As we know, any proposition can translate to Godel numbers, such as  $$( \exists x)\space (x=sy)$$Its Godel numbers is $2^8 × 3 ^4 × 5^ {13} × 7^ 9 × 11^8 × 13^{13} × 17^ 5 × 19^ 7 × 23^ {17} × 29^ 9=a \space big \space number $  We also could get the character sequence "$( \exists x)\space (x=sy)$" through prime factorization of 243,000,000.  Such as$$243,000,000=2^6×3^5×5^6 $$ $$6\space\space\space\space\space 5\space\space\space\space\space 6 $$ $$\downarrow\space\space\downarrow\space\space\downarrow$$ $$0\space\space\space=\space\space\space 0 $$ So the Godel numbers of "$0=0$" is $243000000$  There are four axioms in Godel's PM system, here are two of them: $$(p\lor p)\subset p$$ $$p\subset (p\lor q)$$ through these two axioms, we can prove "$(p\lor p)\subset(p\lor q)$" is a true theorem.   And these two axioms and a theorem all have a corresponding Godel numbers, can we prove this theorem is true through Godel numbers? If it works, how?  Many people think it is a question unrelated with Mathematica, but what I think is that if Gödel numbers could make a proof, then we can build an Association between characters and numbers in Mathematica, and add some rules, then we can prove propositions. But I’m not sure Gödel numbers can make a mathematical proof.
As we know, any proposition can translate to Godel numbers, such as  $$( \exists x)\space (x=sy)$$Its Godel numbers is $2^8 × 3 ^4 × 5^ {13} × 7^ 9 × 11^8 × 13^{13} × 17^ 5 × 19^ 7 × 23^ {17} × 29^ 9=a \space big \space number $  We also could get the character sequence "$( \exists x)\space (x=sy)$" through prime factorization of this big number.  Such as$$243,000,000=2^6×3^5×5^6 $$ $$6\space\space\space\space\space 5\space\space\space\space\space 6 $$ $$\downarrow\space\space\downarrow\space\space\downarrow$$ $$0\space\space\space=\space\space\space 0 $$ So $243000000$ is the Godel numbers of "$0=0$"   There are four axioms in Godel's PM system, here are two of them: $$(p\lor p)\subset p$$ $$p\subset (p\lor q)$$ through these two axioms, we can prove "$(p\lor p)\subset(p\lor q)$" is a true theorem.   And these two axioms and a theorem all have a corresponding Godel numbers, can we prove this theorem is true through Godel numbers? If it works, how?
Why doesn't the frequency of light change during refraction? When light passes from one medium to another its velocity and wavelength change. Why doesn't frequency change in this phenomenon? When estimating variance, why do unbiased estimators divide by n-1 yet maximum likelihood estimates divide by n? I am totally confused: On the one hand you can read all kinds of explanations why you have to divide by n-1 to get an unbiased estimator for the (unknown) population variance (degrees of freedom, not defined for sample size 1 etc.) - see e.g.  or .  On the other hand when it comes to variance estimation of a supposed normal distribution all of this doesn't seem to be true anymore. There it is said that the maximum likelihood estimator for variance includes only a division by n - see e.g. .  Now, can anyone please enlighten me why it is true here but not there? I mean normality is what most models boil down to (not least due to the ). So is the choice "division by n" yet the best choice for finding the best estimation for the true population variance after all?!? How can one derive Schrödinger equation? Total combinations from number of unique combination? How can you find count of the total combinations possible if you have the count of unique combinations? Suppose I need to form a sum of $3$ using $\{1,2\}$ Unique combn. $(1+1+1)$, $(1+2)$ hence '$2$' unique but total is '$3$' i.e, $(1+1+1)$, $(1+2)$, $(2+1)$ I know the elements used to make the combination Note: Number of unique combinations (ie. 2 here) is known. How can I find the total number of combination given the unique combinations? Merging two incoming broadband lines for faster Internet We have a broadband line coming into our building which delivers around 8-10Mbps, which is okay but not great. Fiber is available in our area (30-100Mbps) but it's not available where we are just yet.   Is there some way we could connect two lines (which we would take from two different providers so we would have redundancy if one failed) and merge them into one signal which we could distribute?  Is there some sort of setup that can do this?  Of course we have the option of a leased line but for 12Mbps symmetrical upload/download we'd be looking at around £500 per month compared to £15 per month for normal or fiber broadband.  I made a small pseudo flow chat of the setup below: Consider square matrices over a field $K$. I don't think additional assumptions about $K$ like algebraically closed or characteristic $0$ are pertinent, but feel free to make them for comfort. For any such matrix $A$, the set $K[A]$ of polynomials in $A$ is a commutative subalgebra of $M_n(K)$; the question is whether for any pair of commuting matrices $X,Y$ at least one such commutative subalgebra can be found that contains both $X$ and $Y$.  I was asking myself this in connection with frequently recurring requests to completely characterise commuting pairs of matrices, like . While providing a useful characterisation seems impossible, a positive anwer to the current question would at least provide some answer.  