How does one derive the Einstein Field Equations?

Question: I've been trying to teach myself General Relativity recently, I've came across two problems whisled doing so; I don't really understand why $R=g^{\mu\nu}R_{\mu\nu}$, why couldn't I contract the indices with a different two index tensor eg. $R=\gamma^{\mu\nu}R_{\mu\nu}$, and I don't understand how the Einstein Field Equations are derived. I've searched online and most people derive it by using the Einstein-Hilbert action, but my understanding on Calculus of Variations is a bit shakey and I don't understand what Einstein and Hilbert were trying to minimise. So if you can explain an easy way of deriving the Einstein Field Equations or explain the Einstein-Hilbert action and deriving the equations through it that will be much appreciated.
Comment: To address the first point (why we contract with the metric and not any other two-index tensor) – that is the definition of $R$.
Comment: To understand the construction of the Einstein's field equations is a remarkable conceptual achievement. You can not think to get them explained in a few lines of comment. My advise is to learn the argument in a general relativity textbook.
Comment: Possible duplicate: https://physics.stackexchange.com/q/219681
Comment: you can postulate the Einstein-Hilbert action and derived it using the principal o minimum action or you can postulate the equation
Comment: A wonderful textbook is "Spacetime and geometry - An introduction to general relativity" by Sean M. Carroll.
Comment: @MicheleGrosso does it also have a derivation of Einsteins Feild Equations?
Comment: Of course, it is a comprehensive and extensive coverage of general relativity theory. If you look for it in Amazon, you can have a view to the Contents.
Comment: Try http://www.preposterousuniverse.com/blog/2014/03/13/einstein-and-pi/
Answer: Once the geometric context of gravity is clear (which is really kind of independent of the specific field equations), one can reason as follows.
The field equation must reduce to the field equation of Newtonian gravity, which is $$ \nabla^2\phi=\kappa\rho, $$ where $\rho$ is the density of the source ($\kappa$ is an irrelevant constant).
We know from special relativity, that mass/energy density is by itself not a scalar quantity, but (in $c=1$ units) is the $00$ component of the stress-energy tensor, $\rho=T^{00}$.
If we want a geometric description and the equivalence principle, then particles moving under the effects of solely gravity must obey the geodesic equation, which is $$ \frac{d^2\gamma^\mu}{d\tau^2}=-\Gamma^\mu_{\kappa\lambda}\frac{d\gamma^\kappa}{d\tau}\frac{d\gamma^\lambda}{d\tau}. $$
In the nearly flat spacetime ($g_{\mu\nu}=\eta_{\mu\nu}+h_{\mu\nu}$) and slow particle ($d\gamma^0/d\tau$ dominates over the spatial components) we get $$ \frac{d^2\gamma^\mu}{d\tau^2}=-\Gamma^\mu_{00}\left(\frac{d\gamma^0}{d\tau}\right)^2, $$ where in this limit the connection is $\Gamma^\mu_{00}=\frac{1}{2}\eta^{\mu\rho}(2\partial_0h_{0\rho}-\partial_\rho h_{00})$. We further assume that the metric doesn't change in time (Newtonian gravitation has no time evolution), then we get $$ \frac{d^2\gamma^\mu}{d\tau^2}=\frac{1}{2}\left(\frac{d\gamma^0}{d\tau}\right)^2\partial^\mu h_{00}. $$
Due to staticness, the 0 component of this equation is just $d^2\gamma^0/d\tau^2=0$, which means that $d\gamma^0/d\tau$ is constant, so we can divide by its square (and use that $\gamma^0$ is basically the $t$ coordinate time), we get $$ \frac{d^2\gamma^i}{dt^2}=\frac{1}{2}\partial_i h_{00}, $$ which is the equation of motion in a Newtonian gravitational field if $h_{00}=-2\phi$.
So going back to the Poisson equation $\nabla^2\phi=\kappa\rho$, we know that $\rho=T^{00}$, and $\phi$ is related to $h_{00}$ and as such to $g_{00}$, and second derivatives of $\phi$ appear, so the Newtonian equation has the form $$ K^{00}=\kappa T^{00}, $$ but we want a tensorial equation, so the full gravitational equation should be $$ K^{\mu\nu}=\kappa T^{\mu\nu}, $$ where $K^{\mu\nu}$ is some kind of tensor field constructed out of the metric's second derivatives.
But we know from differential geometry, that all tensor fields depending on the metric's second derivatives are derived from $ R^\kappa_{\ \lambda\mu\nu} $, the curvature tensor.
There are not many second-rank tensors that can be made of this, one of them is $R^{\mu\nu}$ the Ricci tensor. Here we could also look at the Ricci tensor's Newtonian limit to see how it is related to $h_{00}$ but I don't want to do that here.
Unfortunately $K^{\mu\nu}$ cannot be proportional to $R^{\mu\nu}$ because the stress energy tensor must satisfy $\nabla_\nu T^{\mu\nu}=0$, but we have $$ \nabla_\nu R^{\mu\nu}=\frac{1}{2}\nabla^\mu R $$ from the Bianchi identity, so by rearranging we get $$ \nabla_\nu R^{\mu\nu}=\frac{1}{2}g^{\mu\nu}\nabla_\nu R \\ \nabla_\nu\left(R^{\mu\nu}-\frac{1}{2}g^{\mu\nu}R\right)=\nabla_\nu G^{\mu\nu}=0, $$ eg. the Einstein tensor $G^{\mu\nu}$ is automatically divergenceless. So we should have $$ G^{\mu\nu}\sim T^{\mu\nu}, $$ which is the Einstein field equation.
Answer: As it was already mentioned in the comments, deriving Einstein's field equations needs a good foundation in differential geometry. But anyway, I will try to give a more physical intuitive explanation which might help you understand something about "the way to the field equations".
First we want to review classical fields. A field associates each point in space with a physical quantity mathematically expressed as a scalar, vector or tensor. Now one might ask: What's the source of the field? From classical mechanics or electromagnetism, we now that the source of a gravitational or electric field is the mass or charge distribution abbreviated as $\varrho$. The remaining question is: How do we get a mathematical expression of a field for a given $\varrho$? The answer (which can be obtained by doing some math): Solve Poisson's equation. That is, find solutions for
$$\Delta \Phi = 4\pi \varrho.$$
So now imagine we are in 1905. A young physicist named Albert Einstein publishes a new theory, called special relativity. His theory is based on two postulates:

The speed of light is constant in all inertia systems.
The physical laws are the same in all inertial systems.

What has this to do with fields? Special relativity shows that electrostatic and magnetic fields are basically the same, they depend on the frame of reference.
Now Einstein begins to wonder if there is a way to incorporate gravity into this new "relativity" framework. He begins to think: Imagine a man in an elevator. If the elevator rests on earth, the man feels the gravitational force that pulls him onto the ground. But if the elevator is somewhere in outer space and accelerating a force would pull the man to the elevator's floor. The consequences: If the elevator has no windows, the man is not able to distinguish between the gravitational force of earth and the acceleration of the elevator in a "gravity-free" space.
But as fate has it, gravitation proves to be quite difficult to incorporate into relativity, so after years of thinking and mathematical hardship (which mostly involves differential geometry, multilinear algebra and topology), Einstein comes up with an idea: Gravitation is a consequence of the curvature of spacetime. The curvature itself is induced by mass. He comes up with an equation:
$$R^a{}_{bcd} = 4 \pi \varrho_m.$$
What he did was to substitute the so called Riemannian curvature tensor for $\Delta \Phi$. Now  form special relativity, he knew that energy is equivalent to mass (remember $E = mc^2$). So instead of using the mass distribution, he came up with the stress-energy-tensor $T_{ab}$. So the field equations become
$$R^a{}_{bcd} = 4\pi T_{ab}.$$
It basically tells out that mass is the curvature of spacetime, which seems fine, right? But it isn't. This equations disregard the continuity equation. So another year of hard work (again a lot of math was involved) was needed for him to derive  the final result in 1915:
$$R_{ab} - \frac{1}{2}g_{ab} R = 8 \pi T_{ab}.$$
Here $R_{ab}$ is the Ricci curvature tensor (obtained by contraction of the Riemannian tensor), $R$ is the curvature scalar (obtained by anther contraction of the Riemannian tensor) and $g_{ab}$ is the metric.
I hope I could shed at least a bit of light on your issue. A formal derivation of the field equations involves lot of advanced math (remember Einstein needed several year to get a grasp of it), but nevertheless, there is some analogy between classical fields and general relativity.
Comment: Is it fair to say that the field equations were arrived at with a somewhat ad hoc approach rather than having a formal derivation?
Comment: No but the question asked for an easy way to arrive st the field equations and I tried to give somewhat intuitive way to look at the issue. What I don't want to say is that this approach can replace a formal derivation, this is definitely not the case, but it's sometimes helpful to view a topic more intuitively to get a grasp of it.
Comment: The equation $R^a{}_{b { \color{red}{cd}}} = 4\pi T_{ab}$ equals tensors of different ranks, is that really what Einstein derived, or did you mean to say $R_{ { \color{green}{ab}}} = 4\pi T_{ab}$?
Comment: @Yukterez these equations serve as an illustration of the concepts in question, not to be taken strictly mathematical.


Publish relationalship data from arcsde

Question: I have feature classes and tables having relationship shored in them.(Feature classes has id as which is primary key in other table  ). I want to publish the Feature class as a service on arcgis server 10.1. I want to show the value in place of the id's.what is the best way to do it.
Answer: I'm not 100% clear on your intentions, but

I want to show the value in place of the id's.

makes it sound like you should create a join between the featureclass and table. This will add the table's VALUE field to the featureclass's attribute table, allowing you to access the VALUE via ArcGIS Server.
Comment: I meant ArcSDE View .Could please suggest some links to create SDE View in 10.1.

Publish relationalship data from arcsde


Can you still get the M1 Garand Veteran weapon?

