Best way to play as low level in the city full of enemy?

Question: i just started to play ingress. In my town, there are 14 portals and 12 are maintained by Enlightened. I am Resistance and there is no way to fight against them, I was trying to destroy resonators, but with no luck. Resistance is here at a disadvantage.
What is best way to get portals, destroy resonators of level 3 and more, when I am level 1 or 2? Do I have any chance? What is the best strategy?
Answer: You should be able to grind your level up to 2 pretty easily just by hacking enemy portals for 100 AP each time, which is tedious, but past that you're going to have trouble and get bored with 100 AP at a time.  Here's a list of things that earn AP - I know most of it is impossible to do at low levels.
You're really not going to be able to do much solo at low levels, so the best option I can think of when you're starting out low level and at a team disadvantage in your area is to organize your local faction and destroy enemy portals in groups.  Get at least a couple other people together and attack an enemy portal that is well linked (you'll get more AP for destroying links and fields), low energy and in a location that will allow you to link to the few portals you do control (you get even more AP for creating links and fields than destroying them).  It might be hard to find the combo of the those, so take what you can get, but work together.
If you can't find people local at a higher level to help, you might try asking high level players from neighboring towns.  Some people are pretty into this game and will take long field trips to help take over outlying regions.
If your opposition isn't very active, you can let your portals decay so that your links and fields disappear, and then relink for a bunch of points.
Finally, a couple options that requires vigilance:
Submit portal suggestions and check to see if they or other portals in your area get added, then be the first to them when they do.  You'll need to check http://www.ingress.com/intel and zoom in until it says "Showing all Portals" to be able to see the new portals when they're added.
Another option is to watch your email like a hawk for portal destruction notifications  when the enemy destroy a few of your resonators on the few fields you do control, then as with the decaying strategy mentioned before, swoop in and recharge, redeploy and relink your portals.  You can do the recharging from a distance if you have the portal key, which will buy you some time to get there to redeploy and relink.  I find it helps to leave one low level resonator in an easy to destroy location (like on the sidewalk), and try to place a least one higher level resonator in a hard destroy location (like inside a building).  Then you should get notified of the lower level resonator getting destroyed hopefully before they can get to the higher level one.
Comment: I'd like to mention the the decay rate has been increased from 10% points per day to 15% points per day. This makes is more difficult to maintain a lot of portals and you should encounter weakened enemy portals more often.
Answer: What I did-
I too started in a highly enemy controlled area.  Simply hacking portals is a very slow grind, and not usually worth the gas/time to move around.  However I did need to do this at first to stock up on supplies.
Once I had plenty of lvl 1 supplies, I went home and got onto the ingress.com/intel page and zoomed into my area.  I zoomed in until it said "showing all portals" and SCOURED the map one screen at a time.  This way, I found about 6 portals that nobody had touched.  Swoop in, deploy 8 resonators, move on.  Also, I paid attention to the intel map, clicking on enemy portals to see the resonator levels.  Keep them in mind and check them over several days, and you should be able to find some that nobody is recharging.  When a portal was badly decayed (10% energy or less on all resonators)  I would swoop in, and start firing off bursters while standing on top of the resonators.  Even at lvl 1, the xmp bursts will take out resonators if they are low enough.  You may have to use 20 or 30, so make sure you're stocked up.
It's like any other scavenger...  Look for the weak and unprotected ones, and go for the kill!  Even as a lvl 1 I took out 2 different lvl 4 portals, one percent at a time!
4 days later, I'm lvl 4.  War of Attrition.
Answer: Use the table in  What is the level significance in XMP bursters and resonators? to figure out how many XMPs you need to take out a lower level portal.
Find a few portals with several control fields around them, wait for one to decay if necessary, and then attack it. If you destroy enough resonators, so that the lowered range unlinks the portal, you gain massive amounts of experience compared to grinding by hacking. In a city dominated by enemy portals, it's likely they cannot keep all portals charged all the time, so finding a decayed portal should be reasonably common.
XP gain:

187 * broken links + 750 * disabled fields

I jumped from just over lvl 2 to just below 4 during a weekend where I was lucky enough to find several badly-decayed portals with multiple links around them.
Answer: first you need to level up, before trying to destroy enemy portals.
you can feed off them for a while by hacking them.
the bigger the portals you hack, the bigger items you'll get. (limited by your own level)
once you have enough XMPs and are at least level 3, attack a small green portal first, small level ones without too much protection.
keep tabs on green portals regularly, as the others may forget to recharge some of them, and attacking portals that are already decayed is cheaper in XMPs.
good luck.
Answer: I grinded to lvl5 in a few days in a city. my advice is to hack hack hack. the points are low, but the real value is gathering the portal keys. a lot of times players run out of keys, time, whatever and leave loads of free links and fields to make for those who hold the portal keys. also make sure to look out for dropped items on the map and scoop them up.
look out for open resonator slots and fill them, or upgrade the lower level ones (remember to only deploy resonators when it's at the edge of your action circle to make increase the radius between resonators)
remember you can stash keys in a capsule to help you hold enough to make a bunch of quick fields in one pass before the high level opponents come and level you. don't bother wasting high level resonators, just plant some lvl 1s, quickly link and create your fields and bounce. think guerilla warfare until your around level 8.
lastly make Very Small Fields. the smallest possible. if you shoot links across town you will be torn down quickly. it's annoying, and it makes it harder for everyone to play, including your own team/faction/wutevr


is it necessary to use after in the case below?

Question: Is it correct to say: It is just five o'clock when the time is 5:03
OR It is correct to say: It is just after five o'clock ??!
is AFTER necessary??
Answer: Both are grammatically correct, but the difference is in the semantics.
The former means that the time has just reached five o'clock, or that the time is only five o'clock.
The latter means that it is a small amount of time after five o'clock.
Both may be valid in your situation depending on context, though I get the feeling you mean the latter.
Answer: I don't think this is really a language question; it's a precision question.
In many situations, a certain level of imprecision is quite acceptable:

"The party's at seven, should we start getting ready?"
"Oh, no, it's only five o'clock."

This is fine anywhere between around 4:55 and 5:05 or so - it's entirely subjective.

"Oh no, am I late? What time is it?"
"It's just after five o'clock."

This is also fine giving an approximation of the time.

"All right, I'm calling it. Time of death?"
"Five-oh-three, doctor."

This is a situation where you really want the precision.
In general, in most situations you don't need to state the exact time. There are a lot of different ways to say the same time in English, and not much consensus about which is 'correct'. 5:15 can be said as:
Five-fifteen;
A quarter past five;
A quarter after five;
A quarter of five;
Seventeen-Fifteen;
...among others. All are correct, and generally a matter of personal preference or, in some cases, organisation policy.


Surge Protection for MIL STD 704A

Question: I have to protect a switching DCDC converter for MIL STD 704A standard especially overvoltage 70V 20ms and 80V 50ms. TVS diodes does not protect because they protect for voltages <1ms. Do you have any suggestion about surge protection circuit?
Comment: What about a varistor or MOV?
Comment: There are specialized "surge stopper" IC that can be used for these kinds of applications. https://www.analog.com/en/product-category/surge-stopper-overvoltage-overcurrent-protection.html
Answer: For military and space applications we use surge suppressors just downstream of our power connectors to provide over voltage, under voltage, reverse voltage, and current limiting protection. An example of one is the Analog Devices LTC4364 (https://www.analog.com/media/en/technical-documentation/data-sheets/ltc4364-1-4364-2.pdf):

With the right mosfets, the LTC4364 can block up to 100 V continuously, and higher voltages for a short amount of time. It can be configured to either shut down the load in the event of a surge, or the gate voltage of the mosfet can be modulated to allow some voltage though to keep the load powered (typically for a short period of time). As an added bonus, you can set the timer capacitor (CTMR in the figure) to slowly ramp up the gate voltage. This allows you to control the inrush for large capacitance loads.
One thing to note, the LTC4364 seems to have a boost converter inside it to provide voltage to the mosfet gates (it's not mentioned in the data sheet). We've failed conductive emission tests because of this converter, and I'd recommend putting a ferrite bead between the TVS diodes and Vcc of the surge suppressor.

Surge Protection for MIL STD 704A


How to create a views of channel

Question: In about us, i added different data and I want to display it in view.
How to create a view of our about us channel.
I newbie in expression.can we use the template in it
Comment: You need to use templates here. Can you let me know with version of EE you are using?
Answer: Expression Engine is a powerful CMS that has gives you great flexibility when it comes to displaying content on a web page.  However one consequence of this flexibility is that you need to define how any / all information is presented - there are no 'default views' to work with.
The mechanism for defining these views of your content is through the creation of page 'templates' which combine instructions for how / what data to be retrieved from the CMS with HMTL information that defines the web page(s) that will be used to display it.
The way in which these templates work, and how you build a site with them is beyond what can be done in an StackExchange answer... however the EE Documentation contains two resources that might help.
First there is a description of how the EE system 'works' which is helpful background.
Second there are some worked examples of how you go about creating a simple EE site - which covers much of what you need to know to get started.
HTH

How to create a views of channel


Is it possible to make approve and transferfrom in single transaction?

