ZiLOG Z80 fan-out (CMOS version, Z84C0006PEC)

Question: I am building a z80 based 8 bit computer, and I really don't want to fry my chip. I was looking at this datasheet, but I couldn't find the fan-out specified. There are some electrical characteristics on page 30, but that's about it. In the book called "Build your own z80 computer" by Steve Ciarcia, his outputs were buffered. If I'm not mistaken, CMOS should be able to drive quite a few inputs, but I'd appreciate if someone who knows better could tell me whether or not should I buffer them. I would like to reduce the price of the whole project if possible, and pack it in a box that is as small as possible, so it would be cool if I could avoid it.
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Comment: If you'd use 74HCT (CMOS) type logic and CMOS or NMOS other big chips you probably shouldn't worry about fanout until you have say 1 to 10 fanout.
Comment: If it's all CMOS and Z80 you can ignore the issue for anything but a large system. RC014 systems with 10 slots are unbuffered and work fine at 8MHz (and usually way faster). Some of the other CPUs need more care - WDC 65C02/C816 setups often rely on capacitance to cover the lack of write hold, 6803 has a ground bounce if you don't buffer the address lines but CMOS Z80 to CMOS is fine.
Answer: 74xx245 are cheap and/or can be salvaged from computer scrap (if you are capable of desoldering) in large amounts.
Unless you are driving TTL circuitry, the reason you want a buffer is more about capacitive than DC loading.
Mind that NMOS and TTL outputs can be rather asymmetric regarding high vs low driving power, and that TTL inputs are asymmetric regarding consumption!
Comment: All the components I'll be driving are CMOS. I do have a few of those 245 thingies saved for another another project, guess I'll use those. Damn, this thing just got complicated. Thank you for your answer :)
Answer: According to Z84c datasheet, all timings are measured at 100 pF capacitive load. Since your surrounding chips are CMOS, you shouldn't worry, as already said, about DC loading, only capacitive one. The capacitance of single IC pin is in range of 5..10 pF, so if your board has fanout  less than ~10..15 IC pins per any Z80 output, here you have nothing to worry about.
But there is still some thing inherent to CMOS Z80 -- it makes very sharp signal changes so that even 20 cm of a PCB track connected to Z80 output could have significant ringing on every state change, up to the level other chips would accept that ringing as an extra signal changes.
Comment: That depends on how do you treat strobes from the CPU. For example, if any of them used just to trigger write into latch or flipflop (like 74373/374) then usually ringing does not cause problems even if corresponding latch or flipflop triggers twice. Same for read operations, enabling kind of data bus buffers (74245 or 74373 with its outputs in databus) cause no problems. In contrast, when there are some complex ICs on the bus, that for example do some counting on read/write strobes, or merely resync that strobes on its own, too fast, internal frequency, chances are that you'd get problems.
Comment: "even 20 cm of a PCB track connected to Z80 output could have significant ringing on every state change" ... interesting; knowing this, I'd have assumed that Grant Searle's 7.2MHz Z80 computer on breadboard would be effectively impossible, but it clearly works.  Perhaps he's only using peripherals that are particularly tolerant.  Or maybe the capacitance of breadboard is preventing the issue.


Rav Yosef Caro's lack of Sefer HaMitzvos

Question: A friend of mine once claimed that Rav Yosef Caro didn't have access to Rambam's Sefer HaMitzvos. It's apparently evident from comments he makes in Beis Yosef and Kessef Mishnah. I was hoping to see some examples where this is evident.
Answer: Perhaps he meant to share the claim regarding a number of Rishonim.
From Halachipedia

There's a debate regarding if specific Rishonim such as the Samag,
  Maggid Mishneh, and Migdal Oz saw the Sefer HaMitzvot, as it wasn't
  translated from Arabic to Hebrew for some time. Later Acharonim culled
  lists of instances in which each of them cites the Sefer HaMitzvot to
  dispel those claims.[38]

Regarding your friend's, claim, I wonder how tenable it is given R' Yosef Karo had all these Rishonim.
Also, a quick search on Bar Ilan for "ספר המצות" in Kesef Mishneh, Beit Yosef, and Avkat Rochel yield over a hundred results. True, some are for the ספר מצות גדול\קטן, there are a bunch that clearly are citing the Rambam in the sefer and in the shorashim.
Comment: Indeed, SH was translated durring the Rabmams lifetime. In his responsa he even directs non arabic speakers to it
Comment: Those are also good reasons to say this claim is incorrect.
Comment: @Mbrevda I think he even referred them to it before it was translated, on the grounds that so much of it is quotations from the Gemara that even someone who doesn't understand Arabic would be able to get the general gist.
Comment: Didn't Rav Yosef Caro know Arabic? Also, wasn't Sefer HaMitzvos translated by ibn Tibbon in Rambam's lifetime?


If a woman has only had 1 partner since her last period (i.e not three months) is that sufficient to determine the father?

Question: I've heard that a woman should separate from men after a divorce for 3 months, so that there isn't a question as to who the father is. If this didn't occur, but a single period elapsed, would this also be sufficient bidieved?
Comment: No. Women can bleed during pregnancy sometimes.
Comment: Actually 90 days - excluding the day of divorce and the day she remarries.
Comment: @DoubleAA sounds like you ought to write an answer.
Comment: Similar: http://judaism.stackexchange.com/q/62069
Answer: There is a wealth of material in this issue in the mamzerus responsa beginning from around the period of the Noda B'Yehuda. Basically, the Talmud states clearly that a woman may continue to have her period until 3 months into her pregnancy (Niddah 7b and 8b):

מעוברת משיודע עוברה ... וכמה הכרת העובר סומכוס אומר משום רבי מאיר שלשה חדשים
A pregnant woman [may assume she will not be getting her period] from the time the pregnancy is known... and how much is that? Sumchos says in the name of Rebbi Meir three months

In the times of the early Acharonim, it became a topic of discussion that experience showed that women stopped getting their period before this time, possibly due to a change in female physiology.
The Noda B'Yehuda in Even HaEzer 69, who was asked whether a woman who had gotten her period since her husband had passed away and then had a child could be assumed to have had the child out of wedlock, argued that it was still common for women to bleed during the first three months (and even afterward in some cases), and that the reports to the contrary were not to be relied on to defame a woman in good standing.
Rebbi Akiva Eiger (born 1761), in a responsum (#128) regarding a woman who was now pregnant and whose husband had been out of town since the last time she had her period, wrote that even though it is uncommon for women to bleed once they conceive, it is still a plausible enough occurence that one cannot rule out the possibility that she was pregnant from before she had her period (and the child is therefore not a mamzer).
Chasam Sofer (born 1762), in responsum YD 169, makes the assertion that bleeding is reduced upon conception but still continues until three months.
Rav Vozner (born 1913) (Shiurei Shevet HaLevi 189:33 #2) assumes that one cannot be lenient due to this reality and obviate concerns of vesst. One would assume that would apply to the prohibition of havchana (abstaining for three months after divorce) as well.
There are a few opinions with which I am familiar that entertain taking the "new reality" into account. The Sidrei Tahara to YD 194:2 cites the opinion of the Avodas HaGershuni (born 1618) and the Shvus Yaakov (born 1661) that the age of a miscarried fetus can be calculated from the date of the most recent period, implying that the pregnancy can be assumed to not have begun before then, and the qualification of the Talmud was merely defining the point at which one reliably knows a woman is pregnant. [Teshuva Me'ahava (1:70) implies that he in principle agrees to this.] Sidrei Tahara himself rejects this approach, as do the Chavas Da'as and Chasam Sofer.
Be'er Moshe 1:48:3 refers to a lenient opinion which is based on the assumption that the three months was only in the times of the Talmud when women continued bleeding until three months. However, he leaves the matter unresolved, and is only discussing vesstos, not havchana.
So the overwhelming consensus, certainly of the later authorities, is that, despite the fact that we see that women do not necessarily have their period during the first three months of pregnancy, the halacha does not change to reflect that fact.
Comment: Even nowadays women can bleed sometimes intermittently during pregnancy, but it's not menstruation, and probably wasn't back then either.
Comment: `reports to the contrary were not to be relied on to defame a woman in good standing.` Along similar lines, see *Y'vamos* 80b ("הא דעבד רבא תוספאה עובדא באשה שהלך בעלה למדינת הים ואישתהי עד תריסר ירחי שתא ואכשריה"), followed by Rambam ([*Hil. Issurei Bi'ah* 15:19](http://hebrewbooks.org/rambam.aspx?rid=4367)) and Shulchan Aruch ([EH 4:14](http://beta.hebrewbooks.org/tursa.aspx?a=eh_x7499)).
Answer: I don't think that such a question is for a blog. the accepted psak is that as Double AA has written, however just for the sake of something different here's a Chidah (Chomas Anach Shmuel 2 11 4)
ותבא אליו וישכב עמה וכו' פי' ואין כאן חשש אסור משום הבחנה כי היא מתקדשת מטומאתה מנדתה וכיון שהיה לה ארח כנשים ודאי אינה מעוברת וגם היא פנויה כי  היה לה גט מבעלה דכל היוצא למלחמת בית דדוד גט כריתות כותב לאשתו מפרשים


How does onion routing work?

Question: How does onion routing work? How would it be explained to someone who has a basic understanding of networking?
Answer: Onion routing is the heart of Tor.  In the most basic sense, Tor programs running on multiple computers across the internet each make a random decision as to where to route the individual packetflow of a given connection.  There is typically constant flows of encrypted traffic between Tor programs, so it is an electronic and encrypted version of the old 'which cup is the ball under' trick you might have seen at a circus.  Some Tor programs are configured to permit traffic to 'exit' the Tor network and head to standard internet websites.  These are called exit nodes.  A given particular destination must have at least one exit node willing to permit traffic to
flow prior to the connection being made.  To make a connection, an application will talk socks5 to a local Tor instance which will then relay the connection through the Tor network to the exit node.  This process is known as onion routing.  It seems aptly named because one would have to peel many layers to find out exactly where the packetflow has been and is going to ultimately reach an exit node ;-)

How does onion routing work?