Question: Is there any way to earn the Garand without the code, like by points maybe? I've already tried the veteran page but it's always down.
Comment: I can't answer your question, but your tags need fixing. BF2 was never released on ps3, it was a PC exclusive. Which battlefield title are you referring to?
Answer: I recieved the M1 Garand on Xbox 360 after without playing many other BF games, it requires your account to have veteran status to unlock.
Veteran status is a stat that is tracked throughout all battlefield games between BC1 and BF4 (potentially hardline as well but I can't remember, and I would not know about BF1). On my xbox I had only played BFBC2, BFBC1 Demo and about 1/2 years later I recieved the M1 Garand linked to my profile, if you really want it then I would recommend trying to obtain another BF game, alongside playing all the demo's you can. Of course the same method works on Ps3 if you are playing on that.
Answer: According to the Battlefield Wiki page on the M1 Garand:

Only available in multiplayer, the WWII M1 Garand is available to Battlefield veterans.

Then, checking the Battlefield Veteran page under Bad Company 2:

Those who have played two previous installments will be rewarded with the M1 Garand for use in multiplayer.

The only way to earn the M1 Garand in Bad Company 2 is by being a Battlefield Veteran, by having played at least 2 previous Battlefield games.
You cannot earn the M1 Garand by progressing through a specific class, leveling up, or any other means in-game.
So I would keep checking the Veteran page if you still want the weapon.


Remove overlapping buffers

Question: I have put a buffer around points from a point data set. But some buffers overlap each other. Now I would like to remove  all the buffers (from the data set) which overlap another buffer. Does somebody have a suggestion, I would like to use the modelbuilder as I dont know how to use python.
Comment: Can you share what software you are using? I would guess an ESRI license for (9.X/10.X?) as you mention modelbuilder. You could also add these options to your listed tags if that is the case. For example 'arcgis-desktop' or 'arcgis-10.1' might help generate more views/answers. If so, perhaps the Dissolve Type parameter on the buffer tool might be your answer. Type "ALL" or "LIST" might suit your needs.             Buffer tool Documentation can be found [here](http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//000800000019000000)
Comment: Sorry, yeah I am using ArcMap 10.2. I do not want to dissolve though, I would like to delete the points/buffers that overlap another buffer..
Answer: Try a three step approach:

Intersect your buffers with themselves
Use the buffer_intersect layer to select layer by location and specify "INTERSECT"
Delete the selected buffers

The image to the left shows the original buffers.  The image to the right show the intersected buffers in red and the selected original buffers in turquoise.
Comment: Thank you, I think that is exactly what I want.
I will try it..

Remove overlapping buffers


Which churches regularly give altar call (like "raise your hand if you accept Jesus") in their service?

Question: I have seen testimonies of converts from America describing they raised their hands when the preacher asked "who wants to accept Jesus as their Lord?" or something like that. Which churches have this practice on a regular Sunday service? If so, do these churches always have new visitors, or some of the regular members themselves raise hands in these altar calls?
Answer: Altar Calls The evangelist Charles Finney is said to have started the practice of having public "altar calls". A practice utilized by Billy Graham, and other modern evangelists, later on. But it should be noted that denominations used to hold revivals in the 1700s and 1800s with sermons climaxed by "tarrying at the altar" with crying and tears, until there was unmistakable assurance of salvation. Sometimes this took all night in earnest prayer. (See autobiography of Peter Cartwright, Backwoods Preacher, and Religious Affection by Jonathan Edwards)
Local Churches As far as doing such in the small local church on Sunday, this has been adopted by mainly Evangelical-type churches, Charismatic and Pentecostal churches, and independent churches. Note that most altar-calls are for "salvation," but occasionally the minister would apply them to the topic addressed in the sermon, and have the people come forward for prayer: addictions, character problems, healing, etc. This for the congregates who are already members.
Usually there are just a small handful of the congregation who are there as "seekers" or "unsaved." But altar calls are made each week anyway.
Mega-churches Mega-churches, because of logistics, often have the people raise their hand and say a prayer, staying in their seats. (e.g. Joel Osteen's church) But they are encouraged to join a small Bible Study group for discipleship and mature growth in the Christian walk.
Some churches have people come forward to meet with the minister after the sermon, who are already Christian, but wish to join that particular church as members, having moved into the area recently. (Church of God, e.g.)
Formal or High Churches On the other hand most Mainline Churches, such as Lutheran, Anglican, United Methodists, etc. rely on a process of discipleship involving Catechism classes. After a seeker attends several Bible studies, and decides to convert (become a Christian), he/she is Confirmed and baptized, and made a member of the Church. The decision for becoming a Christian is made after thoughtful contemplation, and attending several church meetings.
Comment: Are there new visiters every week, and does it even happens that nobody raises their hand bec nobody is a new visitor? Or members themselves raise their hands for the sake of it. The new comers must be very occasional
Comment: @Michael 16 -(1) In large cities, and in large churches, there are indeed usually "new visitors" in the congregation on Sunday. (2) Members would not raise their hands if they were required to come down to the front altar; that would be "too revealing"!  (3) In much smaller local churches in small villages, the occasional visitor would be more scarce.  But not out of the question, because even there people come and go into the locality. Thanks for these clarifying questions.
Comment: Is there a name for the legalistic formal churches which imitates Jews by their priest robe and rituals etc, and those who reject these ritualism
Answer: The Churches of Christ offer what we call the "invitation" at every service where some kind of homily is spoken. Those present who wish to be saved, those who wish to make a public confession, and those who for whatever reason would like the prayers of the congregation, are invited to come forward. There is usually song sung at this time.
Answer: While not a verbatim expression of the above altar call, there is a practice in the Church of Jesus Christ of Latter-day Saints having a similar appearance to some degree, wherein members of the church are regularly invited to express their willingness to sustain their church leaders in their callings in public church meetings. This occurs whenever new callings are made and again in every conference of the church and at each level and unit of organization; there are several such conferences throughout the year. If there are any contrary votes, they are also asked to be made manifest. The vote to sustain is generally understood to be applicable to members of the church.
As for its relevance to the intent of the altar call ceremony in question, the Lord said to His Apostles:

He that receiveth you receiveth me, and he that receiveth me receiveth him that sent me. (Matthew 10:40)


In DSP - is it true that the only type of interfering wave occurs when frequencies match?

Question: Therefore, any other frequency, no matter the amplitude, won't interfere or degrade original wave in the DSP frequency domain systems?
Comment: I'm voting to close this question as off-topic because because this is physics or electrical engineering, not signal processing.
Comment: Also, you already got an answer to the underlying physical question on https://electronics.stackexchange.com/questions/286185/would-the-electric-fields-of-simultaneously-transmitted-waves-affect-eachther. DSP is really just **Math**. You can write down any equation involving a discrete signal and call it DSP, including an equation that breaks or upholds a non-interfering condition.
Answer: This is a question where theory and practice will diverge somewhat.
Theory
I assume that by "frequencies" you mean sinusoids of varying frequency. In that case, yes, you're correct, for any amplitudes $a_1$ and $a_2$ and frequencies $f_1$ and $f_2$ (where $f_1 \ne f_2$), the signals
$$
x_1(t) = a_1e^{j2\pi f_1 t} \\
x_2(t) = a_2e^{j2\pi f_2 t}
$$
are orthogonal to one another. The exact meaning of orthogonal varies from context to context, but in this sense, you can take it to mean that you could isolate the two signals from one another. If you had infinite time and infinite resources (and perhaps the ability to bend physics to your will if you want to realize it in hardware), you could build a filter that would pass $x_1(t)$ and eliminate $x_2(t)$, or vice versa. In theory.
In our theoretical utopia, the relative amplitudes of $x_1(t)$ and $x_2(t)$ don't matter either. Your theoretical processor can have unlimited dynamic range and/or numeric precision, so having one really large and one really small sinusoid isn't a problem.
Practice
Practical systems are hamstrung by several limitations that can invalidate the above claims. Among them:

If $f_1$ is really close to $f_2$, it can be difficult to resolve them as two separate sinusoidal components. If you were trying to do so digitally using short-time Fourier analysis, for example, you need a longer observation time in order to properly separate the two frequencies in your spectral estimate.
Another similar way of looking at it: if $f_1$ and $f_2$ are close to one another, and you wanted to build a filter that would pass one and eliminate the other, you would need a filter with a very sharp transition band. It can be shown that, qualitatively, the width of a filter's transition band is inversely proportional to its overall delay. So, as your transition band gets smaller and smaller, the delay required to implement the filter gets larger and larger. Furthermore, such a sharp transition band would require higher- and higher-order filters, possibly beyond the point of being practically realizable.
Practical systems do not have unlimited dynamic range. There will be some finite ratio between the maximum and minimum amplitudes that a system can reliably measure. This can limit your ability to resolve sinusoids that have widely-varying amplitudes, even if their frequencies are far apart. A very strong signal can desensitize your observer to the point where you can't see smaller signals.
Try shining a bright flashlight and a small LED at your face at the same time. Even if they are comfortably spread apart in space, you'll have a hard time seeing the LED due to the bright flashlight; your visual system has limited dynamic range.
Comment: Aha, this is where I get confused. Because I'm trying to marry the theory with real life practice, and although I do get the typical answer of "no" interference, research shows, there are issues which still cause problems, and are outside the frequency match!. Thank you for this explanation


How to deploy from sandbox to production in salesforce?