Question: I know that owner should call approve in advance then allowed spender call transferfrom to make transaction. We need two steps(tx).
Iam curious is it possible to make it by single tx! single step.
Comment: Does this answer your question? [Is it possible to batch approve + transfer an ERC-20 token?](https://ethereum.stackexchange.com/questions/79653/is-it-possible-to-batch-approve-transfer-an-erc-20-token)
Comment: Depending on your use-case, you might be able to use an ERC-223 token: https://ethereum.stackexchange.com/questions/31643/erc223-gas-savings
Answer: The purpose of the approve() -> transferFrom() flow is that you want a different account to do the transfer than the one that is allowed to approve it. If it's a single account, you'd normally just do a transfer().
If you wanted to make both these in the same transaction you'd need the approver and the approved user to both be contracts, with logic that allowed them to be called from as part of a transaction from a third contract. You could send a single transaction to the third contract, and that would tell the owner contract to call approve(), then tell the approved contract to call transferFrom(). It's not immediately obvious what use-case would be served by a setup like this, but it's technically possible.
Comment: "You could send a single transaction to the third contract, and that would tell the owner contract to call approve()"

Who must be "YOU"? Is approver(owner) can be YOU?
Comment: And also is it possible contract makes EOA call function?
Comment: And also is it possible contract makes EOA call function?
Answer: Yes, this is possible, but token holder has to be a smart contract, e.g. wallet.  Such contract should implement a function similar to this one:
function approveAndCall (IERC20 _token, address _to, uint256 _value, bytes memory _data)
public payable returns (bytes memory _returnData) {
  require (_token.approve (_to, _value));
  bool success;
  (success, _returnData) = _to.call.value (msg.value) (_data); // Supposed to call transferFrom
  require (success);
  require (_token.approve (_to, 0)); // Wipe out any unspent allowance
}

Comment: wallet is not a smart contract right?
Comment: Terminology is not strict, but here by "wallet" I meant wallet smart contract, either multisig, or with single owner.
Answer: I kind of had the same question but in a slightly different scenario:
I wanted to make a "burn4token" contract where an user burns one ERC1155 collection's token in order to mint another in a different ERC721 collection; all in the same transaction but I had to get my "burn4token" contract approved as operator in order to do this;
I sorted it out changing my flow
I implemented the IERC115Receiver interface into my ERC721 contract; then in the "onERC1155Received" hook I put a burn and mint steps; So now the user can transfer an old ERC1155 token to the ERC721 contract and get a new nft :)
I hope this gives you ideas!
function onERC1155Received(
        address operator,
        address from,
        uint256 id,
        uint256 value,
        bytes calldata data
    ) external override(IERC1155Receiver) returns (bytes4){
        //burn
        (bool burned,bytes memory burnedData) = address(erc1155contract.address).call(
            abi.encodeWithSignature("burn(address,uint256,uint256)", address(this), 1, 1)
        );
        //you can use this to handle or print errors
        require(burned, string(burnedData));
        //then mint here
        mint(1);

        return bytes4(keccak256("onERC1155Received(address,address,uint256,uint256,bytes)"));
    }

It works because after the user sends the ERC1155 to the 721 contract; the 721 contract calls the ERC1155 contract burn function as the owner of the token so no "setApprovalForAll" is required


Superlative + noun + "possible": why does the adjective come after the noun?

Question: Does someone happen to have an explanation or theory for why in phrases like "the best method possible" the word 'possible' comes after the noun?
Comment: I have no theory, but this also works with a few other adjectives: *the sexiest man **alive***, *the best data **available***, *the most bizarre life **imaginable***.
Answer: In English, adjectives normally precede their head, but other kinds of modifiers can follow:

the tallest person

but

the baby [which was] born on 11 June

(the 'which was' can be omitted).
A single adjective modifier cannot usually be placed afterwards, unless there is another modifier with an identifying function, in which case even a single adjective can move after:

the last man standing
  the only site remaining
  the best method possible

I think the construction requires that the two modifiers together are required to identify the individual (i.e. neither "the last man" nor "the man who is standing" would narrow it down). If the adjective precedes its head, as Sean says, it does not necessarily have this narrow meaning.
This doesn't answer "why" — few questions about language that ask "why" can be satisfactorily answered — but it gives some context.
Comment: What difference in meaning do you see between *the only site remaining* and *the only remaining site*? To me they are interchangeable. I'm trying to imagine an actual situation where I could use one, but the other would be iffy or wrong.
Comment: Hmm. I don't think "I think the construction requires that the two modifiers together are required to identify the individual" is right. For one thing, *only* in *the only site remaining* does not help identify the individual; rather it emphasizes that there are no others, which is already at least suggested by the definite article *the*.
Comment: @Jason Orendorff: as regards "identification", I think we are using the word in slightly different ways. I'm not talking about whether the sentence singles out an individual, not whether the sentence helps the hearer know which actually is the individual. In any of these, the speaker is implying that there is a particular individual that is identified, but in all of them the hearer will not know which without extraneous knowledge. As for the difference in meaning before and after, I'm not sure that there is always a difference, as you suggest.
Comment: I think you're saying: "the form 'superlative + noun + *possible*' implies that the phrase identifies a particular individual", but actually the superlative by itself is enough to imply that. In fact, the definite article *the* with a singular noun usually suffices.
Answer: In all of the examples given, the first adjective is either a comparative or an adjective like only or single, which restricts the number of possibilities to one, and it is possible to put a restrictive that is or who is in between the noun and the following adjective.

the best method (that is) possible
  the sexiest man (that is) alive
  the best data (that is) available
  the most bizarre life (that is) imaginable
  the last man (who is) standing
  the only person (who is) present
  the single feasible option (that is) remaining

What is happening is that a very short clause is being shortened to a single word.
This doesn't explain why you can't say the only apple red, since the only apple that is red works fine. English has a lot of different semantic classes of adjectives (see adjective order) and my supposition would be that it only works for some of these classes. It certainly doesn't work for size, color, or age adjectives.
Comment: Good point about the 'exclusivity' of the *first* adjective allowing/requiring the *second* to follow the noun. But whilst I can't go along with @Sean Owen's 'possible' distinction, I have the distinct feeling there might be a usage where [optional] placement of the second adjective before/after the noun really does affect the meaning.
Comment: ...(I'm not nailing my colours to the mast on this one, but) might not **The earliest usage undisputed** imply *more controversy* than **The earliest undisputed usage**?
Answer: I do not think we have much evidence that the position of these adjectives is semantically determined. A few observations:

Short adjectives/participles are less likely to come after the noun. — [Cf. *"the only apple red was ...".]
It sounds a bit old fashioned with certain modifiers that nevertheless cannot come before the noun. — ["Apples so red that..." v. *"so red apples that..."]
But it sounds neutral with other, similar ones. — ["Apples this red are rare."]
I cannot think of an adjective-without-modifier that could occupy both positions but not in the same sentence.
In absolute constructions, the participle or adjective must come late, or it would sound archaic. — ["Her father out of town, she decided to clean the house." / "Her father dead, she left the city." / "Her father brutally murdered, she..." / "Her father still holding a grudge, she decided..."]
Where the adjective is modified by an adverbial constituent, late position seems compulsory, even with adjectives that cannot normally come late. [Cf. *"the fresh from the market apples".]

On this I base four tentative hypotheses:

As an adjective has more verb-like characteristics, i.e. it behaves in some ways like a participle or relative clause, it is more likely to come after. The English/Latinate suffix -able/-ble is a good example. Adverbs have an inherent verbal aspect as well: they are unfit to modify a noun unless we imagine some implicit verb to carry them ("the people [being / who are] here are insane"). I could speculate and suggest that it might have something to do with a predicative function on a subordinate level.
Late position may be (partly) motivated by (syntactical) balance inversion; that is, longer adjectives are more likely to come after the noun.
If an adjective has a modifier that comes after it, the adjective phrase must come after the noun. This is probably related to balance inversion. — ["Apples fresh from the market."]
There is a limited class of inherited expressions with fixed late position, like Secretary General and heir apparent. It may be that those were all formed under the influence of foreign languages, such as French, where late position is the norm, or it may have been more common in older English, or both.
Comment: An excellent analysis, that goes beyond my own. I don't think that absolute constructions belong here, though because they are not NP's (not sure what they are in current theories). And I think your no 4. is very marginal: for many speakers, they are chunked and no longer contain a post-modifier.
Comment: This is really nice. Hypothesis #3 is confirmed by *CGEL*. Modifiers with post-head modifiers of their own do not occur before nouns in NPs, except arguably in some informal cases where the modifier gets strung together into a bespoke compound word: *fresh-from-the-market apples*.
Comment: I don't think we have to "imagine some implicit verb" in the case of adverbs, like *the people [who are] here*, any more than in the other cases, like *the best method [that is] possible* or *apples [that are] this red*. We can imagine all sorts of things; but to me *here* is just a modifier following *people* in an NP.
Comment: Regarding "both positions but not in the same sentence": The adjective *proper* has different meanings before and after a noun, so maybe that sort of fits the bill? *That neighborhood is part of the city proper* vs. *That is not proper behavior*.
Comment: It has nothing to do with shortness of the adjectives. *The tastiest apple red* doesn't work because this construction doesn't work with color adjectives. *The tastiest apple ultramarine* doesn't work either.
Comment: I think adding negation gives the second adjective more scope to 'slide around'. Unlikely, but grammatically, I feel that *tastiest apple* could be *uneaten*, or *unsold* in either position, for example.
Answer: To me, "the best method possible" means "among the methods that are possible, the one which is most good." I assume you are comparing to "the best possible method" which means to me "the method which is as good as any method can be".
If I want to get to Australia quickly, the best method possible is probably an aircraft. The best possible method is a teleporter.
Does anyone agree with that? I must say both are nearly interchangeable to my ears.
Comment: I don't agree with that. They sound equivalent to me.
Comment: To me "best method possible" is less definite than "best possible method".
Answer: My speculation would be that in the pattern the superlative + noun + "possible" the word order is reversed on purpose to emphasize the possibility.
For example, This is the best photo possible emphasize, that a photo is the best that can be produced by any means; better would be impossible. This is the best possible photo mean almost the same, but without emphasis on "possible" it does not sound same.
Answer: A superlative requires a definition of the class referred to; the tallest boy in the class is not usually the tallest boy in the school.  Word order matters in that it attaches the definition to one of the other phrases; the method I know best is not the best method I know.  (The method best I know is ambiguous, and so could only be used poetically, if at all).  With single word definitions it doesn't usually matter whether the noun or the adjective is being qualified, so the best possible method is acceptable, though unusual; when the definition expands it is often confusing, and so wrong, to put it before; 'the best method possible in a Euclidean universe' can't reasonably be rephrased.
Comment: I think OP is focussed on single-word adjective that can follow (and maybe also precede) the noun, so adjectival phrases like *[who is] in the class*, or *[that] I know* don't really count.


Do any of the Babylon 5 cultures use matter/antimatter power sources?

