out of their comfy spot, I do the same to him. Although I'm fighting fire with fire, I make sure to always respond in equal measure and not escalate needlessly. I would rather inefficiently push Alfie instead of "play the human card" that will always trump Alfie.
If you respond too strongly, the cat will see you as the aggressor. Instead of learning a lesson, it'll learn to avoid or dislike you. If you never go beyond the cat's own transgressions, they can't blame you for something that they just did themselves.
What I expect from your cats, long-term.
This is just inference from what you've told us. Take this with a grain of salt, I don't know your cats other than what you've told me (and even then I could've misunderstood.
Faith seems to want to be by herself. Whenever Pip or Kitty misbehave, I expect that you'll end up enforcing a peaceful atmosphere for Faith (e.g. by separating the offender temporarily).
Pip seems to have no issue responding to Kitty in equal measure, so you need to defend him less. But when he misbehaves like Kitty, you should punish him like Kitty. He needs to see that the punishment is directed at the behavior and not just at Kitty.
Kitty is going to need the most work. He is going to have to learn how to back off. As a general rule, you can never stop a cat's misbehavior, but you can steer it. You can't stop a cat from scratching things, but you can redirect them to a scratching pole when they target your furniture or couch. The same is true for any other instinctive behavior.
If Kitty's misbehavior comes from playfulness (which Faith does not reciprocate), try playing with Kitty to redirect his energy. Even if he's not trying to play with Faith (but likes playing with you), you can play with him to tire him out and make him less likely to chase Faith.
If Kitty is being aggressive for the sake of aggression, then that needs to be adamantly addressed. Do not put up with that behavior, even in small bursts. Do not allow him to be unkind to the other cats in any way, shape or form. Make sure that doing so leads to an inevitable punishment that he will regret (but don't overreact either).
5. Fair punishments.
In order to not lengthen an already lengthy answer, I'll simply link to an earlier answer I gave on this topic. Chapter 4 specifically addresses the escalation pattern that I use to teach my cats how to steer their own behavior (by making them want to avoid you steering their behavior for them).
Answer: There isn't really a "one-size fits all" answer to this sort of situation but there are a couple of things you could try, and which I'd go for depends on whether Kitty is behaving purely out of "fun" from his point of view or whether there is any stress in there.
If it's not just Kitty being a bit too exuberant in his play you could try a "Feliway Friends" diffuser. These work like a plug-in air freshener but instead of a faux pine scent they instead put out out a synthetic version of the hormone mother cats release after giving birth which has a calming effect on the cats in the house. I've used them before to help keep my Bengal calm during fireworks and I've seen good results.
There are a couple of gotchas though - if I'm understanding your question correctly your cats are allowed out? If so that will reduce the effectiveness of such a device as they will obviously be spending less time under it's influence as it were.
Likewise if you have a big house you would need multiple diffusers spread throughout the house to ensure good coverage.
If it is all down to Kitty just having too much playful energy and drive then I'd recommend giving him some regular one-on-one play time with you, ideally at roughly the same time every day (before food is best as it fits with the hunt-eat-groom-sleep cycle) and with some toys that he can really go to town on and interact with. This should give him an outlet for his play that isn't poor old Faith and hopefully if that's more fun for him he'll bother the others less.
Answer: Okay, you said giving up one of them is not an option and neither is letting them duel it out. I definitely agree with the latter, but there might be another possible solution rather than the former. :)
Note that if this has been going on for a long time already both Kitty and Faith will have learned and accepted this behavior as the norm and things will be hard to change.
So Faith is about 7-9 which makes her a fully grown adult lady and Kitty is a young male in his prime, about 3 years. Their interests are simply not the same and Kitty seems very intent on wrestling to his heart's content. Additionally, it seems that (judging by what you wrote) Faith is not up to the task of showing Kitty that she isn't into that kind of play (typically, female cats aren't) and instead runs away... Which is great fun for Kitty and he doesn't get that what he is doing is wrong. To him, he is just playing.
Essentially, Faith doesn't have the confidence to stand up to Kitty who in turn is getting more and more frustrated. Even if she was the correct gender and/or loved to wrestle, she probably wouldn't be active enough for Kitty. So what to do?
It's risky in this specific situation as Faith isn't a confident cat, but a solution in many bad 'male cat wants to wrestle with female cat' cases is to get another male of about the same age and activity level as the other male. This is because cats don't usually -want- to terrify other cats for long periods of time, they want to play with someone who reciprocates as that is a lot more fun for both cats. The important bit is that the new arrival has to be properly socialised (grew up with other kittens, accepts hisses or rejection) as otherwise there really could be a scenario where Faith gets chased by two males instead of one.
Again, it's risky, but you said you don't want to give any cat away, so this is the only other solution I can see.
Apart from that, I wouldn't physically punish Kitty for what he is doing - you are basically telling a young adult that he shouldn't play, he doesn't get it.
What you can do is:

Casually walk between the two cats whenever Kitty looks like he is going to jump Faith. Block his path, interrupt the situation peacefully. You can also use a neutral, unknown sound for this (doesn't have to be loud, just distract Kitty for a moment).
Reward good behaviour so the relationship still remains positive otherwise (if it is). Any positive interaction should be rewarded here :)
Build Faiths confidence. Clicker-training is good for this if she is up for that, but also solo play sessions where she gets to have the 'prey' for a little while before it flys off for another round of merry hunting. No laserpointers in this case, they have no built-in reward and can be frustrating.
Comment: For whatever it isn't worth: My two are the same age, and the boy is a lot more assertive about both playing and grooming. But I stopped worrying about it after seeing the girl instigate a few times. And they spend enough time cuddling with each other that I'm not really worried about the relationship.
Comment: Thats great, they seem an ok fit then :) Its really only an issue when one cat can't assert themself or the other doesn't take 'no'/hissing for an answer.
Comment: I'm not sure if adding another cat is a good thing for this situation, and some cats don't react well to physical punishment...Hmm, risky indeed. This is sticky and complicated, a thinker for sure
Answer: I'm dealing with this right now. I have a cat I recently rescued, and he and my cat I've had for over a year aren't really getting along. They like to stalk each other. If one of them takes off running, the other will chase.
I have been using the pillow method. If one of them starts to go after the other, I insert a pillow between them and shake it at the aggressor. This seems to be working. I also hiss at him/her.
I'm trying to train my female cat not to run after she walks past the rescue, because he will always always chase her if she runs.
It's pretty stressful, but until they understand from all this reinforcement that lunging at each other is not appropriate, I can't really leave them in the same room unsupervised. It's tough.
Answer:
Put up baby gates in doorways, this creates an obstacle so the
chased cat can get away unharmed. Put boxes, storage containers under beds to give them a safe space. My 'murder cat' loves our garage because we have stacked containers with a pillow on top for her.

Give high spaces for them to travel along, book cases, shelves, clear the top of frig, cat trees, etc.

Keep a spray bottle handy. A spray bottle that has a distinct sound when spraying. I use a Dawn Power wash bottle.
I've only had to spray a few times, and missed. But now when murder cat sees me with that particular bottle, she settles down in another room.

Keep feeding dishes separate if you can, at least for awhile.
Cats are territorial over the food.

Your first cat 'owns' your property and you. At least in their minds.
Extra special loving for your murder cat, she needs to know she is still the most important feline. After all, you're her most important human.
The newest cat must become submissive to the older family member, they've earned that respect and demand it.
Supervision with a spray bottle, obstacles to aid an escape, high places so they can safely explore, and safe spaces.
The rest is up to them.
Answer: I had a similar situation and decided to consult with a veterinary behavioral specialist. I was able to do it virtually (phone, computer/Zoom). I believe is was $250 but it was the best money ever spent! I would have gone without food or anything else if needed, as it fixed the problem. I thought I knew a lot about behavior modification  (I have a graduate degree in psychology) but one cannot apply human reason (or even canine reason) to cats - they are entirely different! I learned that some of the things I was doing (which others suggested here) were actually making the situation WORSE. Every cat is different and you have three, so a behavioral specialist can figure out what may be unique to your cats and situation to help truly resolve the issues.
Some general guidance:

NEVER "punish" a cat. Of course nothing physical (not even a flick on the nose), but also no spray bottles, yelling, etc. This only increases their stress or anxiousness which is often the cause of their attacks, so it can actually escalate things. Negative reinforcement doesn't work with cats. It just makes them more anxious, aggressive, and/or fearful, and it weakens your bond with them.
Use positive reinforcement. If the aggressor is simply sitting in the presence of the victim and is ignoring him/her, give the aggressor praise and pets. Get aggressor kitty to associate good things (extra attention, treats, etc.) with victim kitty. Reward the lack of bad/aggressive behavior.
Use distraction methods. If there are times when you feel the aggressor is likely to act up, find a way to distract him (e.g., play with a favorite toy).
Consider aids like Feliway multicat. If behavioral methods aren't enough, consult your vet about medications. "Bad" behavior is often a result of anxiety or other negative emotional states. There are meds that can help in combination with behavioral training. Sometimes it's actually the victim cat that benefits most from medication (reducing their fear and increasing their self-confidence makes them less desirable to the bully cat).
Enhance the environment with lots of high places (use vertical space if/when the house is not large), perches to view the outdoors, interactive toys, etc. Be sure to give each kitty plenty of play time, especially the aggressor.
If you're not able to consult with a veterinary behavioral specialist, talk to your vet. Be careful about taking advice from people on the Internet. Mine comes from a trained professional, but you should still run it by your vet. As I said, some of the advice I saw above can actually be harmful to your cat's wellbeing and can make the situation worse.