What can be sprayed over ceiling mastic?

Question: I want to spray a ceiling to seal it and possibly also obscure the uneven surface but am unsure what will stick well.
This ceiling assumably used to have tiles. There is an array of thick glue-like blobs over drywall. I believe this mastic could be unsafe to leave exposed and unsafe to scrape or sand, in addition to being a lot of work.
I have an airless paint sprayer and can rent a large texture sprayer for cheap. The question is, what will adhere to this ceiling and not fall off? Can I spray drywall mud, then primer, then knockdown texture, then latex paint? Are there special products that might bond better than the drywall mud?
Comment: If the mastic bumps are thick it might be better to trowel the mud , I am not sure why you think mastic would be unsafe to scrape. It may be liquid nails or another construction adhesive.
Comment: Up until the 1980s, manufacturers mixed asbestos into many types of mastics. These old ceiling tile glue pods might be unsafe and I wish to entomb then rather than disturb them.
Comment: Asbestos was used in almost all popcorn ceilings but I do not remember it being in products other than black mastic that stuff is full of it.
Answer: Acrylic admix is a liquid additive formulated to increase bonding performance when mixed with joint compounds or texture when additional adhesion may be required or when applying over questionable surfaces. It seems to work for this purpose.


Where can I find styles.xml and themes.xml?

Question: The Android Styles and Themes documentation points developers to the styles.xml and themes.xml files because they say the style references are poorly documented.  However, the links are broken.  Where can I find them?

The R.style reference, however, is not well documented and does not
  thoroughly describe the styles, so viewing the actual source code for
  these styles and themes will give you a better understanding of what
  style properties each one provides. For a better reference to the
  Android styles and themes, see the following source code: [links
  broken]
Answer: themes.xml and styles.xml are now available on github
in the following directory:
platform_frameworks_base / core / res / res / values.


View a file in a tar archive without extracting it

Question: I want to view the content of the tarred file without extracting it,
Scenario: I have a.tar and inside there is a file called ./x/y.txt. I want to view the content of y.txt without actually extracting the a.tar.
Comment: If you use Emacs, you can simply open the tarball in it.
Comment: Er, to view it, you have to extract it.  I guess what you mean is "without writing it to a file"?
Comment: Consider also: using a fuse file-system, to mount the tar as a file-system. If make the files in the tar, just look like files.
Answer: It's probably a GNU specific option, but you could use the -O or --to-stdout to extract files to standard output
$ tar -axf file.tgz foo/bar -O

Comment: Ah works, but I did not manage to print output on new lines. ex; `tar -axf file.tar.gz --wildcards --no-anchored '*read_this_file*' --O` when for example, many files match `*read_this_file*`. Everything gets printed on the same line. From the `man`, I found `--to-command`. so Passing `--to-command="echo '' && cat"` is a bit of black magic but it works :D
Comment: Just this is needed in answer: `$ tar -axf file.tgz foo/bar -O`
Comment: Unfortunately the order of arguments matters and, for BSD tar (e.g. on macOS), this answer doesn't work; instead, `-O` needs to *come before* the entry name, e.g.: `tar xf file.tar.gz -O foo/bar`.
Answer: This prints contents of ./x/y.txt from a.tar to STDOUT.
tar xfO a.tar ./x/y.txt
Note: it's a capital "o", not zero
Comment: hint: it's a capital "o", not zero.
Comment: This works in FreeBSD.
Answer: This is simple as
less  a.tar:./x/y.txt

This magic trick works if you have lesspipe installed and if the env variable LESSOPEN is defined to be | /usr/bin/lesspipe.sh %s which is expected if you have lesspipe installed correctly.
Comment: That's an awesome script - but there is more than one. As I understand it, [this `lesspipe.sh`](http://www-zeuthen.desy.de/~friebel/unix/lesspipe.html) should probably be preferred.
Comment: Will that work on compressed tarballs?
Comment: It should. But I just found it does not work in ubuntu. Go figure. They have broken or removed the feature. You can still view archive list with less but not file content :-(
Answer: Oh, but this is a question about the contents of a file within a tar file. And actually, in some cases this isn't so hard. The thing is, a tar file is just a blocked out stream file - each file within the archive is found after the one before it, and each file gets a metadata header based on a specified format.
Based on that format, I once wrote shitar - which was a few lines of dd and shell scripts which could tar up a stream of block devices on the fly. Based on same, more recently I wrote these few lines of code:
tar --no-recursion -c ./      |
{ printf \\0; tr -s \\0; }    |
cut -d '' -f-2,13             |
tr '\0\n' '\n\t'

... for picking apart a tar file on the fly and performing inline transformations on its component text files. There the cut fields point to fields 1,2,13 of a NUL delimited  line of input. Such things are easy when the tar file contains only text files because tar's record delimiters (as might occur once every 512 bytes) can just be squeezed down to a single NUL per and stripped off - without requiring you to count the occurrences as you do.
tar's header format looks like this:
field    offset   len
name     0        100
mode     100      8
uid      108      8
gid      116      8
size     124      12
mtime    136      12
chksum   148      8
typeflag 156      1
linkname 157      100
magic    257      6
version  263      2
uname    265      32
gname    297      32
devmajor 329      8
devminor 337      8
prefix   345      155

Understand that there is a steep slope between the relative ease of handling simple tar operations with the vastly more complicated aspects of the archive format. While simple things - like packing a small group of homogeneously typed files together or even splitting out an archive containing only members whose types you can predict - can be easily done with a few shell pipes, reliably handling arbitrary archive members is no trifling matter.
It is especially difficult when those members might contain arbitrary binary data - which would certainly preclude any reliable application of tr -s - and this difficulty only compounds when files of various types other than regular and/or charsets other than your native one are used and/or the original archive was created by an implementation with format application idiosyncrasies you are unprepared to handle. And this is only touching on the basic, standardized aspects of the tar archive type - add in extended headers and format extensions and sparse files and compression and... well, good luck with those.
Back to basics, though, the standard record-size for a tar archive is 20 blocks - or 10240 bytes. Given an archive blocked on the standard record-size and containing only standard file types and standard ustar headers, though, you should skip from member-header to member-header by doing reads according to the size header field until you find a member matching the one for which you seek. Once there, read in size bytes from the offset beginning at the tail of your target's member header. And that's your file.
Skipping over the headers isn't terribly easy, though. Different types either will or won't have actual data blocks appended that correspond to size. For example, directories and links will contain no such data block, only a header description, and so you must be prepared to verify the current header's filetype before ascertaining exactly whether you should apply its size field to your skip formula or not.
Also, the record-size factors - depending on whether or not the archive-members' sizes sync up well with the 10240 standard record-size there may or may not be an additional 0-block appended to each. And the record-size can be declared at archive creation time - and so it may not even be 20 blocks at all, though, by spec, it must always be blocked on 512-byte units:

ustar

The tar interchange format; see the EXTENDED DESCRIPTION section. The default blocksize for this format for character special archive files shall be 10240. Implementations shall support all blocksize values less than or equal to 32256 that are multiples of 512.

So if you were working with a tar file which might contain files which might contain arbitrary binary data you would have to skip through the file algorithmically, and according to filetype. The spec says:

The size field is the size of the file in octets.

If the typeflag field is set to specify a file to be of type 1 (a link) or 2 (a symbolic link), the size field shall be specified as zero.
If the typeflag field is set to specify a file of type 5 (directory), the size field shall be interpreted as described under the definition of that record type.
No data logical records are stored for types 1, 2, or 5.
If the typeflag field is set to 3 (character special file), 4 (block special file), or 6 (FIFO), the meaning of the size field is unspecified by this volume of POSIX.1-2008, and no data logical records shall be stored on the medium.
Additionally, for type 6, the size field shall be ignored when reading.

If the typeflag field is set to any other value, the number of logical records written following the header shall be ( (size+ 511 ) / 512 ), ignoring any fraction in the result of the division.

...and, of course, considering also the individual size of each header - which is an additional block per member. So you might skip through read by read from header to header until you land on one matching the header for which you seek, at which time you would then need to check whether the current record merely describes a link to your file or to the actual file. This is especially relevant because when the same file is added to an archive multiple times many tars will only include link headers because the actual file's data can already be found elsewhere within the archive.
Having verified that you'll need to apply your calculations to the chksum field and verify the file you think you have is actually the file you want after all. tar's chksum is fairly simple though-:

cksum

The chksum field shall be the ISO/IEC 646:1991 standard IRV representation of the octal value of the simple sum of all octets in the header logical record. Each octet in the header shall be treated as an unsigned value. These values shall be added to an unsigned integer, initialized to zero, the precision of which is not less than 17 bits. When calculating the checksum, the chksum field is treated as if it were all <space> characters.

Of course, you wouldn't actually have to do any of that, because tar can already do that - that's what it does - and so you should probably just use it to search the archive and extract the file for you. In doing so it won't do anything very differently than you would do if you knew what you were about, except that it will probably do it better and faster because that's its job. And anyway, why should you?
Answer: You can use this line
tar -axf a.tar -O

Comment: This will show any file there is in the tar, not just `y.txt` and it is not clear from the OP's question that that is the only file in the tar.


TRNG and Stream ciphers

Question: How do phenomena such as mouse movement or thermal noise are exploited as TRNG within a symmetric crypto-system  (e.g: Stream cipher)? I will really appreciate if anyone could provide me with a genuine use case in order to well understand the concept. Many thanks in advance.
Answer: I think you're confused about the module boundaries (so to speak) between the symmetric algorithms and the random number generator.
Symmetric algorithms generally expect some of their inputs to be randomly selected.  For example, symmetric algorithms generally expect secret keys to be randomly selected.  Some algorithms also want you to supply non-secret, randomly selected nonces or initial values (IVs).
But those are preconditions that the algorithms expect you to meet before you call them.  The mechanism for generating random values is external to them.
Here's a silly car analogy.  If symmetric algorithms are like cars, then random numbers are like fuel and random number generators are like refineries:

Cars use fuel, and fuel is made in refineries, but neither the fuel nor the refinery are parts of the car.
The car's function depends on the composition of the fuel, but not really on the design of the refinery—as long as it produces appropriate fuel, any refinery is fine.

