Why Interfaces in Java if we have abstract class?
Interface vs Abstract Class (general OO)
In The Last Jedi:     The Resistance fleet is tracked through hyperspace by the First Order after they make their jump away from their old base.   As soon as the FO ships appear, the Resistance immediately come to the (correct) conclusion as to the tech that was used.  Why would they assume the use of this tech, that they have never encountered before, as opposed to the more common technique of     tracking a ship through the use of a homing beacon. Is the composition of two injective functions injective? I'm struggling a bit with functions(limits, squeeze theorem, etc).  I have done some research and found a list of books on calculus but I'm not sure which one would be better suited for me, so I would appreciate if some experienced people here could advise me which books would be better to master the basics so I can move on with derivatives, etc. Limits is my main problem, so I need a book which explain this in depth so I can master it. Then I'll keep going on with the same book for other chapters.  Below is the list which I found :   Apostol T.M. Calculus Vol. 1 Burn R.P. Numbers and Functions: Steps into Analysis  Courant R. and John F. Introduction to Calculus and Analysis I - One of the better calculus text in print.  Crowell B. Calculus  Dawkins, P Calculus I and Calculus II  Garrett P. Calculus  Ghorpade S.R. and Limaye B.V. A Course in Calculus and Real Analysis  Gill G.S. Calculus Bible  Guichard D. Whitman Calculu.  Hardy G.H. A Course of Pure Mathematics  Hwang A.D. Calculus for Mathematicians, Computer Scientists, and Physicists: An Introduction to Abstract Mathematics  Santos D. Differential Calculus  Shapiro B.E. Calculus and Analysis  Spivak M. Calculus  Strang G. Calculus  Thomas G.B. and Finney R.L. Calculus and Analytic Geometry  Tranter C.J. Advanced Level Pure Mathematics  Tranter C.J. Techniques of Mathematical Analysis  Veeh J.A. Lectures Notes on Calculus  So far I narrowed my list down to Hardy, Spivak and Apostol based on further research, but I haven't got my hands on these books yet to compare them and I'm not sure if these books will be good for me.  Thanks. Do Americans remove the "t" in "wanted"? When you pronounce the phrase "I got a message you wanted to see me", is the "t" usually deleted in the word "wanted"? Thank you. What makes frictional force the centripetal force of a car turning along a curve? As friction is the opposing force and acts anti-parallel so there is no component of frictional force towards the center,right? Then how can frictional force be centripetal force? Where to securely store API keys and passwords in WordPress? I'm looking to use a few APIs and many come with keys, secret keys and passwords required to work. Where in WordPress can you store that information? Assuming anyone can hack your DB is there anyway for WordPress to make saving that information more secure? Also, consider the ability to change these keys ever so often so I would need to update the keys on an options page.  UPDATE Is every sufficiently large positive integer of the form $ab + ac + bc + 1$? Is every sufficiently large positive integer $A$ of the form $ab + ac + bc + 1$   where $a,b,c$ are some positive integers larger than some given positive integer $d$ ?  How large is sufficiently large , in other words how do we know our number $A$ is sufficiently large ( as a function of $d$ ) ?    Update :  I read the following about positive integers that are not of the form $ ab + ac + bc $:    And I quote about that sequence above : " Note that for n in this sequence, n+1 is either a prime, twice a prime, the square of a prime, 8 or 16 "  This is why I considered the weird +1 in $ab + ac + bc + 1$  So composites larger than 16 not a prime , twice a prime or the square of a prime are of the form $ab + ac + bc + 1$.  And I wonder why !  I guess that relates strongly to the Original posted problem. Maybe I should have mentioned that before , sorry.  Will Jagy's answer gave the same OEIS link which I already knew. This also explains some of the comments.  Im no expert in genus theory or Galois theory but I assume this has a simple answer. On  the delete notice is above, instead of below the close notice. Why is this happening? I understand this was made on purpose, but would like to know the reasoning. SEO preference: does a site with WWW rank better than without WWW? Bug Identification: Tiny Bugs sed -i - edit files in place does not work in compound commandq Why does an electron shell further away from nucleus has higher energy level? Using electrical potential energy $V=\frac{1}{4\pi \varepsilon_0} \frac{Q_1 Q_2}{r}$ , a particle further away from nucleus has lower magnitude of energy.  Using Coulomb's law, a particle further away from nucleus experiences weaker attraction, hence less energy is needed to maintain orbit$^\star$ around that e-shell compared to a electron shell closer to nucleus, hence the one closer to nucleus supposedly should have higher energy.  $^\star$I know in reality $e^-$ does not orbit around a atom, but its position exists as a probability density or radial probability function. How do you detect a network disconnection when downloading a file in java? Apparently ;-) the standard containers provide some form of guarantees.  What type of guarantees and what exactly are the differences between the different types of container?  