Good luck!


How to get equivalent behaviour of NOR and Nand gates, by making changes on the input side, when their inputs are attached

Question: I have two input NAND and NOR gates, I have attached their inputs and they both give the same result as the NOT gate, that is the inverter. Now i want to make further changes at the input side, so that their result is equivalent. I have tried alot making changes in the input side, adding gates and inverters, but I am not getting the same output. Can anyone suggest something?
Comment: What do you mean by "making further changes on the input side, so their result is equivalent"  What kind of changes?  Please show an example.
Comment: Useful search term : "de Morgan".
Comment: By further changes i mean, adding more gates or inverters, so that their out put is same.
Answer: To make the NAND gate act identically to the NOR gate you have to attach an inverter to each input  of the NAND gate and an inverter to the output of the NAND gate.
To make the NOR gate act identically to the NAND gate you have to attach an inverter to each input  of the NOR gate and an inverter to the output of the NOR gate.
In order to understand why this is the case you need to understand how a Truth Table works for logic gates. If you don't know how a Truth Table relates to logic gates and logic design, you need to learn this. Lots of on-line resources to do so.
Once you learn how to use a Truth Table to represent logic gate functionality, then you will appreciate DeMorgan's Law. DeMorgan's Law explains a basic illusion of logic gates. Many beginners are fooled by this illusion. DeMorgan will explain this illusion and allow you to think logically about digital logic gates. The illusion relates directly to your question and more generally to the effect of inverting inputs and outputs of logic gates.
Who is this guy DeMorgan? He was a French mathemetician that died about 200 years ago. Did he have Nor gates and Nand gates too? Not likely, since it is known that he did not have a DigiKey account.
Comment: FiddyOhm's and Andy aka's answers both seem to explain what you want. Read them both carefully and you should learn something useful.
Answer: I suspect you mean what can you do to the 2 inputs to get equivalence at the output. Well the short answer is you need also to put an invertor on the output. Thie is deMorgan's rule: -

Basically, if you invert the inputs and outputs of an AND gate you get an OR gate. Likewise if you do the same to an OR gate you get an AND gate (that should be obvious once you know one).
The same applies with NAND and NOR - invert all the lines on a NAND and you get a NOR and vice versa.
See also this on stackoverflow


Choose which Airport to connect to

Question: I have an Airport Extreme on one floor and another one on the second floor as an extender. For some reason my device keeps connecting to the first floor Extreme - I know this because I can see it in the Airport Utility. How can I change this?
Comment: Do both of your Airports support 2.4 and 5Ghz?
Answer: What you could do, is give them the same name, but dial down the transmit power for each.
The current AirPort Utility (the 6.x version) doesn't support this, but the old 5.6.x version can still be downloaded here:
http://coreyjmahler.com/2013/10/24/airport-utility-5-6-1-on-os-x-10-9-mavericks/
Comment: Until I added a WiFi card to my Mac Pro, that's what I used to try to make the phones/pads pick the right connection. They still insisted on trying to connect to the far one, even when it was struggling to even find it at 25% power. Adding the WiFi to the Mac allowed me to swap priority & the phones have followed suit. [Ugh, just checked... they haven't - but the Mac does]
Answer: You can set the priority in System Prefs > Network - select WiFi on the left, then Advanced... bottom right.
Drag the available connections into the order you prefer...

Choose which Airport to connect to


Import the seed phrases generated in Electrum

Question: To which wallets can I import the seed phrases generated in Electrum?
Answer: Probably none? Nearly none?
So far as I know, Electrum uses its own system of seed phrases which it's developers consider to be better than the BIP39 system used by many other wallets.
I believe Electrum has some compatibility features that must be specifically enabled. Most users probably don't do that unless they anticipate the need.
The developers of Sparrow Wallet commented

Although it would be relatively simple to implement (as you note, Sparrow already supports the Electrum seed format internally), I've held off on doing so because:

it might be confusing to have two types of seed lists to choose from
I'm not sure it's a good idea to keep supporting such an isolated 'standard'

BlueWallet might have support for Electrum seed phrases.


Extension of solutions of PDEs with constant coefficients

Question: Let $\mathcal L$ be a differential operator with constant coefficients and $\mathcal{L} f=0$ for some $f\in C^{\infty}(\mathbb{R}^n).$
Under what conditions on $\mathcal {L}$ the function $f$ extends to an entire function in $\mathbb{C}^n$?
Comment: If $f$ admits an extension to an entire function, then $f$ must be real analytic. Requiring $\mathcal{L}$ to be elliptic will guarantee real analyticity.  However you need to phrase your question more carefully.  Should we expect *all* functions in $\ker \mathcal{L}$ to extend or only *some* of them?
Comment: Solutions of Laplace equation extend to entire functions in $C^n$.
Comment: For the main question should we expect solutions to be automatically entire ?or that is too good to be true?
Comment: Is not "entire" the part of your question? Can you decide what are you asking?
Comment: Laplace operator is an example of an operator for which the answer is "yes".
Answer: If that property is satisfied, then "hypoelliptic analyticity" holds, which means that $\mathcal L f$ analytic implies $f$ analytic. For constant coefficient operators that property is equivalent to ellipticity, a sharp contrast with the $C^\infty$ case, where hypoellipticity was characterized by L. Hörmander and holds for instance for the heat equation (which is not elliptic).
As a reminder, the operator $$P=\sum_{\vert \alpha\vert\le m}a_\alpha D^\alpha$$ is elliptic means that
$
\sum_{\vert \alpha\vert= m}a_\alpha \xi^\alpha\not=0\text{  for $ \xi\in\mathbb R^n\backslash\{0\}$}.
$
The Laplace equation or $\overline{\partial}$ equation are examples of elliptic operators.
Comment: Could you give the exact reference on Hormander where he proves that analyticity holds in the WHOLE $C^n$. I only found "in a complex neigborhood" of the real subspace.
Comment: I should have made my answer more precise, since it deals indeed with real analyticity (I have modified my answer). Of course, ellipticity alone does not imply the sought property globally in $\mathbb C^n$: take for instance for $n=1$, $\frac{\partial}{\partial z}$ and the elliptic equation $\frac{\partial u}{\partial z}=0$, which has no (non-trivial) holomorphic function as a solution.
Comment: thanks.I did not know about hypoelliticity at all. Now assuming the operator is elliptic should we expect that solutions extend to entire functions?entirety is of course much stronger than analyticity.


Can aileron control maintain track in a single engine failure on a twin?