Symmetric algorithms can be fed randomness directly from a random number generator, but very often they're "fueled" indirectly from intermediate sources that use an RNG.  The most significant of these is the Diffie-Hellman key exchange.
You may enjoy reading chapter 9 of Ferguson, Schneier and Kohno's Cryptography Engineering, which happens to be the one available as a free sample, which motivates and details the design of one cryptographic random number generator design, at an informal, introductory level.
Comment: Thanks for your reply :) I want thus to understand the process of random number generation using such mechanisms as entropy sources. I couldn't find any reliable doc or example so far on it. The doc you did mention doesn't cover or explain it as well as I am expecting.

TRNG and Stream ciphers


How to prevent chocolate from melting?

Question: It is tough to store chocolates during summer season. Is there any way to prevent chocolate from melting ?
Comment: Do you have air conditioning? Refrigerator? Freezer? Any way at all to chill things?
Answer: I store mine in the freezer. It keeps it hard and thaws without an issue.
Answer: Fridge or freezer are both good places to keep your chocolate so it doesn't get melty. If those aren't an option for some reason, then try finding a cool, dark place that won't get too much hotter during the day. A couple ideas:
1) A cellar. Cellars often stay 10-15 degrees cooler than the house they're attached too.
2) Cupboards - if choosing this option, pick a cupboard that has a tight fitting door, is somewhat large (so there's more air in there to stay cool) and that is close to the floor. All of these will help it stay cooler during the day.
3) Pack in an insulator - putting it in a cooler or surrounding it with styrofoam packing, for example. Alternately, you could try to find another insulating material/food already in your kitchen (maybe packing it in rice or flour might work? You might just end up with chocolate-y rice/flour).
Really, the fridge is your best bet if you have one.
Answer: For optimal chocolate storage, you shouldn't really store it in the freezer or the fridge. Chocolate is sensitive to temperature and the taste does change if it's cooled too far. Not to mention condensation usually occurs on the chocolate afterwards, and water and chocolate are generally non-mixy things.
But then again, you might not care too much about that, and I suppose it depends on what you're using the chocolate for (baking, eating, decorating, etc.) as to whether it would matter all that much.
Personally, I would suggest a microwave. It can act as an insulator of sorts. You could also use it to keep things warm too, sort of like a vacuum flask. Works for me.
Answer: Never, ever keep chocolate in a pocket. Avoid keeping chocolate in a backpack. If you have air conditioning, you should be fine leaving it out. If you don't have air conditioning, freeze your chocolate and then defrost it an hour before consuming.
Answer: You will need an isolated box, that you keep cool. This can be done by adding ready made ice, or by a closed-circuit pump system, that removes hotter gas molecules before feeding them back in the system, so inside of the box is it colder, and outside, where the pump is, it is hotter.
Comment: In laymen terms: use a fridge, freezer, or cooler with ice.
Comment: Never been that good with laymens' terms answering complicated questions. Thank you.
Comment: I have refrigerator but iam a chocolate manufacturer in india..so the weather is too hot in summers so even its tough to make a chocolate in sunny days.That is the reason people stop making chocolates in summers here.Is there any way to run business smoothly without any weather hassles?

How to prevent chocolate from melting?


What is this plant with green leaves and white marks?

Question: This plant started popping up behind my garage and it's growing out of control.  What is it, and what should I do about it? I live in Ohio.
Answer: It's Lamium galeobdolon, commonly known as Yellow Archangel or deadnettle. The latter name is because the plant resembles common nettle, which stings, but Lamium has no sting. Classed as a wild flower or a weed, depending on whether you want it growing or not! You can dig it out, or remove some of it, if you want. Does well in shady places, but, as you can see, it can be rather invasive.
Answer: Lamium galeobdolon 'Variegatum' sometimes called aluminium plant although it has many other names, and there are other plants called aluminium plant too.
In some areas it is classified as an unwanted or noxious plant and in such cases should be removed if possible. The problem with it is that it creates a dense covering that prevents the growth of native seedlings and other plants.
In other areas, you can deliberately buy it as a ground cover or to use in hanging pots. It is particularly useful as a ground cover in shady areas, as it can be hard to find good plants for those conditions and this one grows well and looks pleasant.
See this site for information on removal if desired King County, Washington .gov


Why is violating "the only x and y principle" a problem in personal identity theory?

Question: The only x and y principle states that the relation between "x=y" only depends on facts about x & y. So if we say something like "Bob is this particular human being" then this relationship is only determined by facts about "Bob" and facts about "this particular human being". Many people seem to say that violating this principle is a problem for personal identity issues. But I really don't see how this is an issue. For instance, facts about "Bob" are contingent on facts about "Bob's mother" who named him. What's the main problem about violating this?
Comment: I am not sure bringing his mother into it is violating the condition.  A 'fact' can involve a whole chain of proof and any number of constants -- as long as it has only one parameter, it is still 'about' the thing that fills that slot.
Comment: It's the relation of **identity** between `x` and `y` that only depends on facts about `x` and `y`. Other facts about `x` and `y`, except identity, may depend on `w`, `z`, etc.
Comment: I think you're [confusing "Bob" with Bob](https://en.wikipedia.org/wiki/Use%E2%80%93mention_distinction) in your example.
Answer: The 'only x and y principle' was introduced by Harold Noonan. Whether this is the best name for the principle is questionable but here is what Noonan says :

A plausible principle governing identity is that whether a later individual, y, is identical with an earlier individual, x, cannot ever merely depend on whether there are, at the later time, any better candidates than y around for identity with x. I shall call this principle 'the only x and y principle'. ( H. W. Noonan, 'Wiggins, Artefact Identity and 'Best Candidate' Theories',  Analysis, Vol. 45, No. 1 (Jan., 1985), p.4.)

Noonan here and elsewhere ('The Only X and Y Principle', Analysis, Vol. 45, No. 2 (Mar., 1985), pp. 79-83) has complex arguments for the principle but I think it can be explained by means of a fairly straightforward example.
Take X at time t. Over time, X will change : at time t+1 X might have significantly different features from X at time t. Now suppose that at time t+1, there is a Y which has more resembling features to X at time t than X himself or herself has at time t+1. The claim might be made that Y at time t+1 is the closest continuator of X at time t and therefore the 'best candidate' to be X at time t+1.
X at time t and Y at time t+1 stand in no 'ancestral' relationship. There is no causal relationship between them, only the fact of resemblance. There clearly is resemblance, similarity, between X at t and Y at t+1 but according to the principle, X is not the same person over time as Y because X and Y have independent histories and there was no prior time at which they were identical. Whether they are identical or not depends purely on facts about each of them - facts about their histories over time. Their histories are distinct; and they are not identical at t+1 merely in respect of the contingent fact that at t+1, Y has more resembling features to X at time t than X himself or herself has at time t+1.


X.509 strong authentication protocols

Question: Is there also a protocol definition in X.509? In "Handbook of Applied Cryptography" by A. Menezes there is a reference to X.509 strong authentication protocol, nevertheless, I could not find the definition of the actual protocol, i.e. which massages are passing between the partners, in the source quoted, i.e. ITU-T X.509. Can any one help with this?
Thanks
Comment: Doesn't ring a bell. Could you share that quote (or page number) from HoAC?
Comment: Page 511 protocol 12.40
Answer: The strong authentication protocol referred to in the book is defined in the Annex N of "X.509 (10/16)" published by the ITU https://www.itu.int/rec/T-REC-X.509
Here is an excerpt of the standard for the two-way and strong three-way protocols:

Annex N
Considerations on strong authentication
Strong authentication makes use of PKI as specified by this Specification, which
  provides the basic approach to authentication. However, many authentication procedures
  employing this approach are possible. In general, it is the business of a specific
  application to determine the appropriate procedures, so as to meet the security
  policy of the application.
NOTE – This Specification does not specify the procedures in the detail required
  for implementation. However, additional standards could be envisaged that would do
  so, either in an application-specific or in a general-> purpose way.

Two-way authentication, described in clause N.3, involves, in addition, a reply from
  B to A. It establishes, in addition, the following:

that the authentication token generated in the reply actually was generated by
  B and was intended to be sent to A;
the origin, integrity and timeliness of the authentication token sent in
  the reply;
(optionally) the mutual confidentiality of part of the tokens.

Three-way authentication, described in clause N.4, involves, in addition, a
  further transfer from A to B. It establishes the same properties as the two-way
  authentication, but does so without the need for association timestamp checking.

X.509 strong authentication protocols


Business visa validity after I switch organization

Question: I have a valid 2 year business visa issued in July 2014, I changed jobs in June 2015 and now my current employer wants me to travel to UK for a business trip. My visa is showing valid till July 2016. Can I travel on this visa ? Do I need to do anything to reflect change in employer? In my opinion organization is not related to business visa.
Comment: Is it this kind of visa: https://www.gov.uk/standard-visitor-visa/overview   ?
Comment: @GayotFow Sounds like your comment could be an answer?
Comment: @GayotFow Weren't those all converted to standard visitor visas?
Comment: You have a great answer below, please show courtesy towards the site's rationale by accepting it
Answer: You should not try to enter UK with your current visa and instead re-apply with an application reflecting your changed circumstances (i.e. change of employer).
Source: comment by expert user @GayotFow to your Q:

[...] What you have described is a material change of circumstances and that falls as mandatory grounds under Paragraph 320. So if you get caught, expect big time trouble, up to and including removal.