Question: To my knowledge and memory, I don't recall humans or aliens (with the exception of the invaders from the tv-movie Thirdspace) on Babylon 5 making use of force fields or deflector shields so I don't see how they could safely contain antimatter for use as a power source- has this ever been discussed or mentioned on the series?
Comment: Most importantly Babylon 5 is not about technobabble. Your post is a false premise but a correct conclusion.   Vorlons and technomages have force fields. Also regular magnetic fields can suspend antimatter.  Surely the Shadows have almost anything that is technologically possible.  That said I have a vague recollection that there are claims of zero point energy being used by the first ones but that may be from the technomage trilogy which is severely out of continuity garbage.  Fusion generates plenty of energy regardless.
Comment: Ulkesh appeared to have a force field, or something like it, around his encounter suit when they tried to attack him.
Comment: There was a kinda forcefield used in the first season the barrier created by Jason Ironheart
Answer: In a word, no.
The terms "antimatter" and "anti-matter" aren't used in any of the episode transcripts for Babylon 5, nor the spinoff movies or Crusade TV show.
It isn't used in any of the interviews on Midwinter.com, nor does JMS use the term in any of the AOL chats that he did during the time it was airing.
Additionally, the licenced RPG doesn't make any reference to anti-matter.


Bathroom plumbing -Is it right?

Question: My 1940 bathroom plumbing was just replaced and I am questioning whether the set-up is proper. Here is a picture. Long 2" pipe at bottom is for the bathtub/shower, fixture at top left is the toilet and then sink at the pipe on the wall. Sink connects into the main vent stack which the kitchen fixtures and an upstairs bathroom connect to as well. The main vent stack starts in this bathroom at the right of the bathtub drain. So sink has to drain into the vertical vent stack and around past the bathtub connect, down to the bottom 3" horizontal pipe then into the main drain stack that the 3" connects into. The sink is the only fixture in the house that drains into that vent stack.
Is this right?
Answer: That's fine, at least under the codes I work with, since you havn't specified your applicable code. Probably fine, period.
The vent stack is a dry vent above where the sink connects, meaning nothing drains into it. Below that point it's a "bathroom group wet vent" and correctly picks up all the fixtures, with the toilet being last in line. This is explicitly allowed in code.
For example, Section 912 of International Plumbing Code.
Comment: What about that T from the really long capped pipe? If I'm reading the question correct, that is a shower drain line. I thought Ts could only be used in vents and all wet lines needed to be wye's.
Comment: Hadn't noticed that, was mostly addressing the vent concern. Yeah, that's not right.

Bathroom plumbing -Is it right?


How to get steam icons in the Ubuntu dash? Specifically numix

Question: I installed the numix icon pack and noticed the really nice steam game icons (Seen here: http://me4oslav.deviantart.com/art/Numix-Circle-Linux-Desktop-Icon-Theme-414741466). How can I get icons for my steam games in the dash? When I click create desktop shortcut in the game properties it just makes an icon on my actual desktop, kinda lame. Or are the icons for these desktop icons? Thanks for any help, sorry for a confusing question. I'm on Ubuntu 14.04.
Comment: You may be able to drag and drop the desktop icons to the dash.
Answer: If you have activated the PPA and installed the package numix-icon-theme-circle via
sudo apt-add-repository ppa:numix/ppa
sudo apt-get update
sudo apt-get install numix-icon-theme-circle

then the icon theme is already installed in your system. Enable the icon theme if not already done so.

You can find the icons in the folder
/usr/share/icons/Numix-Circle

You could check the installed files/icons with the command
dpkg -L numix-icon-theme-circle

Search a concrete icon with, E.G.
find /usr/share/icons/Numix-Circle -iname '*minecraft*'

or
dpkg -L numix-icon-theme-circle | grep -i minecraft

To use an icon, you need to edit the corresponding desktop file for the game / program. Because the files are located on your desktop, open a terminal via Ctrl-Alt-T and edit the file with your preferred editor, EG
gedit ~/Desktop/minecraft.desktop

or
nano ~/Desktop/minecraft.desktop

Find the line that starts with
Icon=some_icon

and replace it with, EG
Icon=minecraft

without any extension and/or path.

That's all.


Why does the second Weyl scalar describe electromagnetic radiation?

Question: I've been reading about the null tetrad, the Weyl tensor, and the Newman-Penrose identities, and so I found out about the Weyl scalars. While the zeroth, first, third, and fourth scalars describe ingoing and outgoing transverse or longitudinal waves, the second ($ \Psi_2$) describe electromagnetic radiation? Why not something completely different?
Comment: I've never seen such statement, could you provide a reference? Additionally it appears that the statement is wrong. Consider the case of Schwarzschild spacetime.  It is well known that there exists a null tetrad such that all Weyl scalars are null except $\Psi_2$. So we have no electromagnetic radiation and non zero second Weyl scalar. In fact, since $\Psi_2$ decays as $\lambda^{-3}$ (where $\lambda$ is the affine parameter of a null geodesic) it cannot contain information on radiation of any kind, gravitational or electromagnetic.
Comment: I've taken a look and McMahon gives zero reference to the connection between $\Psi_2$ and electromagnetic radiation. As for the latter, the Nariai (is a typo in the book) is the Schwarzschild-de Sitter metric, a black hole in a universe with positive cosmological constant. Since the solution is static there can be no electromagnetic radiation of any sort. In fact by being a solution with zero stress-energy tensor it cannot have any kind of electromagnetic field. This statement in the book is wrong. I'll try to post a detailed answer on the Weyl scalars latter
Comment: It was mentioned in the book Relativity Demystified (by David McMahon).
Comment: It is worth noting that the Wikipedia page on the Weyl scalars mentions an interpretation by Szekeres of $\Psi_2$ as a "Coulomb term", which could mean something completely different - or something very similar. Unfortunately, I haven't been able to et to the original paper.
Comment: I could interpret that to mean something like telling you about the total mass causing the curvature (see the Schwarzschild case, for instance, which is characterized only by the mass), the way the Coulomb field is characterized by only the total charge of the point charge.
Comment: So would you agree or disagree with the physical interpretation? By the way, McMahon mentions later on the Narai space-time, which he says is characterized by no matter (it's a solution to the vacuum equations but with a positive cosmological constant) but electromagnetic fields, because the only non-zero Weyl scalar is $\Psi_2$. Unfortunately, I can't find the paper proposing that.
Comment: McMahon says that $\Psi_2$ represents electromagnetic radiation on page 294, but nowhere else in the book. Thank you for looking into it.
Answer: For the benefit of brevity I'll give a straight answer: the second Weyl scalar is not related to electromagnetic radiation in any way. As per my previous comment, a simple proof is the Schwarzschild (or Kerr) spacetime. It describes a stationary black hole, without electromagnetic (or even gravitational) radiation, yet there is a basis in which all Weyl scalars are identically zero but for $\Psi_2$.
A more extensive answer would require a lot of the the NP formalism, so I'll just point the way. First of all you should note that the Weyl scalars (or any other scalar in NP) are just components of tensors (or Christoffel symbols) in a given basis (i.e. a coordinate frame), and therefore they don't have any meaning in a general situation, since they are not invariants. Nevertheless sometimes the manifold has a special structure and one can find coordinates adapted to this fact, and interpret them accordingly. A simple example is the usual coordinates for Schwarzschild solution, which are adapted to the fact that the spacetime is both static and spherically symmetric (the coefficients of the metric are independent of $t$, and $\theta$ and $\phi$ except for the solid angle part).
Now for the Weyl scalars all spacetimes, at any point, possess one special structure, the so called principal null directions. Consider a general null tetrad $\{l^a,n^a,m^a,\bar{m}^a\}$, we would like to perform a linear transformation that preserves the vector $n^a$. The most general one is given by $l^a\rightarrow l^a+\bar{b}m^a+b\bar{m}^a+|b|^2n^a$ and $m^a\rightarrow m^a+bn^a$, where $b$ is a complex parameter that defines the transformation. Now the Weyl scalar $\Psi_0$ will, under this change of basis, transform as
$\Psi_0\rightarrow \Psi_0+4b\Psi_1+6b^2\Psi_2+4b^3\Psi_3+b^4\Psi_4$.
Since the transformation is quartic in $b$ there are exactly four roots for the equation $\Psi_0=0$. Therefore, in any given point in the spacetime, there are four null directions $l^a$ such that $\Psi_0=0$. This are the principal null directions. The physical relevance of this idea is that if you consider a null geodesic congruence with tangent vector $l^a$ the derivative of the shear along this direction is
$l^a\nabla_a\sigma=\sigma(\rho+\bar{\rho})+\Psi_0$
and therefore if the congruence has zero shear the principal null directions are the ones in which the derivative is also null. Now the interpretation of the Weyl scalars is connected with the Petrov classification. For Petrov Type I (algebraically general) we have four distinct roots $b$ for the transformation of $\Psi_0$. In Petrov Type II there are 3 distinct roots, one of multiplicity 2 (and the corresponding principal null direction is said to be of multiplicity 2), in which direction we have $\Psi_0=\Psi_1=0$. In Petrov Type III we have 2 distinct roots, one of them of multiplicity 3, in which direction we have $\Psi_0=\Psi_1=\Psi_2=0$. In Petrov Type N there is only one root, necessarily of multiplicity 4, in which direction we have $\Psi_0=\Psi_1=\Psi_2=\Psi_3=0$. And for Petrov Type D (the case of Schwarzschild and Kerr) there are two distinct roots, both of multiplicity 2, in which directions we have $\Psi_0=\Psi_1=\Psi_3=\Psi_4=0$. This exhaust all possibilities. The interpretation for the scalars relates what scalars are not null in a given direction, and the interpretation of Petrov classes described in the paper by Szekeres you mention (which is reproduced faithfully in wikipedia, and also in the book Introducing Einstein's Relativity by Ray D'Inverno).
The idea is that since, by Szekeres argument, in Type N we have transversal waves we connect $\Psi_4$ for a given principal null direction with transverse wave modes. For Type III Szekeres establishes that we have transversal and longitudinal waves. Since we identified $\Psi_4$ with the transverse modes we are induced to relate $\Psi_3$ with the longitudinal ones. Now when we change $l^a\leftrightarrow n^a$ we change $\Psi_0\leftrightarrow \Psi_4$ and $\Psi_1\leftrightarrow\Psi_3$ (and $\Psi_2$ is unnaltered). Remember that $l^a=t^a+r^a$ and $n^a=t^a-r^a$ so changing the real null elements of the tetrad amounts to changing the sign of the radial coordinate. Therefore if $\Psi_4$ describes transverse waves going in a certain direction, $\Psi_0$ describes waves going in the opposite direction, and the same for the longitudinal modes.
As for $\Psi_2$, in Petrov Type D Szekeres showed that a spherical arrangement of test particles is deformed in a elliptical one with increasing eccentricity, exactly the same behavior of test particles under an inverse-squared potential, as in Newtonian gravity. Therefore we relate this class, as well as $\Psi_2$, with the presence of Coulomb-like potential. This interpretation is furthered by the peeling off theorem for asymptotically flat spacetimes.
One can only speculate as to why McMahon wrote that $\Psi_2$ is connected to electromagnetic radiation. Maybe is the folklore that says that the Weyl tensor encodes the gravitational radiation (which is not true as showed, beyond grav. radiation it also encodes tidal forces from Newtonian-like gravity) and not finding a radiation description for the second scalar was induced to error. Nevertheless his analysis of Nariai spacetime is totally incorrect, this is a static blac khole in the presence of positive cosmological constant and there is no radiation of any kind whatsoever. Overall I've found his approach to the subject very poor and wouldn't recommend it as a source for Newman-Penrose formalism.
My answer was based in the standard literature in the subject. A classical account, with all the math one could wish, is in Chandrasekhar's Mathematical Theory of Black Holes. For the more physical interpretation I've used Penrose's Structure of space-time, in Battelle Rencontres, edited by Wheller and Dewitt-Morette. Although a bit more mathematical, I've found the approach in O'Neill's The geometry of Kerr black holes very elucidating.
Comment: This is a great answer. Just one addendum. There is one possible type for the Weyl tensor not mentioned: it can be 0, called Petrov type O. Type O is not very relevant to vacuum metrics though, since all type O vacuum metrics are known: they are (anti-)de Sitter or Minkowski.
Comment: @RobinEkman, thanks for the comment, I should have mentioned the Type O, in which case all Weyl scalars (i.e. the Weyl tensor) are identically null. The reason for my omission is that some references do not include it in the classification, since, as you mentioned, the Petrov classification is more interesting in the vacuum case, and for Type O the possibilities are trivial. Nevertheless I must add that all those types assume strict vacuum, that is $R_{ab}=0$, or no cosmological constant. With a non-zero cosmological constant one cannot separate Types I from D and Types II from N
Comment: @cesaruliana ... I love you man...No homo
Answer: To add to what cesaruliana wrote, I think the confusion lies in the fact that apart from the gravitational NP scalars $\Psi_0$ through $\Psi_4$, the NP formalism also defines analogous electromagnetic scalars, $\Phi_0$ through $\Phi_2$. These are contractions of the tetrad vectors with the electromagnetic field tensor $F_{\mu \nu}$. In this case, with the standard choice of null tetrad (the same one that encodes outgoing gravitational radiation in $\Psi_4$), the outgoing e-m radiation is contained in $\Phi_2$.
A useful reference might be Teukolsky's early-1970s papers on perturbations (scalar, electromagnetic, and gravitational) on a Kerr-black-hole background. The earliest of these is Physical Review Letters vol. 29, pages 1114-1118 (1972).


