Will water planets have ice caps?

Question: Considering both full water worlds mostly out of water and ice, and rocky planets that just have a bit too much of it - would polar caps form on them if the conditions are roughly similar to modern Earth? Or the lack of large landmasses would turn the whole hemispheres of the planet into one giant Gulfstream due to there being no obstructions for the warmer water to replace sinking cold polar water and equating the whole planet's temperature, much like the atmosphere currents of a tidally locked planet can prevent its atmosphere to liquify on the night side?
Answer: Yes.
Polar ice caps would defnitely form, if the world is such the the average polar air temperature is well below freezing.
The question is whether the ice would remain in the polar region or drift away and melt.
For a deep ocean world, without any shallow geographic features, the waterflow will be very similar to the airflow around it. i.e. Alternating regions of laminar east-west flow, with regions of more turbulent cyclic flows inbetween.
Something like this:

This will mostly keep the polar water just spinning in place, with limited circulation to lower latitudes, keeping the ice cap in place.
If however the ocean world has significant underwater geographical features (sunken continents), and especially if some of these continents approach or breach the water surface, the oceanic currents will adopt the large, circulating Oceanic Gyre patterns familiar on earth.

With these Oceanic Gyres in place, the polar ice will want to be pulled along with the current, and require either a solid rock foundation(Antartica) or being sufficiently landlocked (Arctic regions) to stay in place.
Comment: "The question is whether the ice would remain in the polar region or drift away and melt." But if the caps did float away and melt wouldn't water at the poles continually re-freeze to make more cap?
Answer: The Arctic Ocean has an ice cap despite the Gulf Stream's access through the Greenland and Barrents Seas. As long as the polar surface waters are cold enough for part of the year for frazil or "grease" ice to form, your ocean world can have ice caps.
Water planets can be ice-free, have ice caps like Earth's Arctic Ocean, be mostly (a cold "eyeball" planet) or entirely covered in ice (like Ganymede), and, if deep enough, have icy floors below their liquid surface.
Not only can ocean worlds have ice caps, they may be more habitable than those without:

Traditional definitions of the habitable zone assume that habitable planets contain a carbonate-silicate cycle that regulates $\text{CO}_2$ between the atmosphere, surface,and the interior. Such theorieshave been used to cast doubt on the habitability ofocean worlds. However, Levi et al (2017) have recently proposed a mechanism by which $\text{CO}_2$ is mobilized between the atmosphere and the interior of an ocean world. At high enough $\text{CO}_2$ pressures, sea ice can become enriched in $\text{CO}_2$ clathrates and sink after a threshold density is achieved. The presence of subpolar sea ice is of great importance for habitability in ocean worlds. It may moderate the climate and is fundamental in current theories of life formation in diluted environments. – The Ice Cap Zone: A Unique Habitable Zone for Ocean Worlds, Ramses M. Ramirez and Amit Levi[PDF]
Comment: The Arctic Ocean is expected to be ice-free in the summer beginning sometime between 2030 and 2050. Polar ice caps in conditions like Earth are *not* guaranteed to be a thing.
Comment: @KeithMorrison It's not going to be ice-free simply because of the Gulf Stream or it would have been ice-free through recorded history.
Comment: I never said it was because of the Gulf Stream. Look up the latest climatological modelling.
Answer: There would be two main variables I can think of. Firstly the amount of radiation received by the planet from its sun (temperature). Assuming you take an Earth sized planet orbiting a sun like star then that basically comes down to how far out from the star the planet orbits.  Note: there are people on this forum who can and probably have already calculated what happens if you were to arbitrarily increase/decrease the Earths distance from the Sun and the impact on climate. Push the Earth far enough out (e.g past Mars) and it would be a ice ball.) So do a search of the site.
Secondly there is the depth of the oceans. If you have large continental sub surface land masses at polar latitudes then depending on the depth of the water covering them and their geography they might interrupt oceanic convection currents enough to allow permanent ice caps to form. But the water level above the continent would have to be shallow, very shallow for this to come into play. If the ocean over the land mass is more than a few meters deep it probably wont be enough to let ice settle in place.
So that just leaves Sea Ice! depending on its distance from the sun and its degree of inclination (seasonal period) you could have a water covered world where, during the winter period masses of sea ice form in the extreme polar regions then drift off on the prevailing currents. Get enough ice forming quickly enough and depending on how strong the wind and water currents are you might get temporary floating ice fields forming during winter in the northern and southern polar regions, alternating so that only one pole as ice at any one time.
Answer: ...And from my search online, the results are confusing. I've found a scientific article about atmospheres on water worlds:
https://web.archive.org/web/20180825152625/paoc.mit.edu/paoc/papers/aqua.pdf
It says the following:

Millennial time-scale simulations yield a mean state in which ice caps reach down to 55° of latitude and both the atmosphere and ocean comprise eastward- and westward-flowing zonal jets, whose structure is set by their respective baroclinic instabilities.

But then there are these guys:
https://journals.ametsoc.org/view/journals/clim/19/18/jcli3874.1.xml
And they say it will be essentially the opposite of that:

The WaterWorld climate is significantly warmer than that observed for the modern reference (Fig. 3). The annual average air surface temperature is 27.5°C, with a shallow equator-to-pole temperature gradient from 33°C in the Tropics to 13°C over the polar islands. Only small areas drop below freezing in winter, and those are over the polar islands—there is no sea ice formed at any time of year.

So I guess the real answer is that it can be both and the writer's free to choose whenever his Waterworld has ice caps or don't?
Comment: All of these simulations (which are not a lot better than guesses as we have no ground truth to compare them against) depend on insolation. A plant further from its Sun or with a dimmer sun will be more likely to have ice or will have more ice.  Closer and hotter, less ice. No one can meaningfully give you an (accurate) general answer,
Answer: Simple: You need a region that has a maximum mean temperature of 271 K. Seawater freezes at a lower temperature than freshwater, and we already have polar regions of this very world with the needed temperature. But as the surface is covered completely by water, that'll be a double-edged sword. Low-lying clouds work to reflect energy back into space and higher up ones help trap it. Water has a low albedo and high emissivity, so it absorbs lots of energy during the day, and loses a good amount of it as well.
Water vapour is a greenhouse gas, but water having a high specific heat capacity means that it takes a good amount of time to heat up and cool down. So, you don't want to place your planet too close to its sun(s). It's that simple.
Comment: Ah, but here's another interesting question, would water on an ocean planet be actually seawater or freshwater? Since there are no continents to wash salts and minerals into the sea from...
Comment: That's definitely a factor.

Will water planets have ice caps?


Can't find Square pencil/brush on Photoshop

Question: All brushes/pencils are rounded shape. Where did the square shapes go?
Answer: Photoshop moved square brushes to a new section called:
Legacy brushes.
You can reach it by choosing the brush/pencil option, then hover over the canvas and right click.
Click on the upper right corner where the little cog with the small arrow is.
At the bottom of the list choose "legacy brushes" and click OK.
The Legacy brushes set will be added to your brushes list.
Open them and look for the square brush sets and choose the appropriate one.
Good luck!


(Blender Game Engine) How can I make scene/layer visible through a certain object without using python?

Question: I am needing a python-free way to make a layer or scene ONLY visible through a certain object. I want to make like a portal effect where you look through an object into another layer or scene. If anyone knows how to do this, please let me know! I would very much appreciate it!
Comment: I don't have time to write a full answer now, I'll do it later if nobody else has until then. What you could do is basically just having a second camera seeing what the portal should show. Then you put the camera output as a texture on the portal. It would be a similar technique to the one used to create a minimap in this tutorial https://youtu.be/HgXAv9csJu0 (still a bit different since the portal wouldn't be in an overlay scene)
Comment: Why does it need to be python-free? It's impossible without python.
Comment: Because I'd rather not take the time to learn how to use python with the Blender game engine. Too busy.
Comment: You won't need to know python if somebody posts an answer explaining step by step how to get a script getting you what you need to work. And by the way, is it possible not to have what you want to see on another layer or I'm another scene but rather in the same scene on the same layers, just a place the player can't reach and see without the portal? This would make things a lot easier (I don't know if it would be possible otherwise).
Comment: That might work. This is the effect I was wanting to achieve in the game engine (WARNING LOUD AUDIO):
https://youtu.be/ol0__bZHcU4?t=3m45s
If it's not possible, that's fine. I was just wondering.
Answer: General idea of making a portal: you have a camera that sees what the portal should show. Then the output of the camera is rendered to the other portal as a texture.
Without python this is not possible though.

So here's a python way to do it (who would have thought? ;) ):
First we create the portal:
You can use whatever mesh you want but a plane mesh is probably the best. Name it something fitting, e.g. "portal". Make sure that when the object isn't rotated (=> rotation of 0,0,0) the object's normal(s) is/are facing towards the -Y-axis.

Next create a new image in the UV/image editor. Make sure that the aspect ratio of the image is the same as the one of your camera. E.g. if your resolution is 1920*1080 the image's resolution could also be 1920*1080.

Then select the portal, create a material and make it shadeless. Head over to the texture tab and create a texture. Assign the created image to it and unwrap the portal (U in edit mode). Set the texture's source to Generated and its mapping coordinates to UV. If the portal's dimensions aren't the same as the image's you should also move the UVs to the image's center.

Last thing left for the portal is the logic. Add an Always Sensor and a Python Controller. Activate the sensor's true level triggering. Now the controller: Choose Module instead of Script and type portal.main into the text field. Add a new text data block in the text editor and paste in the portal script from below. Name it portal.py. Then add two string game properties. Name one camera and the other one image. The value of the second one is the name of the image you created. We'll fill the other one later.