That basically sums it up nicely. If you want to know more, this is a material change of circumstances in the sense of paragraph 321(ii)

A person seeking leave to enter the United Kingdom who holds an entry clearance which was duly issued to him and is still current may be refused leave to enter only where the Immigration Officer is satisfied that:
[...]
(ii) a change of circumstances since it was issued has removed the basis of the holder's claim to admission, except where the change of circumstances amounts solely to the person becoming over age for entry in one of the categories contained in paragraphs 296-316 of these Rules since the issue of the entry clearance; [...]

in addition to paragraph 320(7A) False representation, false documents and/or non disclosure of material facts:
Your refusal notice could look something like here in the guidance for paragraph 320(7A) here on page 12:

Failure to disclose material fact
In your application, [you or another person] failed to disclose the following facts
[state facts].
I am satisfied that these facts were material to the application because
[state reasons].
Appeal rights depend on category application
You should note that because this application for entry clearance has been
refused under paragraph 320 (7A) of the Immigration Rules, any future
applications may also be refused under paragraph 320 (7B) of the Immigration
Rules.
A refusal under paragraph 320 (7B) of the Immigration Rules attracts an
automatic refusal period of up to 10 years. The period starts from the date of the previous event in which the deception or submission of falsified documents
or information was employed.
Comment: +1, succinct, cites the appropriate rules. pure use of authoritative links. This answer should hold up as accurate for a long time.
Comment: @GayotFow thanks, that is very kind of you. Obviously would have been unable to put this together without your help, much obliged!
Comment: The OP should accept this, I will leave a comment


Is the quoted ask and bid the best ask and best bid?

Question: I'm wondering, if you're on a brokerage site that offers "real-time" ask and bid quotes, are those ask and bid quotes you see flashed on the screen necessarily the best ask and the best bid available, or are brokerages limited in their ability to find the best ask and best bid at any given moment? Are some brokerages better than others in this regard? Or is it a matter of paying an extra fee to see the best ask and bid?
Thanks
Answer: If you are looking at a security trading on a United States exchange, then the SEC's rules for National Best Bid and Offer apply.

Brokers are required to execute customer trades at the best available ask price when buying securities, and the best available bid price when selling securities.

In practice, however, it is hard to enforce this rule (see Nanex, courtesy of RYogi on quant.SE).
For other unregulated markets, such as FX, quotes may vary widely between brokerages and even between major trading platforms (e.g. EBS, Reuters, which are the ultimate destinations for most orders).
Comment: Thanks. In theory, the NBBO regulation is supposed to guarantee a trader/investor the best ask and bid, but I'd still be wondering if there are _practical_ differences between brokerages in their ability to actually find the best ask and bid due to factors like broker competence, the quality of the brokerage's technology, etc.
Comment: @Jack see edit.
Comment: At least for stocks, you're going to be pretty close.  They do move fast, given all the program trading, but that also helps keep things in-line.  Any odd quote is going to be "taken out".  So you're pretty safe for Equities.  Other assets like FX, or even worse, Bonds, are much more broker specific; and there is no such thing as NBBO.


Boomerang physics and aerodynamics

Question: I posted a question before on this website asking how to build a boomerang for a project which can fly at least 200 feet, so how would I increase the flight path. A user responded with: slow down the rate at which it turns in its path: this means that the linear velocity needs to be small compared to the wing velocity (less differential lift), so we need fast rotation and a wing with small angle of attack. Second, we need a large moment of inertia so the boomerang maintains rotation during flight - @CuriousOne's suggestion of a tungsten tipped blade is certainly interesting (consider a 3 legged boomerang to give a more favorable construction). Finally - throw it hard. After researching some of the concepts he was talking about I still don't really understand how it will make the flight path longer. Can someone explain to me or direct me to a website which can, keep in mind I'm only 16 and I`m pretty new to physics. Thank you very much for your time. Please also state why you are a reliable source for my bibliography.
Answer: Let me try this another way (knowing that you are 16 helps in tailoring the reply...!).
I don't know if you are familiar with a gyroscope: that's a fast spinning disk that appears to be able to "stand upright". If you tilt it, then it doesn't fall over - instead its axis slowly spins around in a motion called "precession". You can see a nice demonstration of this in this youtube video
Now a boomerang is a bit like a gyroscope: it is spinning around and has angular momentum. Because it's shaped like a wing (in cross section) it creates some "lift" for itself which would normally keep it flying. BUT: there is something that makes the boomerang change its angle. That "something" is TORQUE: torque that is caused by the fact that the bit of the wing that is moving in the same direction as the boomerang "feels" more wind (and thus experiences more lift) than the wing moving backwards. This is actually a problem that helicopters experience all the time, and in fact in helicopters they change the angle of the rotor between the forward and the backward part of the motion to counteract this (fun fact that I am guessing you did not know - see for example

which can be found on http://avstop.com/ac/basichelicopterhandbook/ch2.html .
Now since the boomerang does not have the ability to change the angle of attack, it does experience a torque - and just as in the gyroscope, this torque causes precession. This precession is a good thing for the boomerang: if you throw it in the right way, it will change the direction it is pointing so that over time it returns back where it came from. Now the key to "flying far" is to slow down the rate of turning so it flies a long way (makes a big circle). To do so, we need for there to be relatively little torque compared to the angular momentum of the boomerang, because that's what gives rise to precession.
The torque is a result of the difference in wind speed experienced by the forward moving "wing" compared to the backward moving "wing" of the boomerang. If we consider a boomerang of radius $r$ moving at angular velocity $\omega$, then the wing tip moves with a velocity $v_{tip}=\omega r$, and if you have a forward velocity $v_f$ the relative difference can be found by looking at this diagram:

from which you can easily see that the wing will have greater lift when moving in the forward direction. The difference divided by the mean (relative difference in lift) then becomes:
$$\frac{v_{tip}+v_f}{v_{tip}} - \frac{v_{tip}-v_f}{v_{tip}} = \frac{2v_f}{v_{tip}}$$
Obviously, when $v_{tip}>>v_f$ the difference becomes small. This is why you want fast rotation of the boomerang relative to the forward motion - it slows down the precession and therefore the rate at which the boomerang "turns around and comes back". You can achieve this by making the boomerang larger - the tip moves faster, and as an added bonus you increase the moment of inertia.
The only other thing we need is that the boomerang must not lose its rotation too quickly, so it needs a large moment of inertia; and though we want it spinning quickly, we don't want it to get too much lift (just enough not to fall to the earth) - that means we need a low angle of attack so it gets the right amount of lift for the rotation speed.
As for "credible reference": this is of course a"the interwebz" so you have to be skeptical about any "advice" you read. Take a look at how this site works, and how it has developed its own version of "peer review". Then decide if you can cite me as a "credible source".
You may find the article at http://hyperphysics.phy-astr.gsu.edu/hbase/brng.html quite helpful - it has some diagrams explaining all of the above, and it's generally considered quite a reliable source of information about physics. They use the same simplified model of motion for the boomerang that I used (although I had not found that page when I wrote the above).
One final thing. You can easily make a miniature boomerang by cutting a piece of thin cardboard in the approximate shape of a boomerang - if you take an old playing card and cut it like this, it is just about right:

Bend the edges of the card like so (just a few degrees, to give it some lift):

Finally, lay the card on the back of your left hand and give it a flick with your right index. When you point it up a little bit, it will fly off and return to you. A genuine indoor boomerang.
Comment: What happened, man? It's pretty obvious (at least to some of us) that you usually know what you are talking about. No need to get into advertising for yourself. :-)
Comment: @CuriousOne - I was specifically (and uncharacteristically, I admit) responding to the request "please state why you are a reliable source for my bibliography". In the broad light of day I agree it is out of place.
Comment: thank you so much you`ve been very helpful :),but where did you get that equation and what does the 1 at the start mean?
Comment: I derived the equation from the diagram I included in my latest edit. I changed it slightly so there is no '1' any more but the basic story is unchanged.
Comment: but why would a larger boomerang rotate faster, since when you increase the radius the velocity decreases.
Comment: If the boomerang spins with the same number of revolutions per second, the tip will move faster. I didn't try to estimate whether using the same throwing technique will result in slower spin for a larger boomerang. In general, for projectiles to fly further, bigger is better.
Comment: but I thought this is based on the conservation of angular momentum,which explains why figure skaters spin faster when they pull their arms toward themselves.
Answer: Throwing harder means you need to achieve two things:
1) Increase of the velocity of the boomerang.
2) Increase of the efficiency with which you can convert kinetic energy of your body into boomerang kinetic energy.
Both goals can be achieved with a spear thrower kind of extension http://en.wikipedia.org/wiki/Spear-thrower of your arm. The total mass of the boomerang also has to be matched to your particular physique. If it's either too light or too heavy for you, then you won't be able to transfer an optimum amount of energy into the device. That's exactly the opposite of what happens in sports, where the regulations force athletes to have a certain body build to be efficient. In this case you can adjust the device to your needs, which means that you may gain an advantage over a stronger/faster guy who uses the wrong weight boomerang.
Comment: I was more thinking about the gap between the simple explanation and the actual calculation. I am more of a glass is half empty guy... you may have noticed. I think we can both agree that it's a fun thing to think about ways to make boomerangs "better", though. ;-)
Comment: @Floris: Distance is a function of kinetic energy no matter how hard he optimizes the aerodynamics. He needs to start with a simple problem (how to throw a stone the farthest) before he can even dream about building a tungsten tipped boomerang. Just my two cents.
Comment: @Floris: he said that he is 16 and that he knows little about physics. You think he can handle that actual equations of motion of an aerodynamic object? As for the boomerang not changing height... did you ever look at a real boomerang trajectory??? And why are you putting words into my mouth now? I a the guy who said something about tungsten tips for larger moments of inertia. Let's end this here, before it gets ugly. Have a good one.
Comment: Agree to drop it. Do look at http://hyperphysics.phy-astr.gsu.edu/hbase/brng.html - that's the motion I had in mind (I realize there are many other, more complex, trajectories that are possible).
Comment: thank you, so much CuriousOne for taking time to try and help me :)

Boomerang physics and aerodynamics


How can I tell if an item is duped in TF2?

