What is the opposite of compulsion?

Question: If I am acting based on a compulsion, then I am acting because I am forced to do so.  I have given up my power to chose.  What is the opposite of this?  If I decide to act after careful analysis and decision, what is this called?
Comment: You want an antonym for compulsion or one word for "acting after analysis?"
Comment: acting with deliberation is **not** the opposite of acting from compulsion.
Comment: Out of your free will, maybe.
Answer: You are acting of your own volition, which Merriam Webster defines as "the power to make your own choices or decisions".
Or you could say you are acting of your own accord, which Wiktionary defines as "voluntarily, without being told to or asked to".
Comment: Volition is exactly the word I needed
Answer: You can say you're acting "deliberately" or "with careful deliberation".
From the MacMillanDictionary.com:

"Deliberately":
1) with a definite intention, not by chance or by accident
Examples: "You did that deliberately, just to annoy me.",
"Police believe the fire was started deliberately."
"Her voice was deliberately cool."
2) in a slow careful way
Example: "He spoke deliberately, considering each word carefully."
Answer: There are several notions of compulsion.
One opposite of compulsion not yet mentioned is agency.  Another is autonomy.

What is the opposite of compulsion?


Close live agent chat window when the user click on end chat

Question: Is it possible to close the chat window when the user click on "end chat"?
Comment: Welcome to SFSE! Please take the [tour], and read [ask]. Generally speaking, questions that can be answered with a simple yes/no aren't a good fit for this site. Showing the community what research you've done, and what you've tried to do prior to asking a question usually goes a long way to getting a positive response on this site. Probably the biggest key to success here on this site is to provide plenty of details, because more details usually means the other contributors have to assume fewer things (which leads to more more accurate and tailored answers). Just some things to be aware of.
Answer: We can not override end chat button behavior on standard chat window, so we can not do that.
However, a way to get it by creating your own chat window, of course, not a practical option but you can give a try: https://rajvakati.com/2017/11/02/salesforce-custom-chat-window/


Jetpack Joyride device sync

Question: Is it possible to sync between an android device and a Windows 8 PC?  I have a lot of progress on a lot of games on my Droid Razr M, but I just got a new all-in-one, which is great for my daughter.  I want to put my Jetpack Joyride progress on there, especially, but I don't think it's possible.  Is there any way to do this?
Answer: No, this is not possible, as the platforms use different and incompatible means of storing application data.

Jetpack Joyride device sync


Can I get alignment with only replacing 3 tires?

Question: I have an obvious alignment issue and I need to replace the front 2 tires because of the wear that it has caused.  One of my rear tires is brand new (replaced a matter of weeks ago), but the other has maybe 40% tread on it.  Do I need to replace that one before I have the vehicle realigned?
Comment: Welcome to Motor Vehicle Maintenance & Repair!
Comment: IMO buy the fourth tire, you can either sell the 40% tire, keep it as a spare, or the shop may give you a credit if they sell used tires.
Answer: You can have an alignment done at any time - with or without replacing tires.
But it is worth getting an alignment done so the new tires last as they should.
Edit: even if the rear is a solid axle, an alignment will show if it is bent and needs replacing / repairing depending on its construction.
Comment: @ThisTooShallKickAss - It shouldn't make a difference in the alignment, but may have other implications for such things as the differential if on a drive axle, or a transfer case if the vehicle is all wheel drive. These factors depend on how much different the size is of the old tire (how much wear is there on it, which would make it physically smaller than the others). The alignment will take into account any differences due to the old tire, so is not an issue.
Comment: Does that last old-ish tire make a difference in the alignment if I don't replace it?
Comment: Also, not all axles can be aligned, particularly the rear. Solid or live axles generally can't be adjusted for camber or toe, and they don't have caster, so what's left? Unless your car has independent suspension at all four corners, the rear doesn't have much/any adjust-ability.
Comment: @TimNevins So if I'm in a 2-wheel drive sedan and I only replace the front 2 tires (assuming it's a front-wheel drive), they're essentially only going to perform alignment on the front axle?


Is it reasonable for a fixed contract cost to a client to be based on the average salary of the role from the Stack Overflow Developer Survey?

Question: Say for example it's a two month iOS development project, would it be sensible to set the fixed cost at what two months of a mobile developers salary in the U.S. would be using this data? I'm in the SF Bay Area. I want to avoid low-balling myself while also making what I charge more objective and backed by data. This would be the only project and client I work on during this period.
Comment: According to that survey over 70% of respondents are **employed full time**. In fact, the *overwhelming majority* of respondents are employed in some manner, not freelancers. Using employee salaries has never been an adequate measurement of a *freelancer's* rate in my personal opinion. "Employees" in my field (not iOS development) earn *substantially less* than I do as a freelancer... they simply have more *steady, predictable* income.
Comment: Do you mean that the survey would be a conservative metric for freelance, given that benefits and stability are factored into full-time employee salaries?
Comment: It would be a *very* conservative estimate. But that's merely my opinion.
Comment: So the potential client told you the "2 months" time frame? Until you look at the requirements you don't know if you can do this job by yourself in that time or if you will need additional developer(s). And I guarantee you that if you add a guy you will want a guy who is super reliable and he's not going to low-ball himself so you providing any numbers to the client before you know all of the details is not a good idea.
Comment: @HenryM Currently figuring out the time frame. It doesn't look like I'll need additional developers, since this specifically UI app development in the scope and he's been writing the lower level code himself.
Answer: So as the developer survey states the average salary of an iOS developer, you definitely need to charge little more. Let's suppose a developer earns 100% for a two-month job, so you need to charge 120%-140% of it. A freelancer always charges more than a full-time developer, as you might be giving more time than a full-time developer or you possess more experience than an average developer.
Secondly, once the task is complete, the client will be asking for revisions too that might extend the duration to more than two months. So you need to take this into consideration too.
Answer: No
Your situation as a freelancer typically places additional burdens upon you that aren't reflected in that salary survey, which is primarily geared toward people seeking employment.
Very often, billing rates for a professional are the professionals' salary plus some sort of multiplier. The multiplier varies a lot depending on the industry, but it's purpose is to cover the costs of things like overhead.
Overhead, in a general sense, are costs like office space (even if that's a room in your house), utilities, software license fees, hardware, expenses for marketing, health insurance, taxes, etc. Effectively, anything and everything that's necessary to do the work. For a company, it would also include things like profit. Presumably as a freelancer your salary is the profit; however, if you're thinking to expand your freelancing into some sort of business, then you should also include profit.
In my industry of consulting engineering, we have a lot of overhead costs and I've seen multipliers as high as 3.2. So if my salary is $35 per hour the client's getting billed at least $112 per hour.
As a freelancer, you can probably curb a lot of the costs and bring that multiplier down a bit (perhaps a lot). If you haven't done a rough cost analysis on your overhead costs, it's going to really hamper your ability to determine an appropriate multiplier since you're just guessing at that point.


Is it possible to prove that the off-diagonal blocks of the Cauchy matrix have numerical rank $O(\log n)$?

Question: Suppose we have a $n\times n$ Cauchy matrix of which the $ij$-th entry is given by:
$$
A_{ij} = \frac{1}{a_i - b_j}
$$
the assumption is that the distance between $\{a_i\}$ and $\{b_j\}$ is greater some positive constant, for example:
$$
a_k = e^{i\frac{(2k+1)\pi}{n}}, \quad b_k = e^{i\frac{2k\pi}{n}}
$$
then we have $\displaystyle \min_{i,j} |a_i - b_j|\geq O(\sin(\pi/n))$, and we want to prove something like the off diagonal entries can be represented by:
$$
\frac{1}{a_i - b_j} = \sum^{O(\log(n))}_{k=1} C_k + O(\varepsilon)
$$
Is this possible? or there is another type of rank bound for Cauchy matrix?
Comment: @Paul Thanks for the comment, the Cauchy or Cauchy like matrices arise from various computational problems, such as solving displacement equation, and computationally, as Jack Poulson said, Ming Gu claimed the bound is $O(\log(n))$ but he never proved it, I am just curious about the theory behind this numerical linear algebra problem.
Comment: I agree that something is wrong there. Judging from the question, Shuhao wanted something like $A \approx \sum_{k=1}^{O(\log n)} u_kv_k^*$, where the approximation is accurate up to an entrywise error of $\mathcal{O}(\epsilon)$.
Comment: Does this question relate to low-separation rank approximations of operators, like the work done by Beylkin and Mohlenkamp?
Comment: Would this question more suitable for the math.stackexchange.com site?  It seems more of a theoretical question in linear algebra rather than a computational one...
Comment: @Paul: The thought crossed my mind. First, I want to see if a connection to numerical methods can be made before we discuss migrating.
Comment: I think there is a problem with your last equation: what is $C_k$? How does it depend on $i$ and $j$?
Answer: I know there has been a significant delay here. I also hope that it is not inappropriate to advertise personal work in this situation. I am making an exception to my usual policy because of the time frame.
The short answer to your question is: "yes" if $a_i$'s and $b_i$'s are real. (For complex $a_i$'s and $b_i$'s it is still true in practice, but more thinking is required)
Bernhard Beckermann and I just submitted a paper [1] that shows that if $AX-XB = F$ with $A$ and $B$ normal matrices, then the singular values of $X$ can be bounded as
$$
\sigma_{1+\nu k}(X) \leq Z_k(\sigma(A),\sigma(B))\|X\|_2, \qquad \nu = {\rm rank}(F),
$$
where $\sigma(A)$ and $\sigma(B)$ are the spectra of $A$ and $B$, respectively, and $Z_k(E,F)$ is the so-called Zolotarev number. More precisely,
$$
Z_k(E,F) = \inf_{r\in\mathcal{R}_{k,k}} \frac{\sup_{z\in E}|r(z)|}{\inf_{z\in F}|r(z)|},
$$
where $\mathcal{R}_{k,k}$ is the space of degree $(k,k)$ rational functions. This is called the third Zolotarev problem.
In the case of the Cauchy matrix, let $A = {\rm diag}(a_1,\ldots,a_n)$ and $B = {\rm diag}(b_1,\ldots,b_n)$ then
$$
AC - CB = \begin{bmatrix}1\\\vdots\\1 \end{bmatrix}\begin{bmatrix}1&\cdots&1 \end{bmatrix}.
$$
Hence, $\nu=1$ and
$$
\sigma_{1+k}(C) \leq Z_k([a_1,\ldots,a_n],[b_1,\ldots,b_n])\|C\|_2.
$$
Unfortunately, discrete Zolotarev number (like above) are very difficult to bound.
If we make the additional assumption (which is appropriate for off-diagonal blocks) that $[a_1,\ldots,a_n]\subseteq [a,b]$ and $[b_1,\ldots,b_n]\subseteq [c,d]$, where $[a,b]$ and $[c,d]$ are disjoint intervals, then
$$
\sigma_{1+k}(C) \leq Z_k([a,b],[c,d])\|C\|_2.
$$
In Corollary 7 of [1], we showed that this leads to the singular value bounds
$$
\sigma_{1+k}(C) \leq 4\left[{\rm exp}\left(\frac{\pi^2}{2\log(16\gamma)}\right)\right]^{-2k}\|C\|_2,
$$
where $\gamma$ is the absolute value of the cross-ratio of $a$, $b$, $c$, and $d$. Namely,
$$
\gamma = \frac{|c-a|\,|d-b|}{|c-b|\,|d-a|}.
$$
This gives you a bound on the numerical rank of $C$ as
$$
{\rm rank}_\epsilon(C) \leq \Bigg\lceil \frac{\log(16\gamma)\log(4/\epsilon)}{\pi^2}\Bigg\rceil = \mathcal{O}(\log\gamma \log(1/\epsilon)).
$$
For a case where $\gamma = \mathcal{O}(n)$ and a $\log(n)$ term appears see the Hilbert matrix: https://mathoverflow.net/questions/137059/the-singular-values-of-the-hilbert-matrix/251187
Edit: In your case the $a_i$'s and $b_i$'s are complex valued. This is more tricky and more related to Section 5.1 of [1].
Answer: This is perhaps dodging your question, but why do you want to insist on low-rank off-diagonal blocks? Displacement structure is also data-sparse and allows for stable quadratic-complexity factorization algorithms. There is also a nice survey paper on the subject.
Ming Gu gave a talk which uses Hierarchically Semi-Separable (HSS) matrices to compress the Cauchy-like matrices which arise from a Fourier transform of Toeplitz matrices, resulting in an $O(n \log n)$ approximate solution, but I'm not certain if the compression technique will work for general Cauchy matrices.
Comment: Ming Gu also used this property of off-diagonal blocks having a $O(\log(n))$ rank to get that $O(n \log(n))$ complexity, rather than using the displacement rank. Here I am just curious how did he get this $O(\log(n))$ rank theoretically, but he never mentioned any proof in his paper, merely a numerical result.
Comment: @Jon: If you look at the seventh slide of his talk, you can see that there is a very special structure to the Cauchy matrix: $a_i = \omega^{2i}$, and $b_j = \omega^{2i+1}$, indexing from zero.
Comment: The Cauchy matrix is the same matrix as I pointed out there. I also tested some random Cauchy matrix, and found that the singular value decays very quickly for off-diagonal block part. Ming Gu chose this matrix because he used 1 and -1 when he transformed the toeplitz matrix into Cauchy-like matrix using the displacement equation. By doing this he could maximize the minimal value of the denominators of the entries for the Cauchy matrix, and enhance the stability of the numerical computation.
Comment: @Jon: Ah, good point. $\omega := \exp(i \pi / n)$. I would have to think more; maybe plotting the singular vectors would help...


