How does p-factor affect jet engines?

Question: How does p-factor affect jet engines, if at all? If it doesn't, why?
Comment: @Bianfable there's going to be some P factor in a turbofan, since it's just a fixed pitch prop with oodles of blades, to the extent that flow at the blade AOA varies with body AOA. How much this occurs will be a function of the cowl inlet's efficiency at redirecting the inflow. Downstream guide vanes shouldn't make any difference; it's the asymmetrical lift from differing AOAs of the blades which affects the strength of the downwash from the blades so the fan is making more thrust on one side, just like any propeller. I think there is SOME P factor, but not enough to have a noticeable effect.
Comment: @Bianfable I saw that answer before posting up the question, but it's a little bit vague and does not really address p-factor entirely in my opinion.
Comment: Think about what P-factor *is* and you'll know the answer to this.
Comment: @JohnK OK, I retracted my close vote. Feel free to add that as an answer...
Comment: This question is relevant and could lead to discussion of **gyroscopic effects** when pitching (a rapidly rotating) jet engine.
Comment: Does this answer your question? [Why is rotating engines in opposite directions more important for a propeller-driven plane than a jet?](https://aviation.stackexchange.com/questions/24193/why-is-rotating-engines-in-opposite-directions-more-important-for-a-propeller-dr)
Answer: Since P Factor is primarily variation of AOA of a thrust producing blade when its rotation axis is not aligned with the air stream, you should see some P factor in a turbofan engine, to the extent that the inlet flow strikes the front of the fan from an angle.
I say "some" because the cowl inlet is going to align the inflow to be more perpendicular to the fan face than if the fan was in the freestream (where it becomes just a propeller with lots of extra blades), but it's hard to say how much. In any case, the flow is probably not quite 90 degrees to the fan face at higher body AOA, so it's safe to say there is probably a little bit, but since P factor is not accounted for in turbofan installations, what P Factor exists is not enough to matter (maybe if you stuck a turbofan on the nose of a Cessna 172, you might notice some P Factor effect).
On a turbojet there will be no P Factor at all since the thrust is coming form flow accelerating out the burner can, hot section and tailpipe, not from lift producing propeller/fan blades at the front.
Comment: With a turbofan on the nose of a -172, P Factor certainly won't be the only interesting new "effect" that you're suddenly going to notice!
Comment: "Maybe" is not an answer.
Comment: "Maybe" just jokingly refers to a "turbofan 172".  The answer is, logically there has to be some, but not enough to matter, and "some" might be a 5% effect, or a .5% effect, but in any case it's not enough to matter on aircraft equipped with TFs.

How does p-factor affect jet engines?


In the Spider-Man films of the last 20 years how do Spider-Man's feet stick to walls?

Question: In the Spider-Man films with Tobey Maguire they showed close-ups of Peter's hands with the little jagged spikes growing out of his skin, showing that this is how he sticks to surfaces. But how would that apply to his feet? The spikes are not long enough or probably strong enough to burst through his boots.
Answer: From Wikipedia  entry of Spider-man's power -

The ability works through thin layers of cloth, such as the fabric of
his costume, but not through materials such as the soles of shoes.
When Peter Parker needs to crawl without changing into the costume, he
removes his shoes first.
Spider-Man's wall-crawling ability has increased with time, most
notably in a backup story in Amazing Spider-Man #365 by Tom DeFalco,
in which Peter explains his abilities to Mary Jane. Peter reveals that
every part of his body has gained or increased in attraction to other
surfaces, and he can stick people or objects on his back. Over time he
realizes he could keep his mask firmly fixed to his face (previously
used to prevent undesirable mask removal), protecting his secret
identity.
The Spider-Man entry in the Official Handbook of the Marvel Universe states that Spider-Man is able to enhance the flux of inter-atomic attractive forces on surfaces he touches, increasing the coefficient of friction between that surface and himself.
Electro had a revelation during a battle about Spider-Man's clinging ability and can disrupt this power somehow, claiming that it was based on some sort of electromagnetic bonding. It is ineffective after the events of "The Other".

So it can be concluded that they used Spider-Man's wall-crawling in the comics first and then they tried to explain it time by time. Same with the movies too.
Note: Answer is from the comic universe because movie don't have any elaborated answer for this.
Answer: Specifically in the films, Parker grows a hooked chitin from his fingers. Assuming he cannot do the same with his feet, he needs only to apply compressive stresses (think arches) to transfer the weight to his hands. So his feet would supply minimal support where his upper body would compensate for every movement.
Comment: This would make really good sense, if it wasn't for that ridiculous scene where Spiderman run's horizontally across a building without the use of his hands...
Answer:
A panel of biologists and physicists on the History Channel's
Spider-Man Tech suggested the barbed-hair on his fingertips (from the
movie) could have the same effect as the miniature scopulae hairs on
the ends of a spider's feet. Spiders are able to climb up seemingly
impossible surfaces like glass because the scopulae interact with the
glass's atoms causing a form of atomic static cling via the Van der
Waals force

there is a possibility that this Van der Waals force may pass through his clothes thus not requiring the micro thorn structures to pass through the gloves.

Van der Waals force is nothing is the sum of the attractive or
repulsive forces between molecules (or between parts of the same
molecule) other than those due to covalent bonds, the hydrogen bonds,
or the electrostatic interaction of ions with one another or with
neutral molecules or charged molecules as given in
wikipedia

which indicates the presence of dipole.So what electro essentially does that is change the charge (he essentially neutralizes the dipole induced by spidermans fingers ) using his powers. as given here
Note:
the full answer is given in this link here


Refresh uuid of physical device

Question: I'm trying to clone a LUN on a server and present it to the same server and am running into UUID oddities. I don't create partitions on the LUN, just create a PV and then LVM on top of that so I can grow it online as needed. The cloned LUN has the same UUID as the source LUN for the LVM and the PV, which is to be expected. I change the UUID of the PV using vgimportclone and then change the UUID of the LVM with tune2fs, no problems with those. What I'm running into is the UUID of the physical disk, which should have the same UUID of PV, doesn't update until I reboot the server. To deepen the mystery, if I run blkid on the LUNs underlying disks, it shows me the old UUID, but if I run udevadm on those same disks, there is no UUID.
So I have 2 questions.
1) Anyone know whey blkid and udevadm would show different information?
2) Is there a way to refresh the UUID of the physical disk without a reboot?
Server:
RHEL 6.5
device-mapper-multipath-0.4.9-87.el6.x86_64
lvm2-2.02.143-7.el6.x86_64
Thanks,
DarkSide
Answer: You can reread partition informations with command
<code>partprobe /dev/sda
</code>
UUID is partition-specific, so it also refreshed.

Refresh uuid of physical device


Who is the Centralized authority for E-Passport?

Question: I suppose every country that issues an E-Passport has a member of some central authority or organization which manages the visa information worldwide. The reason some citizens can use e-Passport in Airport e-Gate/Automated passport control is due to this central database that's shared among the countries. I want to know what that authority is? Some countries (like mine) cant use this e-Gate, even if we have an e-Passport, due to being unapproved yet by this authority.
Answer:
I suppose every country that issues an E-Passport has a member of some central authority or organization which manages the visa information worldwide.

No, this is not the case.
Each country must collect this information for themselfs and often do so by reading/scanning the information contained in the machine readable zone of the main page.
Some countries, like Germany, don't even have a national database where the information of their own passports are collected and therefore cannot pass this information onto some other 'central authority'.

Some countries (like mine) cant use this e-Gate, ...

The eligibility who may use an eGate is determined by each country and is often based on the likelihood of citizens of a specific country to be interviewed by an immigration official upon entry.
Comment: @Tanvir "machine readable" refers to the two lines of characters at the bottom of the passport's ID page.  Most visas are machine readable, too, and countries that allow people with visas to use automated passport control might have them scan the machine readable visa when using the passport control kiosk.  Alternatively, the visa-issuing country can look up the visa in its own visa database using the traveler's passport details (which is in fact the only option if it is an e-visa, which are becoming increasingly popular).
Comment: @Tanvir I think I missed a logical step in my previous comment.  You asked "if you have a visa for X country in your passport, then how does that country know your passport and visa therein?"  Country X knows about the visa _because it issued the visa!_  Furthermore, when you applied for the visa, you showed them your passport, and they essentially "remember" your passport by recording it in a database along with a record of the visa they issued.  When you scan the passport, they find the database records.  Because it is their own database, there's no need for a central authority.
Comment: @Tanvir the same process happens when you go to a traditional-style passport-control desk.  The officer scans your passport and gets a bunch of information from various sources, including the airline's passenger manifest (if you arrived by airplane), police arrest warrant databases (if only to say that you are not a wanted criminal), previous immigration records, and, indeed, your visa application file.  The only external authority universally consulted is Interpol, which will tell them whether you have an international arrest warrant and whether your passport has been reported lost or stolen.
Comment: If u have visa for X country in ur passport, then how does that country know ur passport and visa therein? How does the e-gate know ur visa?
Comment: @Tanvir From the information contained in the mashine readable zone of the passport which was collected when the visa was issued.
Comment: is it possible that the visa issuing country embed the visa information into the chip/machine-readable part of the passport?
Comment: @Tanvir No. The machine readable zone is the printed text at the bottom of the main page (<<last-name<first-name<< etc.). The chip itself is read only.
Comment: I just wonder why my country moved to Machine Readable Passport to a chip-based E-Passport. To me, the MRP passport wud solve the problem of E-Gate, since its machine readable.
Comment: @Tanvir That's a different question. If you'd like it answered, please use the "Ask Question" button on the upper right of each page, and ask a new question.
Answer: You have a fundamental misunderstanding of the way all of this works.
The chip in an e-passport contains information about you and your passport. Mostly the same stuff you can see on the bio page of your passport (names, place and date of birth, citizenship, place and date of issue, expiry…), along with some optional biometric data, which can be fingerprints or a photo (or other data which enables facial recognition IIRC). Most importantly, it also contains the passport number.
Note that data in the chip can be protected, with various levels of protection. Basic data is usually protected using a key based on the contents of the MRZ (the machine readable zone, the two lines at the bottom of the bio page, which is read via OCR), which ensures that only a reader than can see the bio page can talk wirelessly to the chip. Biometric data is often protected further with a key which needs to be exchanged between the issuer of the passport and each country wanting to use it.
E-Gates are there to verify:

that you have a valid passport
that you are the person for which the passport has been issued (see below)
that you are allowed in (see below)
that you are not on a list of people who need "special treatment" (wanted by the police etc.)