Now we take care of the camera. So just add one. Again, choose a name that makes sense like "portalCam". You can put this name in the camera property we created moments ago.
Now the logic. Create a new script and paste the camera script in it. Name it something meaningful like portalCam.py. Add an Always Sensor and a Python Controller and activate true level triggering. Instead of choosing a module just choose the script that was just added. Create three string game properties, the first named camera, the second named portal and the third named reference. Put the name of your player's camera in the first property, the name of the portal in the second one.

The third property's value is the name of a reference object. It's the object the camera is referring to when calculating its position, so it's like another portal. The portal we created will look from the point of view of this reference object. You can use whatever you want, a plane (=> another portal) or just an empty would be fine. Add it and put its name in the reference property of the portal camera. All that is left to do is positioning and rotating it. The object's local Y-axis is the direction the camera will be looking in. To check the local transformation just hit ⎇ AltSpace and select local.

And now you're done! The portal shows the output of the camera which moves relative to the player camera's movement.

Here's an example .blend:

Scripts:
Portal Script:
from bge import logic, texture

scene = logic.getCurrentScene()

cont = logic.getCurrentController()
own = cont.owner
portalCam = scene.objects[own["camera"]]

matID = texture.materialID(own, "IM" + own["image"])
map_png = texture.Texture(own, matID)

source = texture.ImageRender(scene, portalCam)
source.background = (scene.world.backgroundColor[0]*255,scene.world.backgroundColor[1]*255,scene.world.backgroundColor[2]*255, 255)

map_png.source = source

def main():

    map_png.refresh(True)

Camera Script:
from bge import logic
from mathutils import Vector, Euler, Matrix
from math import pi

#getting all the objects
own = logic.getCurrentController().owner
scene = logic.getCurrentScene()
cam = scene.objects[own["camera"]]
portal = scene.objects[own["portal"]]
empty = scene.objects[own["reference"]]

#calculating the position of the portal camera
rotmat = empty.worldOrientation*portal.worldOrientation.inverted()*Matrix.Scale(-1, 3, portal.worldOrientation*Vector((1, 0, 0)))
diff = cam.worldPosition-portal.worldPosition
diff = rotmat*diff
own.worldPosition = empty.worldPosition+diff
own.worldOrientation = empty.worldOrientation
own.localOrientation = [own.localOrientation.to_euler()[0] + pi/2, own.localOrientation.to_euler()[1], own.localOrientation.to_euler()[2]]

Comment: Wow this is a very thorough answer. Thank you so much!
(sorry for the late reply, I've been busy with college)


How do one teach thermodynamics to Higher dimensional beings?

Question: Suppose an entity from a higher dimensions who can experience time flowing both forward and backward wants to understand our world and how far humanity have advanced in tech and science, is it possible to explain entropy to them since I suspect that it will be similar to telling them about a solid wall when there is an extra spatial dimension on top of x, y and z dimension.
Comment: Enrol the higher dimensional entity in a university degree course. What could be more more education than that? Nice to see another one of your challenging questions.
Comment: I see no World Building here. only straight physics.  (Also, "*I suspect that it will be similar to telling them about a solid wall when there is an extra spatial dimension on top of x, y and z dimension* ignores the fact that humans are 3D (4D if you include "time") and yet perfectly understand 2D and 1D.  You should probably read Flatland: A Romance of Many Dimensions.)
Comment: @RonJohn:  im creating a scenario where two different worlds meet and tries to understand each other, then there is this challenge that have to be overcome
Comment: VTC retracted.  The rest of my comment still holds.
Comment: They would understand the concept of a solid wall, but where you can go round it or over it, they can also go before it or after it.
Answer: It shouldn't be too hard in practice.
Consider that you're having a conversation with one of these beings. It'll take a while for them to work out what's going on, but once they understand that they have to jog along at a steady pace in time to be able to communicate with you, they'll understand a lot more about our perception of the world.
As soon as you can talk to them, they'll understand our perception of time, and that to us, time has a fixed direction and "speed". Once you're past that, it's just a matter of explaining it to them as you would to anyone else.
Answer: Sure, you simply describe it according to it's time axis. You give it point 1 (now) and point 2 (again, now, but the later now than before). Now you can calculate a vector from point 1 to point 2, that is the positive direction of time.
So then you change the scalar of time in the thermodynamic equations for the vector of time. Tada, you added the extra dimension to your equations.
The other question is, at what point of time does the 4 dimensional being want to know about our advances? It can move in both directions of time, right?
Answer: This being might have already observed that for some reason unknown to him all things in our world move always and only in one direction in the dimension we call time.
Explain that entropy is what defines the direction of time: "we" can only move in the direction where the entropy increases. So we can let a bottle of perfume open to evaporate and fill a room, we will never have a scented room spontaneously fill a bottle of perfume.


Add Tier Price Programmatically Could not save group price Error - Magento 2.2.x

Question: I am having issues setting tier prices in Magento 2.2.1, I have looked at setting the tier prices and used below to get myself started:
Magento 2: How to set Tier Pricing Programatically Group Wise
This seemed to work sometimes but am getting the below error which seems to suggest that something has changed since the update to 2.2.x. Upon some research the old method is depreciated and suggests to use another interface:
Warning: Invalid argument supplied for foreach() in /var/www/mage2/public_html/vendor/magento/module-catalog/Model/Product/TierPriceManagement.php on line 103

https://github.com/magento/magento2/issues/10884
But cannot work out quite how to fix am currently using the below:
\Magento\Catalog\Api\ProductTierPriceManagementInterface $tierInterface
$this->tierInterface = $tierInterface;
$this->tierInterface->add($sku,$groupId,$price, '1');

From the github issue i have changed: \Magento\Catalog\Api\ProductTierPriceManagementInterface $tierInterface to \Magento\Catalog\Api\ScopedProductTierPriceManagementInterface $tierInterface
Update
Ok so i injected that class into my construct and use like below:
$tierPrice = $this->tierPrice;
$tierPrice->setQty(floatval(1));
$tierPrice->setValue(floatval($price));
$tierPrice->setCustomerGroupId($groupId);
$this->tierInterface->add($sku,$tierPrice);

I was getting errors about values not being valid so used floatval to fix this. Still errors occur for a lot of products however always the same products but others work fine everytime. New error:
Exception #0 (Magento\Framework\Exception\CouldNotSaveException): Could not save group price

Not sure why this happens will keep working on it but seems similar to what i was experiencing with the previous method. These seem to be products that i got to work previously as have prices set bet they cannot be set again. I have tried removing the old price and it works after that but need it to just update programaticaly.
Still unsure how to set these prices for a specific website however but may ask this in a separate question.
Answer: Do you have any solution? I got same issue.


Can you catch Keldeo in Pokemon X/Y?

Question: Can you tell me what event you can get a Keldeo in?
Answer: You cannot catch a Keldeo in Pokemon XY.
But there are currently only 2 ways to get Keldeo:

Trade: Be it with a friend, acquaintance or passer-by, you can get a Keldeo if someone's willing to trade one to you.
Transfer from PokeBank: There have been a few events throughout the world during the past two years where Keldeo were being given. You can find a list here.

Unless there's another event where you can get Keldeo in Pokemon XY, those two above ways are the only ways to get Keldeo.
Comment: I don't think it is possible to get event-only pokemon from the GTS anymore. They cannot be uploaded to the GTS for trade, and (apparently) it will not let you complete trades with them. The only way to trade them would seem to be using the PSS Friends/Acquaintances/Passersby lists.
Answer: GTS trade doesn't allow special or mythical pokemon trades anymore, the only free way to get Keldeo is trading with PSS friends, acquaintances, and passersby.

Can you catch Keldeo in Pokemon X/Y?


How to find if partition is soft partition or not - Solaris 10

Question: How to find if partition is soft partition or not in solaris 10?
Comment: Are you talking about `slices`?
Answer: A hard partition will always have its device name as /dev/dsk/cXtYdZsN, where X, Y, Z and N are numbers.
If the device name is of the form /dev/md/dsk/*, then it's a Solaris Volume Manager metadevice - and that category includes soft partitions, mirrors, software RAID and other things. The output of metastat will tell you whether a metadevice is a soft partition or something else.


Windows 10 - Network Sign-in and cached credentials

Question: I'm managing remote laptops connected to our self-hosted domain (Samba 4, in case it's importante, but I don't think so).
Windows PCs have a IKEv2 VPN configured, so from the login screen they can login using the Network Sign-in button. This works great, until for some reason we reset the password.
If the password is reset, Network Sign-in will connect to the VPN with the new password, but Windows 10 will fail to log-in as it has the old password cached.
The workaround, so far, is to login with the old password, and then execute something using runas to cache the updated password.
Is there a way for Network Sign-In to update the cached password when it properly connects to the VPN?
Answer:
Is there a way for Network Sign-In to update the cached password

No. This is an education issue. Users should change their password when they are connected, then CTRL-ATL-DELETE and lock/unlock the computer. Done.


Using regularization with logistic regression

Question: I have a data set of 3000 observations with 9 variables, and I'm trying to predict whether water are safe for drinking.
Regular multivariate logistic regression isn't that good at forecasting, and also none of the coefficients is significant, even if I run univariate logistic regression.
This is why I thought of regularization, but I wasn't able to found an explanation of this and when it is appropriate to use.
Also, if it exists, if be happy for a reference to R functions.
Comment: Statistical significance has nothing to do with regularization and forecasting. What doesn't work about forecasting with logistic regression for you?
Comment: Is your measured response variable binary or some measure of contamination (e.g., 6 parts per million coronavirus).
Comment: It is binary: safe or not safe
Comment: plenty of regularised glm out there i believe. Glmnet is quite popular and has vignette.
However, do you have any expectation of what the relationship is between inputs and "safe"eg I could imagine not safe to drink is "legally" defined as chemical 1> conc1 or chemical2 > conc 2 or chemical 3 > conc3. I don't believe you can fit this in a logistic regression (without adding some nonlinearities).
Comment: This is mostly an exercise at class. There are all kind of substances and measures like Chloramines and pH levels. The prediction is around 58% accuracy, which is quite poor in such cases, as it is health issues.
Answer: Regularisation aims at reducing the effects of design matrix being overdetermined or underdetermined, recall solving $Ax=b$, $A \in \mathbb{R}^{m \times p}$. Regularisation is appropriate to use if $p>>m$ (underdetermined) or $p<<m$ (overdetermined). Here the case is $m>>p$, overdetermined (m=3000, p=9 in this case).
Using LASSO or elastic net regularisation are recommended instead of plain logistic regression. Without regularisation, solution may not be correct. glmnet's introduction will give a good idea how to use LASSO and elastic-net regularisations.
See also rank deficiency.
Comment: Agreed on glmnet—and just to add on, for a general background into what the LASSO does conceptually, I recommend *Introduction to Statistical Learning* by James et al. The R labs are outdated, but the concepts are explained in an intuitive and accessible way.
Comment: It's just been updated with a new edition. Haven't checked the R labs but presumably they've been updated, too.