Question: Let us say I am flying a twin-engine aircraft and I got a engine failure during cruise, without any control on rudder and use the ailerons only, can I still maintain the target track or heading? Will I got any sideslip?
Comment: @RonBeyer: According to [this page](https://www.flightliteracy.com/engine-inoperative-flight-principles-in-multiengine-aircraft/), 8/10°.
Comment: Why are you asking this question? Is there a particular reason you don't want to use rudder? What type of "twin-engine aircraft" are you asking about? There is a big difference between a light piston twin with a 160hp versus a B777 with 115,000 lbs of thrust on one engine.
Comment: You would need a pretty significant roll to counter the yawing effect of the dead engine...
Comment: Without aileron you may not be able to maintain control, much less keep on your track.
Comment: Are you asking whether you can maintain the SAME heading and track as before the engine failure, or whether you can maintain A heading and track?  Because yes, you will be able to maintain a heading and track, but it obviously won't be the same!  And of course you will have sideslip with an engine out and no rudder input.  If you understand what sideslip is, and you know what a rudder does, why would you even ask such a question?!  DV for asking a question where the knowledge indicated w/in the question is completely out of synch with what is being asked.  (i.e. unclear, not useful)
Comment: ...sorry, I'm not being rude here, but really you seem to have enough basic knowledge to answer your own question.  (with an engine out you get a bunch of yaw!)  If you can clarify what specific confusion you have about when and why to use rudder vs ailerons to make this question useful I would be happy to remove my DV.
Comment: I'd say if you lost one engine *and* your rudder, you either hope you have enough altitude to bail out, or start praying. If the ground underneath is fairly flat, I would idle or feather the second engine and look for a place to make an emergency landing.
Comment: Found a link that gives an answer, at least for some a/c-- see page 5 here https://www.cast-safety.org/pdf/5_asymmetric_flight.pdf
Answer: What will happen is the airplane will yaw into the dead engine, with a certain amount of roll into the dead engine, depending on how strong the dihedral effect, the roll-yaw couple, is.  A swept wing airplane will want to roll upside down and it will take lots of aileron just to keep it right side up.
A straight wing airplane with moderate dihedral effect will require some moderate amount of aileron to keep wings level, and with wings level you will find yourself in a flat skidding turn into the dead engine with some aileron applied in the opposite direction - as if you were flying with both engines and applied rudder with opposite aileron to get the same effect.
To stop the turn you will need to introduce side slip that offsets the lateral lift coming from the skidding fuselage, and this side slip needs to be induced by bank, so enough extra aileron will be needed to lower the wing into the live engine until the change in heading stops.  You normally lower the wing into the live engine about 5 degrees even with rudder being used normally, but in this case it'll take a lot more.
You will now find yourself flying more or less straight, that is, not turning, but with the nose skewed toward the dead engine, and quite a lot of bank into the live engine, whatever it takes to stop the skidding turn. You will be all askew but more or less proceeding in a single direction and it will be more or less the same as if you just applied rudder with opposite aileron induced bank to stop the skidding turn caused by the rudder application.
Adverse yaw from the aileron application will aggravate the situation since the down aileron is on the dead engine side, so adverse yaw is helping the live engine.  And obviously, more power aggravates the condition compared to less power and you might find that wide open throttle causes more skidding than can be offset with bank and you run out of aileron and are force to reduce power.
So it might take most of the aileron travel to maintain this configuration, and of course the drag is so high that if you are in a piston twin you may not be able to maintain altitude at any height, even if you have ample power, because you might be forced to reduce power to keep the thing from rolling over as you run out of aileron.
If you're in something like a Seminole at 5000 ft, you might be able to maintain level flight in that condition, maybe if down around blueline speed, but I wouldn't be surprised if the best that you can do is a shallow descent as you're forced to lower the nose to keep from stalling in that skewed profile.
If you're taking multi-engine training, there's only one thing to do; go try it and see what happens and report back here.
Answer: You can use ailerons in cruise, but it gets dangerous if airspeed decreases towards stall (aileron reversal and live engine will cause roll/yaw into the dead engine).
With ailerons there will be a slip towards the live engine, which helps counteract the yaw away from the slip.  The configuration will very much resemble a forward slip, with the accompanying increase in drag.
As the plane is banked and side slipping, an increase in throttle and some nose up trim will also be required.
Depending on the aircraft model, additional rudder (more coordinated) may work better than ailerons alone.
Comment: The last paragraph shoud be deleted, as q defines no rudder rudder control. It is also somewhat misleading as it implies it is ok to fly some planes in sideslip during eo situations. It generally is not...
Comment: You want to ensure you make it to a safe landing spot with minimal fuel consuption. Remember, enough fuel means you have time, and in emergenciens, having time is luxury.
Comment: And after reading the reference by mins, I stand uncorrected. The question specifically asks about a situation *" without any control on rudder"*. What I mean with the latter part of my previous comment: rudder input is necessary in eo situations (unless there is an automatic system providing it). I am unaware of multiengine acft that fly better without it. In engine out situations in multiengine acft you do not need to start lookin outside for spots to land, you check your map/gps for closest suitable airfield, and calmly proceed there, in an controlled manner.
Comment: @jpe61 well, I'll settle for a better understanding, which you seem to have.  Please keep in mind a pure aileron input will produce a slip.  In this case a forward slip (asymmetric thrust does the "rudder" part, ailerons hold the course).  As you correctly pointed out, the last paragraph does not address the question, but was added as a sensible alternative (coordinated) to handle the situation as per reference.


How can Arthur Schopenhauer explain homosexuality, if the ultimate goal of the 'will-to-life' is procreation?

Question: If the ultimate goal of the 'will-to-life,' as outlined by Arthur Schopenhauer, is for the benefit of the next generation, then how can homosexuality be understood?
Comment: See [Schopenhauer's Addendum on Homosexuality](https://www.oxfordscholarship.com/view/10.1093/0198237227.001.0001/acprof-9780198237228-chapter-16).
Comment: Honestly I don't spend much time thinking about these things but I have seen all kinds of pairings work and not work.
Comment: Schopenhauer's argument is misunderstood. "although they (gay people) have sexual urges and can procreate, it is undesirable that they should do so, and therefore the urge is diverted."  Ask yourself Mauro why do gay people find it undesirable to procreate?  Because they do not fall in love with those who they can procreate with. Furthermore, many gay people marry and have very happy families, but when you ask the gay spouse if they have ever really 'loved' their partner, they ought to say 'no.'  And so they had a family from societal urging and rationality, rather than from love.
Comment: I understand what you mean now, marry a straight person. Of course a gay man could marry a gay woman.  I have seen it happen. They have kids perhaps, then they both throw off the denial.
Comment: And Gordon, I bet you all my estate that a gay man marring a gay woman or heterosexual couples marring without either being in love with each other make the best and happiest families.
Comment: Genes having Will? Bosh. Pathetic Fallacy strikes again. People with similar genetic traits wanting to help each others with similar traits is better explained by Sociology than philosophy. That ideas are proven because it is 'intuitively helpful' to evidence anything...is dangerously misguided. Anyway, according to Gide, pleasure not procreation is the goal of the Will.
His Corydon is probably right. Procreation is a by-product of the pursuit of pleasure. It then becomes rationalized as a primary goal.
Answer: Arthur Schopenhauer offered two solution to the problems caused in us by the 'will-to-life;' one is art and philosophy, but the better choice is becoming a sage.  But Schopenhauer does not say who becomes these sages.  Since gay people do not fall in love with members of the opposite sex, they do not lose their minds and then have children in the process.  This frees them up to do the things that sages do.  Being gay is actually an advantage.
Comment: is falling in love not losing ones mind straight or gay?
Comment: I don't quite understand your question another_name.  Whenever someone falls in love with someone else, the former is said to have "lost their mind"  because the needs and desires of the individual according to the person's intellect no longer matters.  All that matters now to that victim of "Cupid's arrow" is satisfying the needs and desires of their will-to-life.
Comment: do you mean [casual](https://en.wikipedia.org/wiki/Casual_dating) dating? i don't think these work great in n the long term, but each to their own
Comment: No, another_name, I did not mean casual dating or anything of the sort.  Now I must ask you a question because without an answer, I cannot help you to understand.  The question is simple, "what happens to you when you fall in love?"
Comment: i don't think there's one universal response
Comment: what are you trying to get to me understand? just say what you mean.
Comment: When someone falls in love with someone else another_name, the "victim's" mind floods with passion for the other person and not just sexually. His  or her    thoughts about being with that person preoccupies every moment of his or her  days and he or she shall find it very difficult to stop the obsessions.  People do incredibly bizarre things in the process and some even break the law to satisfy their demands of the 'will-to-life.'  That's as best as I can explain it.  Has it ever happened to you?
Answer: It's suprising that in the proliferation of philosophical reflection on homosexuality or gayness, Schopenhauer has been left in the cupboard or, as some might say, the closet.
The following extract might throw light on your question. Briefly, while there is a 'will to life as it presents itself in the entire species', this will has both a general and an individual component. The individual component is the source of the paradoxical phenomenon, as Schopenhauer sees it, of homosexuality.

Although Schopenhauer clearly
   has vestiges of an "ethical" approach to homosexuality [He calls it time and again a "vice" (Laster)], he actually
   abandons completely any view of a divine order against which man sins
   either willfully or by immoral weakness of will. Rather, all sexuality is
   nothing but "the will to life as it presents itself in the entire species (627)."
   This drive becomes split into a general and individual component (the
   survival of the species demands my partner selection which I merely
   interpret, illusorily, as my choice or love) as well as into a variety of  vicissitudes (as when it gets "diverted" [irregeleitet] into "perverse"
   forms). This fundamental splitting of the will accounts for the
   "unheard-of paradox" (unerhörtes Paradoxon) (659) or "paradoxical
   idea" (paradoxen Gedanken) (664) of sexual and in particular
   homosexual love - a paradox that is no longer moral but "scientific" in
   nature.