To check you are the right person, the e-gate needs to be able to get your biometric data and match it to that on the chip. This raises a first issue, as not all passports contain the same data and not all gates have all readers. If the gate can only do facial recognition while the passport contains only fingerprints, or vice versa, it won't work. Some e-passports do not contain any biometric data at all, or data which is recognised only by gates on the same country. And as stated above, each country needs to get keys from each other country they want to recognise to read the relevant data.
Once the gate knows who you are, it can try to determine whether you should be let through. The two most common cases are that you  don't need a visa at all (you are a citizen or permanent resident of the country, or come from a country whose citizens don't need a visa to enter that one), or that you have a visa for the country.
For that last case, the visa does not need to be stored in the chip in the passport (it can't, actually). The gate reads the passport number (and the rest of your data) and looks up in the database of the country you arrive in to see if the visa is in there (most countries don't care about other countries visas, only the ones they issue, which they (should) know about, no need for any centralised database).
Further, there are lots of cases when it won't work. Many countries may have special procedures for the first visit on a visa (especially the longer term ones). They may not trust people of certain countries even with a visa. They may not have compatible gates, they may not have the keys, and so on.
Just as an example, in France the first generation of e-passports did not contain the same biometric data as later ones. This makes those older passports unusable in current French e-gates!
Comment: This leaves out the very important point of the digital info being digitally *signed* by the issuing country. Otherwise, you could readily create your own fake digital passport - and way easier than a paper passport as you do not have to take care of all those fine wiggly lines and holograms.
Comment: Doesn't the standard specify that the chip must contain a digital copy of the same photograph shown on the identification page?  Certainly all of the passports I've ever checked contain this.  Furthermore, the use of a key comprising data from the MRZ to control access to the RFID chip is also part of the specification, so "usually" is a bit misleading.
Comment: thanks a lot, now all make sense.
Answer:
I suppose every country that issues an E-Passport has a member of some central authority or organization which manages the visa information worldwide.

There is no such organization.

The reason some citizens can use e-Passport in Airport e-Gate/Automated passport control is due to this central database that's shared among the countries.

This is incorrect.  Each country decides who can use its automatic passport control and under what circumstances.  Countries may have bilateral or multilateral agreements about this, but I am not aware of any.  The systems i am familiar with are all implemented unilaterally.

Some countries (like mine) cant use this e-Gate, even if we have an e-Passport, due to being unapproved yet by this authority.

Most countries I'm aware of extend this benefit to those who can enter without a visa.  In some cases, it is only a subset of those who can enter without a visa who can use automated passport control.
Countries make these decisions based on risk assessments.  If you want to understand why citizens of your country cannot use a particular country's automatic passport control, you should consider the other country's attitude toward visitors and immigrants from your country.
Comment: Depends also on wether the country (our group of countries as in the Schengen zone) has a system that registers entries/exits. If they do not then passports still need to be stamped.
Comment: that means it depends on the visa issuing country, if they want to allow some nations to use their E-Gate.
Answer: Other answers have done a good job of explaining how e-passports work and why a central database of identity information isn't required, but have only hinted at the need and existence of a centralised authority known as ICAO: https://www.icao.int/Pages/default.aspx
ICAO provides two important roles in managing E-passport infrastructure:

Publishing and maintaining standards (both for the physical documents and the chip protocol) so documents can interoperate between countries: https://www.icao.int/publications/pages/publication.aspx?docnum=9303
Distributing and revocation of signing public keys so countries can trust that documents issued by other countries are valid: https://pkddownloadsg.icao.int/

The second point in particular could explain why a valid e-passport wouldn't be trusted by another country if it's not yet on the approved signers list.
Comment: For those not familiar with what ICAO is, it's the International Civil Aviation Organization. It is an inter-government treaty agency whose primary responsibility is cooperation between countries for the safety and efficiency of international civil (i.e. non-military) air travel. This is mostly on subjects relevant to pilots, air traffic controllers, airport operators, and aircraft designers and mechanics, but it also includes some government-related passenger issues, such as passport standards. Most passenger-related issues are handled instead by IATA, the international trade org for airlines
Comment: The main distinction between ICAO and IATA is that ICAO is an organization of government agencies (the civil aviation authorities of the member countries... which is nearly all countries,) while IATA is an industry organization among the airlines. This is why your tickets and bag tags will have IATA designators on them, while flight plans will instead use ICAO (and/or local aviation authority) ones.
Comment: Yes, I was referring to this very organization (ICAO) indeed, I just forgot their name.
Answer: To address the question of a "central authority or organization" from a technical perspective, there is also no need for such centralised control.
The information in the passport is digitally "signed" (using cryptography) by the issuing country. This is what prevents anyone from forging the information.
Each country uses their own certificate to sign their passports. At the border, the eGates will have a list of certificates to trust stored in memory, one for each country. So there's no need for a centralised authority to sign every single passport in the world.
Comment: You could mention that there is a bit of coordination on technical standards (what information the chip should contain, how to read information from the chip, etc.) though this is certainly far short of what the OP suggested.


What counts as a paid app?

Question: As you may know Google doesn't allow developers in certain countries to publish paid applications to Google Play. However, they don't exactly clarify what counts as a paid application?!
Let me clear this up a little bit:
On one of their support page they have a list of countries what tells us where can developers publish paid applications but they don't mention anything about what counts as a paid application, and on one of their other support page (which clarifies that in which country can users download/buy applications) they do tell us that applications with in-app purchases fall under the paid category. Now I'm absolutely confused, and can't decide if this applies to developers too or not.
I mean, I can't decide if this means that developers who live in a country where they can publish only free apps can actually publish a free application with in-app purchases and only the users are restricted to this rule, or even developers who can only publish free apps can't put apps with in-app purchases to Google Play? If that's the case, is there any workaround for this? Also, applications with ads count as a paid app?
So, my real question would be that developers who can only publish free apps simply just can't get any money out of their work?
P.M.: Sorry for my bad English.
Answer: Maybe Google-support is better fit to answer your question.

What counts as a paid app?


How can spectroscopy be used to find the composition of exoplanet atmospheres?

Question: How can astronomers use light spectroscopy to measure the composition of different planets when they can't directly image the planet because the star that they are orbiting is so much bigger and brighter than they are?
Answer: When an exoplanet transits in front of a star it blocks a fraction of the starlight that is given roughly by the ratio of the exoplanet radius to the stellar radius squared (i.e. the ratio of the projected areas).
The effective exoplanetary radius is then defined as the radius out to which the planet is opaque to the starlight attempting to "pass through it" on the way to us.
However, if the exoplanet has an atmosphere, then the edge of its occulting disk is is "fuzzy" and some light may be able to pass through its atmosphere on the way to us. The extent to which this happens will depend on the wavelength of the starlight and the composition of the exoplanet atmosphere.
In the image below (from https://casa.colorado.edu/~desert/GMOS-Survey.html) the exoplanet has a larger effective radius at blue wavelengths and this results i a larger transit depth at those wavelengths.

As an example, consider an exoplanet which has an extended atmosphere that contains lots of sodium atoms. If you observe the transit through a filter (or with a spectrograph) at the wavelength of the prominent "sodium D lines" (589 nm) then the effective radius of the exoplanet (including its atmosphere which will be opaque to photons at this wavelength) and hence the transit depth will be larger than at wavelengths either side of 589nm.
Thus by measuring the transit depth as a function of wavelength you are effectively performing transmission spectroscopy of the exoplanet atmosphere. These measurements along with a model of the physical conditions in the atmosphere (temperature and pressure) lead to an estimate of the chemical abundances.
Answer: When a planet is detected using the transit method, the passing of the planet in front of the star causes the star's luminosity to decrease by a little bit, and this phenomenon repeats itself every time the planet revolves back around its star. If the planet has an atmosphere, some of the light from the star first passes through that atmosphere before reaching our telescopes. Different particles in the atmosphere will preferentially absorb light of different wavelengths, so we can infer the composition of the planet's atmosphere by the periodic decrease in light detected from the star at various combinations of wavelengths.
It is also possible that some of the star's light reflects off of the planet, so the different wavelengths having different fractions of their light being reflected will lead to differential amounts of light reaching our telescopes. All of these are rather small fluctuations, which should give you some idea of the precision capabilities of modern telescopes!
There's a detailed example with real data here.
Answer: There are just additional absorption lines in the observed spectrum of the star when the planet is in front of it (meaning between the star and the observer).