TortoiseSVN: how do I undo the last Update command?

Question: The last TortoiseSVN Update command loaded work-in-progress code from the trunk which shouldn't have been committed yet, and overwrote the files that I had on my branch.
How do I undo that last Update command and revert back to the versions that I had on my machine before the Update?
Comment: exact duplicate of [Tortoise Subversion - undo update](http://stackoverflow.com/questions/6593758/tortoise-subversion-undo-update)
Answer: There is no "undo" or "revert", but if you update to the previous revision, that is the recommended way to accomplish what you are asking for. In TortoiseSVN, select "Update to revision..." and enter the revision number you want to go back to.

revert back to the versions that I had on my machine

If you had local changes on your machine that were never committed (i.e., no revision number), and you updated over them, then you are out of luck. The only way to get back your local changes is if you have a backup (check the "Previous Versions" tab in Windows Explorer...)
Comment: Loosing local changes for Updating to latest is time waste. I dont know if `Git` also works same way. `TFS` is best in this case.
Answer: Check out this guide for undoing revisions on TortoiseSVN. The entire concept of version control supports the issue you've created for yourself so it is a popular action.
Answer: You can update to a specific revision by right clicking on your working copy, Under the TortoiseSVN menu item, there is a update to revision menu option that will allow you to change your working copy to the revision of your choice.  If you do a show log on the repository you can get an idea of when the offending code got committed and revert back to the prior repository revision. As a precaution, I would make a copy of and working copy, or important files in your working copy, to ensure that if things go bad, you can always get back to where you are now.
Now this changes your working copy, not the repo, so if you are wanting to revert the repository back, then that is a different issue, but I do not think that is what you are asking for.
Answer: This is called update to revision... In the ui you can choose which version you want to go back to. Make backup copy of your local svn tree first to be safe!


Streaming fullscreen with Open Broadcaster Software gives black screen

Question: After a long while i figured out how to stream in HD. However, every time I stream in fullscreen, the stream will show a black screen instead. If I stream a game in Windowed mode it will show it without any issues.
How can I stream a game in full screen mode for free so I can prevent them from seeing my desktop and give them the optimal quality resolution?
I'm using Open Broadcaster Software and have set my settings to entire screen. or are there better solutions?
Comment: @EBongo the viewers?
Comment: well, you can use Shift window (http://grismar.net/shiftwindow/) for making windowed games look fullscreen. And it works with streaming. Used it for streaming League of Legends.
Comment: When you say "prevent them" - who is them?
Answer: Per Jim (listed as a Dev) on the Sourceforge Open Broadcaster Software Forum, OBS does not support fullscreen mode, but it is a planned future improvement.

Currently, fullscreen games can't be captured via software means without using DXTory or something.
Fortunately, I do have plans in the future to add fullscreen game capture, but because it's a rather difficult problem to tackle and will require a lot of time and focus, it's a bit lower on the priority list and won't be in for quite some time.

DXtory looks to be a non-free screen scraping application, and I'll go out on a limb and assume that's not what your looking for, as it would be a pretty advanced technical solution.
Others in the thread suggest borderless fullscreen windowed mode as a workaround.  This would accomplish many of the goals you are shooting for, and would get around the OBS limitation.  If you follow that link there are a number of applications listed, including Shift which was mentioned in comments.  There is also some mention of potential drawbacks of each, such as compatibility with Punkbuster.
Finally, as this PC Gamer article points out, be sure that your source content is not minimized when you experience "black screen".

You can now preview the stream or start broadcasting, but you won't see anything if the game is minimized

If you are willing to entertain other options, this video describes how to setup Xfire and Twitchtv for fullscreen mode games, and towards the end he shows proof that it works with Fifa '12.  Both are free services, but with Twitchtv your viewers may have to sit through some ads before watching, if they don't want to pay for the ad free service.
Comment: Borderless fullscreen windowed seems to work. But its not the perfect solution since not all games have that option.
Answer: You can pull up task manager in windows and go to hearthstone right click maximize and you will not see your desktop when you stream in obs. This does not let you stream full screen mode but it hides your desktop from being seen while you stream and play.
 Hope this helps.
Answer: I don't know much about it myself, but I know a place that does: Twitch.
Their support page has more info, but the bit about software and how to set it up reads:

Step 4: Decide on a streaming application. Your main options are XSplit (free and paid), OBS, FMLE, and FFsplit. XSplit, OBS and FFsplit includes built in desktop capturing while FMLE requires a 3rd party capturing software such as SCFH DSF. We recommend starting with XSplit if you are new to steaming or OBS if you are looking for a totally free solution.
Step 5: Set up your streaming software. There are already a large number of really good guide for this so I will just link them here. The basic steps include creating your connection to the TwitchTV server, configuring your quality and bandwidth settings, and configuring a capture of your desktop (PC) or from a capture card (console).

Streaming 101 by TheGunrun [Hardware recommendations, XSplit, and capture card options] ** Recommended

Complete XSplit Streaming Walkthrough by Fire [XSplit Screen region/Gamesource, Dxtory]

FFSplit Install Guide

Beginner's Guide to Desktop Streaming by Destiny [FMLE, VHScrCap, VAC]

Our Tutorial Section - Lots of misc. tutorials that are not listed here.

Also, you may want to check out our FAQ as well.
Answer: All you need to do is to open the settings menu of the game (for example, League of Legends) and change the video output from full screen to windowed.
Specifically for League of Legends, I recommend borderless instead of windowed for best looking streaming.
Answer: You can either use Windowed Borderless mode in your game, or you can try to add the game via adding a new Game Capture from the sources pane.
Answer: Just launch OBS in administrator mode and use game capture, some games in fullscreen also will require that to show it in window capture. But administrator mode for the win.
Answer: Change how OBS is launched.  Right click OBS shortcut -> run with graphics processor -> integrated.  Done.
Answer: I've been able to fix this issue a few times simply by refreshing the game window preferences.  Tutorial here:
Comment: Please summarize the video's content in your answer.  That way, if your video ever goes down, this answer can still help people.