Better Spam Protection than reCAPTCHA?

Question: I hope this is a valid question to ask here... but knows anyone a better spam protection than reCAPTCHA? We've now using reCAPTCHA for some time but got now the situation that obviously more and more spam comes through. Is there any better but still more or less user friendly spam protection you can recommend? It can be just a concept or also a (free) service.
Answer: One alternative to CAPTCHA and less intrusive with better UX is the honeypot method. It's rather simple: you create an input field in the HTML that you hide with javascript. You then check on the server side whether it was filled in or not, if left empty the visitor is human, if filled in a bot.
The reason it works is because javascript is still too computationally expensive for spambots to use (spam is about quantity not quality) so they don't bother parsing it. Originally CSS was suggested to hide the field, however many spambots do parse CSS to detect hidden elements on a page as it's not that expensive.
A friend of mine who told me about the method reported so far a 100% success rate in protecting his sign-up forms using this little neat trick.
Comment: We already use this but with CSS not js yet, I'll give it a try and mark your answer as correct if it is going to work fine.
Comment: Thanks for posting this reply. I knew the term honeypot (but not what it meant, exactly), and I knew of the technique but like burzum I was familiar with it as a CSS based solution. Clear and concise answer, +1!
Answer: I'd like to suggest you have a look at a new technology my company created.  It's called "VouchSafe" and is available free at http://www.vouchsafe.com
We've created plugins for Wordpress, Joomla, Drupal, and PHPBB, or you can roll your own integration with our APIs for PHP and C#.NET.
Comment: great product :)
Answer: Depending on your environment you could use Akismet too. It was created to fight blog comment spam.
Answer: I'm suggesting you cloudflare services, see http://www.cloudflare.com/features-security
I have the CloudFlare Free plan and it suit my needs perfectly, Fast DNS (geo), Caching and Threats blocking.
Answer: Another alternative - http://mollom.com/
Quoted from their site:
Mollom actively protects 51,278 sites from spam currently. Mollom is very accurate, only 5 in 10,000 spam posts were not caught.

Better Spam Protection than reCAPTCHA?


Does ASP.NET guarantee the integrity of a page's controls on a post back?

Question: Say you have an ASP.NET page that lists the prices for certain items in labels. I know it's cringe-worthy, but say you took the price of the item from the label on the page to determine what the user is charged for their order. Does ASP.NET guarantee in any way that the text in those labels has not been tampered with?
Answer: Viewstate is what you're looking for.
It secures the page cryptographically, although it has known limitations that are located elsewhere on this site.
An optional MAC should be enabled for sensitive pages, such as yours, that allows the client to post back prices that are used by the business layer.
Comment: Controls that store values that go through viewstate would be protected, BUT if the control doesn't actually use viewstate for persistence then you're in trouble. It would be worth investigating if Label controls use viewstate.
Comment: Can't you just set the viewstate property for that label control?


Link of order state with Order status

Question: I wanted to confirm what is the link of order status to order state..?
Means if i link multiple Order status to order state e.g. to Processing, what does it mean?
Comment: For order status, see also this answer: http://magento.stackexchange.com/a/516/231
Answer: An order state can have multiple order statuses, whereas one order status is the default of one order state.
You can check out the Order States and Statuses here: System > Order Statuses as shown in this picture:

Standard order status as of Magento CE 188.8.131.52 (see table sales_order_status) are:

Canceled
Closed
Complete
Suspected Fraud
On Hold
Payment Review
PayPal Canceled Reversal
PayPal Reversed
Pending
Pending Payment
Pending PayPal
Processing

The order statuses are already explained in detail here: https://magento.stackexchange.com/a/516/231

Link of order state with Order status


How to retrieve BCC mistakenly sent to BTC Electrum wallet?

Question: I sent my BCC from blockchain wallet to my BTC Electrum wallet and now I cannot trace them. Does not show in either of the wallets. How do i retrieve them?
Comment: Attach link to the transaction on the chain where transaction happened.
Comment: Welcome to Bitcoin.SE! You can help the site by marking answers as accepted if they are correct and address the question so that the question does not remain as "unanswered".
Answer: Can you find the BCH on a block explorer website? What address are they currently in? Is it one that you control the BTC keys using electrum? If yes, then do the following:
You'll need to take the seed from your BTC Electrum wallet, and use it to generate a new BCH wallet using BCH software. Your BTC Electrum wallet will not show a BCH balance, since BCH is a different network (that uses the same addressing scheme).
It is a good idea to send ALL of the bitcoin out of your wallet, before you generate the BCH wallet using the seed. This is important, it ensures the keys that control your bitcoin remain safe. If the BCH software you use is malicious for some reason, it might check the BTC balance of the addresses you control, and steal the bitcoin from you.
Even if there is a small chance of this, you can be safe by just transferring all of your bitcoin out of electrum first, and then deal with the BCH.
Also, don't transfer the btc back to that wallet afterwards. You can still use electrum, you just have to make a new wallet with a seed that is different from the first one.


Fastest way to Dalaran from Orgrimmar

Question: I am looking to travel to Dalaran without a mage port from Orgrimmar as that is where my hearth stone is set. What is the fastest way to get there?
Answer: If we asume you got no other transportation device then your flying mount, you'd simply jump on the "shark" Zeppelin to Borean Thundra and fly from there.
If you visit Dalaran rather often, I strongly suggest you to place your Hearthstone there, since there's a portal to Orgrimmar in Dalaran.
Other ways of getting to Dalaran; the engineering Worm Hole which allows you to chose one of a couple of randomly given zones of Northrend, which is rather close to Dalaran itself. Avoid choosing Howling Fjord and Borean Thundra since they're the most distant ones from Dalaran.
If you've grinded like a madmen with Argent Tournaments in Ice Crown, you can use their tabard to teleport there.
There's no point mentioning warlock summoning, but there's also the guild ability Have Group, Will Travel which allows someone positioned in Dalaran to summon you.
If you got silly much money and for some reason don't like to pay mages a small sum, you can on your next visit to Dalaran purchase an expensive ring which allows you to teleport to Dalaran.
I asume that you're a high level since you ask for the fastest way, and not the safiest way.
Comment: Just a note on the engineering worm hole, its not random, its 5 specific places; Borean Tundra, Howling Fjord, Sholazar Basin, Icecrown, and Storm peaks from which you can choose. There is a random proc though that apparently takes you directly to the underground sewers of Dalaran, but I can't confirm that, I've never seen it happen on my device.
Comment: your missing [the ring](http://www.wowhead.com/spell=54406#used-by-item)
Comment: The Dalaran sewers portal does exist, I've seen it myself. It's useless in terms of actually getting to Dalaran though as you get locked in a small room with a vendor with engineering stuff and no way out except hearthing. With respect to the other portals, you get to choose one, but it places you in a random place in said zone. It's still quicker than taking a zeppelin though.
Comment: Yeah I've gotten the Dalaran portal through Wormhole, and it is quite useless. I updated my answer regarding the Wormhole.

Fastest way to Dalaran from Orgrimmar


How quickly will bread yeast take to create an ethanol concentration of....?