Sobolev Space Notation Question

Question: Hi my lecturer for Functional Analysis has used the following notation when talking about Sobolev spaces:
$$ \overset{\circ}{W}\vphantom{W}^m(\Omega) = \overline{\mathcal{C^{\infty}_0}(\Omega)}^{W^m(\Omega)} $$
$W^m(\Omega)$ is the notation for a Sobolev space but I get lost when the above notation is introduced to define a new space.  I know that it consists of functions which are zero at the boundary but I just dont understand what is happening in the above notation when defining it.  Could someone please give me a rundown of what this notation actually means?
Answer: $V:=\mathcal C_0^\infty$ is a subspace of $W^m(\Omega)$. There is a topology on $W^m(\Omega)$ given by a norm. Then we take the closure of $V$ with respect to this norm, and this gives by definition $W^m_0(\Omega)$.
Comment: A norm $N$ gives a metric, namely $d(x,y):=N(x-y)$, and a metric gives a topology.
Comment: Ok I think I mostly understand this.  However I am not sure what you mean by a topology given by a norm.  According to all the definitions I have encountered a topology does not have an associated norm.  And the definitions do not require one.  I may be going down a rabbit hole here but how is a topology given by a norm?

Sobolev Space Notation Question


Problems with xmodmap

Question: I'm using Ubuntu 17.04. My backspace key is not working, so i used xmodmap to use the key "Home" as BackSpace. The problem is: after i use xmodmap, the key "Home" works as backspace only in FireFox. In the other Linux programs, the key
"Home"  doesn't work as backspace.
Comment: Your usage of `xmodmap` only affects processes created after `xmodmap`. I suggest invoking `xmodmap` from a script in `$HOME/.config/autostart`, or earlier.
Answer: I also use Ubuntu 17.04, 64 bits, with a HP Compac laptop and needed to exchange CONTRL and CAPS-LOCK keys, as recommended for use with Emacs. Since I've seen many posts about xmodmap not working, I'll contribute my 2 cents, even though the remapping I did is not exactly the one you intend.
Following the advise in the man page of xmodmap, I invoked this script from the terminal (I called it 'remapkeys' and put it in home directory):
remove Lock = Caps_Lock
remove Control = Control_L
keysym Control_L = Caps_Lock
keysym Caps_Lock = Control_L
add Lock = Caps_Lock
add Control = Control_L
(without spaces in between)

xmodmap -verbose ~/remapkeys

Everything worked as intended, even if Emacs is already running when I do it. CONTROL key and CAPS-LOCK keys are exchanged.
The same procedure should reverse the changes, but you can always re-start and the keymap turns back to normal.
So in my case, xmodmap worked right but the parameters (in their correct order) are tricky, and the the man page is rather obscure for a beginner.

Problems with xmodmap


High level explanation of Similarity Class for Lucene?

Question: Do you know where I can find a high level explanation of Lucene Similarity Class algorithm.  I will like to understand it without having to decipher all the math and terms involved with searching and indexing.
Answer: Lucene's built-in Similarity is a fairly standard "Inverse Document Frequency" scoring algorithm. The Wikipedia article is brief, but covers the basics. The book Lucene in Action breaks down the Lucene formula in more detail; it doesn't mirror the current Lucene formula perfectly, but all of the main concepts are explained.
Primarily, the score varies with number of times that term occurs in the current document (the term frequency), and inversely with the number of times a term occurs in all documents (the document frequency). The other factors in the formula are secondary, adjusting the score in attempt to make scores from different queries fairly comparable to each other.
Comment: -1 The book says less than the JavaDocs so don't bother buying.
Answer: Think of each document and search term as a vector whose coordinates represent some measure of how important each word in the entire corpus of documents is to that particular document or search term.  Similarity tells your the distance between two different vectors.
Say your corpus is normalized to ignore some terms, then a document consisting only of those terms would be located at the origin of a graph of all of your documents in the vector space defined by your corpus.  Each document that contains some other terms, then represents a point in the space whose coordinates are defined by the importance of that term in the document relative to that term in the corpus.  Two documents (or a document and search) whose coordinates put their "points" closer together are more similar than those with coordinates that put their "points" further apart.
Answer: How was mentioned by erickson in Lucene is Cosine similarity Term Frequency-Inverse document frequency (TF-IDF). Imagine that you have two bags of terms in the query and in the document. This measurement only match exactly terms and after in the context include their semantically weights. Terms with very frequetly occurence has smaller weight (importancy), because you could them find it in lot of documents. But the serious problem what I see that Cosine similarity TF-IDF is not so robust on more inconsistent data, where you need to compute similarity betweens the query and the document more robust e.g. misspeling, typographical and phonetical errors. Because the words must have exact match.


Boundary Layer in aerofoil

Question: I want to know how the top and bottom boundary layer interact at the trailing edge of an aerofoil (zero angle of attack) and what happens to the boundary layer after a small distance from the trailing edge. Does the region at the back of aerofoil have lesser velocity due to the boundary layer? How is it carried by freestream?
Comment: This sounds a lot like a question that would appear on a homework assignment or something... What do you think would happen when two boundary layers meet? Is there a particular concept that you are unclear about in viscous flows that could narrow down where to focus an answer?
Comment: I was confused whether the boundary layer from top and bottom surface interact such that it always form wakes at trailing edge even though the aerofoil is perfectly stream lined and having zero angle of attack or do the boundary layers just join up and form something like a bigger patch beyond the trailing edge.What happens to it as we move farther from trailing edge of the aerofoil?
Comment: Not sure what you mean by "form a bigger patch" but it sounds like you already know how to find the answer to your question. I'll give you a little hint: When the boundary layers join up, there exists a velocity gradient again between the top and bottom layers. Think about how viscosity works and what it does in a fluid and you'll be able to answer your own question.
Comment: Appreciate your help.By bigger patch I meant the joining of top and bottom boundary layer into one  single patch where velocity gradients are signifificant. How does this velocity gradient vary as we move farther downstream from trailing edge?
Comment: Back to my original question -- how do *you* think the flow behind an airfoil would look? I have no problem helping to clear up any misconceptions about what you think it should look like, but I would rather not just tell you what it looks like. Sketching a picture (and heck, finding a picture online) should be pretty simple; coming up with some math for it is a little more complex but something who is taking classes in viscous fluid dynamics should be able to do.
Answer: In every boundary layer (except for exotic hypersonic cases), the speed at the wall is zero. At the trailing edge, the upper and lower layers meet, and if you imagine a plane which extends from the trailing edge backwards and follows the streamlines, the speed at the trailing edge is equally zero. The more you now move away from the trailing edge along this plane, the more the speed increases, as now the inhibiting effect of wall shear is missing, and only the shear of the layers above and below the plane acts upon the air in this plane. If you measure the speed orthogonally to this plane, you will see a speed drop near the plane which gets wider and more shallow the more you move away from the trailing edge.
This speed drop can be measured and gives a very precise value for airfoil drag. See the picture below for a rake of pitot tubes which is used for this kind of measurement.

Boundary Layer in aerofoil


how i find the list of files which should contain both string patterns are present in a given directory

Question: i have active directory which contain multiple subdirectories.i want to find the list of files which should contains the string patterns as "String1" and "String2" respectively
Comment: Could you give more details and an example?
Comment: I'd use `ag` for that.
Comment: i have different files like t1.js and t2.js and t3.js.only the file t3.js has two string patterns as "hello" and "world".i want to find the list of file should contain both the strings.which means my output become t3.js while using grep and find unix comands it results where ever the stings "hello" and "world"
Answer: Why don't you try something like this with find command:
<code>find . -type f -exec grep -q 'String1' {} \; -exec grep -l 'String2' {} +
</code>


Webmin users can see system information: how to disable it

Question: How to prevent Webmin users from seeing the system information, currently they are able to see system info:
Answer: Have you configured Webmin users or groups? Under Webmin\Webmin Users you can set who can see this info or use the system. I would not want general users able to access any portion of Webmin, so I would have it locked down pretty well.
If you have done this, please provide more configuration and security info about the server.  Hope this helps!
Comment: Sorry I am slow to respond; have some family issues that are tying me up.  I am new to Webmin, but my understanding is that the only people to be added in the Webmin\Webmin users area are people who you want to have access to the system information and server administration capabilities of Webmin. Users of the server services, such as email or SQL, need to be added to the appropriate areas of those server services. I am unable to provide screenshots as I am away from my systems. Hopefully next week I might have something for you.
Comment: I configured groups/users, but unfortunately I couldn't find any option for not letting users/groups see the "System Information"
Comment: When you say you configured groups/users; did you add them under the Webmin\Webmin Users, or did you set up groups/users under the server services areas that they would use? You should only put users/groups that need access to Webmin in the Webmin\Webmin Users area.  Wasn't sure from your answer which you did.
Comment: Thanks. I add users under "Webmin > Webmin Users" not "System > Groups/Users"
Comment: Can you provide a screen-shot showing exactly which section of "Webmin > Webmin Users" allows me to set who can see "system info"? I cannot find it


Compiling 3D asymptote diagrams in a LaTeX document on TeXShop (Mac)

Question: I am capable of compiling 3D diagrams from the command line with the asy command, but they will not compile within a .tex file. Is there any way to get these 3D diagrams into a pdf relatively easily?
Comment: Run `asy` with the `-k` option to keep the intermediate files. In one of the generated `tex` files, you will find the code that embeds the `prc` file, or use package `media9` for that yourself.
Answer: There is an Asymptote engine for TeXShop. Us the TeXShop->Open ~/Library/TeXShop menu command and then open the Engines folder and then the Inactive and Asymptote folders for instructions on its use.
Alternatively you could use the pdflatexmk engine along with the platexmkrc file found in ~/Library/TeXShop (as before) and then go down to the Engines/Inactive/Latexmk/platexmkrc samples/for Asymptote folder for a copy of the necessary platexmkrc file. Place a copy of that in the folder with your document and add the line % !TEX program = pdflatexmk to the top of your file and typeset using the Typeset->Typeset (Cmd-T) menu command; you document will be completely typeset.