(John H. Smith, 'Queering the Will', Symplokē, Vol. 3, No. 1, special issue: The Next Generation (Winter 1995), pp. 7-28: 10-11; A. Schopenhauer, Die Welt als Wille und Vorstellung II. Zurich: Diogenes, 1977: 627, 659, 664.)
On one reading, Schopenhauer regards a general or species-wide drive towards procreation or 'partner selection' as simply a feature of the will to life. Sexual love is no part of this drive but the individual misperceives the nature of the drive and believes her- or himself freely to choose a loved one as if the drive weren't in control. One thinks one chooses to fall in love, and might not have fallen in love at all, but the impression is illusory. These are features of the individual component.
Schopenhauer acknowledges but doesn't really explain, or even attempt to explain, how the individual component may also express itself, not in the choice of woman for man or man for woman but of woman for woman and man for man in the 'perverse' form of homosexuality. Homosexualty is merely located among a 'variety of vicissitudes' with which sexual life is beset.
Yet this is not to criticise Schopenhauer's omission of explanation. After all, the 'paradox' of homosexuality is, he holds, a question 'scientific' in nature. He hands the question over to science; and as Smith points out, not immediately but in the second half of the nineteenth-century, both in Europe and the United States 'science' took it up. The names of Freud, Havelock Ellis, Richard von Krafft-Ebing, and Magnus Hirschfeld are familiar to us all.
Answer: Old Arthur is plain wrong. The goal is not human procreation. It's the genes that want to procreate, or multiply. On the level of Dawkins's Selfish Gene, homosexuality fits perfectly. Read his book.
Or have a look at https://www.newscientist.com/article/dn13674-evolution-myths-natural-selection-cannot-explain-homosexuality/
Comment: Arthur Schopenhauer is colourfully right and it seems highly unlikely that genes possess any kind of consciousness to 'want' anything at all.  They just reproduce as they do because that is their nature.  If gene's want something, then I could argue 'trees want to grow' or 'mountains want to erode' and we all know that's nonsense.  I have never heard of "Dawkin's Selfish Gene" and I'm laughing hysterically at the title of it.
Comment: @MichaelLee - While genes do not "want" things themselves, they are presumably responsible for a lot of the brain circuitry that gives us our more innate desires. The reason Dawkins called his book "The Selfish Gene" (not 'Dawkin's Selfish Gene') was that it can be intuitively helpful to think of genes acting *as if* they "want" to propagate themselves, even though they don't literally have conscious desires and the real explanation is that those genes whose code gives them a higher chance of propagating themselves tend to displace those with a lower chance under natural selection.
Comment: @Hypnosifl  I don't think it's intuitively helpful to think of genes wanting anything anymore than we think rocks want to fall to the Earth; that's terrible reasoning!  Furthermore, "those genes whose code gives them a higher chance of propagating themselves tend to displace those with a lower chance" is very true but only trivially so because you need to tell me what kinds of genes increase the likelihood of propagation given the environment they are in. Dawkin's argues that beings will help those who possess genes similar to their own and shun others away.  Is that what you mean?
Comment: @MichaelLee "that's terrible reasoning" Terrible in what sense? Do you think it leads people to specific incorrect conclusions, even if they understand it's a metaphor? "Dawkin's argues that beings will help those who possess genes similar to their own and shun others away. Is that what you mean?" -- I wasn't thinking specifically about that, but yes, the "selfish gene" metaphor seems to mainly be used to help students think about altruism in sociobiology, that some behavior which is self-sacrificing at the level of the organism may be determined by genes which the behavior helps propagate.
Comment: @Hypnosifl. It's terrible reasoning because metaphor is appropriate in arts and literature, but not in science because it opens the door to all kinds of misunderstandings.  Dawkins claim is there are genes that make us want to help those who have genes similar to our own and thus make them more likely to be reproduced.  That's true to some extent, but it does not explain why some people help those with very dissimilar genes.  A sibling, for example, is genetically more similar to that individual than even his own children, but the parent will always help out their children over the sibling.
Answer: "If the ultimate goal of the 'will-to-life,' as outlined by Arthur Schopenhauer, is for the benefit of the next generation, then how can homosexuality be understood"?
Fact: There is homosexuality. Plenty of it. It exists. Therefore any philosophy that claims homosexuality cannot exist is obviously wrong. So you have two choices: Either you demonstrate how Schopenhauer can be right when homosexuality exists, or accept that he is wrong.
Some genes lead to the carrier of the genes duplicating in increasing numbers and the population of carriers to grow. Other genes lead to the population of carriers to shrink. The second kind of genes usually die out quickly. The genes themselves have no will, but genes that don't lead to population growth die out.
This is until we have carriers of the genes that have their own will as far as population growth is concerned, like humans. Humans can decide that it is more fun to enjoy their life than to raise children. Humans can decide that it is a good idea to kill lots of other humans for whatever reasons. Humans can decide to financially support humans who have children. And homosexual humans can decide to have children. It's not difficult actually. A gay man may think that having a child with a woman is not quite optimal, but better than no child. But that is in modern times, what about older times?
Quite simple. For genes to lead to population growth, it's not necessary that an individual has offspring, it's only necessary that a group of people has more surviving offspring. And in past times, when live could be a lot harder than today, and food was harder to come by, in bad times children with an uncle or auntie who wasn't interested in procreation themselves had better chances to survive. That has been investigated by mathematicians and biologists, and the optimal survival rate in seriously difficult times happened with a population of around 5% gay people. Which unsurprisingly matches what we actually have.
Gay men often have no sons and daughters, but their nephews and nieces have better chances to survive. An additional confirmation of this theory is that second children are gay more often than first children. Simple biological explanation; the gay person in a more supporting role for survival is not very effective if they are the only son in the family but much more effective as the second or third son.


Add shipping fee if product is a certain SKU AND going to certain state

Question: I'm looking for a solution or extension that will add an additional fee on top of the already existing table rate shipping costs BUT only if the product is from a certain category (or SKU) AND going to a certain state.
My client ships products all over the US but wants to add an additional $100 to products shipping to the states west of the Mississippi river (these certain products are fragile).
Hope that makes sense.  Thanks.
Answer: This is something we support fully in our shipping extensions on WebShopApps.com, I don't want to advertise here, so if you are interested drop us a mail and we will point out the relevant extension to you.
To do this programmatically you have to override the rates being returned then perform some logic to calculate the rates based on SKU's states, etc. Its do-able, just obviously there are solns out there already.
Comment: Hi Karen, which extension offers shipping price based on product SKU
Comment: I'm not allowed and do not want to advertise here.


Is visiting HTTPS websites on a public hotspot secure?

Question: It's often said that HTTPS SSL/TLS connections are encrypted and said to be secure because the communication between the server and me is encrypted (also provides server authentication) so if someone sniffs my packets, they will need zillions of years to decrypt if using brute force in theory.
Let's assume I'm on a public wifi and there is a malicious user on the same wifi who sniffs every packet. Now let's assume I'm trying to access my gmail account using this wifi. My browser does a SSL/TLS handshake with the server and gets the keys to use for encryption and decryption.
If that malicious user sniffed all my incoming and outgoing packets. Can he calculate the same keys and read my encrypted traffic too or even send encrypted messages to the server in my name?
Comment: I think this is appropriate, as understanding the risks of such an environment is within remit of security professionals. Some of us have had to run public hotspots within corporate sites, and some need to work from a variety of locations - which can include public hotspots.
Comment: What about sslstrip ? (http://www.thoughtcrime.org/software/sslstrip/)
Also see [this video](http://bit.ly/1BylXv) about sslstrip
Comment: Note that while the HTTPS sites may be secure in this scenario, if at any point during your browisng session an HTTP site ever gets loaded (even in an iframe or seperate tab), a malicious hotspot can leverage that to steal your cookies on _all_ popular non-HTTPS sites (even ones you're not currently browsing), and install backdoors for those sites which persist even after you're no longer connected to the malicious hotspot: https://samy.pl/poisontap/

So... yeah.
Comment: To add to the answers below [it depends on the security of the website you are visiting](http://security.stackexchange.com/a/80363/8340) - [also see my tips here](http://security.stackexchange.com/a/44976/8340).
Answer: HTTPS is secure over public hotspots. Only a public key and encrypted messages are transmitted (and these too are signed by root certificates) during the setup of TLS, the security layer used by HTTPS. The client uses the public key to encrypt a master secret, which the server then decrypts with its private key. All data is encrypted with a function that uses the master secret and pseudo-random numbers generated by each side.
Thus,

the data is secure because it is signed by the master secret and pseudo-random numbers
the master secret and pseudo-random numbers are secure because it uses public-private key encryption when the TLS handshake occurs
the public-private key encryption is secure because:

the private keys are kept secret
public-private key encryption is designed to be useless without the private key
the public keys are known to be legitimate because they are signed by root certificates, which either
came with your computer
or were specifically authorized by you (pay attention to browser warnings!)