Question: How quickly will bread yeast take to create and ethanol concentration of ~15%~,~10% and ~5%~ in at 30°C.I believe bread yeast is designed to rapidly fement(at first at least) but is not that ethanol tolerent so will progressively slow down as the ethanol concentration climes.I appologise if a similar thread has been posted already somewhere else on here by somebody else.
NOTE:I am not asking for a precise answer just a rough estimation from experience even  from wine yeast.
Comment: Maybe it's not your answer but I've tried to brew beer with baking yeast from non alcoholic beer! I tried two times and gain even less than 5%, but my friend with another breed yeast (a french yeast in a blue container which I don't know the name) reached to approximately 8%. So I'm gonna to try once again with his yeast.
Answer: I don't know if any bakers yeast that is tollerant to much more than 5% Alcohol By Volume.
Hypothetically: A fermentable solution can fully attenuate in a matter of hours with enough adapted yeast and oxygen. But will never exceed the ABV tollerance of the strian.


How to securely access web api from Html5 Website

Question: So the thing is I have created a Asp.Net web api and am using oauth to secure the access.
The problem is that the clients want to make a pure html(angular) website from which they will access the api. So basically they will call my webservice oauth endpoint using their credentials to get the token and refresh token and they will store that somewhere in the localstorage to make subsequent request. My problem is that if they will use pure html and angular, their call to the api and point and request data will be visible to a malicious user and he can use that client id and password to generate tokens and can use that for his use.
 I searched a lot and didn't find any help . Since it is my first web api project, I don't lot about these. Can any one help me regarding this
?
Answer: Your use-case is the public client as described in RFC 6749 Section 2.1. In this case you have the option to implement the implicit grant. This implies that the user interacts with your site to grant access to the data.
However if your use-case forbids the redirection of the user the OAuth framework can't solve your problem.


Why won't a Stealth Bomber fire on cities anymore?

Question: My stealth bomber won't fire on an enemy. It fired in the previous turn and now it won't attack my enemy's city. Does anyone have any ideas as to why this is?
Answer: Perhaps you have lost sight of the enemy city (is it greyed out)? Units can only attack other units / cities if you currently have them in sight. A Stealth Bomber has a huge range but it has to have some other unit(s) providing sight to the intended target.
Comment: I've got the same issue and can attack a unit beyond the untargetable city, and I have no units between the bomber's location and the target city and target unit - and this is years later, doubt there's any patch coming.


is there anything wrong with "from my perspective"

Question: Is the expression "from my perspective" good English? I was always under the impression that "perspective" refers to what someone else can see (i.e. a third person), and that if you wanted to refer to what you yourself can see you should say "in my view". What do you think?
Comment: Sounds like you need to look up the word *perspective*
Answer: I guess it can work well, from my perspective:
Perspective:
 a way of regarding situations, facts, etc, and judging their relative importance.

my view
my point of view
my viewpoint

are all possible alternatives.
Answer: It's absolutely fine, and entirely natural English (to my native British ears). It can mean roughly the same as 'in my view', though there are cases where 'perspective' might sound better, eg:

from my perspective, as a student with mounting debt...

In this case the position of the speaker clearly informs the opinion, so it would be odd to say 'view' instead.
Answer:
I was always under the impression that "perspective" refers to what someone else can see (i.e. a third person),

This is what dictionaries are for. There are always senses and meanings that one isn't aware of, so even if you weren't mistaken (though you were) you couldn't be sure there wasn't another meaning without checking at least one good dictionary.

and that if you wanted to refer to what you yourself can see you should say "in my view".

They're pretty much synonymous.

What do you think?

It's valid but overused and often misused. It's commonly used as a weasel word when the opinion stated doesn't actually relate to their perspective at all, but is simply their opinion. The same is true of "in my view".
If though you actually do have a genuinely different perspective rather than merely a different opinion, then it's perfectly fine.
Answer: first, visit online dictionaries and see its usage. "from my perspective" is just fine. if you say "it looked good from my perspective" and "it looked good from my point of view", they are the same. your perspective is just how you look at things.
just like everyone perceives things differently, everyone has different perspectives.
how you recognize, discern, distinguish and grasp things, along with your actions, is related to your perspective or view of the situation at hand.


Startup Disk is full but can't do Dropbox

Question: My Mac says that the Startup Disk is full. I know I have way too many pictures in Photos so I got a huge Dropbox subscription to add pics before deleting from my hard drive to free up space. However, now it won't let me download pics since there is not enough free space on the hard drive to do it, so I'm caught in a Catch 22. My ultimate goal is to put pics in Dropbox and external hard drive and delete from computer so that I can have a place to access them when I need them.
Comment: Start by moving pictures, and videos if you have them too, to an attached external drive. This will begin to free up disk space on the Startup Disk.
Comment: What you could do is turn on [selective sync](https://www.dropbox.com/en/help/175) and upload files to dropbox using the web interface. Alternatively you can use a third party service like [Arq 5](https://www.arqbackup.com/), which lets you back up to dropbox and other services. You can use it without the dropbox app and also if you have the dropbox app installed, you can use selective sync to make sure the backups don't get synced back to the computer.
Comment: Also, dropbox earlier this year announced a new upcoming feature called [project infinite](https://blogs.dropbox.com/business/2016/04/announcing-project-infinite/). It allows you to see files in dropbox without taking any disk space.
Answer: Are you syncing your Photos app using iCloud? Using that in conjunction with the setting "Optimize Mac Storage" should solve your problem (provided you have enough iCloud storage space). When set up this way, Photos stores your entire library on Apple's servers, and only downloads full-resolution photos and videos onto your local hard drive as you view them. If you have a lot of empty space, it'll cache more of them locally, but if you start to run out, it will get purge the high-res versions to make room. The thumbnails remain for browsing.
The big advantage to this over moving things manually onto Dropbox, external drives, etc etc is that your photos and videos stay right there in Photos, and are available on all your Apple devices without any real work on your end. Everything is backed up, and you aren't spending all this time manually shuffling things around. The one (and only, IMO) disadvantage is that you need to pay for whatever level of iCloud storage suits your Photos library size.
Answer: Photos is storing all your photographs in a local file in your user folder on your main hard drive. Even when you move a photograph to another location, Photos keeps the photograph in this file unless you delete the photograph from Photos. When you delete a photograph from Photos, the photograph goes to "Recently deleted", but it is still in this local file! You have to delete the photograph from Recently deleted to get that space back. If you do not delete the file, it will delete itself after 30 days — this is the same way Photos works on iOS. If you have a lot of photographs, this file can get really huge.
To locate this file:

Open the Finder
Press Command + Shift + G to open the "Go to the folder:" dialog box.
Type ~/Pictures and click "Go"

This will show you your Pictures folder inside your user folder on your Mac.
The file I am talking about will be called "Photo library" followed by an underscore ( _ ) and the date the library was created (for example: "Photo library_2015.11.05"). However big this file is, it is most likely the source of your problems with not having the free space you expect.
The quickest way to safely free up space is to:

Copy your Photo library file to an external hard drive (if it is a big file, it might take a while to copy over)
Rename the Photo library file so there's no confusion. For example, something like "Photo library_external_drive".
If Photos is open, you will have to quit Photos (Command + Q)
Before opening Photos again, hold down the "option" key — the easiest way to do this is to hold down the "option" key and then click the Photos icon in the Dock. This will open the "Choose Library" dialog.
From the "Choose Library" dialog, click the button that says "Other library..." and choose your external drive, then the Photo library file you just copied over (this is where having renamed the file is useful)
Once you have selected your renamed Photo library file, click "Open"
Verify that all your photographs are there and that everything is right before doing step 8
Go back to the Pictures folder inside your user folder (you could use Command + Shift + G and ~/Pictures again to be sure) and delete the Photo library file (Command + delete). Because you renamed the file on the external drive in step 2, you know that you're not deleting the file you just copied.

To get your free space back you will have to empty the trash. Once that is done, you will get back however much space this file has been taking up on your system.
The next time you open Photos, it will use the new library file that you copied and renamed on your external drive without issue.


What kind of neural network architecture do I use to classify images into one hundred thousand classes?

Question: I have an image dataset where objects may belong to one of the hundred thousand classes.
What kind of neural network architecture should I use in order to achieve this?
Answer: Classification tasks with a large number of classes are usually handled with hierarchical softmax to reduce the complexity of the final layer. This is useful, for example, in applications such as word embedding where you have hundreds of thousands of classes (words), like in your case.
Answer: A large one!
In all seriousness, imagenet had roughly 1000 classes and did not require anything special from the top submissions. Depending on how deep(contextually) these classes are, you may want to do something like multi-label classification. Your biggest problems will likely be differentiating between classes, as well as class distribution.
Good luck!
Comment: I agree with @hisairnessag3 also remember to apply one-hot encoding, multi-hot encoding or sparse categorical depending on your dataset structure.
Answer: As you can imagine and as it has already been said, a large one for your network to tune weights and biases. But I wanted to nuance this statement with two points
First :  you can use an Autoencodeur to pre-process your images. It can reduce dimensionality and so improve the learning capability and efficiency (in a generalization point of view).
This kind of NN takes your images as inputs, encode and then decode them to provie new representation of your initial images. Dealing with the decoded dataset can allow you to consider less hidden layer with less hidden nodes and then speed your work up.
Second : architecture is sure a thing to deal with image recognition, but you can also play on the input representation (that is what the autoencoder aforesaid is about). You can look at PCA (Principal Component Analysis). It allows to reduce dimensionality to a certain number of components (that you specify). It is often used in face recognition where inputs and targets are various.
All that to say that architecture is sure a thing when dealing with large datasets, but there also few tools to reshape the inputs so that then can be more easily learnt.
And by doing so you can improve the capability of your network as much in term of time computation as in quality and accuracy of prediction
Answer: Alexnet (2012), Overfeat (2013), VGG (2014) and ResNet (2016) are cited in many image recognition or segmentation applications. There is also GoogleLeNet (2015).
The lastest is the publication the denser is the network.
The ResNet publication comments on how the network density affects accuracy depending on the image data set size. The article tends to give a motivated answer to the question

Is learning better networks as easy as stacking more layers?

You might consider the training time since you have your own image data set depending on the kind of hardware you can you use ( see this benchmark for instance ). The denser the more time it will takes.
You also have to consider the size of the traning data set w/r to the expected accuracy. If the set is too small the net will probably overfeat. In that case you migh consider a data augmentation strategy (one of the answers mentions auto encoding, I m not sure but this might help for this purpose).
All these publications refer to the ImageNet data base and the associated image classification/detection contest which has 1000 classes.