Why do some objects not move when there is no external force while others move at a constant velocity?

Question: Newton's first law says that an object remains at rest or moves at a constant motion if there is no external force. Why do some objects remain at rest, while the others move at constant motion?
Comment: Hey, in my frame of reference the "others" are at rest and "some object" is moving...
Answer: Being at rest is a special case of *moving at constant velocity", namely moving at the constant velocity =0. In fact, you could leave out this part of the sentence without losing anything.
Alternatively you could say, that, if there is no force, the object is not accelerated (i.e. acceleration=0).

Just a note:
While Newton's laws are sometimes stated as you did, you need to realize that it is a bit of a lazy shorthand notation of the law. Two things need to be expanded on:

The "force" is actually the total effective force, i.e. the (vector) sum of all external forces. Of course, if you apply two equal in magnitude forces acting in opposite directions, the object would not accelerate either.
It is actually not the velocity that is constant, but the momentum (mass times velocity). Similarly, in the absence of a force, it is actually not the acceleration that is zero, but the time derivative of the momentum. However, this is only of relevance in cases where the mass changes (e.g. in rockets). If the mass is constant, you end up with the simple version of the law (constant velocity, zero acceleration).
Comment: Well... this cleared something I've been searching for months. Can't accept the answer yet though, StackExchange tells me to wait 3 minutes. But I still have one doubt. My book says, in a Tug of War, when both teams pull at the same force, the object remains stationary? Does stationary mean constant acceleration?
Comment: Stationary means V = 0 and constant.  That is no net force is applied to the body and as such a = 0 as well.
Comment: And the answer to your title question is that if there is not force the momentum mv  of the body does not change (rocket like motion apart). Because in ordinary words is weird to say that something move with V = 0. Is the state of motion (the momentum above) that doesn't change.


Protractor e2e test case for downloading pdf file

Question: Can anyone tell me how to write test case for a link to download pdf file using jasmine framework ?
Thanks in advance.
Answer: I can currently set download path location
Chrome
capabilities: {
    'browserName': 'chrome',
    'platform': 'ANY',
    'version': 'ANY',
    'chromeOptions': {
        // Get rid of --ignore-certificate yellow warning
        args: ['--no-sandbox', '--test-type=browser'],
        // Set download path and avoid prompting for download even though
        // this is already the default on Chrome but for completeness
        prefs: {
            'download': {
                'prompt_for_download': false,
                'default_directory': '/e2e/downloads/',
            }
        }
    }
}

For remote testing you would need a more complex infrastructure like setting up a Samba share or network shared directory destination.
Firefox

var FirefoxProfile = require('firefox-profile');
var q = require('q');

[...]
getMultiCapabilities: getFirefoxProfile,
framework: 'jasmine2',

[...]
function getFirefoxProfile() {
    "use strict";
    var deferred = q.defer();
    var firefoxProfile = new FirefoxProfile();
    firefoxProfile.setPreference("browser.download.folderList", 2);
    firefoxProfile.setPreference("browser.download.manager.showWhenStarting", false);
    firefoxProfile.setPreference("browser.download.dir", '/tmp');
    firefoxProfile.setPreference("browser.helperApps.neverAsk.saveToDisk", "application/vnd.openxmlformats-officedocument.wordprocessingml.document");

    firefoxProfile.encoded(function(encodedProfile) {
        var multiCapabilities = [{
            browserName: 'firefox',
            firefox_profile : encodedProfile
        }];
        deferred.resolve(multiCapabilities);
    });
    return deferred.promise;
}

Finally and maybe obvious, to trigger the download you click on the download link as you know, e.g.
$('a.some-download-link').click();

Comment: Yes, of course if you want to use Firefox you need to set browserName:'firefox'
Comment: Hello Leo, i'm trying to set a default path location for Chrome, but apparently your solution didn't worked for me.
Comment: @LeoGallucci With above code, Firefox doesn't spawn and no errors. But if I use browserName:'firefox' in export.config, it opens firefox successfully. What could be reason? Thanks!
Comment: @LeoGallucci I would need to use 1 config file and pass browserName as parameter how can i achieve this,
i see for 'FIREFOX' "getMultiCapabilities" is used and for 'CHROME' "capabilities" is used.
help would be greatly appreciated.
Comment: Has anyone had success developing the same for Internet Explorer?
Answer: I needed to check the contents of the downloaded file (a CSV export in my case) against an expected result, and found the following to work:
var filename = '/tmp/export.csv';
var fs = require('fs');

if (fs.existsSync(filename)) {
    // Make sure the browser doesn't have to rename the download.
    fs.unlinkSync(filename);
}

$('a.download').click();

browser.driver.wait(function() {
    // Wait until the file has been downloaded.
    // We need to wait thus as otherwise protractor has a nasty habit of
    // trying to do any following tests while the file is still being
    // downloaded and hasn't been moved to its final location.
    return fs.existsSync(filename);
}, 30000).then(function() {
    // Do whatever checks you need here.  This is a simple comparison;
    // for a larger file you might want to do calculate the file's MD5
    // hash and see if it matches what you expect.
    expect(fs.readFileSync(filename, { encoding: 'utf8' })).toEqual(
        "A,B,C\r\n"
    );
});

I found Leo's configuration suggestion helpful for allowing the download to be saved somewhere accessible.
The 30000ms timeout is the default, so could be omitted, but I'm leaving it in as a reminder in case someone would like to change it.
Comment: I'm glad my answer helped! :) I also ended up writing a helper `function waitFileExists(fileAbsPath)` ;)
Comment: what about phantomjs?
Comment: @LeoGallucci can you please share what you did in the `waitFileExists(fileAbsPath)`?  are you on that method working to get form the `fileAbsPath` so it can work on any computer?
Comment: What do you mean, the code is right up there, see "Wait until the file has been downloaded."
Answer: it could be the test for checking href attribute like so:
var link = element(by.css("a.pdf"));
expect(link.getAttribute('href')).toEqual('someExactUrl');

Answer: The solutions above would not work for remote browser testing, e.g. via BrowserStack. An alternative solution, just for Chrome, could look like this:
if ((await browser.getCapabilities()).get('browserName') === 'chrome') {
  await browser.driver.get('chrome://downloads/');
  const items =
    await browser.executeScript('return downloads.Manager.get().items_') as any[];
  expect(items.length).toBe(1);
  expect(items[0].file_name).toBe('some.pdf');
}

Answer: One thing I've done in the past is to use an HTTP HEAD command.  Basically, it's the same as a 'GET', but it only retrieves the headers.
Unfortunately, the web server needs to support 'HEAD' explicitly.  If it does, you can actually try the URL and then check for 'application/pdf' in the Content-Type, without having to actually download the file.
If the server isn't set up to support HEAD, you can probably just check the link text like was suggested above.


Recognizing Dell EquilLogic with Nagios

Question: EDIT: All firmware and models are compatible, that is why nothing is posted about it.
Okay, so there will be a lot here, so please bear with me. I've been working on this now for a few hours (reading manuals and such) so I'm not just coming here right out of the blue. I am working on a PRE-EXISTING Nagios server where there are several other existing plugins and checks running and working. Now I want to add another server there to check so I made the following modifications:
First and foremost, I added a file to /usr/local/nagios/libexec named: check_equallogic.sh. The permissions are 755, the same as all others. I have chowned to nagios:nagios and in the listing it shows the Owner as Nagios.
I then added a command to the commands.cfg file in \usr\local\nagios\etc\objects that shows the following:
# 'check_equallogic' command definition
define command{
    command_name check_equallogic
    command_line $USER1$/check_equallogic -H $HOSTADDRESS$ -C $ARG1$ -t $ARG2$ $ARG3$
}

Following this, I created a file named equallogic.cfg in the objects directory and it contains (more or less):
define host{
    use     linux-server        ; Inherit default values from a template
    host_name   172.16.50.11        ; The name we're giving to this device
    alias       EqualLogic      ; A longer name associated with the device
    address     172.16.50.11        ; IP address of the device
    contact_groups       admins
}

Check Equallogic Information
define service{
    use                 generic-service
    host_name           172.16.50.11
    service_description         General Information
    check_command               check_equallogic!public!info
}

After ensuring that permissions are okay for all files, I restart the nagios service, no errors. When I go into the WebGUI, I get the following errors AFTER the check runs:
(Return code of 127 is out of bounds - plugin may be missing)

Extra, probably unrelated problem

Furthermore, when I log into the Equallogic server, under Audit logs I get the following error:
Level: AUDIT
Time:  26/05/2014 3:59:13 PM
Member:  ps4100-1
Subsystem:  agent
Event ID:  22.7.1
SNMP packet validation failed, request received from 172.16.10.11

An snmpwalk receives a timeout, whereas others succeed. I will work on importing the MIBs tomorrow. The reason why I am mentioning it is because I want to make sure that it is only a MIB issue for the SNMP. If it is, then ignore this area.
I am entirely unsure of what to do here.
Answer: 127 means "file not found". Verify that USER1 is set to "/usr/local/nagios/libexec" in the resources file.
Perhaps more importantly: You said the script is named check_equallogic.sh but your command definition says just check_equallogic without the ".sh" extension.