Working from  (about ) I have come up with this:  Container Types: ================ Container:     Forward Container         Reverse Container             Random Access Container     Sequence         Front Insert Sequence         Back  Insert Sequence     Associative Container         Simple   Associative Container         Pair     Associative Container         Sorted   Associative Container         Multiple Associative Container  Container Types mapped to Standard Containers =============================================  std::vector:    Sequence    Back        Sequence                    Forward/Reverse/Random Container std::deque:     Sequence    Front/Back  Sequence                    Forward/Reverse/Random Container std::list:      Sequence    Front/Back  Sequence                    Forward/Reverse Container std::set:       Sorted/Simple/Unique    Associative Container       Forward Container std::map:       Sorted/Pair/Unique      Associative Container       Forward Container std::multiset:  Sorted/Simple/Multiple  Associative Container       Forward Container std::multimap:  Sorted/Pair/Multiple    Associative Container       Forward Container   Container Guarantees: =====================                                                                                    Simp                                                                                   or                           For   Rev  Rand        Front  Back  Assoc        Sort   Mult                     Cont: Cont: Cont Cont: Sequ: Sequ:  Sequ: Cont:        Cont:  Cont: Copy    Const:      O(n) Fill    Const:                             O(n) begin()             O(1) end()               O(1) rbegin()                        O(1) rend()                          O(1) front()                                    O(1) push_front()                                     O(1) pop_front()                                      O(1) push_back()                                             O(1) pop_back()                                              O(1) Insert()                                                                          O(ln(n)) Insert: fill                               O(n) Insert: range                              O(n)                                   O(kln(n)+n) size()              O(n) swap()              O(1) erase key                                                     O(ln(n)) erase element                                                 O(1) erase range                                                   O(ln(n)+S) count()                                                       O(log(n)+k) find()                                                        O(ln(n)) equal range                                                   O(ln(n)) Lower Bound/Upper Bound                                                    O(ln(n)) Equality                  O(n) InEquality                O(n) Element Access                       O(1) Can we use sine and cosine graphs for varying temperatures? I have been researching for a while looking for what to include more than just deriving the equation of Newton’s law of cooling and a website gave me inspiration by stating what if the temperature of the environment is changing (± C). In conclusion, I wasn’t sure if that person carried out the mathematical process in the right way and the only place I could ask for accurate information is here so please let me know what you think about the mathematical process he went for and if it is right or wrong. Thanks a lot. Website: I forgot my password of a TrueCrypt file I used a few years ago.  How would I go about 'cracking' it? I'm not sure if there is a way to attack it or if it can only be done by brute-force. I used it on a Mac. I still have the Mac I used it on, but of course turned that off many times since I used it.   I have a passphrase that I thought would relate to this file. Maybe I used some variant of it. But might be something completely different. I have some other words that I might have used, but would need give it another thought how I could have used those.  Is there an automated way to test variations on what I think the passphrase was to regain access to the volume? Is there any other way I might regain access? How can I remove duplicate values from a multi-dimensional array in PHP?  Example array:  Array (     [0] =&gt; Array     (         [0] =&gt; abc         [1] =&gt; def     )      [1] =&gt; Array     (         [0] =&gt; ghi         [1] =&gt; jkl     )      [2] =&gt; Array     (         [0] =&gt; mno         [1] =&gt; pql     )      [3] =&gt; Array     (         [0] =&gt; abc         [1] =&gt; def     )      [4] =&gt; Array     (         [0] =&gt; ghi         [1] =&gt; jkl     )      [5] =&gt; Array     (         [0] =&gt; mno         [1] =&gt; pql     )  ) Number of homomorphisms from $(\mathbb Q^+ , \,\cdot\,)$ to $(\mathbb Z_n , +)$ How many homomorphism are there from $(\mathbb Q^+ , \,\cdot\,)$ to $(\mathbb Z_n , +)$?   Could anyone give a hint how to proceed? I couldn't even start I have a map of a city and a .csv-file containing 700 geographic coordinates pointing to locations within that city.  How can I plot these 700 locations on the map? Easiest way to move my Windows installation to an SSD? I've taken the plunge and bought an SSD and want to move my existing Windows installation over. The current hard disk is 500Gb, but I've trimmed the contents down to about ~40Gb. I'm transferring it across to a 100Gb SSD and looking for the easiest way just to copy everything across and set the SSD up as a boot device.  I've looked at a few tools like Macrium Reflect, but they don't seem able to restore to a smaller drive. Do I need to go for something like  to do this? I'm trying to avoid scary Linux-based boot utilities if possible, does anyone know of an easier way?