Cycle of actions and voice signals on a multipitch climb

Question: When climbing multipitch, what is the cycle of things you do when climbing each pitch, and what are the associated voice signals?
Comment: This varies hugely between regions (and often within regions, for different styles of climbing).  You're unlikely to get a single definitive answer.
Answer: The German mountaineer and safety expert Pit Schubert advocates to reduce the signals down to two:
(I'm reusing @Ben Crowell's terms since I know only the German words)

Leader to belay: "Alice, off belay" (German: "Stand!") when the leader has built their own anchor point. If there is a line of sight to the belayer, the leader may also show both hands as a clear signal that they don't need them to climb anymore.
Leader to belay: "Alice, belay on" (German: "Nachkommen!") when the leader is ready to belay the second climber.

All other commands are redundant.
Comment: And sometimes, when you can't hear anything, you simply have to start climbing anyway. Because, what else would you do? Especially if it is some significant time after the leader has taken all the rope. It helps if one of the ropes is longer and get's pulled up as well, so that you can be reasonably sure the leader isn't just stranded with not enough rope and can't move any more.
Answer: Alice and Betty are climbing in a team of two. Let's start the cycle with both climbers together at a belay, both tied in to the anchor with clove hitches. Betty is going to lead the next pitch. The standard set of voice signals in America, which I'll use below, are the ones introduced by Paul Petzoldt. When the climbers address each other by name, it's so that there won't be confusion between different teams that are near each other.
Betty gets together all the gear for her lead. If Alice and Betty are alternating leads, then  Betty's end of the rope is already on top, and if they're using two cordelettes to construct their anchors, then Betty already has the cordelette that she cleaned from the previous anchor. If they're not swapping leads, and this will be the second pitch in a row that Betty is leading, then the rope needs to be flipped over, and Betty needs to get the other cordelette from Alice.
Betty: "On belay?"
Alice puts Betty on belay. "Belay on."
Betty unties her clove. As a courtesy or in case it isn't clear, she can say "Climbing," to which Alice would answer "Climb."
Betty does her lead, gets to the next belay station, builds an anchor, and ties in.
Betty: "Alice, off belay!"
Alice takes the rope out of her belay device. "Betty, belay off!"
Betty pulls up the rest of the rope. When it's taut and pulling up on Alice's harness, Alice yells, "Betty, that's me!"
Betty puts Alice on belay. "Alice, belay on!"
Alice takes the anchor apart. When she's ready to climb, she yells "Betty, climbing!" so that Betty has a heads-up to expect that she needs to start pulling up rope. Betty responds, "Alice, climb!"
Alice climbs the pitch, cleaning the gear. When she gets to the new anchor, she ties in. "Off belay."
Betty takes her off belay. "Belay off."
For situations where voice communication may be impossible, and the climbers may not even be able to see each other, they should discuss in advance how to handle that. Some people use systems of rope tugs, but in my experience those don't work very well. If the leader yells down "off belay" but there is no communication, then they can just pull up the rope slowly while the follower continues to feed it out through the belay device. If this type of situation is anticipated, then sometimes they will discuss this in advance, and the leader will just promise to put the follower on belay within a few minutes once the rope feels taut. If the rope may not be long enough to finish the pitch, sometimes people will agree in advance to simulclimb a short portion at the end so that the leader can reach the belay.
Comment: Is this convention standard across the whole of America, or just the US?  Your term is a little ambiguous.
Answer: For Great Britain, with both climbers beginning at the foot of the next pitch, with leader L and second S:

LOn belay: I'm tied to the rope and prepared to climb
SClimb when ready: I am now belaying you
LClimbing: I know you're belaying me; expect to pay out now
LSlack (or rope colour), Take in, or Tight as needed during the climb; also Below! when required
LSafe: I'm attached to something solid, you can stop belaying
SOff belay: Rope is now out of the belay system
LTaking in: I'm pulling up the slack
SThat's me: You've finished taking in
LClimb when ready: I am now belaying you
SClimbing: I've cleaned the stance, and am starting to climb

That's the full set, but there's no need to be so chatty all the time, especially when we can see each other; conversely, when the conditions are difficult, the situation often has to be inferred (see imsodin's answer).
There's a couple of extras I use as a second, especially on winter routes: ten metres! and five metres!! when I'm concerned the leader hasn't yet found a belay stance.
Answer: @BenCrowell describes the sequence nicely and @Jasper brings up reducing commands. Due to circumstances (no visual contact and out of earshot) and simply to reduce the yelling on the mountain (both for the pleasure of silence and security, as there is less that could be misunderstood, especially on crowded sections) it is beneficial to do so. I'll give an explanation why you need no signals in the cycle to stay safe (and also which signals I consider still beneficial and why).
There's two safety-critical moments:

Leader reaches next belay, sets it up and secures themselves ⟶ can be taken off belay.
Leader takes second on belay after pulling up rope, such that they can clean the previous belay.

Now lets assume you can't give a signal as you can't see or hear each other. Rope tugging signals aren't reliable either due to rope drag. How can you ensure swift continuation of the climb while staying safe?

Second just keeps belaying until the rope is up. Tedious, but safe. This reduces calling "off belay" to an efficiency measure.
The leader sets up everything for the belay before pulling the rope up. Then once they feel the end of the rope, they purposely leave some slack. Then finish setting up the belay with the rope and pull in the remaining slack, tightening the rope strongly. Thus the second knows they are on belay once the rope is constantly taught. This is good practice even if you use a command, as it reduces the time the second could make a mistake without being on belay.

Essentially if I can give a signal, I always give the "off-belay" command, as it speeds up pulling up the rope. I don't give "on-belay" commands (except of course my rope partner explicitly requests it, in which they'll have to listen to a sermon about the virtues of not doing so after the climb :) ).
Comment: It requires some level of trust in your partner, but this is truly the best method. It can even be simplified to "Keep giving out slack as necessary---including climbing upwards. Don't climb into a loop of slack."


Expected User Limit of IRC Server on Pi 512 MB?

Question: Does anyone have a clue of the maximum number of users RPi 512 MB could handle if running an IRC server? I'm just looking for an approximate number, is it 100, 10'000 or millions?
Comment: No single computer is going to handle millions of *concurrently active connections*.  I'd guess the pi should handle hundreds spread across a few dozen rooms, kind of thing, though it would be pretty stressed if they are very active.
Answer: Several networks have published minimum hardware requirements for new server applications:

Undernet

Minimum 512MB RAM (1 GB or higher highly recommended)
For for x86-based machines, a minumum of 600MHz is required. for Sparc, the machine should be a Ultra-10 or better. Other platforms should run a comparable, or better, system. The Operating System kernel must be configured to allow a minimum of 4096 open file descriptors per process, and must support at least 4000 undernet user connections.
Server must be a DEDICATED machine to serve as an IRC server.

Newnet

Servers should be reasonably powerful machines. An old 386 running Linux/BSD would not be acceptable, although Linux/BSD running on Pentium class machines works fine.

DALNet

The machine must be a MINIMUM of a Pentium 4 (or equivalent)
      with 512MB RAM.  It is advisable to have a gigabit NIC to mitigate
      some of the effects of denial of service attacks.

So that gives you an idea of what the current servers out there are running. Most, however, indicate that bandwidth is the most important bottleneck.
If you're looking for something smaller or private, bouncers like ZNC would probably run fairly well on the Pi.
Comment: You have to remember though that IRC is a complex mesh of servers that form a global Network. These servers distribute messages across the mesh. A users on server A is most likley to read a message from user B on server B much later than any user on Server A. Sometimes network splits occur and channels get split up. So running 2 Pi's on seperate IP's at the same location will help help with load balancing as the 2 Pi's will relay internally. This network can expand exponentially and a-synchronously. So if limits get met - add more stuff!
Comment: @ppumkin True. But if you're trying to make a public server and you're running on a typical residential connection, you're probably going to hit bandwidth issues fairly quickly. Without more info on what the OP is trying to do (make a new network? Add a server to an existing one? Add private chat capabilities for lan parties? Create a private IRC network for a large corporation?) I'm hesitant to try and guess how effective this will be.
Comment: Yea- This is quite an interesting question. There is no proper way to measure the capacity. He can thousands of users doing nothing or 50 hammering it. So defiantly- At first all his friends and friends friends can connect and chat. But then only 1 or 2 people are active. As an experiment he should post his server here and see how many people can connect before it explodes. :) hehehe
Answer: Used to run an IRC server for 5-10 people on a ~400mhz ARM box with 256mb ram. Worked.
Answer: I'm using hybrid-ircd for a remote device management system...  A botnet for good, instead of evil.
So far, we've tested at scale by throwing a bunch of VM's at it, and with every agent deployed checking in at ~15 minute intervals with a csv string of data to the "master" bot, which collects into SQL...
500+ machines were fine.  The master process bogged a bit when we went over that, but that's about the scale limit we were looking for in regional splits.  Additionally, that's not anything to do with the ircd, but more my crap hack python skills (or lack thereof)
It works, built in python on a raspberry pi 1gb to start with, then moved into "the cloud" (shudder).
By the way, Xen is great for a lab, and can be a royal pain the butt to scale manually.


application as family for uk visitor visa

Question: I and my family had 10 years UK multiple entry visitor visa which expired about 2 years back.  I want to apply for the same again.    have filled up my application online and saved it.  I want to add my wife and son's application, but I am directed to visa fees payment.  How do I do it?  Do I pay for myself first and then ii would be given a choice of adding each family member?  can I fill all 3 applications under 1 account?
Comment: Also https://travel.stackexchange.com/questions/51582/how-should-a-family-with-young-children-apply-for-uk-visas
Answer: You will have to fill up each application separately and pay for each application separately, you can use the same card though. After the payment when you get to the VFS Appointment select the same date for everyone with consecutive appointment times. Then go to the VFS together and let them know you're applying as a family.
It is also helpful if you mention your GWF Number in the applications of your family members.


Has the infamous Sao Paulo runway been made longer or replaced meanwhile?

Question: The TAM 3054 accident occured because the pilots were stressed by the fact they needed to land on a not-so-long runway where an incident occured a day before. This stress has lead the pilots to forget about setting the thrust levers to idle. The link writes that after the accident the airport used an alternative runway for some time, but what happened to the infamous one? Has the runway been either prolonged (that's probably hard because there are roads in the vicinity) or closed permanently in the meantime or do pilots still have to land on it?
Comment: @mins Because of when it's raining, there had been an incident the day before. But this has been dealt with as evident from Bianfable's answer.
Answer: The runway has not been extended. The final accident report lists the runway length for 17R/35L (the accident runway) as 1945 m:

It has two asphalt landing-and-takeoff runways: the main runway (17R/35L),
measures 1945 x 45 meters; and the auxiliary runway (17L/35R), measures 1435 x 45
meters.