Recognizing Dell EquilLogic with Nagios


What is the difference between "girl, lady, and woman?"

Question: When can we use them? Are they the same or different? Are they used in different situations?
Comment: http://www.headbloom.com/index.php/resources/post/girl_lady_or_woman_a_question_of_language_and_gender/
Answer: girl can be used in almost any situation, and just refers to a female of any age, however, it's more common to use the term for females under 30.
lady can be used in almost any situation, and is more respectful in my opinion. There is a common term called "lady like" , which implies that ladies act like females should. They are caring, pleasant, have good manners, etc.
woman is a tricky one, since it can be used in situations to show disrespect, such as a man claiming he is superior than a woman. Woman is fine though when describing a female above the age of 18, as long as it's not in a chauvinistic or demeaning way. And woman is likely preferred by most females that are above the age of 18. However, it's probably more common for women above 25 to be called woman, and under 25 to be called girl.
Comment: All of these can be trick and be used to show disrespect, not just _woman_, and I can see a 24-year-old woman taking umbrage at being called a _girl_, particularly in the workplace.
Comment: https://ell.stackexchange.com/questions/169880/the-meaning-of-half-woman-half-girl


How to create a view based on a company in leads

Question: I have to create a new view in leads based on a single company.How to achieve i don't have any idea regarding this because company contains what to match for this list to get all related companies list
Comment: Do you want one listview for every company? Or one listview for a specific company?
Comment: Is it possible one list view for every company.Means is there any way to do so @Sebastian Kessel.
Comment: Yes, but it is VERY difficult, You'd need to create a trigger on lead, and create a new listview (using the metadata API) every time a new company is detected. Not something I'd recommend doing.
Comment: thanks actually my problem is i created record type for lead and in our company research team is there.When they try to assign leads to a particular company.They are try to mass edit leads at a time.means they didn't get options to edit.they are try to edit manually..@Sebastian Kessel
Answer: I got it i have created a view contains with company contains and forgot to mention record type there so now it's working fine in my production.So now i can get all related company with locations and everything.


What's the position of my tongue when I speak Spanish 'd'?

Question: Comparing with Spanish 't', put the tip of tongue behind the upper teeth and press a bit to pronounce it, when it comes to 'd' sound, just put the tip of my tongue between the upper teeth and the lower teeth, I want to know if this method of pronouncing 'd' is right. Is there any other ways to pronounce the 'd' sound?
Comment: this was asked for the same user, and @guifa answers address the tongue position for the 3 cases.
Comment: @lampe: This really does appear to be a duplicate of your earlier question. This question is slightly more specific, but the answer on the other question answers the question of tongue placement. Is there something specific you feel needs to be expounded upon?
Answer: Depends where the d is located.
In front of the word or after a consonant:

It is pronounced like you how pronounce it in English. Example: day = día, agenda = emprenda.

After a vowel

It is pronounced like "th" as in "the". Example: edulcorante, educación.

At the end of the word

It is pronounced like "th" as in "the". But in many dialects it is very relaxed. Examples:  Madrid, usted, verdad. -But you can still find some people that pronounce the "d" at the end of the word as in Madrid-

Ends in -ado, -ido

It is almost imperceptible. Examples: cansado, comido.

Important: This answer must be analyzed in its entire form. The statements are not by any means independent of each other.
Comment: @leonbloy, Yes I know, It is subtle but exists, I don't see a big difference but every country has different ways to pronounce the "d". In Spain you will find that my statement is true. Also the RAE states the difference and therefore justifies my answer. I can not make a statement based on regional dialects.
Comment: @leonbloy: I think that the difference is that in the first case, the /d/ is _occlusive_ while in the second case it is _approximant_. That is, the tongue is in the same position but one blocks the flow of air while the other does not.
Comment: I don't really agree. The difference between the two first 'd' sounds you mention so subtle that most spanish speakers (myself included) are not aware of its existence. In any case, the first case is not as explosive as the Enlish 'd', and the second case never as smooth. And -above all- it's never pronounced sticking the tongue between/outside the teeth as in the "the".
Comment: I think this is a good answer, for what it is, but it doesn't actually address tongue position, which was what the question was asking about. Do you care to elaborate on that point?
Comment: @Flimzy but it is a duplicate
Answer: I think that the standard Spanish d is the same as the standard Spanish t, but with d being voiced, while t is voiceless.
That is, in both phonemes the tip of the tongue is pressed to the back of the upper teeth, but pointing slightly down. Maybe the part of the tongue just next to the tip is just[touching the alveolar ridge. It is actually a denti-alveolar consonant. Sometimes the tongue does not actually touch the teeth, so the consontant is not totally oclusive (approximant consonant).
If you put the tongue between the upper and lower teeth you will get a sound more like /z/ instead of /d/. That is usual and somewhat accepted for a d at the end of a word, such as in "Madrid" or "id" (imperative of "ir"), but not for other places.
In relaxed speech, particularly in intervocalic positions, the d sound is sometimes relaxed. This is done by keeping a gap between the tongue and the teeth (not lower the tongue) and preventing the stop of the air flow.
In extreme cases the d may even disappear, such as in "estoy cansa(d)o".
Note that the t sound cannot be relaxed this way, maybe because it is voiceless.
Comment: i once heard an argentino prouncing 'd' pair sounds, and i feel that it should be between english 'd' and 'th'. and he taught me put the tip of my tongue between the teeth.
Comment: @lampe: There are a lot of variations in Spanish pronunciations, it depends on location, background and personal preferences. Anyway, I don't think that interdental 'd' is [very frequent](http://en.wikipedia.org/wiki/Spanish_phonology), so you are probably better avoiding it.
Comment: :my teacher taught me the same way of pronouncing 'd' as you told me.however,after some training i find that making the neck virbate is harder in the constant talk while put the tip of tounge between teeth would be easier,the final destination is the make 'd' sound softer, is it right?
Comment: @lampe: Producing new voiced consontants may be challenging (I know, I have difficulties with the English voiced 's'). The question is "why". If you want to soften your 'd' to sound less _alien_, then changing it into an voiced interdental is not the solution, as it will sound even more alien. Note that the hard 'd' does appear in Spanish, at the beginning of a phrase, for example. But the voiced interdental is unheard of.
Answer: The consonant D, in Spanish, is known as a dental consonant, because, well, your tongue presses against your teeth.  Not to be confused with Alveolar consonants which would be above your teeth slightly, touching the alveoli (the back of the gum holes).
There are several other variations of that consonant within different dialects.  Some pronounce it as (th), some pronounce is as a J sound (known as a post-alveolar consonant).  The TH form is a labiodental consonant because the tongue touches both the teeth and the lips.
Comment: *th* is not labiodental, but interdental (you stick your tongue between your teeth, and because of that you also touch your lips with it). A labiodental is a consonant where the bottom lip touches the upper teeth, such as *f*.
Comment: Otherwise known as a fricative.  Ok thanks
Comment: Well, *fricative* refers to the articulation *mode*, not the articulation *place*. *f* is both a labiodental and a fricative (also voiceless, if you want to be comprehensive).


Concept of mean systemic filling pressure

Question: Mean circulatory filling pressure (MCFP) in humans was defined by Guyton as "the pressure that would be measured at all points in the entiere circulatory system if the heart were stopped suddenly and the blood were redistributed instantaneously in such a manner that all pressures were equal".
Now according to many physiologists, there must be a location in the vascular network where the pressure is completely independent of the cardiac activity, and this pressure must be equal to the MCFP. Apparently, this was already noted by Starling in 1897. The entire point is well explained in this article.
I have read this article and some others introducing the concept of MCFP to medical students; many of them report the same observations made by Guyton, Starling, etc., but a point which is never addressed is why there must be a location in the vasculature where the pressure is independent of cardiac activity.
I feel like getting a satisfactory answer to this question would greatly improve my understanding of hemodynamics, so I would be very grateful if you could explain the concept to me.
Answer: You've misread the linked article if you take this to mean there is some place that is unaffected by cardiac activity. It's just that there must be some place that is equal in pressure to the MCFP. The actual position fluctuates throughout the cardiac cycle.
The reason, though, that some place must be equal to MCFP is that pressure is continuous. Somewhere in the vasculature (i.e. proximal arteries) is definitely greater than the mean; somewhere else (i.e. proximal veins) is definitely less than the mean. Because the pressure varies continuously the function must pass through the mean at some point along the vascular path. This isn't anything special about biology, just mathematics.
Comment: Thank you very much for your answer. I am well aware that, if MCFP is comprised between mean arterial pressure and right atrial pressure, then there must be a point in the vasculature at which pressure equals MCFP. But the article I mentioned seems to say that this point does not move with cardiac activity, as illustrated with the fact that the three curves in figure 3 intersect at a unique point. This is stated even more clearly in Rothe (1993), a work cited in the article.
Comment: In Rothe (1993) the authors, citing Starling, state "Somewhere in the circulation there must be a point where the pressure is neither raised nor lowered and where, therefore, the pressure is independent of cardiact activity."
Comment: @user47679 "Rothe (1993) thought that this probably happens in small venules (less than 1mm diameter), and that it is not constant - **present at different points of different organs, and changing constantly depending on prevailing conditions**" - quoted from your source.
Comment: They're talking about a hypothetical location that's constantly shifting. I think you're reading too much into it or taking it too literally.


Could we get free O₂ on Mars?