Question: I am a big fan of trading in TF2 and I have seen people report that some items in the game are duped, which considerably lowers the price of the item. Thus, I want to be able to know how to tell if an item is duped. Is there any easy way to find out?
Comment: What does DUPED mean exactly? and what is wrong with trading for them?
Comment: Would you take a no? There are ways to find out, but they are anything but easy. Essentially, it involves checking against several backpacks and original id, all based on history. Since history is something kept by certain websites and has been cleared at times, there's very little reliability to be sure of a dupe. The idea would be to look for original id for the item (when it was first gotten) and check for other instances with the same, but with cleared/lost history, it's a weak test. [Here's a guide.](http://www.youtube.com/watch?v=ILrdi39a2IM)
Comment: @Neffer_23 it means duplicated - fake, hacked, not legitimate, etc.  I'm not sure how it works in TF2, but in most games duped items will not remain yours forever and will eventually disappear because of their illegitimacy.
Answer: The only "easy way" is to check an item's history on www.tf2items.com or www.tf2outpost.com if you use that site. After that it involves tedious backpack checking to see if the new owner has the item and if other people who owned the item still have copies of the same exact item (which is why you check the history). If the new owner has the same copy of an item previous owners still have, then most likely that item has been duped. Duped items are seen mostly in unusual trades. I would look at this: http://wiki.teamfortress.com/wiki/Community_trading_tips, as it can prevent you from getting scammed and buying duped items.


How should I clean remnants of a melted dish from the heating element of a dishwasher?

Question: The lid from a coffee cup spent a cycle melding with the circular heating element at the bottom of my dishwasher.  While it wasn't too hard to pull it off, there is still some plastic and rubber gasket adhered to the metal.  Googling this suggests scraping in some form or another with various kitchen tools, but as the element feels just like a non-stick frying pan I'm hesitant to go that route without further input.
Answer: I've used a butter knife.  It seems like the melting process hardens the plastic and it flakes./breaks off quite easily.
Think of the heating element like a burner on a simple electric range.  They are exposed, and metal pots and pans rub on them all the time.  The element doesn't have a "coating" that you're going to scrape off like a non-stick pan.
Try to get off as much as you can.  Every little bit that you leave on is going to melt and smell bad again each time you wash until it's gone.
Answer: It's probably fine to just leave it as-is, but I'd start with a non-metallic scrub sponge, and if that doesn't work I'd move to a utility blade at a very shallow angle. It's unlikely that you'll damage the finish if you're gentle. Get the bulk of what's left off and be happy.


what can replace high carbon steel as the raw material for sewing needle

Question: I am working on a project that need to redesign a sewing needle. Because I am planning to use a stainless steel loop to replace a needle's eye, using high carbon steel, which make all modern needles, would not be industrially profitable as it is hard to weld and might take a lot of time. I am wondering is there other material that can replace high carbon steel as the raw material for a needle? Or, is there other materials soft enough to be bented into any shape and can be joint with high carbon steel or whatever material that can replace it? Thanks a lot!
Answer: There are a large number of stainless steels which are hardenable and can be used as a more or less direct replacement for high carbon steel. Common grades like 304 and 316 would be possible candidates and 440 is widely used for cutting tools.
Needles are typically made by drawing form wire stock and punching the eye so welding or other jointing processes are not required unless there is a very specific reason why you want it made from two separate materials.
It is also worth mentioning that without more details of what you are planning to achieve it is hard to give more specific recommendations. Obviously needles have been around for a very long time and as such there is limited scope for improvement unless you have a new and different application.
Comment: I am planning to replace the eye of the needle with a soft stainless steel ring so people would just have to yank the ring open to put the thread in and pin it close so it could follow the needle through the clothings.
Comment: You could achieve that by differential hardening ie only harden the point or harden the whole thing and anneal the eye.


Mount external sdcard after upgrade to Jelly Bean 4.2.2

Question: Yesterday I installed a custom rom on my Samsung Galaxy S2. Before I had ICS 4.0.3 installed. Now my external sdcard does not mount automatically on the phone. How can I get this working?
Comment: Won't mount where -- on your device, or on your PC? Any error messages? Please give a few more details, it's hard to help without them :)
Comment: Thanks. I've clarified the question. Hope this makes it clear.
Comment: OK, first part done. Second part was the "error messages". Can you e.g. get a grep on some `logcat` (e.g. using `adb logcat` from your PC; if you don't have the SDK installed on the latter, see e.g. [Is there a minimal installation of ADB?](http://android.stackexchange.com/q/42474/16575) for an "easy way")? If you cannot find your way through those log messages, you could e.g. upload them to [pastebin](http://pastebin.com/) and link from here.
Answer: A lot of sdcards are formatted as ExFAT (extended FAT32 - which is better than FAT32 in a number of aspects for flash media, including support for larger than 4GB files). Unfortunately, this is a licensed and proprietary technology and custom ROMs can't legally include support. If you have a card reader for your computer, you can use that to back up the contents of your sdcard. Then you can right click on the drive in My Computer and select format. Select FAT32 and let it do its thing. When it is finished, copy the contents back to the drive and put it back in your phone.
If you don't have access to a card reader, then you will either need to use another phone to back up your content on the card, or possibly, see if you can still enable USB Mass Storage in recovery (most likely under the Mounts and Storage menu depending on the recovery used). If you can't do either, then you can just format the sdcard from within recovery, which will put the filesystem to standard FAT32 and make it readable in your custom ROM, although, this will erase all the content on the card.
Answer: It turns out that the location or link to the external sdcard has moved from /sdcard/external_sd to /extSdCard, while the /sdcard/external_sd was blank. So I thought that the card was not mounted, but in fact it was.


Kids clubhouse game?

Question: Do you remember the game we played growing up where you would try to find a red ball, roller skates, ball glove, and teddy bear? There was a clubhouse I think as well. It's definitely a children's game. Maybe it had dice as It was a board game.Everyone plays as kids and if you find everything you first you win.
Comment: Other information that might help.  roughly what year did you play it?  Was it game you played going up in 90's, 50s?  This will help narrow it down for people.
Answer: I believe you are thinking of Clue Jr.: The Case of the Hidden Toys, published in 1996 by Hasbro and Parker Brothers.
Here's a link to it on Board Game Geek. You can look through the pictures and see if it looks familiar, but I'm pretty sure this is it.

Kids clubhouse game?


Induction motor scalar control

Question: I have been developing a software for scalar control of three phase induction motor with squirrel cage rotor. This control method is based on voltage-frequency characteristics of the motor. The used motor nameplate contains 400V, 50 Hz and the stator is in wye connection. Based on these values I expect that the slope of the voltage-frequency characteristics will be \$K_1 = \frac{(\sqrt(2)*400)}{(\sqrt(3)*50)} = 6.5\,V/Hz \$. The problem is that I would like to rotate the motor at 60 Hz. Based on the above given voltage-frequency characteristics the operating point would be in the field weakening region. I had an idea that I can prevent the field weakening of the motor by changing the slope of the voltage-frequency characteristics \$K_2 = \frac{(\sqrt(2)*400)}{\sqrt(3)*60)} = 5.4\,V/Hz \$ i.e. the nominal stator volatge would
be achieved at 60 Hz and not at 50 Hz (please see the attached graph).

Is it possible or is this idea wrong? (I see that below the 60 Hz the magnetic flux of the motor will be lower than the nominal value. But the motor starts with no load and the load torque is applied as soon as the motor achieve the no load speed.) Thanks for any ideas.
Comment: Practically your motor will never deliver the nominal torque, but Tnom/1.2 even at 50Hz, because the flux is weakened at all range.
Comment: But it will deliver nominal torque at 60 Hz or not?
Comment: No, it will never.
Answer: If the motor nameplate says 400 V, 50 Hz, wye connection, that is the line to line voltage that must be supplied for the wye connection 8 V/Hz is required. If you re-connect to the delta connection, you can supply 4.62 V/Hz. That would be 231 V for 50 Hz or 277 V for 60 Hz. If you do not provide the rated V/Hz the motor can not produce the rated torque without overheating. Rated V/Hz is required to produce the magnetizing flux for which the motor is designed.
Comment: Thank you for your answer Mr. Cowie. I am little bit confused by your suggested voltage-frequency characteristics slope equal 4.62 V/Hz. Does it mean that the RMS value of phase voltage is set by voltage-frequency characteristics? I meant that the amplitude of phase voltage is set by voltage-frequency characteristics and not the RMS value. Please can you tell me what is correct?
Comment: The amplitude is obviously determined by the RMS value and the phase voltage by the line-to-line voltage, but RMS and line-to-line values are used for all specification.