My current Jeppesen chart for SBSP lists runway 17R/35L at a length of 6365' (1940 m), so basically the same. The other runway (17L/35R) is listed at 4905' (1495 m), so slightly longer.
The runway has however been equipped with rainwater drainage grooves after the accident:

The airport has been troubled by slippery runways and has had several accidents where water accumulation has been a significant factor (most notably, the one involving TAM Brazilian Airlines Flight 3054 in July 2007). Although the main runway was repaved in June 2007, its new rainwater drainage grooves were only finished in September 2007.

(airports-worldwide.com)
Since 17R/35L is still the longer runway, it is obviously still in use today. It has ILS and RNP approaches on both ends, where the shorter runway only offers RNP.


Dietary Requirements of Gold

Question: Is there any scientific research available on the dietary requirements of gold(if any at all)? The motivation for my question comes from the Edgar Cayce readings(holistic medicine). According to Cayce, several diseases are caused by a lack of gold in the diet or the body's inability to metabolize the gold. As to what foods contain trace elements of gold, that is another topic altogether. As to whether or not you believe in Cayce's supposed psychic abilities is also irrelevant to my question, humor me and assume that there may be some scientific validity to his claim.
Comment: Can you provide a specific reference (including pg if book) so that it can be dealt with directly? I know of no scientific validity to the claim as you have written it.
Comment: The reference is the Edgar Cayce Readings, I can't send you a link because you have to be a member of the Association of Research and Enlightenment to access the readings, but you can Google the readings and use "dietary gold and Edgar Cayce " as a search term. Also, male sterility was predicted by Cayce to be caused by lack of gold in the body. This could be a testable prediction, do a study on men with sterility and measure the gold levels in the body and compare to a control group.
Answer: There's no established lower or upper intake level, but there's data showing that the average human body contains around 3ppb of gold, mostly in the form of nanoparticles.
Here's a long review of the academic literature on medical uses of gold nanoparticles:  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3347577/
In particular, it has a lot of effects on the immune system and it has been used as an adjuvant.

Dietary Requirements of Gold


Why ESD does not affect pacemaker?

Question: ESD from human affects electronics. Why it does not affect the pacemaker? Is it beacuse it is much below the surface?
Answer: ESD problems are caused by buildup of static electricity on the surface of a human or other device. The damage is mainly caused by the sudden equalisation of potential. Like a very small lightning strike. Another is noise from other devices.
You may have experienced electric shock when touching a door handle or a friend. This is on the higher end of the for very small smd components can take. They may be damaged even by shocks humans can not feel. However its not like they will always get destroyed by such shocks, its more of a statistical thing for manufacturing while the component is still separated from the board.
Now the important bit is that this causes damage because it is localized, the shock traveled directly to the component before it starts to spread around. The pacemaker is inside a human so it has a lot of insulating and dissipating medium around it.
Also the pacemaker is also shielded by its casing. It is after all a device that sends shocks. Your remote control or computer is not very likely to break from a shock as the case shields the components from being in the direct line for the shock to ground. Similarily the case may protect from other esd sources.
But yes the pacemaker can fry if you have a big enough shock. But then the human would be dead regardless.

Why ESD does not affect pacemaker?


How to design a steam powered supersonic aircraft?

Question: Using 19th century technology could we break sound barrier with a manned aircraft running on steam engine only? To qualify the said aircraft must cross Atlantic Ocean with a flight ceiling capped at 1000 m above sea level.
Comment: Why does it need to fly below 1000 meters?  Seems like an unnecessary complication.
Comment: Just for your chewing, it is way easier to break the sound barrier at higher altitudes than at 3,000 feet ASL.
Answer: No. Before powered sustained flight was invented, scientists and engineers were sometimes sensationalized in the newspapers as saying it's impossible. The real story is a power vs weight relationship. Locomotives existed. Early research showed that using steam engines to fly was off by orders of magnitude.
Technology had to advance before such craft were possible.  If they were possible with the existing technology at the time, they would have been built!
Comment: I can just imagine workers shoveling coal on a super sonic aircraft...
Comment: In [Harry Harrison's novel](https://en.wikipedia.org/wiki/Tunnel_Through_the_Deeps), the aircraft took coal dust and I suppose it can be automatically fed.
Comment: The Li P13a ramjet interceptor was conceived to use powdered coal as fuel as well, and estimated performance was in the supersonic range. Not bad for a late 1944 design during the death throes of the Nazi regime.
Answer: I am going to give a very qualified "yes" answer to the question. Technically, the aircraft is steam powered, but I don't think this is in the spirit of the OP's question. On a larger scale, the answer is still "no", since crossing the Atlantic at low altitude is not going to be possible.
The answer is to use the Walther cycle of decomposing concentrated Hydrogen Peroxide (H2O2) into steam to power a rocket engine. This is essentially the system used in the ME-163 rocket plane and a few planned successors. A brief description of how it worked demonstrates that supersonic speed is potentially possible, but the downside of doing so with a Walther rocket engine.

Me-163
Early versions of the rocket engine were powered by "Z Stoff", a mixture of  calcium permanganate or potassium permanganate suspended in water and used to catalyze the decomposition of the H2O2. This was considered a "cold" engine, but suffered from clogging of the nozzles and inability to throttle the engine. Later versions used a mixture of "C Stoff" (30% hydrazine hydrate + 57% methanol + 13% water with a small amount of potassium-copper-cyanide), which acts as a hypergolic fuel when mixed with "T Stoff" (the H2O2), releasing a considerable amount of heat along with the steam, and also being somewhat controllable in a rocket engine. Both versions of the motor ("cold" engine with Z Stoff and "hot" engine burning C Stoff and T Stoff) could propel the ME 163 to transonic speeds, where compressibility became an issue in controlling the airframe. With suitable aerodynamic refinements, the ME 163 had the potential to become the world's first supersonic fighter aircraft.

The HWK-509 engine
However, like all rocket engines, the HWK-509A engine consumed fuel at a fantastic rate. In the case of the ME-163, the plane was running on empty once to was at operational altitude, and the pilot was essentially flying into the bomber stream (and away from escorting fighters) in a high speed glider. In addition to that, C Stoff and T Stoff were difficult to handle, incredibly corrosive and prone to explode when subjected to a violent shock, such as the disposable launch dolly bouncing up and striking the aircraft as it took off, or the dregs of the fuel and oxidizer exploding when the plane slammed into the ground on landing.
While little could be done about the nature of the fuel, except for some pretty drastic handling protocols by the ground crew (including having thousands of litres of water available to flush the aircraft and work area in case of a spill), larger versions of the ME 163 were under consideration as the war approached the end. The ME-263 had a much larger and more refined airframe capable of carrying more fuel (in order to have more flight endurance), and some consideration was given to using the rocket motor in flight to assist the pilot in making his attack run and evading marauding allied fighters.