Question: I think that the new Mars rover in 2020 will make oxygen, from thin Martian atmosphere. I think Mars' atmosphere is 90% $CO_2$. Carbon dioxide is made of $C$ and $O_2$. $O_2$ is oxygen, so can't we separate oxygen from carbon dioxide? We only need to release the carbon, don't we?
Comment: Carbon dioxide may be split in carbon and oxygen only when the necessary energy is added. There is no free oxygen. Burning carbon with oxygen releases thermal energy, the reverse process requires energy. The human body gets its energy by oxydizing hydrocarbons to carbon dioxide and water. The plants on Earth producing oxygen from carbon dioxide need energy to do that, the energy is delivered by the sun as light. During the night plants don't release oxygen.
Comment: Additionally, splitting that second oxygen atom off a CO2 particle takes more energy than the first. The current technique doesn't produce carbon as byproduct, but carbon monoxide.
Answer: The experiment in question, MOXIE, "Mars Oxygen ISRU Experiment" uses a process of solid oxide electrolysis, to produce oxygen from Martian atmospheric $CO_2$.
First, as reversal of the process of burning coal, production of oxygen from carbon dioxide requires significant amount of energy.
Then, due to specifics of the process, and problems of solid residue the produced carbon would create, the actual reaction performs operation of splitting carbon dioxide into oxygen and carbon monoxide, and separating them.
While technologically much simpler than detaching both oxygen atoms from a $CO_2$ particle, this leaves the risk of mixing highly poisonous carbon monoxide with the oxygen produced.
Experiments with utilizing photosynthesis of plants or cyanobacteria were performed; these release oxygen while using carbon as a building material of the organism (and using sunlight energy to do the splitting); this comes with a range of different problems though, and isn't to be tested in the 2020 rover.
Answer: I believe your concept is a bit off. I don't think that new Mars rover in 2020 will make oxygen... In order to break apart CO2, into C and O2, energy is required and it isn't as simple as getting a knife and cutting a CO2 molecule in half. Just bring a bunch of plants into Mars where they are in contact with their surroundings but they don't die due to freezing, warm them and give them some light so they will transform CO2 into C and O2. This could work!
Comment: sounds like the script for a film : The Martian anyone?
Comment: I know this comment is a bit delayed but from a 5 years in a future prospective the Mars 2020 rover also know as perseverance did use the MOXIE to spilt C from O2. So to answer  the beginning part, the Mars rover will not just spilt C from O2 but already has,

Could we get free O₂ on Mars?


How do sunflowers rotate?

Question: Is there a physics mechanism to explain how sunflowers rotate to always face the sun?
I tried to find more information or references using google search, but no luck.
Comment: It is not a physical mechanism, but rather a biological one.  See: http://en.wikipedia.org/wiki/Phototropism
Answer: The physics is being explored in the rapidly growing field of biophysics.  The short answer is that plants are able to move, in general, via hydraulic or osmotic processes.
Osmotic process describe the tendency of water to move into a solution via osmosis,  where osmosis is "the movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, aiming to equalize the solute concentrations on the two side."  Hydraulic processes reflect the tendency of water to stay put and not move out of a solution. See here.
For instance, plants can better grow through hydraulic pressure, but respond rapidly to environmental changes (cf sunflowers), or food (cf venus fly trap) via osmotic mechanisms such as pumping across cell membranes (discussed in a recent article on the venus fly trap in Nature). In either case, the exact mechanics are still poorly understood and the subject of a lot of research.  Plants that could move or adapt to environmental changes have obvious economic benefit.
Comment: *::removes moderator hat::* I've always hated the "see here" style of linking, and have edited your answer to reflect the natural reading style I prefer. Feel free to revert that edit if you disagree. I didn't do the second link as it seem to be broken. Cheers
Comment: ""Hydraulic processes reflect the tendency of water to stay put and not move out of a solution. "" Could You explain that? Especially the "water stays put"
Comment: A thought on hyraulics...I can see where you are coming from with the move-through/stay-put distinction, but it does not capture the nature of the hydraulic force very well. Perhaps a phrase like *"...reflect the isobaric property of liquids..."*

How do sunflowers rotate?


How to convert a conventional piano or a violin score to the guitar tabulature using Musescore, Finale, Tuxguitar, Sibelius, Guitarpro softwares?

Question: How to convert a conventional piano or a violin score to the guitar tabulature using Musescore, Finale, Tuxguitar, Sibelius, GuitarPro softwares? Is it possible and how it will handle multiple note positions on the guitar fretboard?
Comment: What is your goal? You can imagine that there are piano scores that are not playable on a guitar.
Comment: I don't know every app that you listed, but in the ones I do know, just create "guitar notation" and "guitar tab" staves and copy-and-paste the music. There is often a "set-up" or "options" page to control the tab notation, define the guitar tuning, etc. Just download some free demos, read the help files, and follow your nose.
Comment: Need a an automatic assistance to produce a guitar tab, not to do it manually, just to edit the result, surely, not all scores are directly convertable, but many passages and lines and chords are convertable
Answer: Here's a general guide for Sibelius: Enter the score either manually or by scanning. Now create new instrument (guitar). You may need to create two guitar parts. Copy and paste the piano/violin stave/s into the guitar stave/s. Edit the parts. Then go to Plug Ins and select Guitar Tab. You will get to choose the highest fret the tab will use. Allow yourself plenty of time, because you will need to deal with stems, voices, and notes out of range. Muse Score? Bless its clunky little head. Someone will have nutted out how to do it, but it's probably best to start by climbing Mt Everest, nude, backwards, carrying a honey badger and setting off from Antarctica. Ok, my experience with MuseScore was limited, and nearly a decade ago, but I've not been the same since.


Can mobs spawn in water near Mushroom Islands?

Question: I want to build a mob free base, and I've found a Mushroom Biome. I want to build my base in the water also, if possible. In the map below, will the piece of water there be mob-free? Or does it count as Ocean Biome and therefore mobs are able to spawn? (The brown area is Mushroom Biome)
Answer: If you press F3 you should be able to see the biome you're in. If you go in the lake and see it's not Mushroom Field or Mushroom Field Shore, then mobs can spawn there, underground, on dark platforms etc.
Ideally you could use that space for a mob spawner, even if I'd suggest going over the ocean for that.
I would like to point out that because it's connected to a normal biome like a forest or an ocean, mobs can still spawn there and move in the biome. The only workaround is to build barriers on the seashore and add torches.


Can I take note or highlight on paper during a TOEIC test in the reading section?

Question: I heard that I cannot take notes during the listening section of the test (correct me if I am wrong) but I am wondering if similar rules apply to the reading section. Can I take notes or highlight texts in the reading section?
Comment: The authority for this is the testing service itself.
Comment: Notice that in the case of questions closed as off-topic, one down-vote is automatically done. No user is involved in that down-vote, which means that only a user down-voted the question, now that its score is -2.
Comment: Thanks for down voting, I just wanted to ask this question but I couldn't find a suitable Stackexchange site to ask it. But do I really deserve a downvote?
Comment: I did not downvote but voted to close as Off Topic. I suspect some people may have downvoted because there was no evidence of research - it took me less than 30 seconds to find the answer.
Answer: According to the TOEIC User Guide, page 5, Test Center Procedures

Books, dictionaries, papers, notes, rulers, calculators, watch alarms, mobile phones, listening devices, recording or photographic equipment, highlighters or aids of any kind are not allowed in the testing room.
Paper of any kind is not permitted in the testing room.
Test takers may not mark or underline words in the test book or make notes in the test book or on the answer sheet.

That would seem to rule out any note-taking during any portion of the test, since you are not permitted to mark the materials provided and are not permitted to bring in anything else on which notes could be taken.
Comment: Thanks for you answer, I've been preparing for the reading section by highlighting keywords and I wished if it is possible during the actual exam too =(..


SQL Server: performance of IDENTITY vs INT as primary key

Question: The most important table of my database is about 300,000 records, and growing. More than 20 tables have a FK to it.
Its primary key is a number, but for historical reasons is defined as nvarchar(6) which is obviously inefficient (and ugly).
Records are sometimes deleted from the record, so the primary key is approaching 999,999 and I must change it.
An identity field would be the obvious choice.
An int key, or similar, with the increment produced either by a trigger or by the software, would be an alternative. This would be feasible because the records are always inserted one at a time.
Would an int key provide better performances with complex queries?
Comment: An `identity` column *is* an integer column.
Comment: Where did you pick up this understanding? Have you looked at the books online topic for 'identity` to see the supported datatypes? Then look at the topic for `numeric`. Whilst `numeric(x,0)` is supported this is not stored as a string.
Comment: I understand an identity column is a numeric, i.e. a string only composed of decimal digits. A numeric (9), for instance, would be about 9 bytes, while an int would be about 4 (actually it is 5, but you get the point).
Answer: IDENTITY is not a column type in its own right, it is a property that can be apply to any integer column type. It is usually applied to an INT and in that case there is no difference (the column is an INT column, with the IDENTITY property defined) though it can be applied to any type that is internally an integer (BIGINT, DECIMAL, MONEY, ...).
While your string type is ugly and an integer would be measurably more efficient, changing to a numeric type will require quite some testing effort to make sure that none of your application's code has oddities that make it sensitive to the data types of these keys. I strongly recommend you don't just make the change and hope for the best! Even just extending the keys to NVARCHAR(10) or such might be problematical if any code assumes that the keys will never be longer than six characters... If you don't switch to a numeric type, at least consider moving to CHAR() - a non varying string format will also be more efficient and if you are only storing numbers you don't need the two-bytes-per-character storage of a unicode type like N[VAR]CHAR.
Comment: For 300,000 rows the difference is practically immeasurable.  If you were talking about 3,000,000,000 rows, you would want to use a BIGINT over pretty much any other data type.
Comment: Testing? Of course, but as you say, we would require it ever for just making the field longer.
Comment: I understand that I was wrong, assuming that IDENTITY fields cannot be INT. But the question can still stand: does an INT primary key perform better than a NUMERIC(10)?
Comment: NUMERIC(10) would be stored as 9 bytes per value, so there will be some performance difference compared to an INT (4 bytes) and to a lesser extent BIGINT (8 bytes). This is in part because when hitting disk you are pulling more pages as the rows take more room, and in part because the CPU has more work to do as it doesn't natively support loading and comparing 9 byte values in single instructions (manipulating 8, 4 2, and 1 byte values is natively supported. Whether either difference will be visible in the performance of you r application is something you'd need to benchmark to be sure.