Note that in many rather likely situations one can in fact take $A$ to be one of the matrices $X,Y$, for instance when one of the matrices , or more generally if its  (so coincides with the characteristic polynomial). However this is not always possible, as can be easily seen for instance for diagonal matrices $X=\operatorname{diag}(0,0,1)$ and $Y=\operatorname{diag}(0,1,1)$. However in that case both will be polynomials in $A=\operatorname{diag}(x,y,z)$ for any distinct values $x,y,z$ (then $K[A]$ consists of all diagonal matrices); although in the example in  the matrices are not both diagonalisable, an appropriate $A$ can be found there as well.   I thought for some time that any maximal commutative subalgebra of $M_n(K)$ was of the form $K[A]$ (which would imply a positive answer) for some $A$ with minimal polynomial of degree$~n$, and that a positive answer to my question was in fact instrumental in proving this. However I was wrong on both counts: there exist (for $n\geq 4$) commutative subalgebras of dimension${}&gt;n$ (whereas $\dim_KK[A]\leq n$ for all $A\in M_n(K)$) as shown in , and I was forced to correct  in the light of this; however it seems (at least in the cases I looked at) that many (all?) pairs of matrices $X,Y$ in such a subalgebra still admit a matrix $A$ (which in general is not in the subalgebra) such that $X,Y\in K[A]$. This indicates that a positive answer to my question would not contradict the existence of such large commutative subalgebras: it would just mean that to obtain a maximal dimensional subalgebra containing $X,Y$ one should in general avoid throwing in an $A$ with $X,Y\in K[A]$. I do think these large subalgebras easily show that my question but for three commuting matrices has a negative answer.  Finally I note that  to the cited MO question mentions a result by Gerstenhaber that the dimension of the subalgebra generated two commuting matrices in $M_n(K)$ cannot exceed$~n$. This unfortunately does not settle my question (if $X,Y$ would generate a subalgebra of dimension${}&gt;n$, it would have proved a negative answer); it just might be that the mentioned result is true because of the existence of $A$ (I don't have access to a proof right now, but given the formulation it seems unlikely that it was done this way).  OK, I've tried to build up the suspense. Honesty demands that I say that I do know the answer to my question, since a colleague of mine provided a convincing one. I will however not give this answer right away, but post it once there has been some time for gathering answers here; who knows somebody will prove a different answer than the one I have (heaven forbid), or at least give the same answer with a different justification. Why does (insert favorite search engine here) get better search results than SO, SF, or SU's own search function? I've lost count of the number of times I've either asked a question or been answering a question only to have it closed as an exact duplicate. But searching for the question turns up  a list of seemly unrelated results using the sites builtin search. Then if I switch to a general web search engine and limit its scope to just SO, SF or SU I find the question within the first two or three results.  Why is this?  I'm pretty sure this isn't an exact duplicate of another question though I did find some that were at least somewhat related (using google).  Update:  I also have gotten (somewhat) better results by simply typing a question and browsing the generated list of related questions. Still a search feature that doesn't return good results isn't going to be used by most people.  Common sense would suggest (though common sense isn't always the best indicator) that a targeted solution to a specific problem domain should be superior to a generalized solution. In most cases this appears to hold true. How to fully backup non-rooted devices? I know I'm asking for the close-to-impossible -- as a real full-backup certainly requires root permissions. But still there are many folks around not wanting to root their devices -- afraid of voiding their warranties or bricking their devices, or just not having their devices supported by any rooting method.  I also know there were a lot of questions asked already on this topic, and I checked all of them having the backup tag -- none covers the entire topic, all just have pieces, and especially people new to Android (but other non-power-users as well) would have a hard time figuring out the best way.  So I want a detailed canonical answer to the question:  Not involving root-solutions, how to achieve a backup as close as possible to a full backup?  Please do not give simple one-liner answers like "Use app xyz [full-stop]." If mentioning an app (and I doubt a single app would be the answer here), include what parts are covered by it -- as well as ideas on how to fill the gaps it leaves. (And to make it absolutely clear: I know the first thing coming to mind is Titanium Backup -- I use it myself, but this does not apply here: This question is strictly about methods for non-rooted devices!) Moreover, the answer should be as generic as possible in terms of applying to a wide range of devices (i.e. it should not be restricted to one device or manufacturer).  I already mentioned having investigated all questions here tagged backup. These will certainly be helpful providing details for answers here, so I will list up the topmost ones concerning my question:            All these provide some details for my question (so feel free to get inspired by them) -- but I feel there are still details missing. Also it would be helpful to have a summary -- say, an answer including the full description of "the maximum possible".  