Induction motor scalar control


Gravitational redshift of Hawking radiation

Question: How can Hawking radiation with a finite (greather than zero) temperature come from the event horizon of a black hole? A redshifted thermal radiation still has Planck spectrum but with the lower temperature (remember CMB with temperature redshifted by expansion of the universe). Now, redshift at the event horizon is infinite (time is frozen for a distant observer) so the temperature of the radiation would be zero for him/her, that is no radiation is detected
Comment: It's a bit more complicated than that because the radiation isn't really emitted in the sense that a hot filament emits EM. A distant observer calculates the QFT vacuum to be different to an observer nearer the event horizon, and as a result the distant observer finds the "vacuum" near the horizon to contain a finite particle density - this constitites the Hawking radiation. I'm pretty certain (but wouldn't swear to it) that the Hawking calculation calculates the temperature at infinity. An observer falling freely into the black hole would not see any Hawking radiation even at the horizon.
Comment: The radiation comes from the region just outside the horizon. If you search the site I'm sure something like this has been asked before.
Comment: related: http://physics.stackexchange.com/q/22498/4552
Comment: @JohnRennie I see - so the radiation comes from the just outside the horizon and the redshift is quite large but finite, reducing the temperature to low values observed but still greater than zero.
Comment: http://physics.stackexchange.com/q/30597/ is related, though not a duplicate. I'm not sure I understand this area well enough to hazard an answer rather than just this comment :-)
Comment: @JohnRennie You are right. Hawking radiation is a phenomena that is measured by an observer at spatial infinity. If an observer is free falling in a given spacetime(following a geodesic) they won't see any temperature.
Answer: Oh, one of my favourite questions. Let me try to explain why black holes radiate:
To understand the Hawking radiation, you need to know about the Bogoliubov-Valatin transformation, which is often use to diagonalise Hamiltonians, and which was actually developed in regards of superconductivity and superfluidity. If you have the creation and annihilation operators $$a^\dagger$$ and $$a,$$ you can define new operators
\begin{align}
b&=ua+va^\dagger\\
b^\dagger&=u^*a^\dagger+v^*a.
\end{align}
Just defining new operators is fun, but only really helpful if certain conditions are met. The Bogoliubov-Valatin transformation is the canonical mapping from the set of $a$-operators to the set of $b$-operators:
\begin{align}
\left[b,b^\dagger\right]&=\left[ua+va^\dagger,u^*a^\dagger+v^*a\right]\\
&=\left[ua,u^*a^\dagger+v^*a\right]+\left[va^\dagger,u^*a^\dagger+v^*a\right]\\
&=\left[ua,u^*a^\dagger\right]+\left[ua,v^*a\right]+\left[va^\dagger,u^*a^\dagger\right]+\left[va^\dagger,v^*a\right]\\
&=u^2\left[a,a^\dagger\right]+uv\left[a,a\right]+uv\left[a^\dagger,a^\dagger\right]+v^2\left[a^\dagger,a\right]\\
&=u^2\left[a,a^\dagger\right]+uv\cdot0+uv\cdot0-v^2\left[a,a^\dagger\right]\\
&=\left(u^2-v^2\right)\left[a,a^\dagger\right]\\
&=\left(u^2-v^2\right),
\end{align}
where we have to set $u$ and $v$ in a way that our transformation is indeed canonical, i.e. that $\left(u^2-v^2\right)=1$. This is a transformation of the phase space. This transformation can be used to transform one coordinate system to another one, coordinate systems which are accelerated compared to each other! If we have one observer in the past and another one in the future, both observing an area where in between them a black hole came into existence, their coordinate systems are accelerated (compared to each other), because the black hole curves space-time. The past-observer sees a vacuum, maybe a star that will become a black hole, but otherwise a vacuum. The future-observer sees a strongly curved space-time full of radiation. But why?
The reason is the uncertainty principle! We don't know the energy state of the vacuum, it depends on our coordinate system! I am not talking here about the position-momentum uncertainty, I am talking about the energy-time uncertainty! The higher the precision of your energy measurement, the higher the uncertainty of your time measurement. And because the vacuum can have different energy states, it can also spontaneously create particles - the Hawking radiation.

Summary: A black holes radiates, because different observers can observe different energy state of the vacuum around a black hole. This has to do with the coordinate system and the uncertainty principle. The vacuum energy depends on both.

So, the redshift precisely at the event horizon might be infinite, but not a bit away from it.
Comment: This comment comes late. I understand the first part of your answer but do not understand how the energy-time uncertainty relation comes in here. Thus how does it imply that `the redshift precisely at the event horizon might be infinite, but not a bit away from it'? Can you elaborate a bit?


Oil/sap from the trees

Question: So we moved to a new home last week. There are two different trees, one in the backyard, one in the front. However, both trees have some oil or sap like things on the leaves, and what's worse, wind spreads the oil all over my car or anything under the trees. See attached picture please. What is causing the oil? The trees themselves or bugs? Are the white dots on the leaves bug eggs? I do see many bugs on the trees. Thx
Comment: Oil? You sure you mean oil - those leaves are infested, not sure with what, could be mealybug, scale insects, woolly aphids. That can cause sooty mould, which is black, sticky deposits, which can end up on the ground beneath, but they're not oily. Post pics of the tree/s, not sure what it is from the leaves you've shown, and what part of the world are you in?
Comment: I think I know what he is seeing.  Honey dew is kinda shiny, sticky and the black stuff comes from the secondary 'infection' of bacteria and their excrement/wastes.
Answer: You are correct!  You've got soft bodied insects that suck from the plants. Could be a number of types; aphids, white fly, etc. The sticky stuff is honeydew...the excrement from insects sucking on your plants.  The only solution is spraying NEEM, mixed correctly and only spraying at night.  Do not want to harm a single bee. Or hiring a company to annually spray your trees. Notice through the season how long this event takes place. And we need to know where it is you live on this planet.  These insects will not kill your plants but icky sticky.  Perhaps just a plastic cover overnight will solve your problems.  Less expensive anyhoo.
Comment: Thanks for the answer. I think you are right. I'm in North California.

Oil/sap from the trees


What would Europe look like if Chernobyl exploded?

Question: In a scenario where the Soviets didn't manage to stop corium from reaching the water reservoirs underneath the plant, causing a detonation equivalent to a nuclear explosion, what would life in the continent look like?
Comment: "_a detonation equivalent to a nuclear explosion_" No.
Comment: The book *Roadside Picnic* by A. and B. Strugatsky as well as the movie *Stalker* by Tarkovsky, which is based on it, as well as the *S.T.A.L.K.E.R.* games that are loosely inspired all shown what it could look like. Although, they feature quite more fantastical elements than just an explosion and aftermath.
Comment: Life in Europe would look exactly as it looks today, with the exception that a part of southern Byelorussia would be considered unsafe for human habitation and kept as a nature preserve... A puny nuclear explosion in northern Ukraine wouldn't do anything noticeable to life outside southern Byelorussia. Radioactive material is indeed dangerous, but it's not some sort of magical poison.
Comment: Can we get more detail? I can understand your question, but more background information is necessary to answer. Also, your wording is a little imprecise; *overheated fuel rods* are not "radioactive magma."
Comment: @thescribe-ReinstateMonica lava might be a better term, but [corium](https://en.wikipedia.org/wiki/Corium_(nuclear_reactor)) has also been used. It is what the [Elephant's Foot](https://en.wikipedia.org/wiki/Elephant's_Foot_(Chernobyl)) was made of.
Comment: The Soviet/Russian scientists made accurate assessments of the blast zone and the nuclear (fissile) material dust coverage zones that would have resulted.  These were mentioned in the excellent HBO miniseries; I do not know how much of these reports are available in open literature.
Comment: Can't be science based. If the lava reached the water the result would not have extended past the station. The resultant explosion would have been, tops, a few 100 kg equivalent. Not kilotons, not megatons. The explosion would have been barely noticeable at the station boundary.
Comment: @AlexP: Re "...with the exception that a part of southern Byelorussia would be considered unsafe for human habitation and kept as a nature preserve", isn't that exactly what it is today?  (Ignoring the fact that people do live in that nature preserve.)  It's also instructive to look at pictures of downtown Hiroshima & Nagasaki today.
Comment: Are you refering to a China Syndrome Event (The core is so hot it melts through the reactor floor and continues until it hits an aquifer or water table and flash boils the the water causing violent and radioactive steam eruptions from the ground)?
Comment: @jamesqf: Sorry, I meant to say a *larger* part. The [Polesie State Radioecological Reserve](https://en.wikipedia.org/wiki/Polesie_State_Radioecological_Reserve) (which is the Byelorussian part of the former Chernobyl Nuclear Power Plant Zone of Alienation -- aren't Soviet official designations a thing of beauty) covers only 2200 square kilometers (850 square miles).
Answer: There would not have been a nuclear detonation as the core materials mixed with debris would not have been fissile. And even had they been it would have been more of a damp squib than a true nuclear detonation.
That said even a conventional detonation caused by steam and chemical reactions would have been disastrous as a much larger part of the radioactive contents of the reactor would have been spread around the environment, probably at least an order of magnitude greater, but it's very hard to be certain as it would depend on the exact circumstances.
The net effect would have been a much larger exclusion zone, greater loss of life due to radioactive contamination and even more money required for the clean-up.
But bad as it would have been life would have gone on across Europe.
Comment: At least 5% of the core went away as smoke and floating micro-particles. A larger fraction than that wound up on the ground nearby.  The first responders were tripping over fuel rods and graphite rods. Much more than 10% of the core was already flung around. There was not enough material left to produce an order of magnitude increase.
Comment: If core materials would be flushed to groundwater, that would mean that the entire Dnieper river would be contaminated downstream, and all cities there evacuated, starting with Kiev.
Comment: @SRM contamination of the river would spread the radioactive isotopes into immediate vicinity. Also, people are not the only ones who drink from the river.
Comment: @Alexander why evacuate? If the river is contaminated, don't drink the water. It doesn't make the air unbreathanle or land unlivable. I don't see a need to evacuate. Am I missing something?
Answer:
causing a detonation equivalent to a nuclear explosion

Such a detonation could not happen.
At most, when the lava1 reached a reservoir of water, it would turn it into steam very quickly.  Depending on how stiff the structure was, and how long that very high-pressure steam could have been contained, you could have a Boiling Liquid Expanding Vapor Explosion (BLEVE).  The most likely case will be that the steam will push a large chunk of the molten concrete and fuel-rod mixture into the air, which would be little different from what had already been happening.
The concussion from the BLEVE might kill people more quickly, but if anyone was close enough to be killed, they're probably among the many who had already received lethal doses of radiation, and this would be a merciful death compared to what awaited them.
Molten fission fuel can not have a "nuclear explosion" in the sense of an explosion from a nuclear bomb. In order to go critical -- to have an ongoing chain reaction -- the fuel needs to have a certain density.  As the fuel becomes more dense, it heats up, which pushes the fuel apart and halts the chain reaction.  This can go on for quite some time, and it did indeed go on for several days after the initial explosion, but it is a slow burn rather than the milliseconds-long reaction that creates kiloton and megaton explosions from fission nuclear weapons.
The real danger was, if the lava did break through to under the foundation of the power plant, the groundwater would be contaminated far more than it already was, much more than the runoff from surface contamination and fallout.  The groundwater flows into the Dnieper River that supplies fresh water to Kiev, and the USSR had plans for at least a partial evacuation of that major city in case the foundation was breached.