What are some efficient ways in using COUNT(*) with WHERE

Question: Instead of calling COUNT(*) with a WHERE clause, is there anything one can do to make this work more efficiently on the server?  I would like to get this count on complicated joins, just wondering the best way to do this. Using SET NOCOUNT OFF? Because SET NOCOUNT ON is enabled by default in a new stored procedure and is suggested I guess by Microsoft.  Thank you.
Comment: possible duplicate of [Does running a COUNT() statement using NOCOUNT ON; consume much](http://dba.stackexchange.com/questions/5293/does-running-a-count-statement-using-nocount-on-consume-much)
Answer: SET NOCOUNT has nothing to do with your ability to perform counts or the efficiency of doing so.  This setting merely enables the reporting of affected rowcounts to the client (e.g. the n row(s) affected. message in Management Studio). The reason it is recommended to be off is because it is unnecessarily chatty and can break some applications / APIs.
As for getting more efficient counts from complex joins, it's possible that your joins could be made more efficient, but we can't see them. It's also possible that you can pay that cost at a different time by using filtered indexes, indexed views, or pre-aggregating yourself at insert/update time, using a trigger, etc. Sometimes this can pay off but other than a general suggestion there's not much else we'll be able to offer you without a lot more details.
Comment: Hello Aaron, in regards to your last response as well from question here: http://dba.stackexchange.com/questions/5293/does-running-a-count-statement-using-nocount-on-consume-much

I am wondering the results in your SQL statement, i cannot see how to narrow down to one particular db/table, just a list of objects with a rowcount. Can you please elaborate on this as well?
Comment: You would add an additional `WHERE` clause, e.g. `FROM sys.dm_db_partition_stats WHERE [object_id] = OBJECT_ID('dbo.tablename') AND ...` - and you would run the query in the specific database or use `databasename.sys.dm_db_partition_stats`...


.net Framewor/architecture

Question: Could some one refer me good article (PDF, web Link or Book Chapter) about ".net Framework Fundamentals". There is lot of material available on web but since i am new to .net that's why i am unable to decide what to study and what to leave.
I have a programing background(C++) and now want to learn about .net, i am expecting to start learning from basic and then move towards more details.
Answer: Have a look at this one "CLR via C# (Dev-Pro)" by Jeffrey Richter http://www.amazon.com/CLR-via-Dev-Pro-Jeffrey-Richter/dp/0735627045/ref=pd_sim_b_1 When i started with .NET it was the first book I've read, it is really helps to understand the core of .NET and how it is works. Pdf version should be here: http://www.google.com/search?client=opera&rls=en&q=CLR+via+C%23+pdf&sourceid=opera&ie=utf-8&oe=utf-8
Answer: MCTS 70-536 Training Kit for Applications Development Foundation by Microsoft Press.
Comment: +1 often overlooked but has a good introduction to a lot of areas of the .NET Framework.  It's a little old in the tooth though, and parts of it are being superseded.
Comment: In-fact i am reading this book and currently i am on Chapter 1 "Framework Fundamentals" but in this chapter there is nothing about framework, its all about Reference Type and Value type. Before reading all such stuff, i want to learn about Framework basics
Comment: J Stephan, for the very basics you might go through http://en.wikipedia.org/wiki/.NET_Framework

.net Framewor/architecture


How did Spectral Graph theory emerge?

Question: I was wondering how did Spectral graph theory, this multidisciplinary area between Linear Algebra and Graph theory start?
How did it emerge?
Was there a certain problem (maybe graph isomorphism problem?) that evoked its formation?
Comment: Once the idea of writing down the adjacency matrix is born, it isn't at all hard to use linear algebra to analyse the property of this matrix (and others).  Also, the problem of random walk on (possibly directed) graphs are already out there and there too you have all these tools already being used.  In a sense, it is just a matter of pulling these together and give it a coherent theory.
Comment: Perhaps math history tag is appropriate here?
Answer: The oldest work on spectral graph theory that I am aware of is the work by chemists on Hueckel molecular orbital theory (https://en.wikipedia.org/wiki/Hückel_method). One principal goal of this work was to relate chemical properties of aromatic hydrocarbons to spectral properties of the underlying graphs.
The work of Coulson was very influential.
Chemists also studied the matching polynomial before graph theorists became interested in the topic. (I view
the matching polynomial as a topic in spectral graph theory.)
Another starting point would be the original paper by Hoffman and Singleton on Moore graphs. It seems hard to overestimate the significance of this work. One thing it did was establish the connection between problems in extremal graph theory and graph spectra.
Certainly there was always the hope that one might use spectral information for graph isomorphism but it does not appear that lead to the development of spectral graph theory.
Answer: Well for one thing, certain combinatorial properties of a graph that are of interest to graph theorists and theoretical computer scientists, can be gleaned from analysing the adjacency matrix of the graph. For example, (put informally) a $k$-regular graph $G$ (where $k \ge 3$) is an expander (i.e., lots of edges between $S$ and $V(G) \setminus S$ for arbitrary nonempty subsets $S$ of $V(G)$) iff every eigenvalue except the largest is less than $(1-\epsilon)k$.
Surely there are other interesting combinatorial properties of a graph that can be gleaned from its adjacency matrix, but this comes to mind to me now.

How did Spectral Graph theory emerge?


How to test a class that wraps methods connect to a remote database

Question: I wrote a class creating connection to a remote database, query and do some more actions on that. Which is the best way/ strategy to test that kind of class? I tried to mock third party functions creating connection but I don't think it is the right way. Any help is appreciated. If you need more info, let me know.
Comment: what is your objective?
Comment: I don't know how to test my class/adapter?
Comment: Where is the relevant code?
Answer: To test something like this you want your test to not care whether it's connecting to the actual remote database or to a mock database that you manage yourself. It can be an actual, if lightweight, database or a coded representation of one. As long as the class you want to test does not use hard-coded values to connect, you should be able to do this.
You can find more information in this blog post (which I found in a quick google search). This Stack Exchange question and its answers also offer a wealth of suggestions.
The important thing is that the method you choose works for your purposes. Depending on what you need your class to do, using it in an application with extensive logging may be sufficient.


Adding new objects in visible plane( not default XY plane)

Question: Blender adds new objects in XY plane ,however if one is  working in other planes(eg. XZ,YZ etc), this can be quite irritating having to transform the object to the desired view everytime a new object is added .Is it possible to  add a new object in the visible plane?
Comment: Not completely sure what you mean. Blender always adds new objects to the 3D cursor which is by default in the origo of the world.
Answer: If you select Align to: View in Preferences, new objects will origin to current view, not world:

But if the view is not top/bottom/side (rotated using MMB), object will be rotated in same direction as a view point.
You can also adjust last operation and set align to view here:
Answer: Yo
From the 3D view-port open the object properties (N),
scroll down and expand 3D Cursor, the Z: axis value can be entered from here and your objects will be added to the new location.
When typing values switch between X: Y: and Z: with (Tab) and (Shift+Tab), or just scrub with mouse over them.


combined random variables and pdf

Question: I need some advice to how I would begin my integration for the following problem (as in, I do not understand the region of integration):
If we have a joint pdf, say $f(x,y)=4e^{-2x-2y}$ for $x,y>0$ and $A=\min(X,Y)$, how do we find $P(A>a)$ for $a>0$?
My idea is to integrate $\int_w^x\int_w^y f(x,y) \, dy \, dx$.
But this does not make sense because the bounds need to be restricted to a. Any suggestions?
Answer: Hint: $P(A >a)=P((X>a)\cap (Y>a))$. Either do the double integral with $x$, $y$ each going from $a$ to $\infty$. Or else note that from the joint density function we can deduce that $X$ and $Y$ are independent, and each has exponential distribution parameter $2$. Thus $P((X>a)\cap (Y>a))=P(X>a) P(Y>a)$, and we can avoid the double integral. (Not that the double integral is difficult, since $4e^{-2x-2y}=2e^{-2x}2e^{-2y}$.)

combined random variables and pdf


Why is " arbeiten " at the beginning of the sentence? "Arbeiten muss ich nicht mehr"

Question: Is it possible that it has another grammatical role besides being a verb?
For example, be an adjective, etc
Answer: Another possible word order is:

Ich muss nicht mehr arbeiten.

The word "muss" is the finite verb, the verb that is inflected according to person ("ich muss" but "wir müssen") and tense (present "ich muss", past "ich musste"). The verb "arbeiten" is an infinitive form.
The most important word order rule is that the finite verb is in second position. Other parts are more flexible. So indeed this is also possible:

Arbeiten muss ich nicht mehr.

This word order can be chosen to put the topic first: we first establish that we are talking about having to work and then supply the information that the speaker doesn't have to do that anymore.
But in both sentences "muss" is in second position.
Comment: This answer is complete and offers a good explanation of why it is valid grammar. I would, however, like to underscore that putting "Arbeiten" first as a style choice is a way of emphasizing it. You could think of it like an answer to a question. "Was musst du nicht mehr?" "Arbeiten muss ich nicht mehr." If the question were "Wie lange muss du noch arbeiten?" you might answer "Nicht mehr muss ich arbeiten." As Carsten says, this order puts the topic first, or in layman's terms, it emphasizes the important bit you want the listener to pay extra attention to.
Comment: @Stuporman, I don't actually agree that the emphasis is necessarily on the part that is in first position. I think it's more complicated.
Comment: You're right, now that I think about it. In the original example, spoken emphasis would likely still be on the nicht. In any case, the semantic meaning is never changed with this pattern, so it is chosen to alter the point emphasis, albeit not as reliably on the first bit as I said originally. The subtlety and nuance to that is something best won through experience with competent speakers, so this should be thought of as a starting point rather than a complete understanding.