A last criterium: While I'd consider cloud-based solutions legit, I'd favour local solutions. If you can provide both: all the better. But some people value privacy quite high (I'm one of them), and thus would not trust their data to some cloud service.    Thanks to Ryan, we now have a solution for a really complete backup of all apps and their data -- which I didn't dare to hope for! The only disadvantage: His solution only works for less than 10% of all Android users; namely those whose devices are running at least with Android 4.x.  So again, I ask the close-to-impossible: Any solutions applicable at least on Gingerbread (2.3.x) or, better, even Froyo (2.2.x) -- so they cover the majority of current devices? These solutions may involve multiple tools (try to keep it as simple as possible though; think of your mother to follow your instructions ;)  And keep in mind: NO ROOT!    To further motivate high-quality answers (and hopefully avoid low-quality ones), I will be a harsh one here: As I rarely do, I will down-vote answers not fitting the mentioned criteria on one end -- but on the other end, I will setup a bounty for the best answer (which cannot be done immediately, but earliest after 2 days). Let K={(1),(12)(34),(13)(24),(14)(23)} show that K is a normal subgroup of the alternating group of degree A4 and find all the members of A4/K and write down the group table for A4/K.   i know that K is the Klein four group and i have already proven it is a normal subgroup but i need a start on approaching the A4/K part of this question no direct answers are necessary,just ideas thank you! :) Remove pseudo nodes in QGIS Notation problem in integration: write $dx$ or ${\mathrm{d}}x$? Why teach linear algebra before abstract algebra? Is there a reason why most undergraduate curriculums put linear algebra before abstract algebra?  I'm asking this because personally it seems to be much easier to understand the architecture behind linear algebra as supposed to simply solving problems after a course in abstract algebra. Decomposition of semisimple Lie algebra via its roots What do the different colors mean in ls? What is $\gcd(0,a)$, where $a$ is a positive integer? I have tried $\gcd(0,8)$ in a lot of online  gcd (or hcf) calculators, but some say $\gcd(0,8)=0$, some other gives $\gcd(0,8)=8$ and some others give $\gcd(0,8)=1$. So really which one of these is correct and why there are different conventions? Why does Benford's Law (or Zipf's Law) hold? Both  (if you take a list of values, the distribution of the most significant digit is rougly proportional to the logarithm of the digit) and  (given a corpus of natural language utterances, the frequency of any word is roughly inversely proportional to its rank in the frequency table) are not theorems in a mathematical sense, but they work quite good in the real life.   Does anyone have an idea why this happens?  (see also ) Why is a normal subgroup of $G_1\times G_2$ with trivial intersections with $G_1$ and $G_2$ is abelian? Let $G=G_1\times G_2$ be a direct product, and let $H\triangleleft G$ be a normal subgroup such that $H\cap G_1=H\cap G_2=\{1\}.$ Then $H$ is abelian.     I considered the commutators of two elements in $H$, trying to show that it must be $1,$ but I don't see how this works. Any hint is very welcomed and appreciated. Thanks in advance. Does a fighter's Action Surge trigger Extra Attack? Context  A player in my game will play a fighter with a greatsword. At 5th level he has Extra Attack and the option to use Action Surge. PHB p.72 states:     Action Surge. Starting at 2nd level, you can push yourself beyond your normal limits for a moment. On your turn, you can take one   additional action on top of your regular action and a possible bonus   action.      Once you use this feature, you must finish a short or long rest before   you can use it again. Starting at 17th level, you can use it twice   before a rest, but only once on the same turn.        Extra Attack. Beginning at 5th level, you can attack twice, instead of once,   whenever you take the Attack action on your turn.       The number of attacks increases to three when you reach 11th level in   this class and to four when you reach 20th level in this class.   Confusion  The questions and answers that I find on here deal with multi-classing or Two Weapon fighting specifically, which don't really give a clear yes or no on my question.  The way I read these two features is that when a fighter takes an Attack action, he gets the Extra Attack. After using Action Surge he gets the opportunity to take another Attack action, and with that another Extra Attack. This results in four attacks with the greatsword in one turn, which sounds amazing but also rather powerful. Is this correct, or am I missing something? What happens to waves when they hit smaller apertures than their wavelenghts? I was wondering this for quite a long time now. Let's say you have a water wave (like ripples, not the ones you see during tsunamis) with wavelength 10 m. Imagine you put a boundary with an opening of 1 m. Will diffraction be observed? According to my research, no. But, then, what would be seen? Freestyle Background color not white? I'm following  and I've set my compositor up in the exact same way as he did, except my background is like a slightly light grey and I don't know why.  In his video's he's getting a bright white as his background and I can't see where I'm going wrong.  Here's the render:   Here's the compositor window:    file.  Please help! I really want to know why it's not happening.