Best practice for queries and WCF Service

Question: What would be the best practice for the following scenario:
There´s a grid that will be filled and must be changed according to each row.  For instance, there´s a grid filled with products, then according to each product one of the columns will be dynamically populated.  Is it better to return from the service all the productdetail table and query it on the client side or have a method on the service that will return only the data needed?  The latter would mean that if there are n number of products on the grid, there will be n requests to that service method.
My dilemma is that for some of the users the table will be relatively small and sending it to the client might not be a big deal, but other users do have a considerable amount of rows that would be returned (more than 15k).
Thanks for any insight you may bring.
Answer: You could make your service implement some sort of paging, and allow the client to request the number of records they want (usually a starting index and a count, or something like that).  You could cap the page-size at a certain limit so that you don't end up having to serve a huge request.
Using something like this, you should be able to find a nice balance between the number of calls being made vs. the size of the data being served in each request.
Answer: The topic you have stumbled upon is warmly debated topic of granularity on service-oriented architecture. SOA is like a mystic mammal being discussed by blind men, but that's not really the point.
The "best practice" of the trend is coarse-grained services, a departure from object-oriented. Consider your services to be exchanges of documents, in which you put in everything you need to get things done.
As Andy White has suggested, if the data gets too big you'd need some sort of filtering or paging.
Answer: Sounds like network latency is the issue here. If you've got 100 products and (say) a 0.2s round trip time, that's 20 seconds to load all the data. Minimize your service calls as much as possible and if necessary remap the data into a more suitable structure in your client.
Edit: Another idea, if it's possible in your situation, is to compress the data between your client and service. Have a look at this forum post. You'll see big gains if you're pushing around heaps of XML.
Answer: I'd suggest creating some sort of ViewModel on the server to tailor the data for the client. This is similar to your second option; however, it's not done on row by row.  It's a batch operation on the server side which preps the columns so the view doesn't need any special logic.
I feel manipulating data on the client side can be tricky because there's so many variables- browser, os, computing power.  You control the server, so leverage what you control as much as possible.
Good luck.


Is it possible to individually update each module using update.php?

Question: I am trying to update the reCAPTCHA module (which is installed on sites/all/modules) in Drupal 7. I could see other 3 modules which have pending updates. Is it possible to individually update each module using update.php?
Comment: What do you mean by update? It means downloading the updates module and place it i modules?

Or just running update.php?
Answer: You can't update single module using "update.php".
You can update single module if you installed drush.
List all available modules (except core) with below command.

drush pm-list --type=module --no-core --status=enabled

Update specified module only.

drush up 'module-name'

or

drush pm-update --no-core "module_name"
Answer: Refer https://www.drupal.org/docs/user_guide/en/security-update-module.html to update single module using Drush and inbuilt functionality.
If you are using composer then run below command.
<code>    composer update drupal/<module_name> --with-dependencies
</code>


nBuLi and tBuLi can take part in halogen metal exchange OR deprotonate. Is there any way to predict which it favours?

Question:
I have included a synarchive pathway: https://synarchive.com/syn/144 where BuLi favours deprotonation over halogen metal exchange
Comment: No direct answer from me, but Clayden's book on organolithiums is an excellent read for these things, and I suspect may contain an answer for you. I don't mean the general org chem textbook, I mean this: https://www.elsevier.com/books/organolithiums-selectivity-for-synthesis/clayden/978-0-08-043262-5
Comment: I recommend looking for Prof. Hans Reich (RIP) and his work covering a broad range of organolithium chemistry, with lots of NMR kinetic studies. He condensed much of his life's work in his personal webpage, which has since been repurposed as part of organicchemistrydata.org. There are [multiple sections on organolithium chemistry](https://organicchemistrydata.org/hansreich/resources/organometallic/?page=intro00%2F), which you can access on the dropdown menu on the left. My suspicion is that deprotonation is generally faster, and only Li/I and Li/Te exchange can compete with deprotonation.
Comment: You need to look at the pKas of the protons that can potentially be removed. If there are any below ~50 then you will get deprotonation.
Comment: The reagents should be written as discreet steps: 1)t-BuLi, 2) 4-iodobutene.
Answer: According to Bordwell $\mathrm{p}K_\mathrm{a}$ Table, $\mathrm{p}K_\mathrm{a}$ of $\ce{PhCH2COSPh}$ is $16.9$, which is compatible with alcoholic $\ce{OH}$. We all know that Grignard reagents and organolithium reagents are very susceptible to acidic hydrogens because they are strongly basic as well as nucleophilic. Between $\ce{t-BuLi}$ and $\ce{n-BuLi}$, $\ce{t-BuLi}$ is considered to be acting as base than nucleophile compared to $\ce{n-BuLi}$, due to its bulkiness. However, both of them are known for their capability of metal-proton exchange (e.g., see Wikipedia).
On the other hand, lithium-halogen exchange reactions are kinetically controlled. The position of the equilibrium varies with the stabilities of the carbanion intermediates involved $\mathrm{(sp >> sp^2 >> sp^3)}$, rather than whether it is $\ce{n-BuLi}$ or $\ce{t-BuLi}$. Keep in mind that lithium-halogen exchange reactions using alkyl-lithium typically employ two or more equivalents of alkyl-lithium reagent. The first equivalent is used for the exchange and the second equivalent reacts with the alkyl-I produced to form corresponding alkane, alkene, and lithium iodide (e.g., Ref.1). Nevertheless, if acidic hydrogen is available, the reaction goes through metal-proton exchange.
Note that the scheme given in Synthesis of Aplysin, part of which OP has copied is somewhat misleading. When you look at the original reference (Ref.2), it is clear that their intention wasn't halogen-metal exchange. It is clearly to use the lithium reagent as a base:

It indicated that $\ce{t-BuLi}$ is added to the compound $\bf{12}$ at $\pu{-78 ^\circ C}$, waited $\pu{10 min}$ before adding 4-iodobutene, and then let the solution warm up to $\pu{0 ^\circ C}$. After $\pu{5 h}$ at room temperature, the yield of the compound $\bf{13}$ was still only $48\%$ (see the red box). The ref.2 did not mention what was the other $52\%$. It may be a combination of nucleophilic addition and halogen exchange. I'm just speculating.

An example for use of $\ce{n-BuLi}$ for halogen-metal exchange is given in Ref.3.
References:

Helmut Neumann, and Dieter Seebach, "Stereospecific preparation of terminal vinyllithium derivatives by Br/Li-exchange with t-butyllithium," Tetrahedron Letters 1976, 17(52), 4839-4842 (DOI: https://doi.org/10.1016/S0040-4039(00)78926-X).
David C. Harrowven, Matthew C. Lucas, and Peter D Howes, "Total syntheses of aplysin and debromoaplysin using a diastereoselective, sulfur mediated radical cyclisation strategy," Tetrahedron Letters 1999, 40(23), 4443-4444 (DOI: https://doi.org/10.1016/S0040-4039(99)00768-6).
Donald F. Hoeg, Donald I. Lusk, and Alvin L. Crumbliss, "Preparation and Chemistry of α-Chloroalkyllithium Compounds. Their Role as Carbenoid Intermediates," J. Am. Chem. Soc. 1965, 87(18), 4147–4155 (DOI: https://doi.org/10.1021/ja01096a025).


setting up a startup login password Fedora 20

Question: Currently when I login I do not have to provide a login password. I simply click on my user name and I get in. I would like to change that. In order to accomplish that I went to settings >> users  and then in Login options there was no password set next to the Password label. So I clicked on it and was prompted to enter my old password and new password. However after entering my new password the change button is still disabled. How can I enable that change button so that I could login using my password.
Answer: It's a mechanism that prevents you to set a password that is too weak. Your password strength should be at least "Fair" in order to unlock the Change button.
Weak:

Fair:
Answer: Have you tried passwd from the command prompt?


What does "load" mean and what are the different types?

Question: What are all the different types of "load" and what are the key important facts to know about each?
Comment: While I'd say that your question shows zero prior research effort (this is easily discoverable with internet searches); this is a good question to have on the EE.SE site.
Comment: Fair point. I got as far as wikipedia and the explanation there on load bank wasn't entirely clear.
Comment: For this question to be useful and answerable, you need to provide some context. What sort of circuits/situations are you referring to?
Comment: Why is this question marked as duplicate ? Load type != load
Answer: According to Merriam Webster's dictionary (definition 8b), a "load" is:

a device to which power is delivered

This may be a circuit, a test device ("dummy" load), or a larger machine or device comprised of many sub-circuits.
The types of electrical loads are typically divided into three categories:

Resistive
Inductive
Capacitive

And of course there are devices which contain a combination of these.