Question: We are currently working in a sandbox in salesforce . Now i want to know how i can switch to production environment in salesforce ?
Comment: You need to log in to the `login.salesforce.com` with your username and password.
Comment: Do you mean move functionality to production ? It may be good if you update your question to explain what you mean by "switch".
Comment: Yes i want to move with functionality
Comment: @Amit Can you elaborate your question you are willing to move your objects,clasees from sandbox to production..??
Answer: Salesforce offers a lot of documentation on this subject. Start here:
http://www.salesforce.com/us/developer/docs/dev_lifecycle/index_Left.htm#CSHID=migrate.htm|StartTopic=Content%2Fmigrate.htm|SkinName=webhelp
Here is another article with 2 videos about different deployment methods:
http://www.oyecode.com/2012/07/how-to-deploy-salesforce-apex-code-to.html
The 2 deployment methods mentioned are:

Use salesforce Change sets (as an administrator, from the Salesforce Setup pages)
Use the Salesforce migration toolkit, using the Metadata API (requires technical knowledge or additional tools such as the Mavensmate or Eclipse IDEs)
Answer: First you have to login to your prod org by going to the
url  :- http://login.salesforce.com
enter your credentials and login
since you have to move your data from sandbox to production
hence in sandbox you have to make outbound change set and in production you have to make inbound change set
Change set is the customization/code you want to deploy to production .
once you select the outbound change set in sandbox click upload make sure that you have selected all of the dependencies and profiles
once you upload it to production go to the production org and click on inbound change set and deploy.
Note :- If you are deploying the Apex code your organization must include 75% code to deploy .


I propose changing the colour of the review button when tasks are due

Question: I was under the impression that something along those lines was already implemented, but I paid close attention to the review button and concluded I can not see wether there are reviewing tasks due or not. Hence, I propose some kind of change of the button state.
Answer:
I was under the impression that something along those lines was already implemented

No. What is implemented is this:

If you have more than 1 k reputation: If there is an outstanding suggested edit to review, there is an orange square that takes you to the suggested-edits queue in the top bar. The number of reviewable edits is shown in the queue.
If you have more than 2 k reputation: If there are three or more outstanding reviews, there the same orange square appears and takes you to the review overview. The number of reviews is shown in the queue. Both, the appearance of the square and the number within is not individualised to you and ignores reviews you already completed or skipped. Thus it is not reliable (see also here).

(The reputation levels are given for betas. It's 2 k and 10 k on graduated sites.)
The latter was implemented in response to this feature request and was probably limited to higher-reputation users to avoid pushing lower-reputation users to review and instead let them explore the review system more slowly (see this answer), which is reasonable in my opinion.
Answer: I assume you are referring to the orange marker in the top bar. Unfortunately, it shows up whenever there are (at least three) items for review by anyone; it does not care whether you personally already had a look at some or all of them, or are otherwise ineligible to deal with them (e.g. if you raised a flag that caused a post to end up in a review queue). Developers cited efficiency reasons for this: It would consume too much time to check for the status of all review queues for each page view.


Can atoms capture beta particles or secondary electrons into their orbits and become anions?

Question: Since beta-particles are just free-moving electrons, shouldn't they be caught by strongly-electronegative atoms and thus create anions?
Why do they instead create more cations by knocking other electrons out of their orbits?
Does it, perhaps, depend on the speed of electron: i.e., slow-moving, non-ionizing electrons are more likely to be caught by atoms?
Comment: Chuck this Wikipedia article : https://en.m.wikipedia.org/wiki/Radical_ion
Answer: Beta particle energies are of order $10^5$ eV, which is vastly greater than typical electron affinities of order $1$ eV. The beta electron is not going to end up bound to another atom until it's had a chance to dissipate the unwanted $99,999$ eV. The energy is typically dissipated by colliding with other atoms and ionising them.
Comment: Secondary electrons will have a large range of energies because their energy depends on whether the beta particle scored a full hit or just a glancing blow. Some will have low enough energies to be captured immediately while others will have much higher energies.
Comment: Thanks. So basically, all beta particles will end up being caught by atoms, but not before they dissipate their energy?
Also, am I right to understand that secondary electrons can be caught immediately because they are slow?


Polygon Surface Area Calculator in different map projection(s)

Question: I'm simply looking for an online tool or a utility to calculate the surface area of the same polygon (i.e shapefile)  in different projection (projected and geographic projection system) systems. It would be great if you could share your suggestions.
Answer: You can do that from command line with ogrinfo http://www.gdal.org/ogrinfo.html and SQLite SQL dialect http://www.gdal.org/ogr_sql_sqlite.html.
Example: Select state called "Colorado" from spapefile, convert geometry into EPSG:3857 and compute the area. Unit corresponds with the base unit of the projection and in this case will be square meter.

ogrinfo -dialect sqlite -sql "select
  st_area(st_transform(geometry,3857)) from states where
  STATE_NAME='Colorado'" states.shp INFO: Open of states.shp'
        using driverESRI Shapefile' successful.
Layer name: SELECT Geometry: None Feature Count: 1 Layer SRS WKT:
  (unknown) st_area(st_transform(geometry,3857)): Real (0.0)
  OGRFeature(SELECT):0   st_area(st_transform(geometry,3857)) (Real) =
  447323265912.94

Change the projection code parameter in st_transform into 26913 for UTM zone 13 http://www.spatialreference.org/ref/epsg/26913/ and you will get quite different number:

OGRFeature(SELECT):0   st_area(st_transform(geometry,26913)) (Real) =
  269618280090.583

ST_Area does not give reasonable results for geographic projections. For example area of Colorado is 28.041027282727 square degrees in EPSG:4326 which is rather useless information.


What Is The Difference Between "root", "user", "group"?

Question: I am new in using UbuntuServer13.10 so during giving permission to a folder to open it on FileZilla on other PC, I got these words and now I want to know about the difference between; "root", "user", "group", "guest" and their security levels. So can anyone explain to me briefly about their function and usage. Just the basics, or provide a link to an external article on them.
Comment: [This](https://help.ubuntu.com/community/FilePermissions) will explain you more.
Comment: @AvinashRaj
Thnaks, you link also helped me a lot to make my sharing secure.
Answer: root: is a special account which is usually locked in Ubuntu. This account is in effect the ultimate super user and can change anything on the system.  If you want to enable this account you can but its not recommended see How to enable root login? For how this can be done and a discussion on why its not recommended.
user: These are the users of your systems there are three types administrators, normal users and guest. Administrators can make major changes to the system while normal users can not. There is a useful summary in the RootSudo Comunity Documentation.  The guest account is used to provide casual access to someone (to play a game, surf the web, etc.) without giving them access to other users files. Any files they create are automatically removed when they log out.
groups A user can be a member of one or more groups groups are used to control privileges within the system for example all administrators are in the sudo group but you can also create you own groups.  You may for example have a sales team who need to share some files but you want to prevent other people from accessing these files.  By creating a special group for these people you can do that. See:How can I share a directory with an another user? for how you can set this up.
Comment: - users: 3. guest is the 3rd.
Comment: @Rinzwind thanks I'd forgotten about guest.  I've edited by answer to include it.
Answer: Ok here goes my best shot at explaining this.First of all  root is like what the System user name would be. It has the highest level of permissions available. Some apps require you to run them as root in order to unlock their full potential (i.e. to carry out tasks regularly unavailable to just an administrator/user)
Secondly, user is kind of a generic term for your login or the login of another on the system (i.e. Joey, or Alice) Users are part of groups, There are different levels of groups such as Administrator, or User. Administrator is second to root, and User would follow Administrator. So inyour case file access permissions can be set to only allow one "User", or an entire "Group" of users. Some items are only accessable by root, which usally means that it is some system file or folder that you should consider avoiding messing with. To access these particular files and folders you can get there several different ways. Including by typing from the Terminal
 sudo nautilus This will start Nautilus with root permissions... Helps? -Bill
Comment: it is a very bad idea to do `sudo nautilus`. Please research, and do not recommend this. Use `sudo -i nautilus` instead


Regression and Time Series

Question: I was posed a problem by a colleague that i am struggling with.  He is interested in the relationship between 10 variables and a single dependent, continuous variable.  This could simply be an OLS problem; however, he also wants to relate it to time.  He has a hypothesis that the relationship between the 10 variables and the dependent variable change over a 7 year period.  What model should I be looking into to help him answer his question?
[EDIT]
The independent variables are measures of a company structure (board size, etc.) and the dependent is firm value (eg shareholder return).  By design these would have a level of autocorrelation I believe.  If it were cross-sectional (ie a single year) an OLS seems like the appropriate approach.  I am looking for the change in relationship over subsequent years.  To add a wrinkle we are also trying to control for firm size.
Comment: If you have the same firm(s) in multiple years there is panel structure you can use. If it is an actual time series, then you need some form of stationary to apply OLS. In any case auto correlation can be dealt with
Answer: You can do this with numerous parameter constancy tests, e.g. Chow test. The idea's to regress over two different periods and compare the parameters, whether they have changed substantially. "Structural break" is another key word for Googling.
You didn't give nearly enough information to help further. There are complications with time series as opposed to [cross-sectional] OLS regression, but it's a big topic. OLS might not be applicable at all in its simplest forms for time series data, it depends on the autocorrelations of variables of which you gave no information at all. So, my answer is the best you can get given the description you gave.
Comment: You are right, Aksakal...tried to get a quick answer but why would i think there would be anything quick in statistical analysis:)  Let me clarify this question and add to the post.
Comment: @WadeGarrett from your edit it's clear to me that you can't rely on pure stat or even econometrics literature. your question belongs to corporate finance field, there's a ton of papers doing similar analysis. you better browse that research. there are so many pitfalls in this sort of analysis, I don't know where to start. e.g. often you have to do pooled regression.
Comment: Thank you very much for the insight @Aksakal.  I tend to agree...I think i'm having some trouble here as the hypothesis presented isn't clear enough.
Comment: @WadeGarrett, yes, you need to have a good hypothesis, of course. However, you'll also run into all sorts of endogeneity issues, e.g. aren't the  measures of corporate structure dependent on profitability?
Answer: Your question raises the topic of "Regression vs Box-Jenkins". This that might help you going forward as you pursue Transfer Function analysis and ARIMAX model's exogenous components?. The relationship/differences between the analysis of cross-sectional data and time series data are discussed here. By the way the "complications" are really "opportunities" as you sort out a possible useful model.