InfoPath from 2010 to 2013

Question: How can I migrate/upgrade my existing InfoPath form 2010 to InfoPath 2013. Will opening the form in IP 2013 and publish do the needful?
Comment: Sorry to ask question in old thread. I am going to upgrade from sp2010 to 2013. Do you think I can simply backup/restore entire site collection to 2013 farm to save to trouble converting each form 1 by 1?
Answer: There are some changes in InfoPath 2013 that you should think about, so I don't think it's enough to open the form via Infopath 2013 and publish that will achieve the goal in all situations ,
So to avoid any issue , try to do the following steps to migrate SharePoint InfoPath form  2010 to 2013

Get the list form template from SharePoint 2010
Extract the XSN as a CAB, then modify the manifest.xsf updating to use new URLs and IDs
Package back up the files as an XSN
Publish the list form template to the destination > Test

For more details check How to Migrate a SharePoint 2010 InfoPath List Form to SharePoint 2013 or SharePoint Online
Answer: To migrate the infopath form to 2013. You need to open the infopath file in infopath designer. Then you need to update the data connection in infopath designer and then publish it to sp 2013 library.
You are now ready to use same form in sp 2013
Here is an article with detailed steps about how to migrate InfoPath forms from SharePoint 2010 to 2013: InfoPath and SharePoint 2013 – A Perfect Blend to Upgrade your Forms
Some general steps to do the InfoPath form migration:
Copy the design form from InfoPath 2010
Go to forms library where we have published the design template and view the library in explorer view and you can see a folder called "Forms" apart from your submitted xml forms
Open the forms folders and copy template.xsn file
Open the template.xsn file in InfoPath 2013 , check for web browser form compatible
Hope this helps!
Answer:
Open Form Library in SharePoint 2010 using SharePoint Designer
Export template.xsn file on local machine
Open that template.xsn file from Info-path form designer
Replace all data connection that currently point to SharePoint 2010 site.
Publish to SharePoint 2013 server.
create new library while publishing or Publish in existing library in
SharePoint 2013 site

InfoPath from 2010 to 2013


HTML5 sound file player

Question: I am trying to make a simple player in HTML5 that will allow the user to select an item from an option list and then play the appropriate sound file.  I am targeting Mobile Safari, primarily.
I am trying to emulate this application I built a while ago in Flash:
http://www.finegardening.com/pguide/pronunciation-guide-to-botanical-latin.aspx
The following code I was able to mostly get to work on my local server, but only in Chrome Dev, and it would crash after playing more than one or two clips.
http://jsfiddle.net/smlombardi/7Wg7Y/
Can anyone give me some ideas on how to accomplish this?  That is select an item, load the sound file associated with that item, play the file onClick with a button, then discard the file and load another onChange of the list.
Answer: try to use this html5 sound library.
http://buzz.jaysalvat.com/
Good Luck.

HTML5 sound file player


Firmware Choice: Marlin vs Repetier vs Other

Question: We are custom building a Cartesian 3D printer to be used in a production 3D printing environment, with the following requirements:

Intended Z resolution 0.13mm;
Dual extruders (primary + support material);
Four spools (two per material with y connector, perhaps add auto switch-over functionality later);
Camera;
Onboard 11" touchscreen for control & interaction, run by a Raspberry Pi 3;
Custom board based on ATMega2560 to control the printer;
Printer should gracefully handle conditions like "out of material", etc.

Question: How to select firmware?
Are there notable differences between the primary firmware options (Marlin/Repetier) for this setup?  From the google code groups, I understand the Repetier has a much cleaner code base, and apparently gives smoother physical performance.  Yet, from what I can gather, 95% of the community uses Marlin - is that correct?
Given that this printer will have the Pi3 to control "higher functions", is it worth considering a compact firmware like Teacup?
Edit 12/April/2017:
For others looking: After much review, Klipper was selected as the most forward firmware - all kinematic calculations are done on the host computer, instead of on the microprocessor, resulting is significantly faster/smoother stepper movement.
Comment: Besides that high-end printer and Atmega sound mutually exclusive to me, many if not all open-source firmware should work after proper configuration. The only exception is some rarely used printer mechanics, like SCARA
Comment: @MikhailZ FYI it eventuates that, if you use firmware that performs all the kinematic calculations on the host, then ATmega is more that sufficient: ~20% cpu usage @ 150mm/s.  Physics quickly becomes the limiting factor, rather than the processor.
Comment: You say the printer is intended to be _high end_, so what do you mean by good speed/resolution? Do you have any particular figures/ranges in mind?
Comment: So you have a custom controller board...you know you will need to heavily modify the firmware to get it working right?  Anyway, I voted to close because you didn't specify what you were looking for in the firmware.  All of them easily hit your standard goals at this point.
Comment: Thanks, I have modified Marlin and it is working on the custom board now.  Just trying to understand if there is a significant difference between firmware options - aka is there a reason that a significant majority of people use Marlin over Repetier?
Comment: This can be a solution, but haven't you checked boards based on processors like STM32? In most cases they have enough capacity to handle all calculations even for more complex devices, like deltas. In any case, for production printer you may want to stretch your speed goal far beyond 150mm/s.
Comment: Just FYI 150mm/s @ 20% cpu, Klipper can run well over 400mm/s without maxing cpu.  Appreciate the faster boards, though it just seems (to me) a much cleaner design (& faster) to pre-calculate on a host (RPi is order of magnitutude+ faster than STM32),  & keep only real-time operations on the real-time hardware.
Answer:
Given that this printer will have the Pi3 to control "higher functions", is it worth considering a compact firmware like Teacup?

A few days ago I came across klipper.
It seems to do exactly that functionality split you mention
Comment: Update: After significant review of Klipper, have found it to be excellent, first choice.  The design (all calculations done on the host) is a significant step forward.
Comment: Thanks for sharing, hadn't seen that, that is the kind of thing I am looking for.
Answer: Your choice of firmware depends on a lot of factors, but to name a few important ones: features, your driver board (RAMPS vs Sanguinololu vs Gen..etc), and G-code support.
Smooth/quiet operation is dependent on the motors and the driver chip's micro-stepping capabilities. The way the firmware interfaces with the driver chip plays a very minor factor in that.
Check this page for a list of firmware, their capabilities and compatible electronics (driver board):
http://www.reprap.org/wiki/List_of_Firmware
Would be useful to check G-code support of various firmwares also:
http://reprap.org/wiki/G-code
Also, as a note, a lot of the community uses Marlin, but for a lot of the commercial printers, the code has been modified slightly to fit their needs.
There is no perfect answer to this. Check the links and compare against your needs. Marlin is the safest bet since it has the most support, though you may need to modify or reconfigure it slightly for your custom board.


How do you find underground pipe and wires to a private well and how deep would they be?

Question: I need to terrace some stairs into a steep hill, but my well is at the bottom of the hill and my house is at the top. Therefore there has to be pipe and wires going up the hill somewhere. I don't know where and before we drive rebar to anchor the containing wall/stair we need to know where the pipe is.
Answer: If your water does not freeze up in the winter, they are presumably buried below frost line for your area - which is typically where they do get put by competent installers (and not terribly deep if there is no frost.)  In my area that's 4-5 feet, typically. In most but not all cases it will probably be the shortest, most direct path from where it comes out of the house to the well. The only way to be sure is to carefully dig down and find it on either end, and then use those locations to try and find it nearer to where you might be damaging it if you blindly pounded things into the hillside.
Once you have found a few specific points, be sure to witness them in one or more of these methods:

take very careful tape measure distance readings to fixed objects
(the well-head if it sticks out of the ground, the corners of the
house foundation, other objects likely to stay put for a long time)
and write those measurements down on a map - you can then construct a
triangle to find the pipe again by measuring from a fixed point as
recorded on the map and making an arc, then doing the same from
anther fixed point - where the two arcs cross, the point you
measured the well-pipe at should be below (within some scope of
error.)
I'm fond of a more direct approach, where I construct an annual
flowerbed or easily movable perennial plants flowerbed as a physical
witness to the item below. You'd still want to save a map with the "important house paperwork" and hope it doesn't go missing, but there's no doubt where to dig unless someone obliterates the flowerbed in question. If you remember not to move it, some sort of lawn decoration could serve the same purpose, but a flowerbed is more lasting.
A section of inexpensive PVC electrical conduit just shorter than the depth to the buried item and marked (both what, and from 0 at the bottom to how long it is at the top as a measuring device) with permanent marker can also be buried in the hole as a guide to future digging. The idea being that you can find it quickly as it's just barely buried, and then follow it down and know how close you are to the buried item.
Answer: One safety note: if you at ask think there is a chance you can hit the line, turn off power to your pump while you are working.  Better safe than sorry.
Depth
First of all, the water lines are almost always buried below the frost line. There are exceptional cases where bedrock prevents this, and if there is no other path the lines may be shallower with insulation and possibly heat line, but this is pretty rare.
The power lines are almost always buried right beside the water lines, often both inside a length of  "big o" (corrugated black 4" HDPE) .
Locating via guess
You can take an educated guess by looking at where the lines enter the house. Generally it will be the most direct route that doesn't have any sharp turns.
Locating using a wire tracer
You can rent a wire tracer that will fairly accurately trace the path of underground wires. There are also companies that will do this for you. This will only work if there are wires (submersible pump), or if you cut the water line and run a fish line down it that can be traced.
Obviously the most expensive, but may also be the only way without digging it up. It just really depends on the distance involved and how close the line likely is to the spot you are digging or installing rebar.
Answer: When in doubt, Call Before You Dig (your gas, power, water and telco companies). Your municipality may have a system in place to make this easier, E.g, 811. They will come out and mark where their utilities are so you don't hit them.
Concrete footings must begin below the frost line. Hopefully you don't find it when digging out for the footings. Rebar gives concrete strength, they're not 'piles' to be driven far off into the ground. If you do find it, make sure X marks the spot as Ecnerwal suggests. My 'archives' have saved me countless headaches.
Answer: You can buy (or maybe rent) a wire tracer like this:

The power wire to the well would have to be disconnected from power and the left box connected to the loose wire.  Then walk around with the sensor near the ground and see where the wire goes.  Its range is reduced by the soil so if the wire is very deep (> 18 inches?), it may not work so well.


How does Ollivander know which wand fits other wizards?

Question: When selecting wands, Ollivander does not seem to randomly pick boxes from his collection. How does he know which wand might be the right one?
Comment: He has them fill out the Pottermore quiz.
Comment: Experience makes him pick a wand which might choose the wizard. If it doesn't, then he will change it. Remember the amount of time it took for Harry to get hands on his Phoenix feathered wand.
Answer: While the wand chooses the wizard, a knowledge of wandlore can help predict which wand would pick which wizard.
Taking from Olivander's notes on the subject:

In  my  experience,  longer  wands  might  suit  taller  wizards,  but they  tend  to be  drawn  to  bigger  personalities,  and  those  of  a  more spacious  and dramatic  style of  magic.  Neater  wands  favour  more elegant  and  refined  spell-casting. However,  no  single aspect  of  wand  composition  should be  considered  in isolation of  all the  others,  and  the  type  of  wood,  the  core and  the flexibility  may  either  counterbalance or  enhance  the  attributes  of  the  wand's  length.
(Pottermore - Wand Lengths and Flexibility)
Comment: @Yehuda - Perhaps he just asks them the seven questions on the Pottermore quiz.
Comment: It would therefore stand to reason that Ollivander should interview his customers prior to recommending wands for them, would it not?
Answer:
Wands choose the wizard or witch in a way that is neither random, nor serendipitous. There is much ancient mythology and botanical knowledge regarding the powers and natures of wood and other natural elements. Wandlore, Harry Potter Wiki

This would be one of the insights an experienced wandmaker would use to know which wand could be a better fit for the wizard. But most of the time, it would be a trial and error. Also, most of the wands corresponds to the birth month's wood in the Celtic calendar, so that should really eliminate most of the extraneous candidates.
Comment: HP wiki isn't a reliable source.