How do you use use cases in a driver design with interrupts?

Question: I'm writing driver-level code that runs on a microcontroller. I've described some of the design with use cases. The users in my use cases are the the other software components that use my driver. However, I also receive input from the hardware itself in the form of interrupts. How do I describe those interrupts in my use cases? Is the hardware itself another user? Do I describe the interrupts as exception cases in my existing use cases instead? That may be difficult since some of the interrupts are asynchronous and may happen at any time during any use case.
Comment: Have a look here: https://www.scrum.org/Forums/aft/1472
Comment: I agree that hardware is a legitimate actor, but interrupts, events and the like are *implementation details.*
Comment: I have removed the comment because I didn't want it to lead to false assumptions. After reading Christopher's answer I understood why Robert Harvey cataloged interruption as *implementation details*. It make sense.
Answer: The system under consideration in your model seems to be your microcontroler with your driver.
The actors interact with this system under consideration.  They are of two kinds :

primary actors:  they use the system to achieve their goal. They are often represented on the left side of the diagram.  Here, it's the software components that use the driver.
secondary actors: they are other actors involved in a use case.  They are usually represented on the right side of the diagram.  Here it's the hardware.

I have the impression that the interruptions are not specific to the one or the other use case, and therefore shouldn't be represented as an extension or an exception to them.
If you want to represent interruption, you'd rather go for a distinct use case. I wonder however if it's necessary at all:

the interruptions do not represent a goal for any of your actors
the interruptions are not created by the external hardware, but by your own system (i.e.microcontroller response to an input signal), so it seems to be rather an internal implementation detail of how some interactions are handled.
the logic of interruption and their timing could better be represented using other behavioral diagrams (e.g. an activity diagram with an interruptible activity region).
Comment: I have never heard of that left-primary/right-secondary rule :-/ I always used stereotyped associations (e.g. `<>`)  or un-/directed associations to distinguish both.
Comment: @ThomasKilian Not a rule, more a common practice. I've seen it quite often. I don't know its origin, but I have a pocket memento and other references that states it (e.g. [this book](https://books.google.fr/books?id=NqGVzs-ySYcC&pg=PA12&lpg=PA12&dq=primary+secondary+actor+uml+left&source=bl&ots=Z_kyFHTLeY&sig=ZWxY8i9eYGjsmVDvPRIZ4D7xGSw&hl=fr&sa=X&ved=0ahUKEwiQ5Ze-x7PQAhWCbhoKHUlEAvA4ChDoAQgaMAA#v=onepage&q=primary%20secondary%20actor%20uml%20left&f=false), or [this paper](http://www.scenarioplus.org.uk/papers/reqts_in_uml/reqts_in_uml.htm) or [SO](http://stackoverflow.com/a/6464534/3723423) )
Comment: One never stops learning :-) At least I see my practice also used in the SO answer you cite.


Compare array with SQl table

Question: For each connection in an array called ALLconn, I would like to compare it to my sql table. If exist, then add to my listview. Here is my code below, but it does not seem to work:
Dim LoginFilter As Object
    Dim SelCurrAllSessions As SqlClient.SqlCommand = New SqlClient.SqlCommand("Select * from CurrAllSessions", LFcnn)
    SelCurrAllSessions.CommandType = CommandType.Text
    LoginFilter = SelCurrAllSessions.ExecuteReader

    For Each conn In AllConn
        While LoginFilter.Read()

            If conn.UserName.ToString() = LoginFilter.Item(0) Then
                ListBox1.Items.Add(LoginFilter.Item(0))

            End If
        End While
    Next

Comment: what is the type of the array? what are the columns in the table? what constitutes a "match"?
Answer: Well you need to change the order of the loops
While LoginFilter.Read()
    For Each conn In AllConn
        If conn.UserName.ToString() = LoginFilter.Item(0).ToString Then
            ListBox1.Items.Add(LoginFilter.Item(0).ToString)
            Exit For
        End If
    Next
End While

This is necessary because in your original code, the internal while run till the end of the data loaded from the database then, when you try to check the next conn, you cannot reposition the reader at the start of the data loaded by the database.
Answer: It's the other way around, use Contains to check if the string is contained in the collection, then you can add it to the ListBox:
Using LoginFilter = SelCurrAllSessions.ExecuteReader()
    While LoginFilter.Read()
        Dim connection = LoginFilter.GetString(0)
        If AllConn.Contains(connection) Then
            ListBox1.Items.Add(connection)
        End If
    End While
End Using

Answer: Actually, your Question is uncompleted but still as per my understanding your trying to read result of SqlCommand.ExecuteReader(). When you read that Result it will reading from first to last and that is only one time you can read. if you try to read again it will show up an error because no more content to read from an object Loginfilter.
So, Either you can store that reading into an array and continue with your Foreach and while logic with newly created array or you can do reading from LoginFilter and compare into forech connections in AllConn array or List.
Please feel free to response me if you need more explaination, I will send you in C# version.

Compare array with SQl table


Homoclinic orbit vs limit cycle

Question: In dynamical systems, what is the distinction between a homoclinic orbit and a limit cycle? It seems to me like a homoclinic orbit is effectively just a limit cycle with a particular fixed point along the way, but in both cases can't we describe the object as an attractor (or source, depending on stability) curve in phase space, periodically traversed?
Comment: And why might we prefer one or the other these in our system? Is there an advantage to making the fixed-point pit-stop?
Answer: There are two crucial differences:

A homoclinic orbit is traversed just once. The dynamics cannot start before the fixed point or progress beyond it. Once it reaches the fixed point, it's … well … fixed. The dynamics starts at the fixed point at $t→-∞$ and returns to it at $t→∞$.

Homoclinic orbits are not necessarily attractive.
In particular, you can have homoclinic orbits in Hamiltonian systems, which do not feature attractors (or repellors) at all.

For example, consider a ball on some geography without friction: The ball starts with zero velocity at a hill top accelerates through a valley, slows down on the other (higher) side of the valley, and then returns through the valley to its starting point, where it arrives with zero velocity at $t→∞$. This is a homoclinic orbit. There is no periodicity. Moreover, the scenario is Hamiltonian and thus cannot have limit cycles at all.

Homoclinic orbit vs limit cycle


Tangent space describes the manifold's first order characteristic. Is there something like tangent space describes higher order characteristic?

Question: I'm learning differential geometry. I'm curious that when we learned analysis, we learned higher order derivative, while in differential geometry, first order derivative is generalized to element of tangent space. Then my question is what's the higher order derivative in differential geometry? Are there some literatures including this area? Or maybe this is just a trivial question because there is nothing interesting in higher order derivative.
Comment: Jet bundle. Or, more generally, there are the Weil functors.
Comment: The derivative is on the tangent bundle $df : TM \to TN$, so the 2nd order derivative is a map of the tangent bundle of the tangent bundle, $d^2f : T^2M \to T^2N$.  And so on.
Answer: There are several possibilities:
1.The most naive one is the following: Consider a smooth map $f\colon M\to N.$ Then its derivative $df\colon TM\to TN$ is again smooth, and you can again differentiate.
Clearly, you take a lot of useless information around, so this is usually not the method used by differential geometers to solve any problems.

Another possibility is by defining so-called jet bundles in a similar way for higher order derivatives as the tangent bundle is defined for first order derivatives. There are many books on this, check for example wikipedia. The "problem" with jet bundles is, that they are no vector bundles, and one somehow loses the (mulit)-linear algebra tools..
Differential geometer often use connections on vector bundles to define higher order
derivatives. As the most basic example, one should mention the Levi-Civita connection $\nabla$ (defined by a Riemannian metric on the manifold). With its help you can
derivate vector fields along vector fields. A nice and simple application of its use is the following: (Locally) shortest curves are exactly those curves which satisfy the second order differential equation:
$$\nabla_\gamma' \gamma'=0.$$
Comment: Concerning your remark 2: multilinear algebra is not lost with jet bundles since jets of consecutive order are affine bundles over symmetric multilinear tensors.
Comment: @Michael: You are of course right, that one can recover the multi-linear algebra. Nevertheless, this is not totally transparent, and quite often people prefer to use connections in order to work with higher order derivatives.


What kind of rice can I use in a salt shaker to prevent clumping

Question: Will brown rice eliminate clumps in the salt shaker as well as white rice?  I know white rice works well in a salt shaker to eliminate clumps caused by humidity, but I only have brown rice on hand.  Will brown rice work as well as white rice?
Comment: I wouldn't use brown since it has a relative short shelf life, 6 months compared to years for white rice. Not that either will help a lot.
Comment: Rice in shalt shakers is a urban myth. Modern table salts contain anti-caking agents; if the rice has any influence, it's due to mechanical shattering of (unlikely) clumps. Brown or white makes no difference.
Comment: Instead of adding rice - consider getting a salt grinder and use rock salt instead. You can even buy disposable grinders with salt already in them.
Answer: I do this at home, and it makes a significant difference.  I suspect that it is the mechanical action of the rice, but since adding a teaspoon of rice to each shaker, I don't have to get out the toothpick ever few weeks to clean the hole.
Brown rice includes the germ (embryo) of the grain, which contains some oil.  This oil goes rancid, which has an unpleasant smell.  In the quantities that you are using, and that the oil is still inside the rice, I don't think you would notice.
Note that it doesn't have to be rice.  a few beans, some barley, lentils, or wheat should work just as well.