Regression and Time Series


views export to xls

Question: I need to export views into xls file. If I use any of possible modules like views bonus or views xls export, I get a simple file with xls extension but inside is clean html code of the table instead of a compiled code as excell would produce. I can load it into excell and save it as proper xls file but is there a straight solution? Thank you.
Answer: Did you try the Views data export module? Here is a quote about it (from the module's project page):

... designed to provide a way to export large amounts of data from views. It provides a display plugin that can rendered progressively in a batch. Style plugins are included that support exporting in the following types:

CSV.
Microsoft XLS.
Microsoft DOC.
Basic TXT.
XML.

This module also exposes a drush command that can execute the view and save its results to a file.
Comment: I've just found this:
[Why are the XLS/DOC export files just HTML tables?](http://drupal.org/node/1820458)

Depending on your needs you may export to CSV, this format is easily imported into Excel.
Comment: yes I did, I use it now. But the outcome is the same - html code inside a xls file.

views export to xls


Creating Stream Order

Question: I'm new user of this software, please can somebody guide me, how to calculate stream order in QGIS or GRASS GIS, step by step.... mean commands and how to calculate watershed in this.
Comment: I think it would greatly facilitate our volunteers to help you if you could review [How to ask a good question?](http://meta.gis.stackexchange.com/questions/3349/how-to-frame-a-good-question) and then use the **edit** button beneath your question to improve it with some of the content you seem to be trying to discuss as comments.
Comment: What have you tried?  Where are you stuck?  Please **edit** your question to provide more details as advised when you initially posted this as an answer to another question.
Comment: I told you m just new user of this software mean, this is my first time to download and installation of this QGIS. I have created watershed area with ARCGIS 10, but ARCGIS can not measure all stream orders, e.g Horton, topology etc. please just guide me step by step to creat Horton stream order and other just like Strahler, Shreve in ARC GIS 10, we follow the steps FILL, FLOW, ACCUMMULATION etc. same procedure are available on youtube for this. But I could not find anthing like this video about QGIS, that can teach me step by step Horton and others calculation QGIS
Comment: If you are only interested in how to do this in QGIS now then please **edit** your question to say that and focus on just one type of stream ordering first. Otherwise I think your question is far too broad.
Comment: well, thanks for your positive reply, guide me step by step how to calculate Horton stream order, and watershed delineation.
Comment: First I add DEM data, then, I go to tool box and flow first step r.fill dir, but at end it shows me black image
Answer: I have calculated stream order with the SAGA GIS module. You can read about the modules here: http://www.saga-gis.org/saga_module_doc/2.1.3/ta_hydrology.html
All this assumes you have an elevation model in GeoTIFF.
In order to get that working you need to install QGIS with the OSGeo4W installer and make sure SAGA GIS is checked for install:

Then you have an assortment of hydrology tools ready in the processing toolbox. In order to calculate a drainage route network on overland flow you need to fill in all the small holes of the DEM (so the stream simulation doesn't get stuck.) I used Wang & Liu's method for this:

Now you will have rasters for a filled elevation model, as well as watershed basins and flow directions. Then you can perform a Sink Drainage Route detection on the filled DEM:

This will give you a shapefile with the drainage routes, with an attribute containing their Strahler order. Very useful. Good luck!
Comment: but I want to do this r.stream, in this all streams available, please can u help me in this egard
Comment: I'm sorry, but I haven't used GRASS for this sort of work. You might want to consider rephrasing your question to describe exactly what kind of analysis you are trying to do.

Creating Stream Order


My Macbook's hard drive is encrypted using FileVault 2. Will adding a new user with a different password break the encryption?

Question: I want to add a separate account or accounts to use as chroot accounts for an FTP server, and I want to use different passwords for these accounts.  I know the AES-128 key used by FileVault is calculated from the password, so will using a new password cause FileVault to try to encrypt using a different key, making the ciphertext inconsistent?  Or does FileVault calculate the key based on the password you used when you first encrypt the hard drive?  I'm not sure about this, and I don't want to break my computer.
Answer: FileVault 2 will encrypt the boot volume and allow you to unlock that volume based upon existing user accounts.
Adding a new user account on OS X El Capitan (10.11.6) will automatically add that account to FileVault 2's list of enabled users.  So no, you won't revert your existing setup or remove an existing account's ability to unlock an encrypted volume.
The built-in fdesetup account is also very useful (requires sudo privileges):

 list       [-extended] [--offline] [-verbose]
            List enabled users, or locked volumes.

 enable     [[[-user username ...] [-usertoadd added_username ...]] |
            [-inputplist]] [-outputplist] [-prompt] [-forcerestart]
            [-authrestart] [-keychain | [-certificate path_to_cer_file]]
            [[-defer file_path] [-forceatlogin max_cancel_attempts]
            [-dontaskatlogout]] [-norecoverykey] [-verbose]
            Enables FileVault.  This command will fail if no recovery par-
            tition was found on your disk.

Comment: If you want to see the nitty-gritty details then check out the [FileVault 2 Whitepaper](http://www.training.apple.com/pdf/WP_FileVault2.pdf).

My Macbook’s hard drive is encrypted using FileVault 2. Will adding a new user with a different password break the encryption?


Is Article One Partners a scam?

Question: Is Article One Partners a scam or not? See this for two conflicting opinions.
Answer: Not a scam. I've been researching for them for several years, and have been very pleased with the experience. You don't get paid for your time...it's more of a contest. You submit relevant research and the highest quality submissions win cash awards. There are numerous types of awards (and one category of guaranteed payments for invited "Experts").
Most of the time you don't win, but when you do, you can earn thousands of dollars.
Hope that helps.
David
Answer: In a strict sense no it is not; if you provide something really good i.e. something that knocks out a patent then you will get paid (in around 3 or 4 months) when you do a validity search.
But as with all things in life, there is a down side to it. Try to avoid state of the art searches as they are the easiest ones and usually a winner is awarded "instantly", if you are a "member". What I noticed is that even if you provide 3 really good references, AOP will not increase your submissions (or will increase it 2 days beore the search ends) and thus you will not have a chance to submit more, whilst "members" will have this option and win the prize. In this way the house always wins. Use these searches to increase your in scope submissions which has to be 10% or more in order to be eligible for the prize.
The validity searches are the most promising. As I said earlier, you have to find a really good reference in order to get the award which is not 4500 but 2000 USD while at the same time beating competition.
As always you never know what other people have submitted or how many submissions have been made; you are always in the dark.
Hope this helps to clarify free labour in the digital world...
Answer: Maybe, not a scam. But the award given may be unfair.
Comment: It would help if you could provide us with some background. What (evidence-wise) makes you think it's not a scam? What makes you think the award is unfair?
Answer: It is basically a scam.  Lot of bait and switch tactics.  The amount paid is rarely the amount listed for each study.  There's no transparency.  The rubric is not used consistently and is very subjective.  Updates to studies will contradict previous "leads".  Website is difficult to navigate and the content within the same webpage will contradict itself.
Basically they're crowd sourcing the work that their clients pay them to do.  Definitely not worth the time.
Answer: It is 100 percent scam.
The submissions limit increase is only to collect more and more documents and to fool the researcher into thinking he is going to get some money. In the end after submitting 20-30 exact matching documents, he gets nothing and the one submitting one or two documents wins the money as if it is a lottery not patent research.
The same people are being awarded every time. Whatever combination you try and select important points in study they still give the money to them same people.
Submitting the same documents directly to the clients will get you more money as the clients are not getting the required documents from RWS - AOP connect.
They reject all important documents citing these reasons and say your money was given to someone else.
"
"Highlights are critical to demonstrate how the response is related to the study research requirements. Unfortunately, your highlights are unacceptable and as a result the response is not eligible for reward.
The reason the highlights are designated as unacceptable is: the highlight does not map to the Research Requirement."
"Here's another tip with regard to highlighting.  Don't ignore the highlight warnings – excessive highlighting will never win a reward.  The purpose of highlighting is to direct the attention of our Study Management team to the most interesting part of the document.  Make them want to "look more closely".  Burying the lead by highlighting an excessive amount of language only exhibits the lack of attention to the study requirements – it DOES NOT exhibit that substantive work has been performed.   The rules of every study require accurate highlighting. Accordingly, should you submit the best reference but highlight the document unacceptably, we will award prize money you otherwise would have won to the second-best reference."

Is Article One Partners a scam?


Warping to a warping pilot, where do you end?

Question: If you warp to a ship that's in mid-warp, where do you end up? At the target ship's destination, current or initial position?
Answer: You end up warping to the pilot's current position. Makes perfect sense: if the pilot would warp 100 AU away, you could start warping before he actually reached the destination.
An exception seem to be deadspaces. You will always warp to the beacon in the first room, i.e. if the pilot would warp in and move 100km away from the beacon, you will still warp to it instead of the pilot.
Answer: You warp to the location of the pilot at the moment you requested the warp. If they are in warp, you will land somewhere in their warp path.
Comment: This is interesting. Is this going to be resolved with the new updates to have state is managed in the game? Apparently, it would be correct to assume *somewhere* as a good approximation, from what I currently understand. If you would run two clients A, B, on the same machine, and you warp both towards a warping pilot C, the greater the differences between the time the request were resolved with the server between the two clients would mean that they would be farther apart, and this is more pronounced when C is moving at max speed.
Comment: Just to leave a note, the state and changes were historically managed in a way that it required clients to synchronized with the server continuously for updates. When you run two clients and have pilot A and B shoot each other, B might be in such a state that it is damaged, but A is still shooting and not seeing any damage performed because it has not yet received an update from the server. When A would warp, it would still see B flying because it's no longer receiving updates from the solar system it left, but in reality B is warping as well. They are improving how state is observed.
Answer: You will land at the point space that they where when you initiated warp.
Comment: This adds nothing that hasn't already been said, and is therefore subject to deletion.