How do I send mined etheruem

Question: How do I send the etheruem I mined? Let's say I mined some etheruem. How do I send it to coinbase so I can sell it? I use Geth. (GPU)
Answer: You can send mined ether in the same way that you send received ether. Assuming you run this on the node where you're running your geth instance, start a geth console using this command (same for Windows/Command prompt, macOS, Linux flavours):
geth console # I assume geth is in your path

Then, in the geth console, run this:
eth.sendTransaction({from:eth.coinbase, to:COINBASE_ADDRESS, value: web3.toWei(AMOUNT, "ether")})

This sends AMOUNT ETH (or ETC) to the address provided to you by Coinbase at COINBASE_ADDRESS. You will be prompted to enter your Ethereum account password if you have one. I'm assuming you didn't start geth with the --etherbase argument; if you did, then you'll want to replace eth.coinbase above with your Ethereum account address in single quotes ('), including the leading 0x in the address.
Comment: Well how am I supposed to know how much I have?
Comment: You can check your balance using instructions from here: https://github.com/ethereum/go-ethereum/wiki/Managing-your-accounts . I just realized that by "mined ether", you might mean rewarded by a pool rather than blocks you mined. Can you please edit your original question to reflect that?

How do I send mined etheruem


Does the event horizon of a black hole increase or decrease by adding mass?

Question: So a black hole is a something that has enormous gravity, therefore mass accelerates faster towards it. But if the black hole double  it's mass, will it's event horizon increase in length , or will it decrease?
Comment: Did you read the Wikipedia article about [the radius of black holes and its dependence on the mass](https://en.wikipedia.org/wiki/Schwarzschild_radius#Formula)?
Comment: Not even a minimal amount of research. As a guide - if you type your headline question into a search engine and the first link it provides directly gives you the answer, then it is not a very interesting stack exchange question.
Answer: The radius of the event horizon ($r_\mathrm{s}$) is directly proportional to the mass of the black hole (M). More exactly:
$$r_\mathrm{s} = \frac{2 G M}{c^2}$$
The black holes whose merger was detected by LIGO would each have been about 90 km in radius, and after merger, a little less than 180km.
Read about the Schwarzchild radius
Comment: So apparently, if my waist line is smaller than $6.5×10^{-25}$ meters, I collapse into a black hole.
Comment: Additional fun fact: If you consider the density of a black hole $\rho$ as its mass over it's volume given as a sphere with a radius equal to the Schwarzschild-radius $\rho = \frac{M}{\frac{4}{3}\pi r_S^3}$, than you′ll find $\rho \sim M^{-2}$. It's getting less dense, the more massive it becomes.
Comment: Wow, thanks a lot for that.Really helpful
Answer: particle and anti-particle pair - basically it has a particle that is attracted to the black hole and one that is repelled by it. Since these particles contain mass, the particle that is repelled is left as radiation, and the one that is attracted is added mass, and by adding this mass it will decrease the total density, since it isn't just an addition problem, there is lost mass increase that makes the density and the mass of the black hole a inverse relationship. This states that the more mass the less dense which means that is will shrink and decrease in length.
Comment: Your answers could be improved by a bit better formatting. Btw, what do you mean by decrease in length? The length of what?


How to get a user's friends' names?

Question: I have a users table and friends table. The user with ID 1 has 4 friends (with IDs 2,3,4,13). I want to get names and surnames of users 2,3,4 and 13. I can to it with standard queries but it will slow down the process; querying for every friend. I didn't get how and what to use of left, joints, right joints, simple joints, ASs, ONs, ...
Comment: basic sql query questions are not encouraged here, please use stackoverflow. You have to post your current query/code here, to get optimization recommendation.
Comment: Do you want 2,3,4,13 or 2,3,4,14?  I think you mistyped one or the other and just need to make them match.
Comment: @Sandy are these question just "not encouraged" or not rather actually off-topic?  The FAQ is quite vague on what actually is and isn't on-topic...
Answer: If you want to get the names of the friends for a user perhaps try something like so:
SELECT u.firstname, u.surname
FROM friend f, user u
WHERE (f.Person1 = 1 OR f.Person2 = 1)

This should get a back a list of names in your user table (providing that is what you have named your friends in the user table) that are friends with userID 1
I'm guessing your friend table contains the foreign key IDs of the friends making up the partnership?
Answer: I am assuming the friends table has a user_id field that corresponds to the id field in the users table.
Select name, surname from users
where id in
(select user_id from friends where id=1)

This way, you can just plug the user id in the place where '1' is, and you will get all the respective friends. For example, if you want to find the name and surname of all the friends of the user id 5740 use:
Select name, surname from users
where id in
(select user_id from friends where id=5740)

How to get a user's friends' names?


zora armor, and how it could actually be made to swim up water falls?

Question: so in the game's legend of Zelda, there is the Zora armor. it helps you swim faster (which is shown by having flippers on the feet so obviously that would help, and it's skin-tight to fit whoever it was made for) and it lets you do the impossible; swim up waterfalls. how could you make something like armor, that can help you go up waterfalls? it sounds impossible and purely based on the magic in the game, but I feel like it could be done considering that waterfall climbing fish exist. it doesn't even have to fit the description or look of Zora armor, it just has to be some type of armor.
Comment: @Halfthawed ok. so just attach some water jet packs on the zora armor, and then you can move extremely fast in water and up waterfalls. not like its actually useful though considering the actual amount of waterfalls.
Comment: I guess if you got really good fluid dynamics you could reduce a lot of the resistance and over come it with sheer force but that is extremely unlikely. Here is one NASA made [link](https://en.wikipedia.org/wiki/LZR_Racer)
Comment: How tall of a water fall?
Comment: Fish can't climb waterfalls directly - the species that are known for it either crawl using fins, or jump the small waterfall entirely.
Comment: @Halfthawed is there a way you can jump a ten meter tall fall, or water jet pack up in one shot?
Comment: @Trevor i'd assume about 10 meters which is pretty tall.
Comment: @michaelgriffin They've got some pretty sweet water hydraulic jetpacks these days, so yes.
Comment: The jetpacks let you move pretty swiftly in water, so they aren't useless. You do, however, need a bulky and inconvenient power source in the form of a tethered jetski, and a hose to supply water to the jets.
Comment: If your local gravity is lower and water falls at a slower speed, it might be possible to swim up a waterfall
Comment: i guess thats fair @StarfishPrime
Answer: There's a kind of goby that is able to climb waterfalls by attaching it's abdominal sucker to the slick wall behind the waterfall as well as it's sucker-like mouth to move in a fashion similar to rock climbing in order to slowly move up the waterfall. So it could have a kind of sucker or otherwise sticky surface on the chest, with perhaps suckers on the hands, which, with the proper strength, could allow someone to climb up a waterfall. Though it may require a little hand-waiving. The flippers would absolutely help with climbing up waterfalls as well, as they help push more water as you switch where your suckers are. The armor could also be better streamlined and allow the water to flow by it more easily so it's easier to go faster. Likely it would have some kind of face protection due to all the rushing water. But other than that the amount of actual protection would vary with materials. Any metal would likely have to be a kind that doesn't rust with water, and cloth shouldn't be very absorbent.
If you want it to look more like the Zora armor in game, the metal on the forearms and shins could contain some kind of jet in them that takes water in and shoves it out even faster than it was going. However the pauldrons(shoulder armor) seem like they would get in the way more than help. But they do add protection, even if they would be heavy.
I made a visual representation of how the movement for pulling oneself up the waterfall could work to help visualize. The red is when the chest sucker is down and wouldn't move, the blue is when the hand suckers are down and wouldn't move. The back fins aren't moving in this due to the fact they are the easiest to visualize, they just go up and down like they usually would while swimming.
Comment: this does work, but thats more of a climbing mechanism with an added protection from water. it would be a neat gimmick which would allow people to climb undetected because they are literally in a waterfall, but its not entirely what i was looking for. but the whole shooting out water faster than it comes in sounds cool!
Comment: theoretically you could also use it to do jumping movements, but that would be less secure.
Comment: true. still, sadly so far there is no way to swim up a waterfall.


Is Certificate pinning different from CA pinning?

Question: I am trying to understand if pinning a certificate essentially means pinning the CA or if they are different how so ?
Answer: The generic processing of a certificate goes thus:

The client builds a certificate chain going from a trust anchor (a given certificate which is known a priori by the client) down to the server's certificate.
The client verifies that all signatures and other information are correct along the chain (each certificate has a subject and an issuer names: its issuer name must be equal to the subject name of the previous certificates; each certificate is signed and the signature must match the public key from the previous certificate; and so on).
The client checks that the expected server name (the one from the https:// URL) appears where it should in the server's certificate.

Certificate pinning is equivalent to turning the server's certificate into a trust anchor with limited scope: the client stores the server's certificate, and will used it as the trust anchor when validating the server's certificate again. In the description above, this means that when coming back to the server, the client will build a chain of length 1, containing only the server's certificate itself, which the client "trusts" in that context because it remembers it exactly.
Certificate pinning can be inclusive or exclusive; the former means that the client will accept the remembered server's certificate as a trust anchor in addition to its other, "normal" trust anchors (its "root certificates"), while the latter means that the client will use only the pinned certificate as trust anchor. The overall effect of exclusive certificate pinning is that the client will accept the server's certificate only if it is bit-to-bit equal to the one it remembers; no other certificate would be deemed acceptable.
Usually, when people talk about pinning, they mean exclusive pinning.
CA pinning is the same process higher in the chain. The client remembers a CA certificate (which may be an "intermediate" CA) as a trust anchor. There again, this can be inclusive or exclusive. Exclusive CA pinning means that the browser will validate the server's certificate against that CA as unique trust anchor; the certificate will be accepted only if a chain can be built coming from that specific CA, and none other.