Resistive loads: Resist current flow linearly and cause heat and light (potentially desirable or undesirable). For example, an incandescent light bulb produces light (desirable) but also heat (undesirable). A space heater's element produces heat (desirable) but may also glow. Resistance is measured in ohms for resistive loads, and power is measured in watts.
Inductive loads: Resist changes in current and as such, when you measure the current, it lags (is behind) the voltage. Electromagnetic fields are the key to inductive loads, and as such all motors (fans, pumps, etc), solenoids, and relays are inductive in nature. Inductance is measured in Henrys. The important thing to remember about inductive loads is that they have two types of power, real power and reactive power. The real power is based on the work done by the device (such as what a motor is spinning). The reactive power is that which is drawn from the source to produce magnetic fields. The total power consumed is real and reactive power combined, which is measured in VAR (volts-amps-reactive). It's rather a complicated topic, so check out terms like Power Factor and Real, Reactive and Apparent Power (under AC power).
Capacitive loads: Are for many purposes, the opposite of inductive loads. They resist changes in voltage, and as you'd expect, the voltage lags the current (or more commonly said "current leads voltage"). A capacitor is two conductive surfaces separated by a insulator, which store charge. When power is first applied, current is very high, but drops as the voltage of the charge reaches that of the applied voltage. Capacitance is measured in farads. Like inductive loads, capacitive loads also have reactive power, but it's opposite the polarity of an inductive load. Therefore, a capacitive load has a negative VAR. Capacitive loads are not very common, but things like a flashbulb or a heart defibrillator might be considered a capacitive load (probably a combination load, but you get the idea).
Comment: @John I believe it's a combination of these, but depends on battery chemistry and charge state. Great question, I suggest asking it as a new question on the site.
Comment: @rrrrr Think of power "consumed" as power converted to something else. For example, nichrome wire in a heater converts electrical power into thermal (heat) and thereby "consumes". The topic of real and reactive power for inductive loads is too big for comments, I recommend checking out the links.
Comment: would charging a battery be considered capacitive?
Comment: when you say that `inductive loads have two types of power`, do you mean that (informally) they "consume" this power? similarly, do resistive loads "consume" power?
Answer: When we look at a system, such as an electronic device, it helps us to isolate that device from its environment, which is complex and full of details that are not all relevant.
If a device has an output, then it has behaviors which are dependent upon the device which that output drives. So we cannot remove the device which is driven.
What we do is we retain a simplified representation of the device which is driven by our output, and that is called the load.
Two important behaviors of a device with respect to a load are its basic reliability (can it drive that load while its performance parameters remain within range, and how robustly can it handle being overloaded?) and stability (can that load make the output of the device oscillate or otherwise misbehave).
The term load refers to

the most relevant aspects of a device which is driven, from the perspective of an output. These aspects involve basic properties like capacitive and inductive reactance, and resistance, or more complex networks thereof. So we can talk about "resistive", "capacitive" or "inductive" loads.
a model of the driven device constructed to match these most relevant aspects, while omitting the rest of the device.

Load also refers to power consumption: e.g. "heavy load on the power grid".
As a verb, "to load" means to present a load to an output, and to "load down" means to present a somewhat excessive load (such as too low an impedance to a device that acts as a voltage source, such that the voltage source is unable to maintain voltage). To "overload" is to load down extremely, ("the power supply couldn't handle being overloaded and burned out").
Sometimes "load" is used incorrectly to denote a signal or current, and the circuit or conductor to be a "vessel" that the load is poured into. Watch out for this in informal usage like "the amplifier's high output overloaded the sensitive microphone input it was plugged into". Or: "those wires are carrying a mighty big load for their gauge".


How to generate Facebook Marketing API access token to use it in Windows application

Question: I am using Facebook as advertising platform to promote my application on Apple and Google stores. I would like to make windows service which will download daily report(s) about advertising status of my marketing campaign running one Facebook, preferably using 60 day token, or some permanent solution so that token is retrieved when required. I think that i understand everything to do this except how to generate access token to use it with Facebook Graph API. Which token for which Facebook account do I need and how to obtain it?
Answer: You'll need a Facebook app and to grant that app the ads_read permission in order to retrieve reports about your advertising efforts via the API (source).
You'll want a long-lived token so that you don't have to re-authenticate very often. The access token documentation details the steps to exchange a short-lived token for a long-lived one.
You may also want to consider managing the app, ad account, and access tokens (via a business system user) with the FB Business Manager.
Comment: If you want to manually retrieve reports you can do so via the FB ads manager https://www.facebook.com/ads/manage/ or via the Power Editor https://www.facebook.com/ads/manage/powereditor/. But to retrieve report data via the API you need an FB app. Authorize the app to read data from your ad account then setup code to automatically retrieve ad report data from the API and output. Checkout the FB guide https://developers.facebook.com/docs/marketing-api/guides/chapter-0-create-flow-overview
Comment: I need a FB app? Never made one, this is something that can be installed to some user profile and than he can do something inside browser? I don't see how to use Facebook app to automaticly download daily reports, i can only imagin someone clicking download report each day. Can you please explain this part please.
Comment: I was under impression that you could dl reports only from your sandboxed FB app running in browser, but if FB ap is just proxy that wires up FB infrastructure and enables you security credentials to invoke API from you native PC app than this is what I need. I'll check it some more. Thanks!
Comment: OMG I was using GET examples from FB documentation. All the time some strange authentication errors were coming, I was sure that I had something conceptually wrong. But now i tried same call 3 days later with short time token generated with Graph Explorer and everything is working OK. I just have to make 60 day token now.
Answer:
Create a facebook app.
Go to the Graph API Explorer in Facebook's Tools & Support section.
Pick your app from the drop down.
Hit Get Token > Get User Access Token.
In Select Permissions choose required permissions or select them from extended permissions.
Use the user access token that will be presented to you in the access token form input field.


I have to reset switch in basement if I don't turn off space heater when I use the blow dryer

Question: My husband says I have too much plugged into my side of the bedroom- 1) lamp  2) laptop  3) cell charger  4) a blow dryer. All are plugged into a Phillips box that plugs into an outlet.  I am no Thomas Edison ( or Nikola Tesla) but I think the only thing that really pulls the power is the blow dryer, followed by the lamp.  My guess is that the laptop or cell charger don't use much power.?  If I forget to unplug a space heater across the room in winter when I use the blowdryer all bets are off and have to unplug everything, go to the basement and reset the breaker.  I want to call an electrician and get (I GUESS) another breaker switch for the bedroom so that I can run two "power pullers" (heater, vacuum cleaner, blow dryer. etc) at the same time.  Is this really expensive? * What can I expect to pay?*  The house is 6 years old- 3 bedrooms 2 baths on a basement.  In Georgia, not Manhattan. Would I just say "I need a second breaker for the bedroom" ??"... Yall?..  Thanks!
Comment: I'm voting to close this question as off-topic because it is about the price of a project.
Answer: A cellphone charger is an inconsequential load.  The lamp would also be inconsequential if you'd put LED bulbs in it.  The laptop certainly uses less than 10% of the circuit's capacity, maybe even 3-5%.
A blow dryer is a huge load, and takes 80% of a circuit's capacity.
A heater is also a huge load that takes 80% of a circuit's capacity.
Those two devices take so much because their mission is to create heat.  They want to make as much heat as possible, so they use the circuit up to its limits.
Most bedrooms have 1 circuit, so it's effectively all the same side.
Now that you know, you could try living with it.
Or you could use the blow dryer in the bathroom, which usually has a separate circuit for this reason.
You could use the house's own heating system instead of using space heaters, gas is cheaper to heat with.
You could install hardwired electric heaters on their own circuit, the heaters are cheap, the power to run them is expensive.
Other than that, yeah, you'd have your electrician pull another bedroom circuit.  We cannot begin to speculate what that would cost, because it totally depends on the particular construction of your home.  It will be at least $100 and likely a whole lot more.  This varies too much to even guess at.

Details...  Household power in North America is 120 volts.  That switch downstairs (called a breaker) will have a number on it, probably 15, which the curcuit's capacity in amps.  Useful power is measured in watts, which is volts x amps.  15 amps x 120 volts  is 1800 watts.  Most devices will state their watts.  Some will state amps, and you must multiply by 120.
Watts are often abbreviated W, or sometimes you see a similar unit called VA.
Your cell phone charger is probably 7-10 watts -drop in the bucket.
Your laptop is probably somewhere between 35W and 150W.
Your lamp - anyone's guess. They made 500W halogens. LED bulbs are as little as 7W.
Your hair dryer will say, but expect to see 1500W.
Your heater, ditto.
Answer: You need 'an additional circuit' installed in the bedroom.  I would recommend you use the new circuit just for the space heater, and have it rated for 20A.
$500ish if you're lucky.  (your breaker panel has capacity to be expanded, your breaker panel is in a completely unfinished basement, your bedroom is on the first floor)
$2500-5000 if you're unlucky.
Answer: I agree with Harper's assement but not the cost,, depending on the the of construction Billy may be closer or even low. Is there room in the panel for more AFCI breakers in a bedroom that is the standard now. How much material (wire) from the breaker box? The cost of time and materials, last the cost of the permit. Some times these items can up to well over 500$ for a job like this. In most cases with a newer house the access is easy compared to older homes but there is still the permit paperwork parts material,,, so the cost goes up as it really wipes a full day out for a small job. Just my professional opinion. The cost may be cheaper in your state but wire boxes and the AFCI MAY be more than 100 + permits and the time to do the job.
Comment: I know.  It varies too much to even guess. I know jurisdictions that don't want you to pull a permit for something that small.


Is my avocado plant looking healthy?

Question:
I'm sorry if this is another one of those useless-for-the-community post but I'm just curious, if anyone could tell me if my avocado plant is looking healthy or not. Perhaps if you're an expert and can tell me what variety of avocado this is so I can keep an eye out for its nutrient needs.
(PS, some of the leaves seem to be curling down, is that a problem I can solve quickly?)
Comment: give this plant more light! It grows outside in the tropics and is considered a tree.  It needs a lot of light or will end up looking very spindly
Answer: Your avocado looks fairly healthy...good soil, color is a bit on the light green side and curling down could be due to time of day or temperature.
The big question is fertilizer.  Have you used a balanced fertilizer?  N P K plus micro nutrients?  How old is your plant?
Sure looks like it is lacking in chemicals for photosynthesis.  Fertilizer.  Let us know what you've added, not added for chemistry so this plant is able to make its own 'food'.  My go to fertilizer if you've never added any fertilizer to this pot is OSMOCOTE 14-14-14.  Extended release.  Timed release and for your avocado just half the amount will make a huge difference.  Less is better than too much none is a slow death.

Is my avocado plant looking healthy?