How to change pdf background color in evince?

Question: White background color is harm to eyes. How to change pdf background color in evince?
Now I use evince 3.4.0 in ubuntu 12.04 x64.
Comment: I suffer also from white background and I used to use xcalib below, but currently mostly I relay on KWin which offers invertion of colors for each window separately. That's why I use on most machines KDE as I have it out_of_the_box, just need to turn it in settings.
Comment: I am only suggesting workarounds, but **Zathura** is another (really cool) pdf viewer, and its background is black. Otherwise, you could customize your global Ubuntu theme (for e.g copy the theme Ambiance from `/usr/share/themes/Ambiance` to `$HOME/.themes/`, and tweak the color definitions in `gtk.css`). Unfortunately, I don't know where are the specific variables for evince, so all your apps would then have a black background XD. Personally I'm anyway much happier with a totally dark theme, but it's matter of taste.
Comment: After inverting the colors there are grey bars are left on both sides - you could get rid of them by just resizing the window to only show the inner inverted PDF content - `alt-right mouse` for the resizing shortcut, then `alt-left mouse` to move the window around. I think this is the simplest and most straightforward solution, provided your desktop background is dark/black.
Answer: I am sure that you can invert the color but I am not sure whether you can use a different color of your choice as a background color.
To invert the color.
Goto: View -> Inverted Colors (Ctrl+I)
Hope this helps.
Comment: Yes it is very much possible. After your comment I just went through the evince source code and found this. [ev-document-misc.c](http://git.gnome.org/browse/evince/tree/libdocument/ev-document-misc.c) holds the inverted color section `if (inverted_colors)
  cairo_set_source_rgb (cr, 0, 0, 0);
 else`. So by changing this i think we can achieve our desired color but i haven't compiled and tested this. You can try your luck.
Comment: I want to change back color to others like light green. I think this is easy to implement for developer of evince.
Comment: Then how to change back color to light green. I am newbie.
Answer: I have have been fighting this issue for quite a while now.  The best solution I have come up with is to use the xcalib command via two easily accessible application launchers on my tool/app bar.  I have named them "Screen Mode" and "Paper Mode".
With the help of xcalib I can activate on-the-fly two different screen configurations without profiles and other bloat. Note that xcalib is the smallest bit of software and won't take any space or memory as it passes the configurations to the display adapter and exits. You can forget about the heavy specialist software like argyll+dispcalgui etc.
Firstly install xcalib with:
sudo apt-get install xcalib

Then set your two launchers with these commands:
To set the "Paper Mode" use:
xcalib -red 1.7 1 64 -green 1.7 1 57 -blue 1.7 1 28 -alter

To (re)set back to "Screen Mode" use:
xcalib -clear

As you can probably tell I am adjusting the parameters per-channel and my effort was to produce a Kindle like result.  Obviously you can and should adjust these according to your preferences and screens.  Of course this adjusts the screen as a whole but, for me at least, this has been a blessing for those all white IDEs.  It will take you a couple of minutes to get used to it, but you'll never go back. It's a true Eye-Saver!
The command usage is xcalib -ChannelName Gamma Brightness Contrast and are all separated with a single space.  The -alter feeds the signal to the adapter. The -clear send a "Clear" message to the adapter to clear all adjustments back to normal.
IMPORTANT TIP: In order not to loose the blackness of letters always keep Brightness to 1 as shown above. Play around only with Gamma and Contrast.  Gamma will produce a darker/lighter result and playing with the Contrast between channels will adjust the resulting color.
Another TIP: Set the "Paper Mode" shortcut to first reset and then set the color (e.g., "xcalib -clear && xcalib -red 1.7 1 64 -green 1.7 1 57 -blue 1.7 1 28 -alter"), so that if accidentally use it multiple times it does not change you display color to black.
I hope this will prove handy to many of you!
Comment: As I noticed that the 1.7 gamma was just about cracking the font anti-aliasing (as well as the fact that I used some really funny numbers above), a more appropriate command for "Paper Mode" would be... `xcalib -red 1.5 0 65 -green 1.5 0 60 -blue 1.5 0 25 -alter`
Comment: At the time of writing xcalib is incompatible with Wayland (window renderer), since each process is only allowed to draw within its respective window.
Answer: I found Okular as better pdf viewer. It let you change from GUI the background color and the font color to any value you want. i.e.
Setting -> Configure Okular -> Accessibility -> Change Color
For example, set :

Dark Color as font color
Light color as background color.

I had Okular version 0.16.5

In Okular version 1.9.3
Setting -> Configure Okular -> Accessibility
Enable the checkbox for Change Color
In "Color mode" select "Change Dark & Light Colors"
"Dark Color" i.e. (font color) to any light color you want e.g. White or light gray
"Light color" i.e. (background color) to dark color e.g. Black Or dark gray
Answer: Change the background color of evince to the light greed and protect your eyes
Config the compile environment and download source
sudo apt source evince

Change the source to your color, such as the light green(R:199,G:237,B:204)
Edit the function ev_document_misc_invert_surface in file: libdocument/ev-document-misc.c at line 467
change
cairo_set_operator (cr, CAIRO_OPERATOR_DIFFERENCE);
cairo_set_source_rgb (cr, 1., 1., 1.);

to
cairo_set_operator (cr, CAIRO_OPERATOR_DARKEN);
cairo_set_source_rgb (cr, 0.8, 0.9098, 0.8117647);