The important points above are the following:

The client remembers a certificate from the server's certificate chain.
The client complains loudly if the actual server's certificate (upon an ulterior connection) does not match what was remembered (subject possibly to some heuristics to smooth out normal renewals).
This process is site-specific: this is the main departure of pinning from the usual "root CA store" model. What the client remembers and requires is for a specific target site. The remembered certificate (CA or not) is not general for all sites, but only for one.
Comment: Your comment was very descriptive and helpful. Thank you! Can you guide me as to where I could find references to read more on exclusive and inclusive certificate pinning ? Google doesn't quite return anything substantial.
Answer: Certificate pinning stores the fingerprint of the certificate for a site, so that (in addition to normal verification) if the certificate changes, this can be used as input to the validation process.  The "pinned" certificate is not the CA, but the one at the end of the chain.  You might display a warning to the user if, for instance, a certificate is replaced with another one 8 months before expiry.
It's way easier to get a fraudulent certificate or forge a signature on one than it is to generate one with the same modulus as an existing one.  Certificate pinning is designed to provide some defense against the former.
Comment: Thank you for the prompt reply. Could you point out some helpful resources to understand in detail what transpires at the browser's end (in order to validate the authenticity of the site while pinning is in place) ?
Comment: There are lots of ways to do it, and lots of scopes (individual and network, etc.).  OWASP gives a good overview: https://www.owasp.org/index.php/Certificate_and_Public_Key_Pinning


I have a WSDL webservice I am trying to access from java/Blackberry

Question: I have a WSDL webservice I am trying to post and receive information from via java within a Blackberry app. I have used axis1.4 to build the stub and other class files within Eclipse. I have have the BB interface already designed. I can run a simulator and navigate to the page to enter the information (in this case Account Registration). I just need to know how to make set up the transfer. I currently have button on the screen as well that shows "Congratualtions" message when clicked but that should be on an "if" I need to make the button call the webservice, post information entered in fields to database, and get back a reply ( in this case true/false). My app is built on giving and receiving info to about 7 different methods within my webservice. Sending normally between 2 and 5 fields of data and receiving 1-3. Seems like I have all the pieces of the puzzle I just can't link them. I have read post on many other sites and this one about implementing wsdl but the examples are too simple to be useful to me. Please help.   Thank you in advanced.
Answer: Have you looked into ksoap2 ? I think what you need to know is how to pass complex objects Complex Objects & Ksoap2


Use PayPal From Website Form

Question: Requirements state that I build a form that collects user's PayPal login info and charges the user's PayPal account a fixed price without having the user leave the website.
I am beginning to think this is not possible.  Can someone kindly suggest a solution that comes close to meeting the requirements?  I am not certain what's possible and what's not possible with PayPal.
Comment: The requirement is not reasonable.  I would never reveal my PayPal login details to a third party website.
Comment: @RichieHindle what is the closest solution I can offer?
Comment: Sorry, I don't know (which is why I left a comment rather than an answer. :-) )
Answer: There isn't a solution for what you are trying to do.  In order for the buyer to login and pay with PayPal, they would have to leave your site and go to the PayPal login page.  You could set up a billing agreement so that they only have to log into PayPal once.  With a billing agreement, you would be able to charge the buyer's PayPal account when you are needing to.  You would just need to have the buyer go through the flow initially and get sent over to PayPal to login and agreen to the billing agreement.  Then when you want to charge them again in the future, you would just need to reference the billing agreement id.
Answer: The closest solution to having the user pay without leaving the website is PayPal digital goods express checkout.
Here is an example to show you how it works:
http://jmsliu.com/1307/integrate-paypal-digital-goods-express-checkout-into-website.html

Use PayPal From Website Form


Can you defer the July self assessment tax payment until January in the UK

Question: Someone told me they don't pay any self assessment income tax in July, they just pay the full annual bill in January.  Is this possible in the UK or is the July payment compulsory?
Comment: According to [this HMRC page](https://www.gov.uk/understand-self-assessment-bill/payments-on-account), it's not always necessary... you should probably read that and if that doesn't answer your question, add more details about your specific case.
Answer: Everyone who self-assesses for income tax is theoretically subject to making three payments in respect of a given tax year. Let's take 2022-2023 as an example:

A first payment on account in Janary 2023, i.e. before the tax year ends.
A second payment on account in July 2023, i.e. just after the tax year end.
A final balancing payment in January 2024, the same deadline as filing the tax return.

You are exempted from the payments on account if either of the following two conditions apply:

your last Self Assessment tax bill was less than £1,000
you've already paid more than 80% of all the tax you owe, for example through your tax code or because your bank has already deducted interest on your savings

Most taxpayers whose income is primarily from employment will meet the second condition because PAYE gets their tax roughly correct, and therefore they will only have to make the final balancing payment 8 months after the end of the tax year.
On the other hand most taxpayers with significant self-employed income won't be paying tax on it via PAYE and will therefore end up being subject to making payments on account.
The payments on account are an very crude estimate: they're based on the previous year's self-assessment return, i.e. 2021-2022 in our example. The final balancing payment corrects that estimate.
Comment: Also while I haven't seen it explicitly stated anywhere, my experience is the payments on account seem ti be based purely on income tax with NI and student loan collected through the balancing payment.
Answer: If submitting your tax return online you may be able to postpone until January the year after the tax year ends. That is the case for rental income "on the side", for example.
HMRC deadlines here.
For income tax for self-employment it appears to work differently though, with July and January payments on account.


What originally spins the intake fan when the engine is started?

Question: I've been thinking about doing a presentation about how jet engines work. I've watched some videos and everything seems understandable to me, but I have one question. What originally starts spinning the intake fan? The intake fan is powered by the turbines at the back of the engine as far as I understand, so what powers the fan before there's air in the engine to spin the turbines?
Thank you for any help.
Comment: This is an excellent video about starting a CFM56 engine.  I think it will help answer your question:https://www.youtube.com/watch?v=0OgEbs3ovOw
Comment: I may have misunderstood your question. Can you clarify: are you asking about how the starter motor works (which is what niels answered) or how the fan starts spinning (which is what I answered)?
Comment: I'm interested in the entire process of the engine start, which is why both answers are useful to me; the more I know the better.
Comment: OK, got it, then I'll leave the answer as it is...
Comment: Thank you for the video. It helped me visualize the answer to my question.
Comment: In Ye Olde Times you literally spun the propeller by hand.
Answer: Most turbofans have a 2 spool design:

The fan and low pressure compressor (LPC) are powered by the low pressure turbine (LPT) and rotate at a speed called N1.
The high pressure compressor (HPC) is powered by the high pressure turbine (HPT) and rotates at a speed called N2.

Only the N2 stage is connected via an accessory gearbox to the starter motor (this starter motor could be powered by different things, see niels nielsen's answer for details). Therefore, the N1 stage will not be moving initially during engine starting. Only the N2 stage starts to accelerate, powered by the starter motor. Note that air can move through the fan and LPT, even when they are not rotating.
The faster N2 rotates, the more air is now pushed through the engine. The air exiting the HPT still has enough speed to accelerate the LPT as it moves towards the exhaust. This is what starts to accelerate the N1 stage. Typically, you'll see N1 starting to rotate a few seconds after N2.
The N1 speed will remain relatively low during the starting process, until fuel is added. With the added energy from the combustion, a lot more pressure is available in the turbines. This will now increase the N1 speed significantly as more energy is extracted in the LPT.
Eventually, the starter motor can be disconnected as the HPT supports the HPC without external help and the engine becomes self sustaining.
Answer: There are several ways this is done. You can use an electric motor running off a battery to spin up the jet engine, or you can use a big fan to blow air into the air inlet of the engine, or do the same with compressed air pipes that feed compressed air into a pneumatic motor that is geared to the engine.
The energy source that runs each of these things can be either built into the airplane, or rolled out to the plane on a cart (called a start cart) and plugged into the engine temporarily.
In some cases you can put the starter motor into the start cart and connect it to the jet engine with a rotating shaft to spin up the engine. The SR-71 used a start cart with two Buick Wildcat V-8 engines in it to start its engines.
It is also possible to use a big brick of solid rocket fuel or a really big shotgun-shell-like cartridge in a cannister that feeds the hot gas from the burning fuel straight into the turbine section of the engine to spin it up. This lets you start jet engines in remote places that don't have start carts available.
BTW shotgun or Coffman starters were also used on piston-engine aircraft and farm tractors, where the hot gas was fed to a pneumatic motor with a piston in it that was geared to the crankshaft.
Comment: Don't forget shotgun cartridges. The English Electric Canberra and Hawker Hunter fired shotgun cartridges to start the turbine spinning. IIRC they had several redundant shots so they have several attempts they can make before the mechanics need to reload the engine.
Comment: If I remember my book on Chuck Yeager's life, he apparently was part of the team that figured out that you could "blow start" a jet, blowing air into it via the exhaust of a previously-started aircraft.  I consider it to be one of the more amusing variants to the "blow air into the air inlet of the engine" approaches you mention!
Comment: Alright, this definitely helped, I appreciate the help.
Comment: @slebetman [Flight of the Phoenix shows a shotgun start](https://www.youtube.com/watch?v=IACjOvyx5hs).
Comment: @Schwern, i remember that movie, will edit!!!
Comment: @CortAmmon, on the topic of amusement, US Air Force jargon for oral sex is an "airstart".
Comment: *"The SR-71 used a start cart with two Buick Wildcat V-8 engines in it to start its engines."* Wow!
Answer:
What originally spins the intake fan [...]?