Stuck In login loop after updating my system to Ubuntu 18.04

Question: I updated my system from Ubuntu 16.04 to Ubuntu 18.04 and after updating when I am starting my machine it's asking me for login password and when I am typing the correct password one blank screen appears and it's again asking me for the password and the same thing happening again and again. I saw some answers on the internet for this problem and everyone says to press Ctrl + Alt + F1 on login screen to enter into console and follow some commands as they mentioned but whenever I am pressing Ctrl + Alt + F1 or F2,F3,.....,F9 nothing is happening and I can't see any console and I am still on the login page. Please help me guys to get rid of this problem. I will be very happy if you can help me
Comment: To test, do you have another account that you can try to log into? Report back. Start comments to me with @heynnema or I may miss them.
Comment: Also, do you use a Nvidia video card? If so, did you also update your Nvidia driver?
Comment: status please...
Answer: A common reason for login loops...
If you've been starting graphic applications from the terminal using sudo, that's what probably caused your problem. Always use sudo -H.

boot to the GRUB menu
choose Advanced Options
choose Recovery mode
choose Root access
at the # prompt, type:
 - sudo mount -o remount,rw / # remount the disk as rw
 - cd /home/your_username # change directory
 - ls -al .*thority* # list some files

You should see something like this...
-rw------- 1 your_username your_username 407910 Nov  2 08:56 .ICEauthority
-rw------- 1 your_username your_username     58 Jun 23  2017 .Xauthority

If it DOES NOT show -rw------- then...

sudo chmod 600 .*thority* # change file protection

If it DOES SHOW root root then...

sudo chown your_username:your_username .*thority* # change file ownership
reboot # reboot the computer

Reboot and see if you can log in.


Connect to Remote SharePoint Farm with C#

Question: I have a SharePoint Farm with 25 servers. I was asked to build a new server out of the SharePoint to pull the information from SharePoint Farm and store in SQL Server in different server outside of the SharePoint Farm within the Same network. I will need to use to C# and SQL Server to pull the information from remote SharePoint Farm. I have already installed SharePoint in my local development box. How do I connect to remote SharePoint Farm with C#. I have the Farm Administrator User Credentials. How do I connect to the SharePoint Farm wihtout being in the actual Farm with C# or with PowerShell?
Comment: You should use SharePoint webservices.
Answer: You should use Client Object Model (.NET Framework redistributable assemblies):
http://msdn.microsoft.com/en-us/library/office/fp179912(v=office.15).aspx
Answer: You need to reference SharePoint client DLLs into your remote VS project:
<code>- C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\ISAPI\Microsoft.SharePoint.Client.dll
- C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\ISAPI\Microsoft.SharePoint.Client.Runtime.dll
</code>
(replace "14" by "15" in these paths if SP2013)
These DLLs are redistribuable (can be deployed with your Web site or exe) and can be used on a development machine without SharePoint installed.
Once these DLLs are referenced, you can write C# code to access/manage your remote SharePoint. Examples can be found in the MSDN, for instance, to read list items: http://msdn.microsoft.com/en-us/library/office/ee534956(v=office.14).aspx (SP 2010, same for SP2013).
Note: these client DLLs are wrappers around SP Web services and REST calls. So, a simple HTTP(S) connection is required between the client and SharePoint.


Can the confusion/diffusion of a hash algorithm be quantified?

Question: I am wondering whether the confusion and diffusion are quantified in any way when we examine hashing algorithms and, if so, how they are quantified and what units (if any) are used to represent them.
Comment: For a _full_ hash function, not really, but at the level of diffusion per round (for hash functions based on iterative block ciphers, at least) you can. Is that what you mean?
Comment: @forest I didn't know that. Is it because making these quantifications for a full hash function would be too complicated? I think that quantifying diffusion per-round would also answer my question.
Answer: It depends a lot of why you want to do this and for what types of hashes and input data but I would think that for some purposes it may be sufficient to do so through sampling, though you would certainly want to be careful about how much you trust this sort of analysis depending on how you're using the hash values.
For example, if you know that the data you're hashing often has a certain format or is always a certain length you could generate a set of test inputs of manageable lengths. For the purposes of this answer let's say 4 bytes but in practice you'd probably want to pick an assortment of lengths that apply to your use-case.
You should include some simple values (e.g. 0x00000000, 0xFFFFFFFF), some patterned values (e.g. 0x40404040, 0x1ACEECA1), and some completely random values.  Then take each one and hash it to get a baseline, $H$, of length $N$ bits (i.e. 32).
For diffusion you could then, for each bit of the input:

Flip the bit, $i$
Hash the altered value, $H_i$ (i.e. the hash with bit $i$ flipped)
Count the number of bits that differ between the new hash and the baseline hash, $d_i$
Calculate $1 - \frac{| d_i - N/2 |}{N/2}$ to get a rating $r_i$ between 0 to 1 (with 1 being ideal diffusion).

For confusion you could do something similar, checking that across $H_0$ to $H_{N-1}$ each bit is flipped relative to the baseline due to changes in as many input bits as possible, creating a score for each output bit.
Using these diffusion and confusion scores, you could then compare hashes depending on whatever criteria you're most looking for in your use-case.
All of that said, this is a somewhat brute-force method that could very easily miss issues in a hashing algorithm that could be important from a security perspective.  So again, I would only consider using a method like this if you're considering the properties of the hash relative to a specific use-case that isn't security-critical.  Making this kind of statistical argument when you don't already have some idea what types of inputs you might be seeing reducing the likelihood that it will tell you anything useful effectively to zero.


USB Drive make Read only

Question: I would like to make USB drive to become read only for users (user with no root access) .Am using ubuntu 14.04.
Comment: What file system is there in the USB drive (ext4, FAT32 or NTFS or some other file system)?
Comment: FAT32 File system only
Answer: [Please be aware, that an advanced user will be able to override this method. It is enough to boot the computer into a live system with a USB/DVD boot drive or to move the USB drive to another computer.]
Mount a FAT32 partition in a pendrive with write permissions only for root
FAT32 and NTFS file systems get their permissions in linux when mounted, and they cannot be changed for individual directories and files. The crucial part of the example is the command line to mount the FAT32 partition on the USB drive.
This example assumes that the pendrive is seen as /dev/sdx. Replace x with the actual drive letter, for example b: /dev/sdx1 ---> /dev/sdb1
<code>sudo mkdir -p /mnt/sd1  # only if you want a new mountpoint
sudo umount /dev/sdx1   # only if already mounted (but with bad permissions)

sudo mount -o rw,users,umask=022 /dev/sdx1 /mnt/sd1  # mount

ls -ld /mnt/sd1                          # check permissions

sudo bash -c "echo 'Hello World' > /mnt/sd1/hello.txt"  # test writing with sudo
cat /mnt/sd1/hello.txt                   # test reading (as user)
ls -l /mnt/sd1                           # check permissions of the content
rm /mnt/sd1/hello.txt                    # test removing (as user)
echo 'I am a user' > /mnt/sd1/user.txt   # test writing (as user)
</code>
Answer: Number of solutions are already, Might be helfull for you
USB devices showing a read only

USB Drive make Read only


Doctor Who recent reference to Tegan

Question: A recent Doctor Who made a comment about an Australian who got stuck on the Tardis, obviously a reference to Tegan. But can anyone tell me which Doctor and episode it was in?
Answer: A seach of the chakoteya.net transcripts gives (source):

The Tardis materialises in a cobbled alley, in sepia with tickly music in the background all the way through.)
DOCTOR: Okay, so. Not London 1893. Yorkshire 1893. Near enough.
CLARA: You're making a habit of this, getting us lost.
DOCTOR: Sorry. It's much better than it used to be. Ooo, I once spent a hell of a long time trying to get a gobby Australian to Heathrow Airport.
  (That would be Tegan Jovanka.)
CLARA: What for?
DOCTOR: Search me. Anyway

Which is from The Crimson Horror with Matt Smith's Doctor.
Comment: Worth noting: the next line here was "brave heart, Clara" - a catchphrase used with Tegan.
Comment: https://www.youtube.com/watch?v=tK4euPP5ebE
Comment: @Will Your comment is everything.

Doctor Who recent reference to Tegan


Self join vs. inner join

Question: What is the difference between self join and inner join?
Comment: http://stackoverflow.com/questions/2458519/explanation-of-self-joins
Answer: I find it helpful to think of all of the tables in a SELECT statement as representing their own data sets.
Before you've applied any conditions you can think of each data set as being complete (the entire table, for instance).
A join is just one of several ways to begin refining those data sets to find the information that you really want.
Though a database schema may be designed with certain relationships in mind (Primary Key <-> Foreign Key) these relationships really only exist in the context of a particular query. The query writer can relate whatever they want to whatever they want. I'll give an example of this later...