Thus, your HTTPS connections and data are safe as long as:

you trust the certificates that come with your computer,
you take care to only authorize certificates that you trust.
Comment: 3. You make sure all of your data actually goes over HTTPS (some sites are partially encrypted and partially not)
Comment: @JoshG The client also has a private key, which only they know about. This is the essence of asymmetric (public/private key-pair) encryption. To decrypt a response from the server, a man in the middle would need the client's private key.
Comment: Oh I forgot that we have asymetric encryption systems.
Comment: My local swimming baths (Hillingdon, UK) return their own certificates, so actually have full sight of what you send, even if the https is shown. 99.9% of users wouldn't notice this.
Comment: Question: Can you clarify how any response from the HTTPS server could be encrypted in such a way that someone listening in the middle could not interpret it? The only key that the client has is the public key which is also available to listener / hacker.
Answer: Short answer: it depends.
SSL/TLS itself is no more vulnerable over a public wifi connection, than over "regular" internet. It was designed to be used in open channels.
However, there a few other aspects to consider:

Often users don't browse directly to the HTTPS site, they start off at the HTTP site and redirect from there. E.g you browse to http://example.org/, and click the Email link, which redirects you to https://mail.example.org/. Since the original HTTP page is not encrypted, that malicious user can modify your traffic, causing the Email link to NOT redirect to HTTPS, but maybe somewhere else. For example, if you clicked the Email link on example.org's homepage, would you notice that it took you to http://mail.exxxample.org/? (as an example). You might, someone else might not.
If the user hijacks your connection, but provides his own bogus SSL certificate instead of example.org's - your browser will show an error, that the cert is not valid. However, most users will just click through this, allowing the attacker to MITM to your secure site, over SSL.
Another aspect to consider, don't assume the public hotspot is securely configured. As this question shows, pwned routers are all too common, and can cause way more damage irrelevant of your SSL.
Comment: All Google sites are HSTS-ed. Better to use `example.org`.
Comment: @Pacerier yeah, *now*, didn't used to be :-) Also their certs are mostly pinned (re my 2nd point), so... yeah. Thanks!
Answer: A session which is entirely over HTTPS is fairly safe, as all requests from the browser, and pages transmitted by the server are encrypted.
However, when accessed via HTTPS, many sites will only carry out the authentication step over HTTPS, and then drop back to HTTP for the rest of the session. So, your password itself is safe, but the session ID used by the server to identify you for that session is transmitted in the clear by your browser. This reduces the load on the webserver (because encryption/decryption is CPU-intensive) but makes the site much less secure. Gmail is safe because it uses HTTPS for the whole session, but Facebook and many other sites do not.
This is how tools such as Firesheep are able to hijack users' accounts when an attacker is sharing an unencrypted wireless network.
You can protect yourself from this attack by either using a VPN to encrypt all session data, or by only using networks which have strong, per-user encryption such as WPA-PSK (WEP uses the same key for every user, and so does not offer protection from this attack).
Comment: As far as I know, [WPA-PSK is not an effective defense against Firesheep-like attacks](http://www.kennynet.co.uk/2010/11/26/ubuntu-firesheep-aircrack-and-wpa/).  It's my understanding that [aircrack-ng can decrypt WPA-PSK encryption](http://www.aircrack-ng.org/doku.php?id=airdecap-ng), if you capture the initial handshake, which is easy to do.  WPA-PSK is [not secure against these kinds of attacks](http://wifinetnews.com/archives/2003/11/weakness_in_passphrase_choice_in_wpa_interface.html) and [provides a false sense of security](http://wiki.wireshark.org/HowToDecrypt802.11).
Comment: Probably worth noting that it's no longer the case that  "encryption/decryption" is cpu intensive. Its important not to perpetuate the myth that CPU load is worth considering when thinking about using HTTPS on all parts of your website. see: https://istlsfastyet.com/
Comment: Also, this answer overlooks the risk of MITM attacks (active attacks).  Firesheep currently does not conduct MITM attacks, but it could, and then there can be risks even if the site uses HTTPS exclusively.  (e.g., if it does not mark its cookies with the SECURE flag.)
Comment: @D.W., thanks - thats why I suggested the original question move from superuser.com to here - this question came from there, not from a "security guy". Btw, when you see bad answers, especially answers that are explicitly WRONG, you should downvote them. That helps float the correct answers to the top and sink the wrong ones.
Answer: Since the answers seem to be all over the place (yes, no, might be, should be), let me first reiterate @waiwai933's answer: it is secure.
To be specific, you asked: "If that malicious user sniffed all my incoming and outgoing packets. Can he calculate the same keys and read my encrypted traffic too or even send encrypted messages to the server in my name?" The answer is no and no. With a footnote.
The reason he cannot calculate the same keys seems paradoxical, and in fact was a major break-through in cryptography when it was first published by Diffie and Hellman. TLS uses a key exchange protocol, like Diffie-Hellman but more sophisticated to protect from man-in-the-middle attacks. The protocol lets two people who have never exchanged information before compute a secret key that no one seeing all the messages can compute.
Once you have a shared secret key, you can use it to encrypt your traffic. Since the malicious user doesn't know the key, he can't encrypt traffic that looks like it came from you.
Now those footnotes.

We are assuming that the SSL/TLS protocol is correct. With most involved cryptographic protocols, bugs are found and fixed from time-to-time. SSL/TLS did have a recent bug (mentioned in a few answers as a reason it is not secure), however it has been both temporarily worked around and now fixed (RFC 5746) and the fix is in various stages of deployment. (In your scenario, the bug allowed a malicious user to send packets in your name but not read your traffic.) It is always possible that the attacker knows how to break TLS/SSL due to an unpublished error in the protocol.
An obvious point but worth repeating: The malicious user could see your request and send his own response using his own certificate. So do check the certificate.
Perhaps another obvious point: you can only be sure SSL/TLS will be used for future pages if the current page is HTTPS. For example, if you are on an HTTP page that wants you to log-in, and from past experience you know that clicking the log-in button will redirect you to an HTTPS connection, this functionality could be stripped out by an active malicious user on your network. So only log into pages that are already HTTPS (unless if you know how to detect if the log-in page has been modified).
Answer: HTTPS is pretty resistant to decryption from packet sniffing. However the server authentication side depends on a somewhat weak method of distributing CA certs and the CAs don't do much in terms of verifying identities. Some years ago a Microsoft cert was issued to an imposter. See Bruce Schneier's article on the subject - in practice, for general public websites, we don't have anything better.
Comment: Invalid certificates is a problem on all types of connection -- not just a public WI-FI hotspot.
Comment: Richard quite right, there's nothing special about public Wi-Fi hotspots regarding certificates. I felt it worth mentioning as it also applies there. A rogue hotspot operator could redirect you to their own webserver using a fraudulent certificate.
Comment: but invalid certs and MITM are much more of a risk on public wifi
Answer: In practice, SSL and TLS both have issues, but they are around the Man In the Middle type of attack. For an example TLS MITM renegotiation issue see here
Of course, this attack requires the attacker to be in the middle of the communication path, which limits the threat a little :-)
Answer: If you're concerned about safely browsing to some site over an insecure network, the best steps you can take to ensure security are:

Make sure that the site uses HTTPS, not HTTP.
Make sure the site has a valid certificate.  Don't click through warnings.  Don't accept invalid certificates.
Use HTTPS Everywhere or Force-TLS to ensure that you are using a HTTPS (i.e., SSL-encrypted) connection for everything related to that site.
Answer: Entirely, in practise. The encryption starts off with the root ssl people (Verisign, Thawte, etc) and therefore it's suitable for insecure lines. AFAIK TLS doesn't have a problem with man in the middle attacks, its only weaker/older handshakes that have that problem.
Comment: TLS is susceptible to MITM
Answer: Most are forgetting the user aspect and how they might use that pc at the hotspot also.  Most people use similar passwords on other sites, may blog, ...etc. those may not be as secure well as HTTPS/SSL gmail you might be connecting to.  Problem I see from security most people will goto other sites expose a packet sniffing program with enough information to get at some of your accounts.  Best is as mentioned if you are using an unencrypted wireless connection hopefully you have a vpn you can connect to after that that will add the extra layer of security.
Answer: The connection is quite secure as far is secure connections(ssl) is concerned. Provided, if it is used with awareness:

There should be no redirection from HTTPS to HTTP
Some sites uses HTTP cmd over HTTPS pages also , there shouldn't be any sensitive info transmitted over that
Weak configured https servers and outdated browsers are still exploitable even over secure sockets
Answer: The sort answer is, as far as we know, if HTTPS is setup correctly you are secure 99% of the time. That's a big if. Its not as simple as just seeing HTTPS in your browser. SSL-Strip still works on some secure sites/browsers.
Lets not forget that before SSL-Strip, no one thought such a tool could work. Cybersecurity is an ever changing field and a game of whak-a-mole so to speak. A new exploit will come out at some point that lets you do the attack you mention on any site. Look at the "unbreakable" WPA2 standard. Not so unbreakable after all. It will be patched but it doesn't do much for the people hacked before then.
Currently there is a way to decrypt properly configured SSL traffic. It requires gaining access to a CA and issuing fully trusted certificates. Your browser will not detect a MiTm attack using that. The good news is, its extremely hard to pull off.....for now. Using a VPN does not guarantee safety because the VPN itself could be the Mitm attacker.
Basically anything online can and at some point will be hacked. There are things you can do (VPN, updated browser etc) to minimize the risk, but you are never 100% secure. Don't let anyone tell you otherwise.