There is no "intake fan" term used when talking about turbine engines. There's a fan, there's compressor, and there's an intake. But you can't mix those terms: they all mean different things!
A fan is used to blow air that bypasses the core of the engine. Turbofan engines have fans. Turbojet engines don't.
A compressor is used to provide the cooling and working air for the combustors and the turbine.
An intake is generally the system used to route air to the engine. Typically it has no rotating parts, although it may have parts that translate (move back and forth as the flight regime changes).
The compressor in jet engines - whether turbojet or turbofan - is usually axial, and has several stages. In the simplest of the engines, the compressor and the turbine are on a single shaft. This trades off performance for simplicity. More complex engines may have two independent compressor systems, each powered by its own dedicated turbine system. The shafts for those systems are coaxial. The first (low pressure) compressor is powered by the last (low pressure) turbine. The 2nd (high pressure) compressor is powered by the first (high pressure) turbine then.
The fan may be powered by the same turbine as that of the compressor. That would only work for very low bypass ratio turbofans. At higher bypass ratios, the fan is too heavy of a load during the engine startup, and cannot be coupled to the compressor shaft. Instead, the fan then has its own, free power turbine located behind the core of the engine. The word free refers to the fact that this turbine's rotation is completely independent from the rotation of the core of the engine. In many cases you could mechanically brake the free power turbine during engine startup, and then release it later, although it's a rather unnecessary exercise typically.
As for how the engine starts: there is a source of mechanical power that spins up the engine core - it is called a starter. Starters may be electric, pressurized air turbines, hydraulic, or even pyrotechnic (so called cartridge starters where a slow burning rocket fuel is the energy source).
First, the ignition (spark) source is usually turned on, then the starter spins up the core.
Eventually the core is spinning about as fast as the starter can make it go - it's called motoring, and modern engine management systems may display the MOTORING flag in the engine speed readout. The fuel is then turned on to the engine, and it ignites in the combustor.
The core then accelerates. The starter energy source may need to be left on for some time to help the core go up in speed faster. The startup process is, perhaps counterinitutively, a very hard operating regime for the engine. That's because the turbine engines use the air from the compressor to cool the combustors and, most importantly, the turbine blades. When the engine is initially ignited, the compressor is providing well over an order of magnitude smaller mass air flow, and the cooling of the hot section of the engine is marginal. If the engine is not accelerating quickly enough during a start, it may overheat - especially if the engine is fed with too much fuel. This requires immediate shutdown and usually continued motoring to cool down the engine before another start attempt.
As a rough idea to where the energy goes in a jet engine: about 2/3rds of the power available for extraction by the turbine(s) is used to drive the compressor. The remaining 1/3rd is used either as a thrust source via the jet nozzle, or powers the free power turbine that then drives the fan, or a reduction gearbox that drives a propeller, a rotor, an electric generator (typical in peaker electric plants), or a turbopump (typical in natural gas applications).
Now you can get a hint as to why the engines not get spinning faster during startup before introducing fuel, to reduce the turbine temperature during startup. It takes an enormous amount of power to get the compressor operating at any significant fraction of the idle speed where turbine cooling is sufficient. This speed would be at least ground idle, but sometimes higher than that, up to flight idle speed. To keep starter size and power demands manageable, the starters bring the engine up to a speed that only just allows startup without overheating and not much more than that.
Another reason for low startup speeds is the airflow limit for reliable fuel ignition. Once the fuel is burning, the engine can be brought up to full air flow. But when the airflow is too high, it's almost impossible to ignite the fuel, so there's an upper engine speed limit for reliable ignition and development of stable flame in the combustor.
Comment: "At higher bypass ratios, the fan [...] cannot be coupled to the compressor shaft. Instead, the fan then has its own, free power turbine [...]" -> this is true for Rolls-Royce Trent engines with a 3 spool design, but other high-bypass turbofans have the fan connected to the compressor shaft (LPC and LPT shaft), sometimes with a gearbox in between (e.g. PW1000G).


Show that the Bell states form a basis

Question: I can't seem to understand how to show that the Bell states for a basis. Should I explain that through the circuit and what gates are used or by the basic proof behind proving a set as a basis?
Comment: what's your understanding of what a "basis" is? See also https://quantumcomputing.stackexchange.com/q/15332/55
Comment: I would definitely check out the link given by gIS in the above comment, but as to your precise question, the most straight forward way is to show it using the basic proof behind proving a set as a basis i.e. showing that the basis set form a linearly independent set and span the total set of vectors.
Comment: I did follow the link provided by gIS only. I found it just after posting my query and it solved my doubt. Thank you for both the solutions!!
Answer: If you are taking the four Bell states
$|\Phi^+ \rangle = \dfrac{1}{\sqrt{2}}\big(|00\rangle + |11\rangle \big) $
$|\Phi^- \rangle = \dfrac{1}{\sqrt{2}}\big(|00\rangle - |11\rangle \big) $
$|\Psi^+ \rangle = \dfrac{1}{\sqrt{2}}\big(|01\rangle + |10 \rangle \big) $
$|\Psi^- \rangle = \dfrac{1}{\sqrt{2}}\big(|01\rangle - |10 \rangle \big) $
and place them as a column of a matrix $U$, for instance:
\begin{align}
U &= \bigg[ \hspace{0.2 cm} |\Phi^+ \rangle \hspace{0.2 cm} \bigg| \hspace{0.2 cm}  |\Phi^- \rangle \hspace{0.2 cm} \bigg| \hspace{0.2 cm} |\Psi^+ \rangle  \hspace{0.2 cm} \bigg| \hspace{0.2 cm} |\Psi^- \rangle  \hspace{0.2 cm}\bigg]  \\
&= \dfrac{1}{\sqrt{2}}\begin{pmatrix} 1 & 0 & 0 & 1\\ 0 & 1 & 1 & 0\\ 0 & 1 & -1 & 0 \\ 1 & 0 & 0 & -1 \end{pmatrix}
\end{align}
Here you can see that $U$ is a unitary matrix. That is, $U\cdot U^*= U^* \cdot U = I$ where $U^*$ is the conjugate transpose of $U$. Since $U$ is unitary, its columns must form an orthonormal basis. These columns are the Bell states.
Comment: +1 In fact, you've shown more: that the Bell states form an *orthonormal* basis (i.e. an orthogonal basis made up of vectors of unit norm).
Comment: How can you deduce this property for unitary matrices ?
Comment: Let $U = [e_1 , e_2 , \cdots, e_n]$ then $U \cdot U^* = I$ implies $e_i \cdot e_j = 0 $ if $i \neq j$ and $1$ if $i = j$.
Answer: The Bell states form an orthonormal basis of 2-qubit Hilbert space. The way to show it is to come back to the definition of what an orthonormal basis is:

All vectors have length 1
They are orthogonal to each other.
The 2 qubit Hilbert space is 4 dimensional and you have 4 (orthonormal) vectors which implies linear independence.
So the only thing you need to be able to do is compute <b|b'> where |b> and |b'> are Bell states.

Show that the Bell states form a basis


Best way to detect high-impedance state

Question: I want to build a logic probe. What is the best solution to detect a high-impedance state?
Comment: While I cannot advise as to best, I once built a system to detect hi-z by biasing the node to halfway between the logic levels and using an ADC to measure that. Software compared the measurement to a tolerance range and classified it as hi-z
Comment: @ElianRussel OK, but you still need to define what is and what is not considered high impedance. Also, what types of logic gates? 5V TTL, 5V CMOS, 3.3V CMOS? Any others? Do you want to have a tester for each logic type or universal for many types?
Comment: That depends as impedance can be voltage sensitive with clamp diodes
Comment: You would have to define how high impedance is the threshold between determining high impedance input and for example input that is pulled down or up to some voltage with a resistor. And how much the probe is allowed to affect the circuit so that it does not disturb the normal operation? Then, when you have a specification, the best way can be determined. Or at least a few good ones, as the best is usually opinion-based or too complex and expensive, so likely you just want good enough solution that fits the specs.
Comment: @justme : just a good solution to test logic gates at hobby level.
Comment: Interesting point, however, I'll start using a "mid point voltage" on 5V TTL LS GATES. It seems easy to do. As soon as I get more needs, I'll find other solutions.
Comment: I get the feeling that this was done to death in the '80s,  look for a circuit that uses LM339 comparitors and has a TTL/CMOS switch.
Comment: https://electronics.stackexchange.com/q/509380/238188
Answer: Logic probes detect voltage either high or low. High impedance is not voltage.
Generally the probe is biased to mid point voltage through a resistance so it can be "pulled" high or low. High-Z is no action, but could be confused with a valid mid point bias although that should not be the case with logic circuits.
To detect high-Z, a small current is applied then the state is measured. Current into the probe point would measure as high. For your indicators you need a way to discriminate between the Logic high, and the High-Z high.
The current must be small enough not to disturb the operation of the circuit being probed.
EDIT1: An approach that I have used is to use a switch to select between TTL and CMOS levels. TTL levels are fixed to 2V high and 0.8V low. So any thing in between is unknown. Use a window comparator with two outputs: One turns on a red light for hi and the other turns on a green light for low. All other voltages the lights are off. So biasing in the middle will leave the lights off.
For CMOS, the levels depend on the supply voltages, so you need a supply voltage reference to the window comparator then set the thresholds to 70% and 30%.
Comment: So, the idea is to connect "a middle voltage" and then verify the final state of the pin. I got your point: if the pin is in Z-state it will not drive the line. Should the "mid point voltage" depend on the logic family? Can I fix it at Vcc/2?
Comment: @ElianRussel: Yes the mid point is based on the logic family. See my edit1
Answer: Always start with full specs or functional expectations as one addition might be implemented differently such as "open Circuit detect"

Logic Probes may use Schmitt triggers to detect the valid logic
state.

Dynamic states or glitches are detected and displayed by a <= 1
second toggle.

Invalid levels may be detected with a dual Op Amp amp window
comparator.

Vdd may be supplied by an internal regulator with switch options or
alligator clips to D.U.T.

Open Circuit may detected by dual OA window comparator with the probe
biased to mid-Vdd

the probe could also be used to inject voltage at high impedance for CMOS with a 1Hz clock.
Indication can be both LEDs dim or OFF or as you specify.

Input impedance ought to be >=100k and input capacitance < 10 pF.

This is a partial solution without glitch detection.
You can switch to simulate an open circuit and then slide the voltage to the probe. Simulation
Comment: Is the oscillator required or I can provide a fixed "mid point" voltage by another solution? OFF TOPIC: The link is broken, too many schemes :) "https://%20http://ww".
Comment: Fix link.  Nothing is required unless you specify it, like I did. It was just a novelty to inject pulses while being high impedance and open circuit detection in one circuit

Best way to detect high-impedance state