Config and make and make install
cd evince
./configure --prefix=$YOUR-PLACE  --enable-nls --disable-scrollkeeper --disable-dbus --disable-debug --disable-tests --disable-nautilus --disable-thumbnailer --disable-previewer --disable-comics --without-keyring --without-gconf --without-gtk-unix-print
```

Then make, and I get the error:
Making all in synctex
make[3]: Entering directory '/home/luopeng/Downloads/evince-3.28.4/cut-n-paste/synctex'
  CC       libsynctex_la-synctex_parser.lo
  CC       libsynctex_la-synctex_parser_utils.lo
synctex_parser_utils.c:106:29: error: format string is not a string literal [-Werror,-Wformat-nonliteral]
        result += vfprintf(stderr, reason, arg);
                                   ^~~~~~
1 error generated.
Makefile:545: recipe for target 'libsynctex_la-synctex_parser_utils.lo' failed
make[3]: *** [libsynctex_la-synctex_parser_utils.lo] Error 1

Of course, fix it by:
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wformat-nonliteral"
    result = fprintf(stderr,"SyncTeX ERROR: ");
    result += vfprintf(stderr, reason, arg);
    result += fprintf(stderr,"\n");
#pragma GCC diagnostic pop

In the version of Ubuntu 18.04, I found several errors as the above case and
I fix them with the GCC ignored. (please add # before the pragma in the following code)
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wformat-nonliteral"
  the code where the errors occur
#pragma GCC diagnostic pop

Then change the config in /usr/share/applications/evince.desktop
change Exec=$YOUR-Evince-PLACE/bin/evince %U

When click the view->Inverted Color, your background color will change to light green
Enjoy it!
Answer: Thanks for Evan's invitation. Here is what I did to achieve the light green background in Evince 2.30.3.
As mentioned in a thread(https://mail.gnome.org/archives/evince-list/2011-November/msg00015.html) posted by me several years ago, I replaced the source code from Line 327 to 329 of libdocument/ev-document-misc.c with the following snippet:
// make sure that we just turn the near "white" back ground to light green
if ((245 <= p[0] && p[0] <= 255) &&
    (245 <= p[1] && p[1] <= 255) &&
    (245 <= p[2] && p[2] <= 255)) {
    p[0] = 204; /* cc */
    p[1] = 232; /* e8 */
    p[2] = 207; /* cf */
}

Note that the code position may vary for different versions of Evince.
Then re-compile Evince:
$ ./configure --prefix=/usr --enable-nls --disable-scrollkeeper --disable-dbus --disable-debug --disable-tests --disable-nautilus --disable-thumbnailer --disable-previewer --disable-comics --without-keyring --without-gconf --without-gtk-unix-print
$ make

After compilation, DONOT run make install, because this may change your locale settings of Evince. Just backup the original /usr/lib/libevdocument.so.2.0.0, and replace it with the modified libdocument/.libs/libevdocument.so.2.0.0 manually.
Then re-launch Evince, open a .pdf file(or other supported formats, whatever), and use the "Inverted Color" option to change the background color into light green.
May this be helpful :)
Comment: it would be useful if you showed the code around your snippet. Otherwise it is not clear how to find the right place without spending too much time.
Answer: I am also trying to change the background color of evince but have failed.
Probably another application qpdfview is useful to you. It can color all PDF documents with green background, including the scanned PDF documents, it is comforting your eyes.
Unfortunately, the edition installed from synaptic is not working to color the scanned PDF documents, but an edition installed with PPA is good for that.
Just do the following commands.
sudo apt-add-repository ppa:b-eltzner/qpdfview
sudo apt-get update
sudo apt-get install qpdfview

You need to change the color parameter.
And as regarding to  scanned PDF documents, you need to hit the button view-composition-darken with paper color, and it will be ok.
Comment: great!  continuous scroll, multi page, change bg-color, all in this one


In a microservice, is the transactional boundary the bounded context or the aggregate?

Question: May be the first question should be: what would help me decide whether my Microservice should contain an entire bounded context possibly with several aggregates, or should it rather contain only one aggregate?
If the Microservice contains several aggregates, then another question rises in my mind: shall the Microservice have several transactional boundaries (one per aggregate) or shall the Microservice have only one transaction boundary (the bounded context)?
Maybe, the answer to these questions is: it depends. But I'd like to have some rationale to make the right decisions.
I've tried to implement an experimental Microservice that implements a bounded context made of 3 aggregates and one transactional boundary per aggregate, and I've faced some pain points:

Eventual consistency within the Microservice itself
Atomic update of the aggregates and publication of domain events within the Microservice in a resilient way
Deciding whether all the domain events shall be published externally (to other Microservices) or only some of them
Deciding whether the domain events shall be published in the form of integration events in order to preserve the purity of the domain model

Then I've envisioned the possibility to split the Microservice into several Microservices, one per aggregate:

It gets more natural to publish all domain events (no clear need to make a difference between domain events and integration events)
The internal complexity of the Microservice is reduced

I've not managed to find some documentation on this subject to guide me in the decision making.
Edit:
Would it make sense to consider that the bounded context could be a set of Microservices working together as partners, and where each Microservice would encapsulate one and only one aggregate with its own scaling strategy?
Then, each Microservice would have one and only one reason to change: the aggregate.
Comment: If you need to use several aggregates (even belonging to the same bounded context or microservice) inside a transaction you are breaking DDD and microservices philisophy. This indicates wrong design and should not be done.
Answer: Of course the answer is always "it depends". In this case though, most of the time, you would want a microservice to contain everything that is necessary for a closed set of functionalities. Note I said "functionalities", as in business functions. Not technical functions, like saving a "Person" to a table, but business ones, like 'transfer money from one account to the other', 'reserve seat', things like that.
In your terms it would mean to contain one transactional boundary, and probably multiple "aggregates".
All of this is because of maintainability. When you modify a given functionality (or fix a bug, etc.) you will want that to be localized to a single microservice. Anything that requires coordination between microservices is usually much more difficult. You have to coordinate teams, coordinate rollout, etc., and that is exactly what microservices are supposed to be a solution for.
Comment: I do like the idea of using the bounded context as the transactional boundary of several aggregates when the Microservice encapsulates the bounded context. After all, if I need different scaling strategies for each of my aggregate, then I guess I'd rather put them apart in their own Microservice.
Answer: You can usualy (it depends, as allways) map one bounded context to one microservice.
A microservice can use several aggregates.
The transaction boundary is the operation that the aggregate performs. This operation can not be rejected/undo/rollback by anything outside the aggregate. If the aggregate say "YES" it is because ALL business rules and invariant are checked. Having to reject/undo/rollback an aggregate operation from outside the aggregate is a smell for wrong design.
HINT: Asides of purely wrong designs, fine grained operations used to be the solution of many potentialy "undo" encounters. A user does not "Pay" when push Pay button. Becasue you have to undo the registered "Pay" operation (the aggregate says that, as far as it knows, the user can pay) if the bank reject the operation. A user raises the "Pay" business operation that consist in several steps:
"Order a payment": register the order and raise UserOrderedPayment event if the aggregate says "OK" to that. After that, everything else is internal events/operations in your sistem.
UserOrderedPayment event launch the charge of money. If it fails, UserOrderPaymentRejected event is raised and the Payment Order is marked as rejected. If the money charges with no problem then a new Payment is registered and UserPaid event is raised.
About

Eventual consistency within the Microservice itself

With a fine grained design as above; there is no eventual consistency within the MS itself because every step is in a consistent status. The payment order could be still in pending status (but it is a valid and consistent status), OK, just wait a few milliseconds and refresh ;-).

Atomic update of the aggregates and publication of domain events
  within the Microservice in a resilient way

Again; with a fine grained design as above; the transacion boundary is just one aggregate so there is no "atomic update of the aggregates". As domain events (within the same MS or not) should not be rejected, they only fails by infrastructure downs (net, persistence, etc) so the best strategy is keep the failed events somewhere and raise them when the outage is over.

Deciding whether all the domain events shall be published externally
  (to other Microservices) or only some of them

You should invert the responsibility here. Always raise the domain events and provide a way to subscribe to that events. This way the resposibility lays on the interested microservice that has to subscribe to the events needed.

Deciding whether the domain events shall be published in the form of
  integration events in order to preserve the purity of the domain model

You only need raise the same domain events, you integration code can subscribe to these events and perform the integration action.
Answer: Assuming we follow the approach of one microservice for each BC, the transactional boundary is the microservice. Each application service method of the BC would be a transaction. An aggregate inside the BC define a group of domain objects that should change together in order to maintain invariants. An operation on an aggregate should be wrapped into a transaction. So each application service method should call just one aggregate operation.


FIR filter phase response

Question: With reference to the diagram below, is it correct to say that at the pass band the phase response is linear. It also remains linear in the stop band but it get chunks of π. These jumps happen when the magnitude response goes down to minus infinity dB (theoretically) ?
Answer: You are right. Even though the phase jumps at the zeros of the frequency response, such a phase response is usually still called "linear". For a frequency selective filter with frequency response zeros in the stopband, the phase always has discontinuities at the zeros. A purely linear phase response (without jumps) is only possible for filters with no zeros in their frequency response.
The frequency response of your filter can be written as
$$H(e^{j\omega})=|H(e^{j\omega})|e^{j\phi(\omega)}\tag{1}$$
where $\phi(\omega)$ is the phase response with jumps at the zeros, as shown in your figure. For such a linear phase system, the frequency response (1) can equivalently be written as
$$H(e^{j\omega})=A(e^{j\omega})e^{j\hat{\phi}(\omega)}\tag{2}$$
where $A(e^{j\omega})$ is a real-valued but bipolar (i.e. positive and possibly negative) function satisfying $|A(e^{j\omega})|=|H(e^{j\omega})|$. The phase $\hat{\phi}(\omega)$ in (2) is now a purely linear function without any jumps.
Comment: i have to dig it up, but i wrote my own `unwrap()` function in MATLAB to fix those jumps.  i would display phase along with **linear** magnitude (not dB) and the magnitude would not have the absolute value applied to the sign. $|H(e^{j\omega}|$ would be real, but potentially bipolar.  my `unwrap()` function also dealt with quadratically increasing phase to accommodate a linearly-swept chirp signal.

FIR filter phase response


Who are the companions of the elephant mentioned in Surah Al-Fil?

Question: Who are the companions of the elephant mentioned in Surah Al-Fil (The Elephant), what was their treacherous plan and how did it go astray?
Answer: The companions of the elephant refer to the army of Abraha, so-called because it included a number of war elephants.  He felt slighted by the Quraysh, and planned to exact revenge by destroying the kaaba.
He didn't succeed.
Comment: he felt because kaaba more popular than his site.
Answer: The Madain Project explains the companions or the people of the elephant in the following passage; I have added the link to the article as well.

The Companions of the Elephant (أصحاب الفيل) are a group of people whose story is recounted in the chapter of the Quran named "The Elephant
Companions of the Elephant are referred to in verse (105:1) of chapter (105) sūrat al-fīl (The Elephant): (أَلَمْ تَرَ كَيْفَ فَعَلَ رَبُّكَ بِأَصْحَابِ الْفِيلِ)
Translation: Have you not considered, [O Muhammad], how your Lord dealt with the companions of the elephant?"

Source: https://madainproject.com/companions_of_the_elephant