How does one identify up-and-coming web trends?

Question: What do you foresee will be the major trends in web site design of 2017? In the past few years we've seen a huge growth in fixed headers, parallax, and other "chrome" elements. How do you identify big web trends (particularly these style-focused elements)?
Comment: I'm hoping this question can be reworked. While "Guess what the next thing on the web will be" is opinion-based, "How can I identify up-and-coming web trends early?" is maybe better.
Answer: Check caniuse.com
The "hot new thing" in web design is still often driven by what the technology will support.
So, checking out http://caniuse.com and looking at what features will soon have near-universal support will give you a good clue about what sorts of things may be coming to websites soon. (That is to websites that target general audiences. If a site doesn't need to support every browser, it may be using these technologies already, see below.)
Blending raster and vendor graphics
Two technologies nearing this point currently are  blending of HTML & SVG elements, and the calc() function. Together, these technologies will make it easier to incorporate vector graphics into a web design. (Check out the links for some examples of what these technologies enable.)
Embedded videos
Two technologies for embedding video into web pages without plugins are
HTTP Live Streaming and WebRTC. MSIE (up to 11) does not support either, but Chrome, Edge, Safari, and Firefox support one or the other.
Remember when Flash animations were everywhere? (Back when Flash has a very high installed base.) With these HTML video technologies becoming widespread, ubiquitous web video may be coming back.
Check out cutting-edge apps
Some web apps have limited browser support, and focus on delivering cutting-edge experience for a short list of supported browsers. Some (but not all) of the features they utilize are on the "coming soon" list for other browsers. These sites are doing now some of the things that will be coming to general audience websites as those features become more universally available.
One of my favorite "cutting edge" web apps is roll20.net, which officially supports only evergreen Chrome and Firefox. You can check out many (but not all) of the tricks they use for free (while a few require a subscription). You inspect their pages and see that they are using all sorts of HTML5 functionality. Their WebRTC video for peer-to-peer connections is a big improvement over older Flash-based systems, for example.
Not everything roll20, or other sites like them are doing, is going to be the "next big thing," but if you watch them (and read their change logs) you can watch the innovation happening.


Example of DNS Compression Pointer Offset > than 12 bytes

Question: I want to write my own DNS server as an experiment to learn more about DNS. I've been reading RFC 1035 and it mentions a DNS Compression Pointer used in the answers of DNS replies.
I read that the compression pointer offset starts from the transaction ID (beginning of response) and goes on until it finds the host name in the question section of the response.
Since the DNS header is 12 bytes long, every request I've sent has had an offset of 12. Can anyone provide an example of when the offset would be greater than 12?
Answer: Prior occurence of the same name can sometimes be only the last part of the previous domain name:
(Example from Wireshark SampleCaptures, mDNS & Apple Rendezvous, mDNS3.cap)


how to uninstall mist permanently for Mac

Question: my mist is not syncing in private network and it did not show the ethers. so I need to uninstall Mist permanently and  again I want to install .But when I install the mist it shows the older mist .
Answer: It is most likely your blockchain not syncing, try
1) delete $home/Libary/Ethereum/chaindata this should wipe out the blockchain itself and let you sync from scratch.
2) delete the entire $home/Libary/Ethereum folder (backup your keystore folder, which is your wallets before deleting, very important!!)
3) Try geth removedb to remove the blockchain and state databases.
Comment: I just did the same as you mentioned but still it shows me the previous mist and also it is not working when I connect to private network and I opened mist by using terminal and mist shows an error  couldn't connect to the node. Retry the connections


Is Stack Exchange immune to DDoS and DoS attacks?

Question: I have heard that DDoS and DoS attacks work most of the time and that they are used when SQL insertion and other methods fail. I know Stack Exchange has a lot of programmers, and that they have this stack which deals with such things. What I want to know is if Stack Exchange is immune to DDoS attacks. Is it?
Answer: Nothing is immune to DDoS attacks, no resource is infinite. It may take a bit more effort, but any service can be taken down if you have enough resources. Upstream connections can be saturated, webservers can be overloaded with requests, etc. So the same goes for Stack Exchange.
Stack Exchange staff actually wrote an a blog post on how they deal with DDoS attacks and lessons learned.
Answer: Stackexchange may not be immune, but it has very good prevention methods. It has a rate limiter which prevents a certain amount of requests from coming through in a certain time frame. In addition, it will automatically block accounts and ip addresses if they are malicious.
Comment: I would call "detonate high explosives inside a datacenter" anything but simple...


Realtime noise-removal with PulseAudio?

Question: Is it possible to do realtime noise-removal with PulseAudio, so the sound output that's coming out is smoother?
Comment: @Vadi - I have added an answer that might help you if you are still using Ubuntu or any other PulseAudio based Distro. Hope it works for you buddy.
Comment: it probably is possible, though it would be intensive, and would require someone write a filter for it.
Comment: This [post](https://cubethethird.wordpress.com/2015/07/26/real-time-background-noise-cancellation-of-microphone-input-on-linux/) seems to be doing something similar
Answer: Pulseaudio module module-echo-cancel
I started reading a lot about PulseAudio and "hidden" options it had so I could find one that was similar to this question. The one I found was the noise-cancellation module, which is one that dramatically lowers any static noise on the microphone and even A LOT of the background noise, basically giving you the benefit of only recording your own voice with excellent quality (For audio recording for example). To do this follow this steps:

sudo nano /etc/pulse/default.pa

Add the following line anywhere on the file, but I recommend almost at the end where you will find a comment about Echo Cancellation stuff (~line 140):
load-module module-echo-cancel

Reload PulseAudio (pulseaudio -k) or simply restart the computer. You should be able to select the new Noise Cancellation option from the Input Device Section:

You can find more information about it on the Echo Cancel Module Page
Set input as default
If you wish to set as default the echo cancel device simply turn the above line into:
load-module module-echo-cancel source_name=logitechsource

and then at the bottom of the file add
set-default-source logitechsource

In this case I named the source logitechsource, but you can name it whatever you want and simply either restart pulseaudio.
Rename device
Lastly, if you do not want a super long name on the Sound Settings (When you want to select an input/output device). My suggestion is renaming the input device like this:
load-module module-echo-cancel source_name=logitechsource source_properties=device.description=LogitechHD

And again, restarting pulseaudio. The end result looks like this:

UPDATE - Full documentation Found Here Thanks to clément
Comment: If you want to avoid the echo module from adjusting the volume slider automatically, you can set `load-module module-echo-cancel aec_method=webrtc aec_args="analog_gain_control=0 digital_gain_control=1"` as per https://gitlab.freedesktop.org/pulseaudio/pulseaudio/issues/224
Comment: Wouldn't it be more appropriate to edit `~/.config/default.pa`?
Comment: You can also load the module temporarily with `pactl load-module module-echo-cancel`
Comment: Adding `module-echo-cancel` to pulse audio makes my voice sound slightly metallic in recordings. It does remove some of the background noise, but not all of it.
Comment: The suggestion of @BipedalShark is good but since the user's configs will overwrite the system's configs, you will need to source the default configs at the top of your file with `.include /etc/pulse/default.pa`. Credit goes to [this awesome answer](https://askubuntu.com/a/446552/69691). Also, I wrote my configs at `~/.config/pulse/default.pa`.
Comment: it isnt working module-echo-cancel.c is not found says the error i get @luis
Comment: @nikoss Hi I can give a hand with that on the askubuntu chat or via google hangouts.
Comment: Do you happen to know how to do this for output? I'd like to dynamically (`pactl`) add/remove echo cancellation during playback of recordings that were made in a church.
Comment: `load-module module-echo-cancel source_name=logitechsource source_properties=device.description=LogitechHD
` doesn't work after restart
Comment: To update my comment above, it seems like you know need to prefix whitespace with "\\" and type out `aec_args="analog_gain_control=0\\ digital_gain_control=1"` for it to work.
Comment: I have to `pulseaudio -k` after every restart to see my microphone with noise reduction. How to solve this?
Comment: Hi @FreePhoenix I would not know anymore, I moved to pipewire.
Answer: This is an old question, but I had the same problem and after some Googling (where I mostly found people who agreed it wasn't possible) and reading some man pages, I have now developed a solution based on user2330377's idea.
First you need to create a noise profile for SoX. Just use any audio recording program to record a few seconds of noise, then cd into the directory you saved it to and do sox noise.wav -n noiseprof noise.prof.
Then you need to create an ALSA loopback device:
sudo modprobe snd_aloop