An INNER JOIN relates two tables to each other. There are often multiple JOIN operations in one query to chain together multiple tables. It can get as complicated as it needs to. For a simple example, consider the following three tables...
STUDENT

| STUDENTID | LASTNAME | FIRSTNAME |
------------------------------------
      1     |  Smith   |   John
      2     |  Patel   |  Sanjay
      3     |   Lee    |  Kevin
      4     |  Jackson |  Steven

ENROLLMENT

| ENROLLMENT ID | STUDENTID | CLASSID |
---------------------------------------
        1       |     2     |    3
        2       |     3     |    1
        3       |     4     |    2

CLASS

| CLASSID | COURSE | PROFESSOR |
--------------------------------
     1    | CS 101 |   Smith
     2    | CS 201 |  Ghandi
     3    | CS 301 |  McDavid
     4    | CS 401 |  Martinez

The STUDENT table and the CLASS table were designed to relate to each other through the ENROLLMENT table. This kind of table is called a Junction Table.
To write a query to display all students and the classes in which they are enrolled one would use two inner joins...
SELECT stud.LASTNAME, stud.FIRSTNAME, class.COURSE, class.PROFESSOR
FROM STUDENT stud
INNER JOIN ENROLLMENT enr
    ON stud.STUDENTID = enr.STUDENTID
INNER JOIN CLASS class
    ON class.CLASSID = enr.CLASSID;

Read the above closely and you should see what is happening. What you will get in return is the following data set...
| LASTNAME | FIRSTNAME | COURSE | PROFESSOR |
---------------------------------------------
    Patel  |   Sanjay  | CS 301 |  McDavid
     Lee   |   Kevin   | CS 101 |   Smith
   Jackson |  Steven   | CS 201 |  Ghandi

Using the JOIN clauses we've limited the data sets of all three tables to only those that match each other. The "matches" are defined using the ON clauses. Note that if you ran this query you would not see the CLASSID 4 row from the CLASS table or the STUDENTID 1 row from the STUDENT table because those IDs don't exist in the matches (in this case the ENROLLMENT table). Look into "LEFT"/"RIGHT"/"FULL OUTER" JOINs for more reading on how to make that work a little differently.
Please note, per my comments on "relationships" earlier, there is no reason why you couldn't run a query relating the STUDENT table and the CLASS table directly on the LASTNAME and PROFESSOR columns. Those two columns match in data type and, well look at that! They even have a value in common! This would probably be a weird data set to get in return. My point is it can be done and you never know what needs you might have in the future for interesting connections in your data. Understand the design of the database but don't think of "relationships" as being rules that can't be ignored.
In the meantime... SELF JOINS!

Consider the following table...
PERSON

| PERSONID | FAMILYID |  NAME  |
--------------------------------
      1    |     1    |  John
      2    |     1    | Brynn
      3    |     2    | Arpan
      4    |     2    | Steve
      5    |     2    |  Tim
      6    |     3    | Becca

If you felt so inclined as to make a database of all the people you know and which ones are in the same family this might be what it looks like.
If you wanted to return one person, PERSONID 4, for instance, you would write...
SELECT * FROM PERSON WHERE PERSONID = 4;

You would learn that he is in the family with FAMILYID 2. Then to find all of the PERSONs in his family you would write...
SELECT * FROM PERSON WHERE FAMILYID = 2;

Done and done! SQL, of course, can accomplish this in one query using, you guessed it, a SELF JOIN.
What really triggers the need for a SELF JOIN here is that the table contains a unique column (PERSONID) and a column that serves as sort of a "Category" (FAMILYID). This concept is called Cardinality and in this case represents a one to many or 1:M relationship. There is only one of each PERSON but there are many PERSONs in a FAMILY.
So, what we want to return is all of the members of a family if one member of the family's PERSONID is known...
SELECT fam.*
FROM PERSON per
JOIN PERSON fam
    ON per.FamilyID = fam.FamilyID
WHERE per.PERSONID = 4;

Here's what you would get...
| PERSONID | FAMILYID |  NAME  |
--------------------------------
      3    |     2    | Arpan
      4    |     2    | Steve
      5    |     2    |  Tim

Let's note a couple of things. The words SELF JOIN don't occur anywhere. That's because a SELF JOIN is just a concept. The word JOIN in the query above could have been a LEFT JOIN instead and different things would have happened. The point of a SELF JOIN is that you are using the same table twice.
Consider my soapbox from before on data sets. Here we have started with the data set from the PERSON table twice. Neither instance of the data set affects the other one unless we say it does.
Let's start at the bottom of the query. The per data set is being limited to only those rows where PERSONID = 4. Knowing the table we know that will return exactly one row. The FAMILYID column in that row has a value of 2.
In the ON clause we are limiting the fam data set (which at this point is still the entire PERSON table) to only those rows where the value of FAMILYID matches one or more of the FAMILYIDs of the per data set. As we discussed we know the per data set only has one row, therefore one FAMILYID value. Therefore the fam data set now contains only rows where FAMILYID = 2.
Finally, at the top of the query we are SELECTing all of the rows in the fam data set.
Voila! Two queries in one.

In conclusion, an INNER JOIN is one of several kinds of JOIN operations. I would strongly suggest reading further into LEFT, RIGHT and FULL OUTER JOINs (which are, collectively, called OUTER JOINs). I personally missed a job opportunity for having a weak knowledge of OUTER JOINs once and won't let it happen again!
A SELF JOIN is simply any JOIN operation where you are relating a table to itself. The way you choose to JOIN that table to itself can use an INNER JOIN or an OUTER JOIN. Note that with a SELF JOIN, so as not to confuse your SQL engine you must use table aliases (fam and per from above. Make up whatever makes sense for your query) or there is no way to differentiate the different versions of the same table.
Now that you understand the difference open your mind nice and wide and realize that one single query could contain all different kinds of JOINs at once. It's just a matter of what data you want and how you have to twist and bend your query to get it. If you find yourself running one query and taking the result of that query and using it as the input of another query then you can probably use a JOIN to make it one query instead.
To play around with SQL try visiting W3Schools.com There is a locally stored database there with a bunch of tables that are designed to relate to each other in various ways and it's filled with data! You can CREATE, DROP, INSERT, UPDATE and SELECT all you want and return the database back to its default at any time. Try all sorts of SQL out to experiment with different tricks. I've learned a lot there, myself.
Sorry if this was a little wordy but I personally struggled with the concept of JOINs when I was starting to learn SQL and explaining a concept by using a bunch of other complex concepts bogged me down. Best to start at the bottom sometimes.
I hope it helps. If you can put JOINs in your back pocket you can work magic with SQL!
Happy querying!
Answer: A self join joins a table to itself.  The employee table might be joined to itself in order to show the manager name and the employee name in the same row.
An inner join joins any two tables and returns rows where the key exists in both tables.  A self join can be an inner join (most joins are inner joins and most self joins are inner joins).  An inner join can be a self join but most inner joins involve joining two different tables (generally a parent table and a child table).
Answer: An inner join (sometimes called a simple join) is a join of two or more tables that returns only those rows that satisfy the join condition.
A self join is a join of a table to itself. This table appears twice in the FROM clause and is followed by table aliases that qualify column names in the join condition. To perform a self join, Oracle Database combines and returns rows of the table that satisfy the join condition.
Comment: So, you mean `self join` is not a a `inner join`?
Comment: @Rahul the `self` refers to both sides of the join usign the same source[table]. the `inner` describes the fact that the a result row can only be formed by both sources containing at least one row that satisfies the condition. The two terms describe separate aspects of the join.
Comment: @LukasKroess A `self join` by itself a `inner join` except the fact that you are performing an `inner join` with the same table
Comment: @Rahul so you are saying that `SELECT * FROM Folder f1 LEFT JOIN Folder f2 ON f1.parent = f2.id` would _not_ be a self join, since it's not a inner join? That doesn't seem right, since I'm joining a table with itSELF

Self join vs. inner join


What to say to someone who is going to eat

Question: In French "Bon appetit", in German "Guten Appetit" or "Mahlzeit", I also know it exists in Arabic and Japanese. As expatriate speaking English daily, I often get confused when I want to say that to my roommates and I end up saying "Enjoy your meal" or "Have a nice meal" which sounds weird to me. Is there anything else one can say?
Comment: In US restaurants, "Enjoy your meal" is a common exhortation from servers. "Bon appétit", while rarer, is also sometimes heard (though perhaps more so in Britain, whose population is generally better acquainted with French than is the population of the USA). In a family or other informal setting, many Britons will enjoin each other to "Tuck in!" or "Dig in!"
Comment: "Belly up to the trough and pig out!"
Comment: Related: ["Enjoy!" Enjoy what?](http://english.stackexchange.com/questions/202321/enjoy-enjoy-what)
Comment: Japanese was mentioned. The word they have, and I am not sure if I have the *romaji* correct is *itadakimas*. But the interesting thing about that, and where it differs from *bon appetite*, (and typically Japanese) it is said by the guest rather than the host. And it is almost an apology for starting - "do please forgive me but I am about to start - and I understand that it is your food I'm eating".
Comment: In British English, saying anything is unusual. That said, a lot will depend on circumstances. The website "Etiquette Scholar" suggests "At a small table of only two to four people, wait until everyone else has been served before starting to eat. At a formal or business meal, you should either wait until everyone is served to start or begin when the host asks you to." The host will usually glance at the guests and say something like "Please..." and start eating. There is no need to say any more.
Comment: In Persian/Farsi saying "Noosh e Jon" or "نوش جان"
Answer: Speaking as an Iowan now living in California, I would simply say, "Enjoy."
Answer: The intransitive use of 'enjoy' is now fairly extensively used by restaurant staff in Britain too.
I agree with the OP that English greatly lacks a suitable term for enjoining your dining partners in the way that 'bon appetit' does in French.
But English, across the centuries, has overcome natural deficiencies, by simply borrowing a foreign expression. Are there really many people in America who wouldn't understand what 'bon appetit' meant?  I alternate it with 'Guten appetit'. But doesn't Spanish have an equivalent which Americans might use?
Comment: *Buen provecho.*