In relativistic electrodynamics , why doesn't the Lorentz force not change its formulation ? And only the electric and magnetic fields transform ?

Question: My question is that when we want to find the Lorentz force acted on a particle moving in an electric and magnetic field , the equation is invariant in any two inertial relativistic frames. Why is that so ? And only the electric and magnetic fields transform ?
Answer: The force f also transforms. In one frame you see f,E,v, and B.  If you boost yourself wrt this frame, you will see f ',E',v',and B'.  In the new frame the primed quantities satisfy the Lorentz force equation just like the unprimed quantities did in the original frame.  The covariant form of the Lorentz force equation makes obvious how everything transforms:
$$
\frac{dp^{\alpha}}{d\tau}=qF^{\alpha \beta}U_{\beta}
$$
The force on the charge q are the 3 spatial components of $\frac{dp^{\alpha}}{d\tau}$.   The EM field tensor $F^{\alpha \beta}$ has the components of E and B in it.  The 4-velocity $U^{\beta}=(\gamma,\gamma v_x, \gamma v_y, \gamma v_z)$.   Please see "Relativistic form of the Lorentz force" down the page in  https://en.wikipedia.org/wiki/Lorentz_force if you would like more details on all these covariant objects and how the vector form of the Lorentz force equation falls out.
Comment: Can you please suggest me a reference from where you studied about tensors and 4-vectors


WCF REST : returning classes and rules / Pascal/Camel case for fields?

Question: Can anyone tell me the recomemended case (pascal or camel) for returning classes with fields... For example, the example that comes with vs 2010 uses Pascal Case like so
<code>// TODO: Edit the SampleItem class
public class SampleItem
{
    public int Id { get; set; }
    public string StringValue { get; set; }
}
</code>
Notice first capital letter on Id and StringValue. I was wondering is this the recommended way? A lot of public services seem to return camelCase as fields.
I must admit it feels more natural with Pascal Case which follows the microsoft naming conventions for Properties etc.
Also the properties are going to be singular because its for 1 record i.e. Id, StringValue etc..  but what about the class name, i presume this will be singular name also as the XML that i return will make an array of SampleItem ??
I am just sort of looking for a bit of confirmation really.
The class i return will contain fields for my specific returned data, is there any fields i should be including by default.... I think not??  As if it fails i just return Error 400 so i don't need to supply any Error Number, Error Desc etc in each class
Any comments really appreciated
EDIT
here is an exmaple of the method i am using to return the xml ... its the default method in the standard vs 2010 template
<code>    [WebGet(UriTemplate = "")]
    public List<SampleItem> GetCollection()
    {
        // TODO: Replace the current implementation to return a collection of SampleItem instances
        return new List<SampleItem>() { new SampleItem() { Id = 1, StringValue = "Hello" } };
    }
</code>
Of course this method returns a LIST (only an exmaple) of SampleItem..
SampleItem is a class  and here it is
<code>// TODO: Edit the SampleItem class
public class SampleItem
{
    public int Id { get; set; }
    public string StringValue { get; set; }
}
</code>
Comment: What are you using to generate the XML?
Comment: I am using the standard template in vs 2010 to return the XML. I update my question with the example item
Comment: Ok question updated, but basically it returns a .NET class which of course is PascalCase :-)
Answer: The Microsoft Naming Conventions suggest PascalCasing for properties, and camelCasing for parameters. Therefore, your current casing is correct, if your goal is to match Microsoft's standards.
Comment: Yes, i know my standards are correct for properties and class names etc i just wasn't sure if i need to change these to match an XML output of camel casing... or it doesn't matter....
Comment: Well, like I said, if your goal is to make the XML output match Microsoft's names, then what you're currently doing (what's in the sample) is perfect. If you're defining the XML format, you get to say what it looks like.


aberration vs aberrance vs aberrancy

Question: All three nouns are derived from aberrant, the latter two are not used often I suppose considering that spell check considered them misspelt.
What are the difference between the three? Are they interchangeable?
Answer: Yes, you could probably get away with using them interchangeably.  However, as always, there are shades of meaning at play.
An "aberration" is something aberrant, like one penny in a jar of nickels.
"Aberrance" is the degree to which something is aberrant.  There would be more aberrance associated with a snail in a jar of nickels.
"Aberrancy" names the concept of being aberrant.  There's so much aberrancy in the world--a jar of nickels with a penny and a snail in it isn't really all that strange, considering.
Comment: I think OP is considering them all as nouns with the first definition, with the latter two taking similar forms to the noun "occurence" and "vacancy".
Answer: This question verges on being off-topic as general reference, given that you can find the answer in a standard dictionary. But making sense of the dictionary data does require some understanding of how dictionaries convey information about (formally) undefined terms, so I'll walk through the relevant material on aberration, aberrance, and aberrancy to illustrate how this works.
In Merriam-Webster's Eleventh Collegiate Dictionary (2003), you'll find aberrance and aberrancy listed in boldface type near the end of the entry for aberrant; aberration has its own entry. Here are the relevant entries (with the dictionary's pronunciations of the term omitted because I can't replicate some of the type characters used for them):

aberrant adj {L aberrant-, aberrans, prp. of aberrare to go astray, fr. ab- + errare to wander, err} (ca. 1780) 1 : straying from the right or normal way 2 : deviating from the usual or natural type : ATYPICAL — aberrance n — aberrancy n — aberrantly adv
aberration n {L aberrare} (1594) 1 : the fact or an instance of being aberrant esp. from a moral standard or normal state 2 : failure of a mirror, reflecting surface, or lens to produce exact point-to-point correspondence between an object and its image 3 : unsoundness or disorder of the mind 4 : a small periodic change of apparent position in celestial bodies due to the combined effect of the motion of light and the motion of the observer 5 : an aberrant individual

If we accept that the meanings of aberrance and aberrancy are somehow limited by the definitions of aberrant (within whose dictionary entry they appear), we can see that aberration has a number of meanings that they do not share. It follows that aberration is not synonymous with aberrance or aberrancy. But in what respect are the latter two terms limited by the entry for aberrant?
When a dictionary parks a related form of a word at the end of an entry for a word (as the Eleventh Collegiate does here with the nouns aberrance and aberrancy and the adverb aberrantly at the end of the entry for the adjective aberrant), it is indicating that the words thus treated have the meanings given for the main entry but converted to the part of speech that the appended word is classified under, mediated by the meaning or meanings of the relevant suffix.
The Eleventh Collegiate offers these entries for the suffix -ance and the suffix -ancy:

-ance n suffix {ME, fr. AF, fr. L -antia, fr. -ant-, -ans -ant + -ia -Y} 1 : action or process {furtherance} : instance of a quality or process {performance} 2 : quality or state : instance of a quality or state {protuberance} 3 : amount or degree {conductance}
-ancy n suffix {L -antia, more at -ANCE} : quality or state {piquancy}

On the face of it, the -ance suffix has more possible meanings than the suffix -ancy, but the range of meanings that a suffix has in a particular instance depends on the root word it is attached to. Theoretically (according to Merriam-Webster), aberrance might refer to an action or process, a quality or state, or an amount or degree of its root word, while aberrancy can refer only to the quality or state of its root word; but in actual usage, the two words are virtually interchangeable.
In any case, whether you reserve the right to use aberrance in certain situations where the Eleventh Collegiate doesn't endorse using aberrancy (such as to connote the process of doing something aberrant), the two nouns share these meanings: "1 : the quality or state of straying from the right or normal way 2 : the quality or state of deviating from the usual or natural type : being atypical."
I doubt that many people consistently maintain a distinction between aberrance and aberrancy; most people either use them interchangeably or use one to the exclusion of the other. In contrast, though the first definition of aberration is quite similar to the sense of those two words, aberration has other meanings that they do not.

aberration vs aberrance vs aberrancy


FX volatility at weekends

Question: Forex markets are closed at weekends. So if a major event happens on a Saturday which impacts exchange rates significantly, is there a way to quantify how much before waiting until Monday?
Answer: The Forex market opens at 5 PM U.S. EST on Sunday night and the futures market opens at 6 PM EST on Sunday. Asia financial markets in general open about 8 PM EST Sunday night but there is pre-market activity.
The markets could open drastically beyond stop-loss orders.
Answer: As S Spring mentioned, Forex markets does have limited hours on weekends, if you want to see market activity before Monday.
Other than that, no, you cannot accurately anticipate how prices will react to news when markets are closed. The market price reflects the combined activity of all market participants, and that activity hasn't happened yet.
While individual traders can determine that their own fair price for 1 CAD is now lower because of news X, you can't easily guess how everyone's opinions will be factored in the Monday price. I bet some hedge funds would be very interested if you developed a system to accurately do this.

FX volatility at weekends


Polishing PacBio or ONT with Illumina

Question: Which tool would be good to polish PacBio or ONT with Illumina?
Thank you in advance
Comment: Do you mean to correct PacBio/ONT reads or polish their assembly?
Comment: Metagenomic or single genome?
Answer: PacBio and ONT are very different technologies; it would be helpful to be more specific. Regardless, I can only give my understanding as it relates to ONT reads....
TLDR: use medaka. Don't use Illumina reads.
Nanopore reads called using the most recent super-accuracy basecallers have a very high consensus accuracy (q40-q50) for SNPs and non-homopolymeric INDELs after nanopore-only correction. ONT provide their own tool, medaka, which uses assembly-mapped nanopore reads to correct the assembly. Medaka uses the base calling model from Guppy to predict the most likely sequence, given the input reads and the assembly.
If you're using Illumina reads to polish a nanopore assembly, it should only be used to correct INDEL errors (and ideally, only homopolymer errors). The two most commonly-used tools that can correct nanopore reads using Illumina reads are Racon and Pilon. Based on the recency of the commit histories, I'd guess that Racon would be the most likely to work well. It looks like Racon can do both assembly correction and read correction.
Answer: Why don't you use a hybrid assembly which pretty much does it by default? Tools like OPERA-MS assemble the short reads with MEGAHIT and then uses nanopore to build bigger contgs. Otherwise, I recommend pilon which probably the most popular tool.
Answer: For PacBio HiFi reads (or other techs of comparable quality), it is not recommended to polish with Illumina at all. You run the risk of collapsing true heterozygosity/variation in your underlying genome and yielding a haploid assembly that does not represent either haplotype.
If that underlying variation is an issue because you need a haploid assembly, you are free to use a deduplication technique such as purge_haplotigs or similar (reviewed here) to obtain a primary set of contigs/scaffolds. Assigning p and q haplotypes is something that you can do with a tool like falcon-phase (if you have Hi-C) or a similar technique (I haven't been following the literature for a couple years, possibly there's something a lot better now!).
For a recent review of recommended strategies you can read the T2T consortium benchmarking paper on the topic.

Polishing PacBio or ONT with Illumina


Profiling option for application developed using PEGA

Question: I am from Performance Engineering background, we have a workflow application developed using PEGA.Is it possible to Profile that application using JProfiler or is there any tool available exclusively to Profile the applications developed using PEGA
Comment: You can use Pega application profiler.
Answer: A long as it's running on the JVM, you can profile it with JProfiler.
In the JProfiler GUI, invoke
Session->Integration Wizards->New Remote Integration
and follow the steps in the wizard. Basically, you will have to insert a VM parameter that enabled profiling.
Alternatively, you can use Session->Quick Attach to profile any running JVM on your system.