This is required because pulseaudio, unlike Jack, cannot directly connect audio software together; we will hence use the loopback device as a proxy.
Now you need to start paman and find the names of both your microphone (or other recording device) and of the loopback device we just created. Once those are found, you can execute the following command to start recording sound from your microphone, piping it through SoX and then playing it on the loopback device:
pacat -r -d alsa_input.pci-0000_00_14.2.analog-stereo --latency=1msec|sox -b 16 -e signed -c 2 -r 44100 -t raw - -b 16 -e signed -c 2 -r 44100 -t raw - noisered noise.prof 0.2|pacat -p -d alsa_output.2.analog-stereo --latency=1msec

(Where you need to substitute the correct device names for the -d parameters -- the input device for the first pacat invocation and the loopback device output for the second.)
There you go, almost done! As a last step, start recording sound with the application of your choice, then start up pavucontrol, change to the "Recording" tab and set the audio device used for recording (displayed as the grey button to the right) to "Monitor of Loopback Audio Device". You should now have a clear and noise-free recording!
Comment: This script does simplify the process https://gist.github.com/tiagoengel/334573f7c3b98c27356297d6b1996336
Answer: So simple solution on ubuntu LTS or debian based distros
this is an online script that activates noise cancellation feature that is already in pulse audio (the default audio on debian).
wget -qO - https://gist.githubusercontent.com/grigio/cb93c3e8710a6f045a3dd9456ec01799/raw/94f07c7d75bcf5dd9b08a9c3034844223ec6fbe1/fix-microphone-background-noise.sh | sudo bash && pulseaudio -k

you can also run these commands in secsetion (they are the same thing)
sudo cp /etc/pulse/default.pa /etc/pulse/default.pa.bak
sudo cat <<EOT >> /etc/pulse/default.pa
load-module module-echo-cancel source_name=noechosource sink_name=noechosink
set-default-source noechosource
set-default-sink noechosink
EOT

Comment: Hey how can I undo this? I executed it twice, and now I see duplicated inputs and both microphone and camera are gone!
Comment: This one worked, thanks Karim!

BTW, @OmarDulaimi  - it's probably too late, but next time just take a look at the script :) It makes a backup in the first line, you had it in `/etc/pulse/default.pa.bak`, so the answer is: move the `/etc/pulse/default.pa` file somewhere (just in case, it's again a backup), then rename '/etc/pulse/default.pa.bak' to `/etc/pulse/default.pa`
Comment: @OmarDulaimi just remove the additional lines you added. or restore the backup file created `/etc/pulse/default.pa.bak` and redo it
Answer: I use NoiseTorch  in Arch Linux ,  is easy to use and easy to install => NoiseTorch Github Link
Answer: There is no any information on module documentation page about noise cancellation. There is only AEC (Acoustic Echo Cancellation) algorithm inside module-echo-cancel, which is have several implementations, like webrtc | speex.
Therefore you should buy headset|microphones with integrated noise cancellation feature as much as possible.
As I tested voice recording in ubuntu I found some particular qualities:

Skype, Telegram uses raw input from the default device (in my case front-in-mic|backward-mic jacks). If you need to cancel a noise in this apps, you should buys headsets|microphones only with integrated noise cancellation feature
If you need to use voice calls in browser, then you should keep in mind that browsers has it's own voice processing algorithms implementations, e.g. WebRTC.
Also web applications (sites like talky, hangouts, appear.in, etc) can have it's own voice processing algorithms implementations, despite the fact that they can be based on webrtc hangout-analysis
Answer: There is a noise suppression plugin for module-ladspa-sink:

https://github.com/werman/noise-suppression-for-voice

Here you can can find a tutorial for installation on Fedora, and 2 sample recordings to get an idea about the effects of the plugin:

https://weblog.lkiesow.de/20200705-ladspa-realtime-noise-suppression-plugin/
https://fedoramagazine.org/real-time-noise-suppression-for-video-conferencing/

On Ubuntu:

Download the linux_rnnoise_bin_x64.tar.gz
From the archive, copy the bin/ladspa/librnnoise_ladspa.so e.g. to /usr/local/lib/librnnoise_ladspa.so or anywhere else.
Find out the name of your input using pactl list sources short. For example my microphone is called alsa_input.usb-046d_0994_4A365E20-02.mono-fallback
Run the commands bellow to activate the noise cancellation (before running replace the /usr/local/lib/librnnoise_ladspa.so and alsa_input.usb-046d_0994_4A365E20-02.mono-fallback to your values determined in the steps 2. and 3.):

    pacmd load-module module-null-sink sink_name=mic_denoised_out
    pacmd load-module module-ladspa-sink sink_name=mic_raw_in \
        sink_master=mic_denoised_out label=noise_suppressor_mono \
        plugin=/usr/local/lib/librnnoise_ladspa.so \
        control=50
    pacmd load-module module-loopback \
        source=alsa_input.usb-046d_0994_4A365E20-02.mono-fallback \
        sink=mic_raw_in channels=1 source_dont_move=true sink_dont_move=true
    pacmd load-module module-remap-source source_name=denoised \
        master=mic_denoised_out.monitor channels=1

In your voice recording application you will configure audio input from Null output or Remapped monitor of null output and test (I used Skype Echo)

To make the changes permanent, append the commands from the step 4 into /etc/pulse/default.pa, omitting the pacmd on the beginning of each line.
Answer: Investigation shows that there is no known way of doing real time noise reduction filtering with any Linux sub system. Some websites point to hardware you can buy which should do the trick much better than doing a software filter.
Alternatively if this is for a recording, you could pass the sound through Audacity and use the noise filter there.
Comment: Pity, I wonder why not. Thanks for investigating.
Comment: Sure! There is no any information on module documentation page about noise cancellation. There is only AEC (Acoustic Echo Cancellation) algorithm inside `module-echo-cancel`, which is have several implementations, like webrtc | speex.
Answer: Here's at leas tone way, implement webrtc-aec in the
http://wiki.gentoo.org/wiki/PulseAudio
webrtc-aec  Yes         Uses the webrtc.org AudioProcessing library for enhancing VoIP calls greatly in applications that support it by performing acoustic echo cancellation, analog gain control, noise suppression and other processing.
Here's a paper dated 2013 on the subject (noise removal with pulseaudio not webrtc-aec specifically)
http://lac.linuxaudio.org/2013/papers/37.pdf
"We have presented rst results of a multi-channel noise/echo reduction solution built on
top of PulseAudio and motivated the design decisions. The work has resulted in a number of improvements in the PulseAudio echo cancellation and signal-processing framework,
which have been contributed during the version 3.0/4.0 development cycle and should facilitate future embedded Linux audio solutions. Further work includes optimizing code for audio stream mixing, more ecient resampling methods, and the implementation of an efficient AEC in the multi-channel processing pipeline."
Comment: What I had in mind for a quick had was to takle a noise profile and do real time noise removal using sox and then play back the audio output which I can then use as an input stream in pulse.

I believe that pulse is able to create and redirect input and output streams in this manner. That is the playback stream can get redirected as an input stream to another application and the playback stream is per application not per device, so it need not be routed to device hardware.
Comment: I'm giving this a go using pacmd to list pulseaudio devices, pacat to record and playback from them and sox to do the real time effects on the stream.
Comment: Would you be interested in posting the scripts for it?
Comment: IS that not what is used in PulseAudio plugin echo cancellation ?
Answer: my amd laptop same problem the below code cancel my noise:
sudo nano /etc/pulse/default.pa

## add below code on bottom
### Enable Echo/Noise-Cancelation
load-module module-echo-cancel aec_method=webrtc aec_args="analog_gain_control=0 digital_gain_control=1" source_name=echoCancel_source sink_name=echoCancel_sink
set-default-source echoCancel_source
set-default-sink echoCancel_sink
### 2nd tips
.ifexists module-echo-cancel.so
load-module module-echo-cancel aec_method=webrtc source_name=echocancel sink_name=echocancel1
set-default-source echocancel
set-default-sink echocancel1
.endif

Hope someone will be helped
Answer: If you are on a distro that uses pipewire instead of pulseaudio (e.g. PopOS 22.04+) you can create an echo cancelling source/mic through the following steps:

cp /usr/share/pipewire/pipewire.conf ~/.config/pipewire/pipewire.conf (create the directory if it doesn't exist already).