How long are files usually kept in a browser's cache

Question: How long are files usually kept in a browser's cache? I'm wondering about the general population - how long would I need to wait to be reasonable certain that most people's caches will refetch the file?
Answer: It depends much on person, browser and settings.
Browsers usually reserve a certain amount of disk space, like 12 MiB, to this task. If a user stops using the browser it is indefinitely. If they uses the browser rarely, it will be until the expiration - either by internal policy or by HTTP headers. If they uses the browser heavily, it can be 12 minutes or even less.
I believe that it is hard to say 'in general' as I had a website on which 50% entries were from Firefox and 49% from Opera when IE at that point had over 75% market share. If the target audience of your site are users who use their browsers heavily, it can be very short. On the other hand if your site is only website visited it can be nearly never.
Comment: Browsers don't keep items in cache "indefinitely" when headers such as "Expires", "Cache-Control", or "Last-Modified" are present.  Rather, browsers will set a specific expiration time for that content.  Your answer implies this, but it should be explicitly stated.
Comment: @StephenOstermiller - thanks. I did meant it by "(i.e. until expiration)" but I hope it's clearer now.
Comment: @StephenOstermiller Well, technically isn't he correct since if a user stops using a browser the cache is going to remain until the browser is used again? Correct me if I'm wrong. I mean, seems like a silly point to make except from a security standpoint there may be cached data recovered off a PC because it's still there as long as the browser isn't opened. Am I wrong?
Comment: There could be cases a browser is no longer used and still has really old cache sitting around.   However, the question is about whether browsers refetch files, not about inactive browsers.
Comment: @StephenOstermiller you seem to reiterate what you wrote 3 years ago. I tried to clarify it by "If he/she uses the browser rarely, it will be until the expiration - either by internal policy or by HTTP headers.". TBH I'm not sure how to clarify it further.
Answer: Typically files will stay in the cache until space runs out.  There is an article that describes some of the defaults for popular browsers:
It's Time to Rethink the Default Cache Size of Web Browsers
As Maciej stated, it's also a matter of how much each person browses.  Think of the user base that will be visiting your site into consideration, as well as how bandwidth-heavy your site is (if the average user downloads tons of content from your site, chances are they will fill their cache real fast).
If a cache refresh is needed, you can force the page to not cache files.
Comment: Here's [the link to last working copy](https://web.archive.org/web/20151130113539/http://kaioa.com/node/74) from archive.org. Note that the document is ~8 years old (2008) which is quite ancient in browser behaviour terms. This question is still in the top 10 when you ask Google.
Answer: As the other answers have stated, it depends upon the browser settings. But note that having a file cached doesn't necessarily mean that changes are not fetched from your site. If configured to do so, the browser will send a request to the server, with details about the date of the cache file. The server than then respond with "ok, use your cache - it's the latest version" when there are no changes. This repsonse is small and quickly sent. If the server version has been updated, then the server will respond with the new version.
How often the browser sends a request to the server is browser-specific - they can be configured to check for updates each time, once an hour, once a day, never or anywhere in between. The server can also specify how often the browser should check for updates.
this is just a sketch - it is simplified and incomplete. For the complete details, see W3C - HTTP/1.1 Caching.
Answer: Depends on how long the server specifies. The server sends back parameters in response header. They can specify max-age (how long to cache the file before it expires in milliseconds) or an expiration date (date that the file will expire). If it has both, max-age will take priority.


Marketing Cloud Business Units

Question: If we would use 2 Business Units, would we still have the main Account with access to all lists from Business Unit level and all subscribers with a possibility to mail them all with a specific sender profile?
Answer: Not sure I  get the full scope of your question, but overall - yes. All subscribers added in the Child Business Unit will also be added to the All Subscriber List in the Parent Business Unit - and thus, if you send an email to the all subscriber list - yes, you would have access.
If your data model is based on data extensions, then this wouldn't be possible from the Parent, given that the parent doesn't have access to data extensions stored in the child business unit.
However, if you use data extensions, you can populate a "total audience" data extension in the child business unit, by selecting data from both the child and the parent bu (i recommend 2 queries), first one selecting data from child, second selecting from the parent by adding ent. in front of the data extension name.
All depends on what you are trying to achieve.
Comment: Hi Michael ! Sorry for dropping the ball on this ! :) Depending on your requirements - it sounds like creating two data extensions would be a better option for you, in the same BU. Are the subscribers overlapping or ? Could you maybe add some more details? :)
Comment: Thank Kanneth! We're actually going to use Data Extensions. You're talking about parent/child BUs, do we definitely need use a BU for the parent to mail them all, or can we archive this without actually adding an extra BU?

Marketing Cloud Business Units


Can I dig a hole under an enemy using Mold Earth?

Question: So, using Mold Earth, can I remove the 5ft. cube of loose earth under an enemy?
Answer: In the description of the mold earth cantrip, it says:

If you target an area of loose earth, you can instantaneously
excavate it, [...]

Now, it depends on what you want to achieve and what your DM rules. The spell specifically states "This movement doesn't have enough force to cause damage." and it also depends on your DM's definition of "loose".
So I would say, yes, you can move a 5ft cube of earth from underneath an enemy, but your DM may rule differently depending on your goal.
Comment: Once it can't cause damage, the maximum result you would achieve is a prone enemy inside a 5ft. deep hole if your DM consider it loose even with some weight (enemy) over it.
Comment: 5 feet isn't enough to cause fall damage anyway
Comment: Which might be enough, if you just want to slow/temporarily sideline an enemy.
Answer: Yes, you could remove a 5-foot cube, assuming it's loose earth.
However, doing so will not cause physical damage to the enemy.
(You could rule, the 'damage' would be to speed, additional movement, the ability check to see if enemy gets caught in the hole or not, etc.)
I'd say the magic is powerful enough to move the dirt, but not necessarily fast or forceful enough to cause damage...the target might end up being stuck in a hole (assuming the DM allows that) or on the wrong side of a 5-foot hole (if the DM rules that you couldn't remove it right from right underneath an enemy or the enemy makes an ability check to not fall in).
I'd assume that loose earth here refers to the earth not being worked (e.g. regular dirt) and not being stone or walls or something special.
Comment: There has to be more to "loose" than "not worked." I would assume that frozen tundra or the earth _under_ a wall, though unworked, are not loose.  (In fact I'd bet the latter case is _the_ reason the designers included "loose" in the description) There are some interesting edge cases that probably fall to GM fiat, e.g. a well-used centuries-old dirt road which was never intentionally "worked" but has incidentally become extremely compacted.
Comment: @Timbo -I agree with you there, hence, "regular dirt".  Dirt under a wall, definitely not under any definition (Unless it's something like a farmers stone wall -- a castle wall, heck no).  Frozen Tundra, definitely a DM call...or at least maybe worth half the volume.  I think you'd be right, they wanted to rule out "special" terrain and give the DM the right to allow/disallow it situationally
Answer: One thing to note with spells like these and targets is that they are meant to target a non moving target.  Which you are by targeting the ground.  But if you want to catch a creature in the area of affect then I'd require you to succeed at a ranged attack since your wanting to execute the spell when a moving target is in the spot you are targeting.  Otherwise the target you intend will not be in the area of affect.  Still won't cause damage but I might require the creatures in the area to roll to say on their feet.


Does quantum field theory exist non-perturbatively or is perturbation inherent to its Nature?

Question: In quantum field theory, calculations generally are made by using a perturbation approximation with the aid of Feynman diagrams. The theory is not well suited for bound states as Feynman diagrams refer to situations where initially and finally free fields are present, which obviously is not the case for bound states (like the hydrogen atom).
If not perturbatively, how calculations on bound states can be made? Does it even make sense to say that there are perturbations in the case of bound states? Does QFT exist non-perturbatively?
I saw this article (MURRAY GELL-MANN AND FRANCIS Low
Institute for Advanced Study, Princeton, Eem Jersey
(Received June 13, 1951)), which treats the bound state of a proton and a neutron interacting through a meson field (as the old strong force is handled) but I'm not sure if this is a good example where QFT can be perturbatively used, as this refers to an old fashioned strong force (the meson field acting between nucleons is replaced by the gluon field acting between quarks in the modern theory of the strong force).
In the comments, reference is made to lattice quantum chromodynamics. It is said that this is a non-perturbative procedure. That begs the question though. Why stepping over to discrete spacetime points? And this doesn't answer the question of why non-perturbative calculations can't be made in real spacetime. In fact, the example is a hint that non-perturbative calculations are not possible in continuous spacetime.
To put it differently: is perturbation inherent to the QFT?
Can't we use (perturbative) QFT in solving bound state problems by connecting (summing?) an infinite (or very high) amount of disconnected (in time) free interactions? Each of these interactions can be treated perturbatively and the total of these interactions represents the bound state). Or do these disconnected free interactions somehow overlap?
Comment: I think the question title should be edited. "Does QFT exist non-perturbatively" verbatim is essentially answered [here](https://physics.stackexchange.com/q/33201) and it is not difficult to search online for "QFT non-perturbative effects". A better question title, which pertains to your main question is, "Can bound state calculations be made perturbatively in QFT?" or something along those lines.
Comment: Comments are not for extended discussion; this conversation has been [moved to chat](https://chat.stackexchange.com/rooms/120752/discussion-on-question-by-deschele-schilder-does-quantum-field-theory-exist-non).
Answer: The conformal-bootstrap, Instantons, Solitons. Fore more: nLab: non-perturbative quantum field theory
Comment: I downvoted because this is not an answer, just some words followed by a hyperlink.


Deleting certain rows in excel

Question: Hi there I have 200K rows in my excel project I need to delete about 30K of them. How can I do that without CTRL+Shifting for 30+mins.
I'm unsure what else I can do. Please help
Comment: Hi Lucas and welcome! And how do these 30 thousand rows differ from the rest 170 thousand?
Comment: If you can describe the criteria for which rows need to be deleted, we can probably provide you with a formula that will do this in seconds.  Can you succinctly describe what it is about a row that makes it a row you want to delete?  Also, do you have excel O365?
Comment: Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking.
Answer: As you're mentioning CTRL+Shifting, I guess the rows are all beneath each other. You may select multiple fields by typing into the name field (below the selected field name is A1, it's on the lower ribbon to the left).