1Pedantic note: The term lava is a correct term for the mixture of molten rock, cement, metal, and fuel rods.  Another term that can be used is corium, which is a lava created by the core of a nuclear reactor melting down.  I use the more generic term because the radioactivity has no significant effect on the behavior of a BLEVE.
Comment: Just another pedantic note:  Working Nuclear Reactors are "Critical."  What happened at Chernobyl was a "Prompt Critical" event.    Most people, (including all the writers for TV and movies ever) don't know this, and assume the former is a bad thing to shout in any emergency involving a reactor.   The latter is the "everyone panic" phrase.
Comment: @hszmv: Good point. I don't know how to explain that nuance in the context of this question. For any worldbuilders who want to get nuclear energy and weapons right: Scott Manley has a very approachable series on YouTube called "[Going Nuclear](https://www.youtube.com/playlist?list=PLYu7z3I8tdEnTQMXpP6gYN9DVm_DjXza9)".
Comment: I'd personally recomend looking at TVTrope's articles on the subject as they have a list of both correct use of terms AND an accurate depiction of what would happen if you nuke a random Mickey Mouse Organization in the L.A. area.
Answer: It would not have been much different than what happened IRL. You see, while hot-in-both-senses uranium contacting cold water will create an explosion, it will be far from catastrophic. This is because those water reservoirs are (a) underneath the plant, and (b) the uranium would come down at a trickle. The net result of these two factors is that, instead of a "Oh hey, 20 tons of ultra-hot uranium just dumped into the cisterns and by the way we are all dead from the resulting explosion", you just have a "Oh hey, there was a minor explosion in the cisterns just now, but it is completely inconsequential compared to the NUCLEAR FALLOUT ARMAGEDDON WE ARE ALREADY EXPERIENCING!!!
The net effect of all this is that the exclusion zone (the area that you can't live in) would have been a bit larger. Honestly, considering all the FUBARing that was happening anyway, I doubt that anyone would have noticed the reservoir contamination until after the fact.
Comment: This is incorrect -- massive heroic efforts were made to stop the core from reaching the water level.
Comment: @CarlWitthoft I did not mean it literally; I was just saying that, compared to the other stuff that was happening, the core reaching water level is relatively minor.
Answer: Bad, but not apocalyptic.
With ionizing radiation, any amount increases cancer risk, etc. There is no safe dosage. On the other hand, spread over Europe or the entire Earth, fallout will be widely dispersed.
According to this, Chernobyl released 5% of the nuclear material to the environment. According to this, Chernobyl equals 1% of the worldwide nuclear weapon tests. Of course not all isotopes are made equal, but a total release of the material in Chernobyl would be on the same order of magnitude as nuclear tests.
Those tests were a bad idea. In all likelihood, they caused plenty of deaths by cancer. More in some places than in others, like the well-known John Wayne example. They didn't make Earth uninhabitable.
Comment: The [linear no-threshold model](https://en.wikipedia.org/wiki/Linear_no-threshold_model) for nuclear radiation is almost certainly wrong.  It's used because it's the most conservative of the three competing models for low-dose radiation, and because it's the easiest to apply (effects of low-dose radiation are so small that they're masked by statistical noise, making it nearly impossible to find a threshold for the threshold model, or the inflection point for the hormesis model).
Comment: The statement about ionizing radiation is at best unproven, and probably wrong, since there is quite a bit of evidence that people living in places with high natural background radiation are no less healthy, and sometimes healthier, than those living in low background areas.
Comment: @jamesqf People in those areas are sometimes healthier but not because of the radiation.  For example, Colorado may have higher levels of radiation and better general health than Louisiana, but there are massive lifestyle differences that make Coloradans healthier in spite of the one increased risk factor.  Actual laboratory testing supports om's  claim  https://www.sciencedaily.com/releases/2005/07/050725185400.htm


Ejected planets during the early stages of the formation of the Solar System

Question: This is similar to a previously asked question, but I am just asking about theory rather than observational evidence. Assuming there were a much larger number of protoplanets in the early Solar System, what percentage of these would theoretically have been ejected (orbital velocity increased beyond escape velocity due to interaction) and what percentage would have been just moved out to a more distant orbit (say, in the Kuiper Belt or the Oort Cloud)?
In other words, should we expect to find rocky worlds in the far outer Solar System that were formed inside the current radius of the asteroid belt?
Comment: note: I did a quick calculation and at the Earth's distance from the sun, the escape velocity would be about 42 km/sec (the Earth's orbital speed is about 30 km/sec).
Comment: qualification: in previous comment, this would be vertical escape velocity (ie. radial direction away from the sun)
Answer: We can identify two distinct realms of planetesimal formation - the inner Solar System and the outer Solar System. The initial group of small bodies1 in the inner section of the protoplanetary disk was rather quickly accreted into planets via various interactions; some bodies grew bigger and thus dominated the disk around them. In the outer parts of the disk, many small bodies were accreted into the cores of the giant planets. However, not all were accreted, and there may in fact have been a second generation of planetesimals that formed via gravitational collapse due to disk instabilities.
We can compute timescales for this "cleanup" phase, the period during which small bodies were either accreted onto protoplanets or ejected to larger orbits or out of the Solar System entirely. In the inner section of the disk, among the terrestrial planets, it was on the order of $\sim10^8$ years, while in the outer section, it was on the order of $\sim10^9$ years. However, the giant planets may actually have finished accreting matter before the terrestrial planets.
This means that the small bodies that were ejected or boosted to larger orbits were largely from the outer Solar System, terrestrial planetary migration effects aside2. Various N-body simulations agree that a total of $\sim300M_{\oplus}$ of small bodies was ejected from the outer disk, due to interactions with Jupiter and the formation and evolution of Uranus and Neptune. We know that the Oort Cloud's mass is on the order of $\sim1$ to $\sim10M_{\oplus}$, and the Kuiper Belt and scattered disk have masses an order of magnitude or two lower. Therefore, only a small fraction - certainly less than 10% - of the small bodies could have been ejected into the Oort Cloud.
If you want to visualize some of this, check out this video from a simulation of the Nice Model3. It shows the orbits of the four giant planets (Jupiter in red, Saturn in orange, Uranus in purple, and Neptune in blue). It shows Neptune and Uranus swap orbits, and also shows the dispersal of a large group of small bodies outside the original orbit of Uranus (which began further out than Neptune). The video shows the simulation over the course of a bit more than one billion years, and you can see a drastic clearing out of the extreme outer disk over this timescale.
The above results came before the development of the Nice Model and its variants, as well as the possible discovery of Planet Nine and the discoveries of many small objects in the outer Solar System, beyond Neptune. They should therefore not be taken as totally accurate descriptions of the formation of the Solar System - the video in particular is likely incorrect - and I'm guessing I'll get some flak for using them as the basis of this answer.
Obviously, many N-body simulations of the Solar System have been done, and over the last decade, a substantial number use some variant of the Nice Model. Some use planetesimal disks - excluding the masses of the eight planets - on the order of $\sim10$ to $\sim10^2M_{\oplus}$; Nesvorny & Morbidelli (2012) (two of the architects of the model) found a sweet spot, so to speak, of $20$ to $50M_{\oplus}$, depending on the exact result. As I understand it, masses of the surplus disk around these values fit the Nice Model well, and so perhaps we should drop the total mass of the ejected small bodies by a factor or about ten.
Footnotes
1 I used this term quite a lot here; it refers to bodies with masses in the planetesimal regime and lower. The reason for this is that a substantial amount of the ejection came after the inner planets had formed, and the remaining bodies were small.
2 This should not be taken to mean that no or very little matter from the inner Solar System was ejected, especially if we assume some movement among the orbits of the inner planets (see Meech et al. (2016)).
3 Originally brought to my attention through a question on Physics Stack Exchange by Kyle Oman.
References

Goldreich et al. (2004) (early N-body simulations)
Krivova & Solanki et al. (2003) (early N-body simulations)
Meech et al. (2016)
Nesvorny & Morbidelli (2012) (more recent results)
Comment: What about our superearth(s)? Isn't our system quite atypical in not having one and instead having a very small first planet?
Comment: @kubanczyk Are you sure you don't mean [Hot Jupiters](https://en.wikipedia.org/wiki/Hot_Jupiter)? The fact that a lot of exoplanetary systems have them is detection bias; they're extremely easy to find, compared with most other planets.


How will using desert sand instead of concrete sand affect my concrete recipe?

Question: I've read that desert sand is too round for concrete mixes. But, I'm not building a dam, I'm building a 3 foot landscaping wall. I'm interested in using desert sand for the color, so this is an aesthetic choice. How will using desert sand affect the strength of my mix? Are there ways to counteract the effect? I've been reading everything. Is desert sand ok to use, just uneconomical for large projects?
Comment: Use the desert sand to provide a facing coat while the wall is made using "proper" sand.
Comment: Dune sand and beach sand are also relatively rounded. Dune sand also tends to be uniform in size so does not "pack" very well . I doubt the affect on concrete strength would be a problem for your wall.
Answer: You can use desert sand ( or use the approved concrete coloring admixtures which will give the tint you need) with the following considerations:

Wash the sand thoroughly to remove any possible dirt or clay mixed with it, roundness is not a big issue.
Add more cement to compensate for possible reduction in strength.
Add concrete acrylic glue as per manufacturer's recommendation.
Generally the following is recommended proportion of concrete materials:

Add 6 bags of 94lbs of cement, water not more than 40% of cement weight, to a cubic yard of sand and gravel. The more water the weaker your concrete. so consider the moisture of your sand as part of water ratio.
The aggregate should be roughly half less than 3/8" and half between 3/8 to 1.25" for your application and lack of slump tests.
Don't forget curing the wall, watering it for at least a week.
One could seek advice from local trades-people as to the ways to make your concrete stronger and as far as plasticity workable.
Comment: Much appreciated. The extra detail on mix ratios also was super helpful. I think I'll probably pay someone to look over my plan and work before I start pouring.