ME 263
So a supersonic "steam rocket" using some form of Hydrogen Peroxide decomposing to steam is possible for short ranged aircraft. H2O2 in high concentrations is quite unstable, so ground crews will be extremely wary of it, and if used in conjunction with other fuels (like C Stoff, but other versions of the Walther cycle using gasoline or diesel fuel were also experimented with) you don't exactly have a steam rocket, but a somewhat more versatile bipropellant rocket system.
Comment: There is also the ammonia engines in the [X-15](https://en.wikipedia.org/wiki/North_American_X-15) which still holds the record for the fastest manned aircraft. Not steam powered like the OP requires, but certainly doable with 19th century technology since all the elements required for superalloys like Nimonic had been isolated by the early 1800s.
Comment: @Tim Going from 'elements have been identified' to 'casting and shaping a defect-free structural piece of Nimonic ' is probably a little harder than snapping your fingers...
Comment: [It's not the knowing that's hard, it's the doing.](http://worldbuilding.stackexchange.com/questions/55871/is-there-a-precedent-for-inventing-a-new-technology-many-years-before-anyone-els/55880#55880). At a guess, I'd say the time difference between 'elements have been identified' to 'casting and shaping a defect-free structural piece of Nimonic ' is more along the line of about 120 years. Just slightly longer than a snap of one's fingers.
Comment: Correct me if I'm wrong here, but as far as I remember the mentioned Junker/Messerschmidt jet was developed in the 20th century, thus it doesn't really fit the *19th century* constraint on technology..?
Answer: The difficulty is in generating thrust from steam power. You can build a steam-powered aircraft that uses propellers to generate thrust. That isn't particularly difficult. But making a propeller-driven aircraft supersonic is sufficiently hard that nobody has ever tried it. Every supersonic aircraft has been propelled by gas turbines, or in a few cases, rockets.
You can build a steam rocket: something that burns fuel to heat water to steam and use the expansion of the steam as a rocket. But it's horribly inefficient, because you need the burner system and a boiler, and you just can't get the temperatures that would make a rocket even slightly efficient. You're also throwing reaction mass overboard at a terrible rate, because water is heavy. You'll never get this to cross the Atlantic.
You might manage this with a steam turbine system that drove a ducted fan, which had an appropriate kind of air intake so that the fan wasn't seeing supersonic flow. But the craft will be huge, and terribly inefficient compared to a gas-turbine engine.
Answer: Low bypass turbofans, turbojets or rocket engines are the only ways we currently use to achieve supersonic flight. Propeller driven engines, whether a Brayton cycle turboprop or a WWII diesel powered aircraft cannot generate enough thrust to effectively maintain powered flight past the speed of sound.
Rocket engines aren't steam powered, we'll just leave it at that.
Low bypass turbofans and turbojets work (very simply) by compressing intake air, burning fuel in it to heat it up, sending that hot gas through a turbine to power the compressor, and then exhausting the rest. In a turbofan, the compressor is also used as a propeller, to drive air around the engine (called bypass). Air must be subsonic when entering the compressor to prevent shock waves from propagating through the engine and 'putting out' the fuel burning in the combustion chamber. If you, say, roll through another aircraft's turbulent jet wash, these shock waves can form, as seen in the documentary Top Gun.
That is the science of a jet engine (again, in super brief). There is no way to involve a steam engine in a jet engine. To make a Brayton cycle work, you need a very hot heat source in your combustion chamber, heat transfer from steam will simply not work. You either need an atomized liquid (or gas, maybe?) fuel burning in the combustion chamber, or something exotic like a super-heated solid surface with very high surface area. So, nuclear powered jets are more feasible than steam powered jets.
So the long answer is No, you cannot power supersonic flight with any variation of a steam-based Rankine cycle. However, as stated before, rockets work just fine. If you want von Braun to make his developments in the 1870s instead o the 1930s, that's up to you.
Comment: Could you boil water in a sealed boiler and build up enough steam pressure to launch a projectile at supersonic speed? Steam boiler doesn't need to tag along..
Comment: @Innovine Yes, but I don't think that meets the letter or spirit of the question.
Comment: It was not supposed to be an answer. I was just curious :)
Comment: That is exactly the system Bob Truax devised for Evil Knievel to jump the Snake River Canyon back in the 1970's. See: https://en.wikipedia.org/wiki/Steam_rocket
Answer: Short: No
Longer: Nooooooooo...ooo?
Although such speeds could hypothetically be reached under the right conditions (vacuum, space, etc..) and maybe even on earth herself. The given time constraint of the 19th century makes this a no no.
While we could easily apply our 21st century science to the problem we'd still be missing the materials we so easily have available today that just were not available in that form and quality 200 years ago.
Thus, as already mentioned by others, we can a) not get past the power-to-weight issue, and b) we don't have materials strong enough to survive the forces applied at supersonic speed.
Comment: Are you suggested steam-powered supersonic flight is impossible today? Your third paragraph almost implies current materials technology might make steam-powered flight work, while the fourth paragraph somewhat suggests otherwise even today because our materials aren't strong enough. Perhaps some clarification is needed.
Comment: @a4android applying currentday science could probably yield some results, but in the 19th century we won't have modern day materials available because producing them requires technology that has to be built and provided over time, which requires a shift in society etc........; e.g. you could apply out physics knowledge and what we learnt from building planes, but you'd still be lacking good materials
Comment: Most steampunk technology is infeasible because 19th century materials weren't capable of sustaining their operations. The infrastructure & social organization to make & use them was missing too. I've read enough about Victorian engineering to know how dramatic the transformation the new steam technology was, but flight was impossible then.
Comment: @a4android that's about what I'm saying, yes
Answer: Not feasible.
It might be possible with unreasonably gargantuan structure with near unlimited resources, maybe main steam source would be not coal, but something different, still to stay at steampunk preferred (basically heavy) materials, it is just over the last edge effort. Some "mystic" electric power source might come in picture, but in the end to get energy from steam engine is too heavy for this purpose. So far this is only for "if it can fly". To reach sonic boom it is again a huge milestone, and pretty risky moment for the structure itself.
Answer: With 19th century technology it is clearly impossible. However, saying it is absolutely impossible is inaccurate. You might want to look up Project Pluto, which was to have a nuclear powered Ramjet at supersonic speeds and low altitude. There were also other programs to have nuclear powered aircraft. Now most of those involved air cooled reactors, however having a high pressure reactor could also work.
Major problems from the 19th perspective would be having nuclear reactors to start with (though doing so would explain why steam is being used), heavier than air flight, the theory of ramjets, and the advanced metallurgy required to build such a craft, which was a problem still in the 1950's. Adding the metallurgy to your world could cause a problems of additional consequences.
Answer: A water tube superheated steam generator of sufficient capacity can power a high speed compact steam engine to drive a ducted fan similar to the turbofan. If the steam is in a closed loop, you can get away with less water so less weight. Torque isn't an issue with steam power. there is plenty. weight management with super alloy boiler with light weight insulation can be used. To be honest, I am working on one such project. Not aiming supersonic as of now, but close to it. In short , a fan that works off steam power. check out the DOBLE car boiler system. As for condensing the steam back to water, you have a lot of surface area on the airplane wings. You could have a near silent flight with two ducted fans that are powered by steam engines. It can be definitely done.
Comment: Hello, *Jai Shetty*, and welcome to Worldbuilding.  Perhaps you could include sources to prove your statements.  Please take our [tour] and visit the [help] to learn more about the site.  Have a nice day!


What is the etymology of "moura" in Mourasuchus?

Question: Mourasuchus is a unique species of prehistoric crocodile so far discovered. It is a filter-feeder and has shared its habitat with Gryposuchus and the more fearsome Purussaurus in the Miocene Era --  reptilian counterpart of the Whale shark and Basking shark.
Answer: The original description of the species was given in 1964 by Llewellyn Ivor Price in:

Price, L. I. (1964). "Sôbre o crânio de um grande crocodilídeo extinto do alto Rio Juruá, Estado do Acre". An. Acad. Brasileira Ciên. 36 (1): 59–66.

As such, it seems plausible that the name means 'crocodile of Moura', Moura referring to Rio do Moura, a river in the Brazilian state of Acre where the animal's fossils were discovered.
Answer: Fossil crocodilians from Colombia and the Cenozoic history of the Crocodilia in South America (Langston, 1965) has the original description of Mourasuchus, so that would be the definitive source and contain the etymology. Unfortunately, copies of that book appear to be rare and the text not available online.
But guessing that "moura" is Latin or greek in origin leads to a promising result. Moura is the feminine of mourus, which comes from the ancient Greek for "black," "dark," or "obscure."


Autocorrelation for periodic signals

Question: Autocorrelation for power signals is defined by $$R_x(\tau)=\lim_{T\to\infty}\frac{1}{2T}\int_{-T}^Tx(t)x^*(t-\tau)dt\tag{1}$$ Is it true that for periodic signals $(1)$ can be computed by $$R_x(\tau)=\frac{1}{T_0}\int_{T_0}x(t)x^*(t-\tau)dt\tag{2}$$What are the other ways for computing $R_x(\tau)$? In this question, it's showed that $$|X(f)|^2 =\sum_{n=-\infty}^\infty |c_n|^2 \delta (f-nf_0)\tag{3}$$
But I don't think that's a valid derivation since, as it's pointed by Matt L, it involves product of  $\delta(f)$'s which is meaningless. So is it possible to prove $(3)$ by other means or it's a false statement? And, in the end, can we conclude that for a periodic power signal $$(\mathcal{F}R_x)(f) = S_x(f) = |X(f)|^2 = \sum_{n=-\infty}^\infty |c_n|^2 \delta (f-nf_0)$$
where $S_x(f)$ is PSD, holds?
Comment: Do you think the derivation is wrong simply because of the use of deltas? The derivation is correct...and deltas are absolutely useful as a measure. A "measure" here being how applying a delta to a function and integrating (or summing) yields useful results.
Comment: @Envidia I said that in a mathematical point of view. As far as I know, in the distribution theory $\delta^2(x)$ is undefined.
Comment: So in (3), where is the product between deltas occurring? None of these deltas are overlapping in the summation. The only thing the deltas are being multiplied with are their respective $c_n$ term.
Comment: @Envidia You are right. I was just looking for a more rigorous proof. Intuitively it makes sense as you said.
Answer: The OP states that for a (deterministic) power signal $x(t)$, the autocorrelation function is defined as
$$R_x(\tau) = \lim_{T\to \infty}\frac{1}{2T}\int_{-T}^T x(t)x^*(t+\tau) \,\mathrm dt\tag{1}$$ and then wonders whether in the case when $x(t)$ happens to be a periodic signal with period $T_0$, then it is true that
$$R_x(\tau) = \frac{1}{T_0}\int_0^{T_0} x(t)x^*(t+\tau) \,\mathrm dt ??\tag{2}$$
The answer is Yes, for the reasons given below.
Let us consider the integral $\int_{-T}^T x(t)x^*(t+\tau) \,\mathrm dt$ in $(1)$ when $x(t)$ is periodic with period $T_0$ and $T = nT_0+\alpha$ where $0\leq \alpha < T_0$. Note that the integrand also has period $T_0$.

When $\alpha=0$, the range of integration is over $2n$ periods of the integrand and so
\begin{align}
\int_{-T}^T x(t)x^*(t+\tau) \,\mathrm dt &= \int_{-nT_0}^{nT_0} x(t)x^*(t+\tau) \,\mathrm dt\\
&= 2n\int_0^{T_0} x(t)x^*(t+\tau) \,\mathrm dt.
\end{align}
It follows that
$$\frac{1}{2nT_0}\int_{-nT_0}^{nT_0} x(t)x^*(t+\tau) \,\mathrm dt = \frac{1}{T_0}\int_0^{T_0} x(t)x^*(t+\tau) \,\mathrm dt.$$
The astute reader will have seen that if $\alpha=\frac{T_0}{2}$, the range of integration is over an interval of length $(2n+1)T_0$ (that is, over $2n+1$ periods of the integrand) and so we have
$$\frac{1}{2(n+1)T_0}\int_{-(n+\frac 12)T_0}^{(n+\frac 12)T_0} x(t)x^*(t+\tau) \,\mathrm dt = \frac{1}{T_0}\int_0^{T_0} x(t)x^*(t+\tau) \,\mathrm dt$$
also.
For $\alpha \in \left(0,\frac 12\right)$ (or $\alpha \in \left(\frac 12,1\right)$, the integral is over an integer number of periods (say $N$) of the integrand plus an interval $\beta T_0$ of length less than $\frac 12T_0$ and so
\begin{align}
\int_{-T}^T x(t)x^*(t+\tau) \,\mathrm dt &= N\int_0^{T_0} x(t)x^*(t+\tau)\,\mathrm dt + \Delta
\end{align}
where $\Delta$ denotes whatever the contribution is from the integration of $x(t)x^*(t+\tau)$ over that little interval of length $\beta T_0 < \frac 12 T_0$.
But then we have that
\begin{align}
\frac{1}{2T}\int_{-T}^T x(t)x^*(t+\tau) \,\mathrm dt &= \frac{1}{(N+\beta)T_0}\left[N\int_0^{T_0} x(t)x^*(t+\tau) \,\mathrm dt + \Delta \right]\\
&= \frac{N}{N+\beta}\cdot \left[\frac{1}{T_0}\int_0^{T_0} x(t)x^*(t+\tau) \,\mathrm dt\right] + \frac{\Delta}{(N+\beta)T_0}.
\end{align}
But $(1)$ says that we need to take the limit of the above as $T \to \infty$ which we can do by letting $N\to\infty$ and so we see that in all cases,

If $x(t)$ is a deterministic periodic power signal, then its autocorrelation function
$$R_x(\tau) = \lim_{T\to \infty}\frac{1}{2T}\int_{-T}^T x(t)x^*(t+\tau) \,\mathrm dt\tag{1}$$
can be computed via
$$R_x(\tau) = \frac{1}{T_0}\int_0^{T_0} x(t)x^*(t+\tau) \,\mathrm dt, \tag{2}$$ which is what the OP asks for.