I guess the functionality is best explained with an example: To delete rows 159k-189k in a table with colums named from A to K, you would type A159000:K189000 into the name field and all specified cells will be selected. Then you delete the selected area either with DEL and all values are gone (the empty cells will be still there), or you use the right click menu, click delete there and choose "Shift cells up" to delete the cells alltogether.
Answer: Make a macro - https://support.microsoft.com/en-us/office/quick-start-create-a-macro-741130ca-080d-49f5-9471-1e5fb3d581a8
From website: If you have tasks in Microsoft Excel that you do repeatedly, you can record a macro to automate those tasks. A macro is an action or a set of actions that you can run as many times as you want. When you create a macro, you are recording your mouse clicks and keystrokes. After you create a macro, you can edit it to make minor changes to the way it works.
As long as there is some sort of sequence to the records you are deleting this should work fine. Another alternative is to sort the records so all of the to-be-deleted ones are on top or bottom where you can easily click the last row, then scroll up or ctrl+f your way to the first to-be-deleted record (note when you select the first one, make sure you select the row. This way, you can hold shift and select the other row  and delete as many rows as you want at once. It's all about getting Excel to order it for you, or to have the macro do the sequence thing for you. For example, a with a macro you can find a particular string, then delete that row, and repeat until done. Or you can record pushing the down button 3 times so it deletes every third record. You could even have it save automatically every 5th delete. Anything you can do can be recorded and saved as a macro.

Deleting certain rows in excel


What is the name of this strange white caterpillar?

Question: My friend found this caterpillar in south-central Tennessee in August. No identification guides or databases I've searched had a similar picture. Does anyone know the name of this species?
Comment: Welcome to Biology.SE! For identification questions please [edit] your post to include the size of this creature. The size can be an essential clue for identifications. ——— If possible clear, closeup photos of the creature from multiple angles would also be helpful. ——— Please also take the [tour] and then go through the help pages starting with [Ask] questions effectively on this site. Thanks!
Answer: It is hard to be sure without clearer pictures, but that looks like something in the genus Eriocampa and it could be a Butternut Woollyworm (Eriocampa juglandis).
Picture taken in Kentucky from BugGuide.net © 2015 by dan40165:

These are the larvae of a sawfly (so not technically a caterpillar) and are about 1 inch long.
You can learn more about this particular species here. To narrow down the exact species it would be helpful to edit more information into your post including the types of plants in the area.


How is powershell commonly used in SQL server?

Question: I am wondering how is powershell used in SQL Server? I heard people saying that powershell makes a DBA's life easier. But I dont know how is powershell commonly used in SQL server. Can anyone provide me insights or links as to how powershell is being used in SQL Server?
Comment: You can use SMO with Powershell to easily script out database objects, databases and data.  Here's an example I did a few years ago:  https://social.msdn.microsoft.com/Forums/sqlserver/en-US/71569b94-ec97-444a-b68c-ec2a87c7b6df/example-scripting-data-with-smo-and-powershell?forum=sqlsmoanddmo
Answer: So the main thing is that you can set these up to be dynamic. You wouldn't have to log into each instance to find out the settings. It does make it easier to perform a health check and just keep track of the lot. I used it for Parsing Excel documents to SQL server. This turned out to be very simple instead of converting it to csv documents and importing from there.
But what I would do is check into this site, it has a pretty good repository. A bit old though...
Additionally, Microsoft is making a huge amount of enhancements to powershell into SQL server. It was ignored previously as it was already a scripting language. Most people now are going into proper desired state configuration where you can spin up a SQL server instance with memory and all the settings you would like inside the SQL server instead of it just setting up the windows box and you needing to log into the SQL instance to run scripts. Pretty good post about it here.
Comment: In a perspective of  multiple servers (install/update multiple instances) it's the best tool.
Comment: thanks for sharing. Do you mind posting a link on a guide how to do that? greatly appreciate it
Comment: Not that this is a bad answer but you only pointed out probably 0.5% of what I use PS for with SQL Server. Also the site you noted, viewing a handful of that author's script is just executing T-SQL via a PS script and didn't see one utilizing SMO which is more common.
Comment: [As well if you are referring to MS putting more into `SQLPS` the community is assisting in this via a Trello site.](https://trello.com/b/NEerYXUU/powershell-sql-client-tools-sqlps-ssms).


Sort folders by size in Finder

Question: Is there any way, to sort sub-folders in a folder or a whole volume (such as the root drive where macOS is installed) by size? Is it possible to do without resorting to a 3rd party app?
Answer: If you open the folder in finder such that your sub-folders are displayed you can click on size at the top of the list. If the folder sizes are not displayed, open finder and under the View menu select "Show View Options". This brings up a pane like an image shown below.

At the bottom of the pane is a checkbox to determine folder size. Check the box and folder sizes will be displays. Then by clicking on the size near the top of the Finder window. the contents should be sorted by size.
You need to make sure size is shown in the Finder window. To ensure this is the case, in the pane we just looked at, be sure to check "Size".
Comment: Thanks a lot.  That's quite useful.  However, I'm still not able to sort all items in the whole volume by size (including folders, with should appear on top).  I tried doing an advanced search for all items modified before today, or for all items of kind: folder, which shows all items (including folders) but folders don't show dates, and the checkbox "calculate all sizes" is greyed out.  Your answer was useful anyway!
Comment: This solution works only in List mode, not Columns. Otherwise, the options isn't even present
Answer: If you need to get an estimate of storage, and not always use the sorted mode, you can use About this Mac.app.
Open it, storage tab, manage, Documents tab, File Browser. This will take some time and CPU but it will sort everything eventually.
Comment: That's a great way to sort stuff in the home folder, thanks a lot.

Sort folders by size in Finder


How would you explain that Ubuntu is fine for many people

Question: How would you explain that Ubuntu if fine for most people who just want to browse the web, write e-mail, and so on?
Comment: [You do everything through a browser now ...](http://xkcd.com/934/)
Answer: People are so entrenched in using Windows that telling them ubuntu is better for their basic tasks is not enough, you have to show them how ubuntu can be better/easier performing whatever it is they are trying to accomplish.
You can set up a fully configured Ubuntu box for them and show the differences on their daily usage.
You can show to them how a particular Ubuntu / linux app is more suited / easier / cheaper
for them.
Comment: You can use a Live CD to demo the possibilities.
Comment: thats a good idea. :)
Comment: Run some basic windows apps using wine to demonstrate that *nix can still run any windows apps they're attached to and don't touch flash video online with a 10ft pole (unless it actually works well on your computer). If you want a safe bet goto YouTube (since it uses HTML5 video).
Comment: you could also set up a virtual machine with their original windows setup as fall back in case ...
Answer: I like to bring along a non-technical person, and have them explain Ubuntu is fine.
You can argue until you're blue in the face, but most people will just think, "Well, you're a technical person, of course it's easy for you."  But when someone that they consider a peer who has their same (or less) computer experience tells them in a heartfelt way, "Ubuntu is plenty good", that's the best sales pitch.
Answer: This is the sort of question better directed to the Ubuntu Marketing Team - not only is it their entire reason for being, but they can also provide materials, mentoring and research data to back up assertions.
In particular you should read the Activism Guide and look at the materials available  on the Spread Ubuntu site.
I personally like to show people the Ubunchu! manga, it goes over well in my social circles.
Answer: Let the people forget about the os! Nobody works with the os. As you write in your question it goes about writing e-mail or browsing the web: there are the applications who are curcial. So, firefox is know very well to browse the web but what email-client would you promote?
If these apps are usable on windows, like firefox, let the people try them and if it comes to the point that they should pay for an app like outlook people reminds that there exists an app for free.
Time comes where a computer must be replaced and that will also be a point of decision. If the experience with the apps are positive it will be a small step to save some money and choose a free os.
Answer: Just show them.
Remember that Ubuntu is different, but you need it to be better for people to consider it.  Much like switching from a right-hand scissor to a left-hand scissor.  Can do the same, but is different, and it needs to be better to be considered.
Answer: I would just show them the efficiency of ubuntu on my dual boot machine with windows and to compare the difference for themselve.
Answer: I like to show them Compiz. :)
Answer: How you did know that yourself? If someone told you and it worked, it's very likely that it works for some other people :) Let them to try it for themselves, help them to clean that «uncleanable» virus in their pendrive, show them that great Compiz effect... and tell them they also can do that easily.
Also, you may want to join a LoCo Team near you and get involved in spreading Ubuntu in your community.
Comment: I'm a programmer, just by the nature of that occupation I'm tech savvy.


How to level an old cement floor?

Question: I have an old garage where I want to make an office. The floor is not 100%  leveled and has some holes. I read about self leveling material for the floor. Is this something one can apply? is it easy? Are there other diy solutions?
Answer: Self-leveling material may be appropriate depending on the condition of the existing floor and the depth of the repair. You'd need to take some measurements and read the product literature to know for sure.
The alternate approach is to rip treated joists as needed to create a level frame, then overlay suitable subfloor sheathing.
Without more information that's about as specific as I can get.
Answer: I used self-leveling product for my basement, and yes it is doable for your typical FIT DIYer, but it is best with two people if you have to mix more than a couple of bags. It is actually a quite demanding task as each 50 pound bag must be mixed and poured and worked in before it dries.  Each of these bags must be mixed with a powerful drill. In my basement I had to use ~12 -  50 pound bags, and I was exhausted after the hour or so it took to apply them. My wife helped for the 1st 8 bags but got exhausted and I had to finish myself. The best tip I can give is to watch a some youtube videos on how to apply the material. In the end I was very happy with my results and I was able to apply laminate flooring to the level surface.
Answer: you can level any floor with self leveling mortars, but they have to be the right ones.  the typical ones you find at big box stores are overpriced garbage.  here is the one we use:
http://flextile.net/products/CIS/p-cisSP-59.html
just check their site for installation and youtube for how it works.

How to level an old cement floor?