How to project (bake) an environment texture onto a mesh as if from a slide projector?

Question:
I have an equirectangular texture applied as the environment. I go to the camera view, match the camera's settings and create my mesh so it somewhat matches the objects' shapes in that texture. I unwrap the mesh.

I deliberately avoid such methods like cutting an image into small pieces and then manually texturing the objects. It's really tedious and time consuming and I have to do tens of these images. Also, the projection, as I've mentioned, is equirectangular, so doing it manually is really not an option.
How do I project the pixels from that env. texture onto my mesh? (like a slide projector)
Comment: Look into the  UV project modifier
Comment: Render the view without the mesh, so you have a non-equirectangular version of the background, then use project from view as blender pony suggested.
Answer: One cheap and fast way of doing it is to use The unwrap option "Project from view" Like in this Picture:

This uses your current 3D viewport as to unwrap it 1to1 into the UV image editor. If you align your camera just right it behaves like a Projected Image. I recommend to do this in the orthographic 3D view as well.
Comment: It would work with a regular texture but not with an equirectangular one. Thanks for the suggestion though.
Answer: There are two ways I know of:
Version a:  If you want it to look like it actually is projected from a slide projector, you can actually put textures on area lamps or spotlights.  The lightsource will project the image onto the surface like you would expect it would from a projector:
Version b:  If instead you want to add graffitti, you would actually combine two UV mappings.  One would be for the texture proper, and the other would be for the graffitti.  This video explains the technique in a myriad of ways:
Youtube link to texture mixing video


Huge List Problem

Question: I have a list with 28,000 items. I want to delete everything except the latest months worth of data. This will be basically deleting 27,000 items. I cannot view this list in data view as the view exceeds the base 5000 items.
How can I delete these items?
After it is done I will set up a retention policy.
Answer: You should use powershell. There you can easily set the filter etc.
To view the list you should either increase the Resource Throttling settings from Central Administration.
[void][System.reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint")

$site = new-object Microsoft.SharePoint.SPSite("http://sitecollectionURL")
$web = $site.rootweb
$list = $web.Lists["Name of the List"]

$caml='<Where>
        <Leq>
            <FieldRef Name="Created" />
            <Value IncludeTimeValue="TRUE" Type="DateTime">2011-12-31T02:32:20Z</Value>
        </Leq>
    </Where>'

$query=new-object Microsoft.SharePoint.SPQuery
$query.Query=$caml | Write-Output

$items=$list.GetItems($query)
$listItemsTotal = $items.Count;

for($x=$listItemsTotal-1;$x -ge 0; $x--)
{
        $items[$x].Delete()
}

$web.Dispose()
$site.Dispose()

powershell to delete items from a list where created date < "1/1/2011"
Changing Resource Throttling

Login to Central Admin
Go to Application Management -> Manage Web Applications
Pick the Web application for which you want to change the threshold
In the ribbon above, click General Settings. That will bring down a menu, from which you should pick Resource Throttling
Change the value

Managing Large Lists
Video: Techniques for managing large lists
Manage lists and libraries with many items
Designing large lists and maximizing list performance (SharePoint Server 2010)
Retention Policies
http://blogs.msdn.com/b/ecm/archive/2011/10/12/policy-and-retention-in-sharepoint-server-2010.aspx
Comment: I get this error when I run the script: You cannot call a method on a null-valued expression.
At C:\Users\administrator\Desktop\List item Delete filter.ps1:17 char:22
+ $items=$list.GetItems <<<< ($query)
    + CategoryInfo          : InvalidOperation: (GetItems:String) [], RuntimeException
    + FullyQualifiedErrorId : InvokeMethodOnNull
Answer: Don't overcomplicate things here... you just need to do it once and there is a simple no-code solution...
Just link the list to an Access database and run a delete query on the list. Access handles large lists quite nicely, plus you don't need to modify your list view threshold, you don't need to write any code, and anyone with write access to the list can do it -- no special privileges required.
If you aren't super comfortable with making a delete query in Access (which is fairly easy I should add), you can just use the table view and delete the individual records you want, similar to using Excel.
You should be aware it will take a while to delete lots and lots of records like that -- Access may look like it's given up on you, but give it a bit of time and it will get things done.
Answer: Just in case someone finds it useful, you can also disable the list view threshold for a single list using powershell.
$web = Get-SPWeb http://server.com/web
$list = $web.Lists["BIG_LIST_NAME"]
$list.EnableThrottling = $false
Personally I would delete everything during out of work hours,  when the administrator time window that temporarily disables the LVT should also be running. In that case there is no need to disable it by code or using the Central  Administration
Comment: If I run this PS on my list will it allow me to see all 28,000 items in the data view? thanks
Comment: Yes. But disabling the LVT can have serious performance implications.
Answer: Since the number of items is large, I would suggest using SPWeb.ProcessBatchD. It is an efficient way of deleting large number of items. Here is a sample server object model code:
private static StringBuilder BatchCommand(SPList spList)
{
    StringBuilder deletebuilder = new StringBuilder();
    deletebuilder.Append("<?xml version=\"1.0\" encoding=\"UTF-8\"?><Batch>");
    string command = "<Method><SetList Scope=\"Request\">" + spList.ID +
        "</SetList><SetVar Name=\"ID\">{0}</SetVar><SetVar Name=\"Cmd\">Delete</SetVar></Method>";

    foreach (SPListItem item in spList.Items)
    {
        deletebuilder.Append(string.Format(command, item.ID.ToString()));
    }
    deletebuilder.Append("</Batch>");
    return deletebuilder;
}

Usage
using (SPSite spSite = new SPSite(site))
{
    using (SPWeb spWeb = spSite.OpenWeb())
    {
        StringBuilder deletebuilder = BatchCommand(spWeb.Lists[list]);

        spSite.RootWeb.ProcessBatchData(deletebuilder.ToString());
    }
}

Ofcourse you will need to set SPQuery object to restrict the number of items in your case.

Huge List Problem


How to record what is being clicked on google sites with google analytics?

Question: I added google analytics to a google site with the hopes of being able to record what people are clicking on (what resources are being used and not being used) in order to better optimize the page in the future, however, it seems as if only events are being clicked without any descriptor of what is being clicked or what page is being viewed. Is there a way to change this so I see what pages people are traversing, what specific resources people are utilizing? See below.
Answer: Google Analytics is tracking mostly clicked links or loaded pages. If you want to track more details you would have to use JavaScript to log each button press and/or any other activity. JavaScript would be invoked by a button click, you would have a function to log this specific click and then do the base activity that has been provided to that button. JS can call your API or PHP file to log anything you want, for example you can log whole form details to a file when a button is clicked or anything has changed on the webpage (ListBox changed selection).
The other way is to track requests in server's logs - Apache2, nginx, IIS. Each request is logged in logs so you can tell which IP at what time has requested a specific file. But this does not tell about clicked buttons (if it is not loading anything from the web server).
Answer: Google Analytics is what you will encounter, as it's the latest
Analytics version of Google.
The previous version is called Universal Analytics and can still be used.
Both Universal and Google Analytics offer similar data to users,
although Universal Analytics gives more in-depth information about user behavior, so fits better your needs.
Universal Analytics events
is described as:

This article explains how to use Tag Manager to set up Universal Analytics event tags that are triggered in response to clicks on links, clicks on other types of elements, at timed intervals, and when a forms are submitted.

As this might change in the future, here is just the main information.

Link clicks
There are two common methods shown here for how to send click event
information to Google Analytics:

Use a separate trigger and tag for each type of link.
Use a single tag with a regular expression to handle all click data.

Clicks on any element
Use this technique to measure clicks on elements other than links.
This includes clicks on images,  elements, or any other item on
the page accessible by the DOM.
Timed intervals
Measure timed intervals when you need to understand time on page when
no events are triggered.
Form submits
This technique will cause a tag to fire every time a form on a web
page is submitted.

Universal Analytics requires additions to every page whose activity
you wish to track.
Answer: You can't edit the google sites generated code as would be necessary when using just straight up analytics.
Google tag manager would be the perfect solution as it was built exactly so you don't have to edit the code. Unfortunately, google sites doesn't let you connect with google tag manager like it does with analytics.
It's crazy they did all the work to let you connect with analytics but didn't support tag manager too given it is almost identical to enable. Maybe if enough of us beg they will add it?


Choosing PCR conditions

Question: How does one choose PCR conditions? Does it depend on the taxon, on the DNA concentration, on the primers or anything else?
Answer: You need to first build an initial protocol and then optimize it.
The most important to determine the conditions is the polymerase used. Some polymerases work at higher or lower temperatures and will work faster or slower. Find the default protocol from the polymerase seller and this will give you a good idea where to start.
The second most important is the primers used. Depending on the GC % in the primers and their length, the times or temperature of each step will change, or if you prefer will need to be Optimized.
The length of the DNA fragments will also determine the length of the cycles. Most polymerase sellers will includes steps to optimize the reaction depending on the quantity of DNA used, primer length and other parameters.
For more information:
Designing primers:
Taq DNA polymerase protocol example:
Optimizing PCR

PCR thermal profile, Deoxynucleoside Triphosphates and Primers are interesting paragraphs
Answer: Use primer3 to design your primers, then just use whatever protocol your lab usually used for PCR.
A high annealing temp solves many problems, and a good enough taq won't mind.
I successfully sanger sequenced thousands and thousands of mammalian exons, with primer3 designed primers, a HotStar Taq, and a 60 degree annealing temp.  Worked almost every time.
Obviously if you are doing long PCR, or working with a very high GC or low GC species, that might requires more optimizing.
Answer: According to my knowledge the main thing in annealing temperature in PCR
Primers annealing temperature should be selected by keeping in 5° less than the lower melting temperature of primers
Secondly MgCl2 working Concentration also plays a major role in PCR optimization
Comment: As you are probably aware there's a lot more to designing a PCR, as the previous answers have suggested.

Choosing PCR conditions