For the important special case when $x(t)$ is a real-valued function, Eq. $(2)$ simplifies to
$$R_x(\tau) = \frac{1}{T_0}\int_0^{T_0} x(t)x(t+\tau) \,\mathrm dt, \tag{3}$$
and this is the only case we consider below.

Note that so far, there has been no whiff of Fourier transforms or Fourier series or Dirac deltas or anything to do with the frequency domain, and so let us bring that stuff in.  Note that a real-valued deterministic periodic power signal $x(t)$ can be represented by a Fourier series
$$x(t) = \sum_{n=-\infty}^\infty c_n\exp(j2\pi nf_0t)\tag{4}$$ where $f_0 = T_0^{-1}$ and $c_{-n}=c_n^*$, and does not enjoy a Fourier transform in the classical sense (where Dirac deltas a.k.a. impulses are not allowed). However, $x(t)$ does have a Fourier transform $X(f)$ in the generalized sense where
$$X(f) = \sum_{n=-\infty}^\infty c_n\delta(f-nf_0).\tag{5}$$
In the classical theory, if $X(f)$ is the Fourier transform of $x(t)$, then $|X(f)|^2$ is the Fourier transform of its autocorrelation function $R_x(\tau)$. Does this result hold in the generalized sense too? That is, for a periodic signal $x(t)$ with Fourier series as in $(3)$ and periodic autocorrelation function $R_x(\tau)$ as in $(2)$, what is the Fourier series for $R_x(\tau)$ and the corresponding generalized Fourier transform of $R_x(\tau)$???
We have that $R_x(\tau)$ has Fourier series $\displaystyle \sum_{n=-\infty}^\infty a_n\exp(j2\pi nf_0\tau)$ where
\begin{align}a_n &= \frac{1}{T_0}\int_{0}^{T_0} R_x(\tau)\exp(-j2\pi nf_0\tau)\,\mathrm d\tau\\
&= \frac{1}{T_0}\int_{0}^{T_0} \left[\frac{1}{T_0}\int_{0}^{T_0} x(t)x(t+\tau)\,\mathrm dt\right] \exp(-j2\pi nf_0\tau)\,\mathrm d\tau\\
&= \frac{1}{T_0}\int_{0}^{T_0} x(t)\left[\frac{1}{T_0}\int_{0}^{T_0} x(t+\tau)\exp(-j2\pi nf_0\tau)\,\mathrm d\tau\right] \,\mathrm dt\\
&=\frac{1}{T_0}\int_{0}^{T_0} x(t)\left[\frac{1}{T_0}\int_{0}^{T_0} x(t+\tau)\exp(-j2\pi nf_0(t+\tau))\,\mathrm d\tau\right] \exp(j2\pi nf_0t)\,\mathrm dt\\
&=c_{n}\cdot \frac{1}{T_0}\int_{0}^{T_0} x(t) \exp(j2\pi nf_0t)\,\mathrm dt\\
&= c_n\cdot c_{-n}\\
&= |c_n|^2.
\end{align}
Thus, for a real-valued deterministic periodic power signal $x(t)$,

$$\text{If } x(t) = \sum_{n=-\infty}^\infty c_n\exp(j2\pi nf_0t), ~~\text{then }
R_x(\tau) = \sum_{n=-\infty}^\infty |c_n|^2\exp(j2\pi nf_0\tau). \tag{6}$$

Finally, what about Fourier transforms in the generalized sense?
Well, we have that
\begin{align}
\mathscr F[x(t)] &= \sum_{n=-\infty}^\infty c_n\delta(f-nf_0)\\
\mathscr F[R_x(\tau)] &= \sum_{n=-\infty}^\infty |c_n|^2\delta(f-nf_0)
\end{align}
and so it is not the case that the formula
$$\mathscr F[x(t)] = X(f) \implies \mathscr F[R_x(\tau)] = |X(f)|^2
\tag{7}$$
of classical Fourier analysis holds unless we are willing to assume that when we multiply out $\left|\sum_{n=-\infty}^\infty c_n\delta(f-nf_0)\right|^2$ into a doubly infinite sum over $m$ and $n$,
$$c_n \delta(f- nf_0)c_m^* \delta^*(f- mf_0) = \begin{cases}
|c_n|^2 \delta(f- nf_0), & \text{if}~m=n,\\
0, &\text{if}~m\neq n,
\end{cases}
\tag{8}$$
holds. In particular, we must treat $|c_n|^2\delta(f-nf_0)\delta^*(f-nf_0)$ as being equal to $|c_n|^2\delta(f-nf_0)$ as stated in my previous answer that the OP has denigrated as invalid and asserted that the claim that, in this context, the square of a Dirac delta should be treated as a Dirac delta, is meaningless because $\delta^2(x)$ is meaningless. The alternative is to insist that $(7)$ holds in classical Fourier analysis only and cannot be applied in the generalized Fourier analysis that allows for Dirac deltas. You pays your money and you takes your choice.....
Comment: @robertbristow-johnson Thank you for reading  my answer carefully and for the edit.
Comment: Thanks a lot for your valuable answer.
Answer: Assuming that the signal $x(t)$ is periodic then it can be described by its complex Fourier series as
$$x(t) = \sum_{n=-\infty}^{\infty }c_ne^{j2{\pi}nf_0t}$$
Where $c_n$ are the complex Fourier coefficients.
There is a  Fourier transform pair that states
$$\mathcal{F}(e^{j2{\pi}f_0t}) = \delta(f - f_0)$$
Applying this to the Fourier series of $x(t)$ element by element you eventually get the Fourier transform $X(f)$
$$X(f) = \mathcal{F}\big(\sum_{n=-\infty}^{\infty}c_ne^{j2{\pi}nf_0t}\big) = \sum_{n=-\infty}^{\infty }c_n\delta(f-nf_0)$$
This is also a common Fourier transform pair.
If we assume that $x(t)$ is also wide-sense stationary then the Wiener-Khinchin Theorem states that the power spectral density $S_x(f)$ is given by the Fourier transform of the autocorrelation $R_{x}(\tau)$.
The power spectral density is then given by
$$S_x(f) = |X(f)|^2 = \sum_{n=-\infty}^{\infty }|c_n|^2\delta(f-nf_0)$$
Take the inverse Fourier transform using the first transform pair above and you get the autocorrelation function $R_x(\tau)$
$$R_x(\tau) = \mathcal{F^{-1}(S_x(f))} = \sum_{n=-\infty}^{\infty }|c_n|^2e^{j2{\pi}nf_0t}$$
Which is the result from the post you linked. You can take this the other way and calculate the autocorrelation first and then take the Fourier transform to yield the power spectral density.
Comment: How can we calculate the autocorrelation at first?
Comment: @S.H.W Use the definition that you provided. Because we're dealing with complex exponentials, there's significant simplification that occurs with the integrand. Try it with a single complex exponential $e^{j2{\pi}f_0t}$ first and then try to form the total sum. Integration is linear, so the operation can be applied to each element of the summation individually which eventually leads to the final sum expression.
Comment: Using $(2)$ I can do that but I don't know how to do that with $(1)$. Can we derive $(2)$ from $(1)$?
Comment: @S.H.W (1) is the more general expression, where as (2) is a simplified version when explicitly exploiting the periodic nature of a signal. In this case either one should give you the same answer. Also keep in mind that there are different convetions with (1): If the limits are taken to be from -T/2 to T/2, then you remove the factor of 2. The expression you have is over a period of 2T, hence the factor of two in the denominator.

Autocorrelation for periodic signals


DEC "CI" bus, better known as?

Question: Looking over a list of various VAX processors, I noticed a reference to a "CI" bus. Poking about in Google I can only find very passing mentions of this, and no details. I suspect this had some other un-acronym name that might work better?
Answer: At a certain point in the 1980s, 'I' things were all the rage in DEC.
BI - Backplane Interconnect (the bus in the 8xxx series VAXen - see Wikipedia)
CI - Computer Interconnect (showed up as star coupler for VAXclusters, also to HSC intelligent storage controllers)
NI - Network Interconnect (ethernet to you).
SI - Storage Interconnect - controller-to-disk.
The point was to define a hierarchy of standardized interconnect structure, with different characteristics of speed, operating area, number of nodes supported, etc.)
Figure 1-3 in this manual shows various interconnects, including the CI.
The DEC Technical Journal, number 5, from 1987, has some CI details.
Comment: That "I" nomenclature was mainly used in architectural diagrams (like that Figure 1-3) - it didn't so much mean it was the name of an actual technology (like "MASSbus").  right?
Comment: There was very definitely one definition of the "CI" protocol, just like "NI" meant "Ethernet" (10 Mb/S at the time) and not just any old network interconnect, and "BI" was one specific backplane technology.  From the point of view of a programmer in DEC, CI was the term for a specific technology.  It showed up in product names too: e.g., the CI780 module that plugged into a VAX 11/780 backplane to allow it to be connected up to a star coupler.
Comment: If you look at the [Software Product Description](http://h30266.www3.hpe.com/PDFs/SPDClusters.pdf) for VAX clusters, you'll see *"OpenVMS Cluster software supports any combination
of the following interconnects:
• CI (computer interconnect) (Alpha and VAX) …."*  so it's definitely an actual thing.
Comment: "just like "NI" meant "Ethernet" - gebus, thanks DEC.
Comment: I think you have a sequencing error. First you decide what busses you need, architecturally speaking - so we needed a local area network interconnect (to replace the point to point DDCMP links that DECnet was previously built on).  So that's your "NI".  Then you make a case that the best candidate for NI is Ethernet (though maybe a 10Mb/s Ethernet rather than the existing 4Mb/s Ethernet, I don't know the timing here).

DEC "CI" bus, better known as?