Add this snippet from the docs to the active module so that you have the following in your pipewire.conf:
 context.modules = [

  # ... Lots of other modules loaded here

  {   name = libpipewire-module-echo-cancel
      args = {
          # library.name  = aec/libspa-aec-webrtc
          # node.latency = 1024/48000
          source.props = {
             node.name = "Echo Cancellation Source"
          }
          sink.props = {
             node.name = "Echo Cancellation Sink"
          }
      }
  }
]

Restart pipewire: systemctl --user restart pipewire.service

You can listen to the results by recording your voice using something like simplescreenrecorder or by running this command gst-launch-1.0 pulsesrc ! pulsesink (make sure you have headphones on or the microphone will pick up the sound form the speaker causing unpleasant feedback noise).
I personally find that the noise cancelling from the echo cancelling modules makes my voice sound more natural compared to using a program capable of more aggressive noise cancelling such as NoiseTorch. However, echo cancelling is mostly good for removing your fan noise and similar whereas NoiseTorch can automatically mute your microphone when not speaking in addition to removing some noise when speaking.

Realtime noise-removal with PulseAudio?


How can I save a commit in Subversion?

Question: If I'm not online, I sometimes want to package some changes to a commit, that is saved in the working copy and can be really committed to the repository once I back online. How can I do that with Subversion (possible with the help of additional tools).
Answer: You could use git-svn and establish a local git repository that you keep in sync with the svn. You can do local commits to the git repository while working offline and transfer the changes to the central svn when you come back online.
Answer: If you're looking to batch up some changes into a single sort of commit, the only option I can think of is to generate a diff of all the stuff that you've done (TortoiseSVN has an option to do this, or you can use the svn diff command if you're command-line friendly), but that only works once really unless you know you'll be working on separate files for each batch.
I've done this a few times in the past when I didn't want to create a whole new branch for a feature that was very experimental but didn't want to check it into the central repo either.  I could then share that patch file with others, but it was still a relatively fragile way of doing it since it was only guaranteed to apply to the revision I generated it against.
Answer: erm... If you have checked out a copy of the code, you just work on that local copy, then when you are back on-line, you commit your changes. Your computer only checks if it has the latest version. Or I have missed something in the question here?
Comment: That doesn't give you rollbacks and later on the versioning history for seperate changes.
Comment: The problem is, that I want to make different commits for different items. If I commit when I back online, I have only one big commit, and lose the history in this.
Answer: Try looking at GIT as repository control. GIT handles a lot of actions offline and in this way is very fast and reliable. GIT however is something completely else then SVN so you'll have to do some research on that. But if you're working offline frequently I would definitly take a loot at GIT if I were you :]
http://git-scm.com/
Answer: SVN does not support this feature, local offline commits by itself. Therefore you would net something like git-svn that extends the working copy with a distributed repository to make local commits possible.
However, you can save your commit as a patch file what goes easily with e.g. TortoiseSVN or simply "svn diff" save every step as a patch and they'll work like iterative commits.
when back online you can simply apply one patch after another and commit every step.
Comment: Saving and applying patch-files by hand sounds time-consuming and error-prone. Exists a tool, to automate this task?
Comment: Dunno, but you could script that quite easily. Or just take git-svn that has some more overhead, but the result should be the same.

How can I save a commit in Subversion?


Denavit-Hartenberg convention in practice

Question: How are the coordinate frames and D-H parameters determined in industry? Does one use the CAD drawing of the arm or physically take measurements of the actual arm?
Comment: Could you please better specify what you are tying to find out? "in industry" is such a broad term that it cannot be used to define use-cases or select one of the two options you have suggested.
Comment: By industry, I mean robot manufacturers (FANUC, Universal, etc.)
Answer: The DH parameters need to be as accurate as possible, and that means using dimensions from CAD. However, sometimes this is still not accurate enough.  (The robot might not match the CAD exactly for various reasons.)  In this case, we turn to kinematic calibration.
Put simply, kinematic calibration is taking measurements of the arm and adjusting the DH parameters accordingly.  Although, this must be done in a very controlled and precise manner.  (We don't just use a tape measure on the arm for example).
This can take various forms.  For example if you have a very precise jig, you can make the arm touch known points on the jig.  Or if you have a high-DOF arm you can fix the end-effector and move the rest of the joints through the arm's null space.  Or you can simply instruct the arm to move to various configurations and measure the end-effector pose with high precision external sensors.  Then you typically have a large minimization problem to solve.  You need to back out new DH parameters that are most likely to give you the measurements you have seen.
To better understand kinematic calibration, the clearest source of information I could find on the subject is in the book "Robotics: Modelling, Planning and Control" by Bruno Siciliano, Lorenzo Sciavicco, Luigi Villani, Giuseppe Oriolo, chapter 2.11.  Which requires a description of the arm in DH parameters, multiplying out the kinematics equation, partial differentiation w.r.t. each DH parameter, then a least-squares fit (with the left pseudo-inverse), then iterate.
But I was never able to get this technique to work for me.  What did work was using PoE (Product of Exponentials).  In PoE, the kinematic parameters of the model vary smoothly with changes in joint axes and can handle kinematic singularities naturally. I followed this paper and was successful in kinematically calibrating my 7 DoF arm:
I. Chen, G. Yang, C. Tan, and S. Yeo, "Local POE model for robot kinematic calibration," Mech. Mach. Theory, vol. 36, no. 11/12, pp. 1215– 1239, 2001.
Answer: If you are dealing with an (existing) industrial robot, all of its nominal dimentions and all axis to axis distances are specified in its manual (e.g. Kuka). All industrial robot manufacturers do this, without exception. All values in the DH table come from this table.
In most cases, positive and negative directions of the axes are defined in the manual and/or are also marked on the robot iself. This already defines the orientation and direction of all Z axes in the DH convention, you just figure out the rest (basically the one remainig possible rotation) by yourself and fill out the table.
These are obviusly nominal values, the real measurments of the robot include assembly and manufacturing tolerances, load and temperature effects. these are disregarded in the DH Table / Convention and are added as compensation odr calibration values later on.
Comment: Note that Kuka is using an old parametrization for frame rotations (coming from CNC) which can be surprising at first.
Answer: CAD is used as a first estimate. Often the kinematics (coordinate frames) are part of the design specification, and you can use those numbers directly. Where they are not, all mechanical design CAD packages have tools included for measuring positions and angles in the model.
Where the first estimate is not accurate enough, a process called kinematic calibration is used. There are many techniques, and as Chuck mentions, it is often difficult or impossible to directly measure distances of interest. A web search on the topic will give you an introduction.
Answer: You generally need to get the measurements from CAD or manufacturing drawings, simply because there's generally no reasonable way to measure something like a joint centerline, especially when the motors and shafting are concealed under body/trim plates.
Comment: @MarciodeQueiroz - That's the entire point of a CAD package. There's no special software or add-on, any basic CAD software should allow you to pick points and measure the distances between. Try searching "how to measure between to points in  ______" and put in the name of your CAD package. Solidworks, Inventor, Sketchup, Creo, NX, etc. - they all have a measurement tool as part of the base package.
Comment: So there are special software add-ons that allow you to pick points on the CAD and measure distances between them?


Are supports for spot lights required?

Question: If I am planning to install this spot light on the ground (in dirt), are support beams or things like that required or can I just place this in the dirt and it would be fine?
Answer: A lot of outdoor light comes with a stake or something to stabilize it.  I have ran a lot myself and basically I have two rules.  Run all of your wiring through PVC all the way up to fixture.  Second, if it doesn't come with something make your own stake.  If this thing is just a rock with wiring sticking out I would get some thick wiring, glue it on the bottom and stick that in the ground like a fork.  For more traditional type of lighting we put half cages over them but since this was made to look like a rock that would probably kill the aesthetics.
Answer: I have put the lights on the ground.  I buried the wires in mulch being sure NOT to bury any connections.
Comment: This just says what you did, but doesn't provide any evidence that it was the correct thing to do. For example, including the model number of your lights and an excerpt from their installation instructions showing this mounting method.

Are supports for spot lights required?