How is a wall boundary implemented (using ghost cells) in a simple 2d euler flow solver?

Question: I understand that you must reflect the velocity of the cell across the wall and store that reflected velocity in the ghost cell (which will then be used for flux/residual calculations), but that is the extent of my understanding. I have tried implementing this in my euler flow solver but I am still seeming to get issues which makes me think there must be more to implementing a wall boundary condition.
Comment: I guess a wall means here that there is no flow through this boundary. Then the BC must enforce zero flow velocity normal to the boundary.
Answer: Specifying velocity boundary conditions isn't enough. If you are indeed solving the 2D Euler equations, you will also need to specify boundary conditions for density and temperature (if you are using primitive variables which I assume you are). Maxim is correct that you need to enforce zero velocity normal to the wall in addition to the no slip condition. For the mass denisty equation, you could use $\rho_{ghost} = \rho_0$ where 0 is the index of the interior cell on the wall boundary.
The wall BC for the energy equation needs a little more information. Is your wall adiabatic? If so, the ghost cell implentation is similar to the one above for the mass density. Furthermore, you could choose the value of the temperature in the ghost cell to represent a prescribed heat flux through the wall. If the temperature of the wall is to be prescribed by the user, then:
$$T_{ghost}=2T_w -T_0$$
This is all valid if you are using one layer of ghost cells, but can easily be extended to multiple layers for higher order solvers.


Unhappy Problem with Price Rules magento 1.8 and 1.9

Question:
When I cancel and create a new price rules with the same parameters, it happens sometimes after 24h a problem once the problem occurs only after three days. The problem I have at the other two sites just as it comes magento ver. 1.8.
Answer: You have to enable the cron in magento for the price rules to reflect every day, its how the system works. Else if you enable the catalog price rules but it will last only for a day and the next day it will disappear.
You can search google for how to enable cron jobs in magento. Good starting point here http://www.magentocommerce.com/wiki/1_-_installation_and_configuration/how_to_setup_a_cron_job. This task is mostly an operations task depending on the hosting provider you are using you can set up the cron job easily. I hope this answers your query
Comment: Thank you very much Vivek Gounder. I did Setup Magento Cron Jobs in cPanel. In a few days I will see the result.


Links in /usr/bin or /usr/local/bin: hard or symbolic?

Question: For the links located in /usr/bin and in /usr/local/bin, are they better to be hard or symbolic? They seem to be all symbolic? Why?
Comment: Why not? Also: 1. While dangling links might be a concern, at least the app is deleted when the source of the link is deleted, unlike with hardlinks. 2. Links across filesystems.
Comment: Symlinks are generally more flexible and useful things than hard links.   I'm not sure if I see much purpose to hard links at all, TBH.
Answer: When links are put in /usr/bin or /usr/local/bin, this is often because the actual binary is living somewhere else. Why is it living somewhere else? Usually because it is a part of a group of files, often in its own subdirectory, which it depends on to run. Why can't all those files be dumped into /usr/bin or /usr/local/bin? Because those locations are only for binaries, the binaries live at the top level of those directories, and no subdirectories are allowed.
So, why don't we use a hard link there? Because a hard link to a file is considered to be of equal status to the original file (it shares the same inode) and the file will not be deleted while there is a hard link to it. In contrast, a symbolic link is simply a pointer to "the real thing". In practice, putting a hard link to a binary file that is living somewhere else wouldn't make much sense, because if those other files in that other location/subdirectory were removed, the file could not function by itself.
An example of this are the various TeX utilities, which live in /usr/bin. If you look at them, you will see that many of them point to files that are actually in /usr/share/texlive/texmf-dist/scripts/. You can see these, for example, by doing ls -la | grep texmf.
As muru mentions, another reason is that hard links don't work across filesystems.
Another situation in which links are used for files in /usr/bin or /usr/local/bin is Debian's alternatives system. In this case, a binary may be a symbolic link pointing to a symbolic link in /etc/alternatives, which then points to the actual binary, often again itself in /usr/bin.
Example:
ls -la /usr/bin/awk
lrwxrwxrwx 1 root root 21 Jul 31  2013 /usr/bin/awk -> /etc/alternatives/awk
ls -la /etc/alternatives/awk
lrwxrwxrwx 1 root root 13 Aug 11  2013 /etc/alternatives/awk -> /usr/bin/gawk

It is fairly obvious in this case that a symbolic link is the correct mechanism, because it is pointing to any of a range of possible binaries.
Comment: While the rest of your argumentation is valid and makes complete sense to me, the following statement is not true: `Also, the hard link might not work in any case, because it wouldn't follow the actual link to its "actual" location (untested).` The binary wouldn't know its location and relative paths would always be in respect to the current working directory, not the directory of the binary (or its hardlink).
Comment: @inVader Ok, I've taken out that bit. It was probably not a good idea to put it in without actually testing the statement.
Comment: @inVader just to explicate what I had in mind - suppose you have a binary `/usr/bin/foo`, which is actually a symlink to `/usr/share/foolib/foo`. Now, the latter file might depend on other files in `/usr/share/foolib`, via say relative paths. If it did, then if `/usr/bin/foo` was a hardlink as opposed to a symlink, it would break, because the hardlink `/usr/bin/foo` would be looking for those files in the wrong place. But off the top of my head I don't have an example of this.
Comment: But if I call /usr/share/foolib/foo from my home directory, than all relative paths are relative to my home dir not to the directory the binary is in. The binary would have to know that it and its depending files are supposed to be in /usr/share/foolib, at which point it should no longer matter whether we have a hard or a symbolic link. I might be wrong, but I really do not see a way in which the binary would actually figure out its own location, then expect files to be in the same directory.


Exp calculation in HearthStone

Question: How is experience calculated in HearthStone? I understand you get more exp for playing against people, but i can't find how to work out the exp gained.
Comment: I'm talking about in general, not specific modes.
Answer: To my knowledge, there is not a definitive answer. However, we do know (from testing and from developer statements) that experience per match is influenced by the following:

Number of cards played
Number of minions killed
Playing against a human seems to reward a modifier to these

A game in which one player concedes without playing a single card or killing a minion, even against a human, rewards very little to no exp.
Some wonderful people over at reddit did some thorough investigation and seem to have reverse-engineered a somewhat reasonable approximation, which can be read more at What determines how much experience I get from a hearthstone match in play mode?
References: http://hearthstone.gamepedia.com/Experience#Experience
https://twitter.com/bdbrode/status/511582329874038784
http://www.reddit.com/r/hearthstone/comments/1nrxww/how_is_experience_gain_caluclated_in_hearthstone/cclklx4

Exp calculation in HearthStone


Recommendation letter combination for PhD

Question: I am currently pursuing master degree in Germany and is applying to US PhDs for 2023Fall. I have heard it for long that for PhD applications letters from professors in class are not of much use. But my situation is that I have done several projects(with decent outcome) under only one professor, and I have no other professors to write letter for me in terms of research potential. Will I be in disadvantageous situation as many people have research experience in multiple groups and multiple letters talking about research?
Comment: @Buffy Sorry I forgot to specify. I am currently pursuing master in Germany but trying to apply to US PhDs
Answer: For US applications I wouldn't worry too much about this. There are a lot of reasons why a professor with whom you'd only taken classes would be able to confidently predict your success. And several projects under your belt is a positive indicator in itself.
But, you are where you are and you use what you have. Don't ask for letters from those you aren't sure about, of course, but one very strong letter and a couple of others that are also strong but focused on more general things than research should be fine. Did you ask a lot of questions? Did you express a lot of interest, generally and/or specifically? Did you stand out in some (good) ways. Those sorts of things make you memorable and increase the likelihood of a helpful letter. Along with good marks, of course.
Note that admissions decisions are broad based in US, though letters are very important.

See: How does the admissions process work for Ph.D. programs in Country X?


How to Search contacts using wildcard characters(*,%,?) in salesforce

Question: Please give me answer for "How to Search(box) list of contacts using wildcard characters(*,%,?)  in salesforce "
Comment: Are you asking how to do wildcard searches in the Global Search? If so, none of the tags you've specified apply to the question.
Answer: If you are asking in standard salesforce functionality here is the answer :
1. * (Asterisks) match zero or more characters at the middle or end (not the beginning) of your search term. For example, a search for john* finds items that start with john, such as, john, johnson, or johnny. A search for mi* meyers finds items with mike meyers or michael meyers. If you're using sidebar search, an * is automatically appended to the end of the search string. You can use an * at the beginning of a search term in a standard lookup search.
2.? (question mark) Question marks match only one character in the middle or end (not the beginning) of your search term. For example, a search for jo?n finds items with the term john or joan but not jon or johan. If you're using global search, the ? is removed from the end of your search term and isn't treated as a wildcard. You can't use a ? in a lookup search.
Limitations
Wildcards take on the type of the preceding character. For example, aa*a matches aaaa and aabcda, but not aa2a or aa.!//a, and p?n matches pin and pan, but not p1n or p!n. Likewise, 1?3 matches 123 and 143, but not 1a3 or 1b3.
You can't search for a ? or * in a search phrase that is enclosed in quotation marks or when Exact phrase is selected in the search scope because they function as wildcards. For example, "my wo?d" matches my wood and my word.
A wildcard (*) is appended at the end of single characters in Chinese, Japanese, Korean, and Thai (CJKT) searches, except in exact phrase searches.
If you're entering search terms in global search, the search box drop-down list gives you the option to search for your term with an * added to the end. For example, type jo in the search box, then select Search for jo* (starts with) to find joan, john, johnson, and other matches that start with jo.
Answer: The documentation that Salesforce provides is very good, take a look.
https://help.salesforce.com/apex/HTViewHelpDoc?id=search_unified_wildcards.htm&language=en
Comment: Yep, Salesforce documentation is a really good read... As is the Google:
http://www.google.com/search?q=salesforce+search+contact+wildcards

What fascinates me is that increasingly more and more people are LAZY to google for pretty basic Q&As, documentation of basic functionalities etc. Maybe I'm just too cranky today, but I guess the questions should reflect minimum of own effort put into the resolution of the problem.

PS. @Michael Welburn - not aimed at you, but at the original poster
Comment: Hi @Michael!
I'm unable to find the documentation at the link you provided. It seems the document ids taken off by salesforce. Can you help me if you have a copy of the same with you. Or if you are aware of the new link. I was unable to find it in SFDC documentation library.


Cobalt chemosynthesis?

Question: I want to create a world where everything revolves around cobalt and I was wondering if there are autotrophs that receive energy due to chemosynthesis involving cobalt compounds, is it possible?
Comment: @Community - Is that you, Bot?  This is a straight up question of fact in the service of worldbuilding: cobalt containing biomolecules and energy metabolism.  Lets see...
Comment: Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking.
Answer: I can't find any extant examples, cobalt rich environments are pretty rare on Earth these days so that doesn't surprise me. It should be possible for a pathway from CoO (Co+2) to CoO(OH)3 (Co+3) to be used in the same way that Iron Oxidising Bacteria use the Fe+2->Fe+3 conversion for metabolic energy but I'm not sure of the exact chemistry and energy of that exchange so it may not be sufficiently efficient.

Cobalt chemosynthesis?