Web Parts for both 2007 & 2010

Question: I'm curious if anyone has seen any guidance/best practices on developing web parts that can work for both SP2007 & SP2010 with minimal conversion?  They wouldn't take advantage of any new 2010 functionality.  Rather, I'm thinking about forward migration - taking parts written for 2007 and ensuring that they can run on 2010.  Recompilation to target new SP DLLs is OK but I'd rather not maintain two completely separate code branches.
Answer: You don't have to do anything. Your 2007 Web Parts will run in SharePoint 2010 as long as you are not using any of the deprecated APIs (such as the old SSP stuff).
SharePoint 2010 ensures (using assembly redirection) that the referenced v12 DLLs will use the new v14 assemblies. So you do not even have to recompile your stuff. Just deploy them...
Comment: And remember not to use the new Visual WebPart template of Visual Studio 2010, they won't work in MOSS 2007.
Comment: Just to elaborate on Wictors comment regarding compilation: as long as it is code running inside IIS you dont need to recompile. For timerjobs, eventhandlers etc. recompilation is needed
Flo: thats a good point!
Comment: errr, all 3 of you now! :)
Comment: Compatibility is from 2007 to 2010 in most cases (assembly redirection etc makes it work, except as Anders says). But you should never use anything developed for 2010 in 2007. The feature framework has changed a lot, which is not supported in 2007 and there are no reverse assembly redirection.
If you need Viusal Web Parts in 2007, make a manual one using LoadControl or use SmartPart.
Answer: The new Visual WebPart designer is really just a "pattern" that makes it easy.  You can use the exact same code + ASCX method with a WSP project today - for 2007.   It's just .NET code that you load the control.
The only bit you DON'T get is the actual visual design / drag+drop within Visual Studio.
Another thing to note is that .NET 4.0 is what would be targetted by VS-2010 - whereas you should remain at .NET 3.5 for SharePoint 2007 AND 2010.

Web Parts for both 2007 & 2010


How do ECMP and OSPF collaborate within Cisco's MSDC network?

Question: Reading the MSDC design guide, it was indicated that OSPF is used for the routing and ECMP for supporting equal cost paths.
Who does the actual routing, is it OSPF using ECMP or the other way arround? And how are they actually interacting?
And it is indicated that the routing decision is made in the leaf node. Does that mean that the routing protocole is being executed only in the leaf nodes?
Comment: Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
Answer: All that we mean by "ECMP" is literally "Equal Cost, Multi Path".  The router installs two same-cost paths to a given next-hop, and they are used equally.
OSPF makes the routing decisions for the network based on its LSA database.  If there are two paths that have the exact same cost from OSPF's perspective, it will consider them both "best".  The router installs both equal-cost paths into the routing table, and the concept of "ECMP" occurs.  Half of all flows will be sent one way, half go the other.
One of the benefits of EIGRP is that it supports unequal-cost multipathing via the "variance" command.  OSPF, IS-IS, and all other IGP protocols I know of in the Cisco world only do equal-cost multipathing.


apache http https redirect server still listenting on port 80

Question: I have an application that is using Apache. I have install the mod_ssl module for Apache, configured an SSL certificate and because the application provides such function configured http to https redirects but the server still is listening on port 80 (on 443 as well). Is this a security concern?
Answer: It depends on the specific setup if this is a security concern or not. If any access to port 80 only results in a plain redirect to the https:// site on port 443 with no cookies set (or at least no sensitive cookies) then there should be no security concern. If instead it is possible to reach different actions by accessing some page within the plain http:// site  or if the redirect to https:// already sets a sensitive session cookie or similar then there is a problem since this cookie is transferred unprotected.
Answer: No - this is just Apache behaving as told.
Apache can't redirect from http to https except by listening for http requests on port 80. @Steffen Ulrich's answer gives some more hints on verifying everything is on the up-and-up, but I see no reason to suspect this is anything more than Apache doing exactly what it is supposed to do.
If Apache wasn't listening on port 80 you wouldn't get your redirect - instead you would get no response at all and a site that appears broken.


how do I clean 4 qts of 10W30 out of the cooling system,

Question: By mistake, I put 4 qts of 10W30 engine oil in my radiator.  How can I clean it out?  I've seen suggestions for using dish washing detergent. Is that safe? I am thinking of using DAWN because it emulsifies oil.  What do you think? I have a 2000 Toyota 4 Runner 3.4 L 5 speed.
Comment: Welcome to the site. I'd *highly* recommend you *not* use DAWN dishwashing liquid to clean out your engine. While it would ultimately break down the oil, it will also foam to no end. This will cause you huge issues in the end. Foaming (to any degree) will critically degrade the cooling system which relies on contact of the liquid to the metal to transfer heat. Also, once the Dawn is in the system, what are you going to use to get it out? Also, running the engine while pouring water into is a very bad thing to do ... great way to warp a head, especially aluminum heads.
Comment: Interesting. I wonder how well the oil cools. Run it as an experiment, use radiator flush kits to flush it out. There is a release valve below the radiator. Add ample water and flush agent in the radiator as the car is running and also, add the same to the reservoir. Use distilled water the whole way.
Answer: Check out how to flush your system on this SE question : How do you clean out blue devil (sodium silicate/liquid glass) out of your cooling system?
DO NOT add DAWN washing up liquid - this used to be added to the fountain in the square in Coventry by students and would put foam down the street for a hundred yards... :)
Flush the system and you can use tap water for the flushing, but once you are done use the correct fluid / water mix and run it and keep an eye on it.
I don't think you want to use your vehicle as an experiment, unless you have a spare...
Do note that some racers do add oil to the cooling system, at least in the past, to reduce friction form the pump but only 10 or 20ml...
A quote from the link (to prevent link rot...)
If it is not sticky, then you can use a garden water hose, as long as you have a better than good flow rate, fit the hose to the bottom rad hose and feed the output from the top hose into a large bucket.
Use the full flow rate but it may be easier to remove the thermostat - otherwise you have to have the engine running hot to open the 'stat... Even if you remove the 'stat then it can help to have the engine running but it won't get hot.
Also, do the same for the rad...
If it is sticky then you may consider a tank and external pump to do the same thing, oh and be VERY careful if you add a chemical to help remove the "sludge". Make sure it won't kill the seals.
Answer: The reason for dishwasher soap is because it will not foam up. If you already ran your engine you will need to flush it a few times most likely. If you didn't run you engine. You could remove your lower radiator hose and flush out the radiator.  But will most likely have oil residue in the coolant no matter what. You'll just need to get as much of the 4qts out. Good luck.
Answer: I know these engines (Toyota 3.4L), so I would suggest you to remove radiator (4 bolts, 2 from each side, 2 hoses to engine, no transmission oil cooler, as it is manual as I understood), and just run tons of water through it. Also remove thermostat (3 bolts, easy to remove) and run tons of water through upper coolant hose. This way you will make sure, that no harm will be done by chemicals. After that I would just use distilled water and change it couple of times in 10, 50, and 100 km intervals, until you see, that it does not contain any oil.


Disable stack protection on Ubuntu for buffer overflow without C compiler flags

Question: I'd like to try some shell codes and I want to disable linux protections.
I know I could compile using flags but I know another way exists to disable these protections in general I just can't remember. Can you help me?
Answer: To expand on what vonbrand has (correctly, +1) said, there are two parts to Linux's stack protection.
Stack canaries
Stack canaries are the compiler-enforced feature vonbrand refers to. These can't be disabled without a recompile.
To prove this to yourself and see how they work take the following code:
#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>

int mybadfunction(char* a_bad_idea)
{
    char what[100];
    strcpy(what, a_bad_idea);
    printf("You passed %s\n", what);
}

int main(int argc, char** argv)
{
    printf("Tralalalaala\n");
    mybadfunction(argv[1]);
}

Now compile that (gcc -fstack-protector -masm=intel -S test.c) into something gnu as would be happy to assemble and read the output. The important point is that on exit from  the mybadfunction function, there is this little piece of code:
    mov edx, DWORD PTR [ebp-12]
    xor edx, DWORD PTR gs:20
    je  .L2
    call    __stack_chk_fail

As you can guess, that's taking a stack cookie from [ebp-12] and comparing it to the value at gs:20. Doesn't match? It then calls a function __stack_chk_fail in glibc which kills your program right there.
There are ways to work around this in terms of writing exploits, but the easy way in terms of building a shellcode test case is to compile your program with -fno-stack-protector.
Non-executable pages
There are some other considerations on modern Linux systems. If you take the usual shellcode testing stub:
char buffer[] = {...};

typedef void (* func)(void);

int main(int argc, char** argv)
{
    func f = (func) buffer;
    f();
    return 0;
}

modern GCC/Linux will map the .rodata section of the PE file read only with no execute permissions. You need to turn that off, which can be done using the code sample from this blog post. Basic idea: you use mprotect to add the permissions you want to the pages in which the shellcode data resides.
Non-executable stacks
If you are going to test a traditional exploit scenario, e.g. my bad code above, with your shellcode then you also need to ensure the stack is executable for the simple cases. The PE file format contains a field for determining whether the stack is executable — you can query and control this with execstack. To enable an executable stack, run
execstack -s /path/to/myprog

This can be done on arbitrary programs without needing a recompile, but won't automatically disable stack canaries as these are baked in on compile.
Added bonus: aslr:
To turn that off, echo 0 > /proc/sys/kernel/randomize_va_space.
Did you just tell someone how to exploit my precious penguin?
No. Any exploit must work around stack canaries (very much non trivial) and either find a program with execstack set, or set it (meaning it can already execute arbitrary commands anyway) or else use more difficult techniques, such as return to libc/return orientated programming.
Answer: Stack protection is done by the compiler (add some extra data to the stack and stash some away on call, check sanity on return). Can't disable that without recompiling. It's part of the point, really...
Comment: ASLR requires the OS to do it at runtime. NX bits also require system support. What part can't be disabled at runtime?
Answer: You can disable some protections (stack smashing detection and making the stack executable) with these options.
--z execstack
-f no-stack-protector

You can, also, turn off ASLR (address space layout randomization) with Bash with the command:
echo 0 > /proc/sys/kernel/randomize_va_space


How Do I Make A Trace Effect Similar to Tron: Legacy Light Cycle Scene

Question: I am trying to make an effect similar to the effect in Tron: Legacy in the light cycle scene. I am aiming for the effect found from 0:04 through 0:12 in the video I linked. I feel like using a wireframe would be good but I don't know where to go from there.
Link: https://www.youtube.com/watch?v=3GBLtnxxtzQ
Answer: Edit: Cegaton already answered a very similar question here.
You'll find a lot of tutorials on Youtube about light streak effects, there are a lot of ways, it really depends on how it is supposed to look like and how sophisticated you want it to be... I guess in the Tron movie they use several combined effects so even if I knew it would be hard to sum it up in few words. One of the most basic way to make a light streak could be this one:

Create a curve, create a second one that will be its Taper Object (its profile from its start to its end).

In the Properties panel > Data, play with the parameters to give your curve its thickness and its profile. The taper allows you to make the streak slightly disappear at its end.

Play with the End value (and also maybe its Start value) and give it some keyframes to animate.

Now in the Node Editor > Compositing mode, give a blur effect or else to give the glow effect.