Here, N refers to the set of all natural numbers, and P refers to the power set of N. Let the number of finite subsets be equal to F. I am having trouble representing the cardinality of the power set. :)
Show that the set of all finite subsets of $\mathbb{N}$ is countable.   I'm not sure how to do this problem. I keep trying to think of an explicit formula for 1-1 correspondence like adding all the elements in each subset and sending that sum to itself in the natural numbers, but that wouldn't be 1-1 because, for example, the set {1,2,3} would send to 6 and so would the set {2,4}.  Multiplying all the elements in each subset and sending that product to itself in the natural numbers wouldn't work either since, for example, {2,3} would send to 5 and so would the set {1,5}. Any ideas?
How to prove the inequality $2\sqrt{n + 1} − 2 \le 1 +\frac 1 {\sqrt 2}+\frac 1 {\sqrt 3}+ \dots +\frac 1 {\sqrt n} \le 2\sqrt n − 1$? Prove that for any positive integer $n$, $$2\sqrt{n + 1} − 2 \le 1 +\frac 1 {\sqrt 2}+\frac 1 {\sqrt 3}+ \dots +\frac 1 {\sqrt n} \le 2\sqrt n − 1$$ Progress I think Riemann sum should be used for the middle term. I got the limit as $n\to \infty$ of the function inside using Riemann sum but what I cannot get are the lower and upper bound. Should individual letters be preceded with "an"? I was reading a comic book and I noticed that the author used "an A" and "an N" in a particular phrase (screenshot below). While I understand the usage of "An A" here (A being a vowel and what not) and "N" having a pronunciation which has a sound similar to "A".  Since most of English alphabets have pronunciations which start with a syllable similar to one of the vowels or semi-vowels; is it OK to prefix the letters using "An"?  For eg. An X, or An Y.        Zenescope's Grimm Fairy Tales - Werewolves: The Hunger, page #8 (Click to enlarge) How can I move an object towards another along a straight line Sending Text Cross Domain By Bookmarklet Isekai manga where the mc is reincarnated through his own magic The MC reincarnated himself through his own magic after his party defeated the demon king. Before he used his magic to do so, he took the daughter from the demon king and traveled with her over the world. After he used his reincarnation magic he was born in a noble family where the daughter of the demon king is now working as a maid in this family. He also has an older/younger (not sure) brother. Through his knowledge he starts his training of body, swordsmanship and magic as a baby. Years later his brother wants to play with him but he denies him every time. But one day he accepts it to play tag. His brother can't catch him. After that, he requests from the mc to teach him his magic. He denied again, but made an offer. Only when he can catch him, would he teach him his magic. I hope I'm not mixing up different mangas. What is a polite substitute for badass (used as a noun)? What's the action of banging two beers together called? When beers in cans or bottles are banged together in rejoice of the company you're with, what is the action called? External hard drive very slow to load after dropping it and can't be repaired with Disk Utility Today as I was using my WD HDD, it fell off my corner. After I fixed the thing, I plugged it into the Mac and it loads the HDD but it's very very very very slow. It also does not work sometimes. The entire laptop just hangs  Tried running Disk Utility First Aid but it won't work.  It says some error and hangs  Can someone advise me on what I can do in this case? "There are several reasons proposed for the collapse of the bridge." There are several reasons proposed for the collapse of the bridge.   Is this present simple tense or the present perfect tense? I thought it might be the latter since there is a retrospective aspect to the sentence. However, I am quite unsure. “situation where” vs. “situation in which” I submitted a manuscript last year to an IEEE journal, and the paper was accepted for publication. I recently submitted a new manuscript to the same journal, but, this time, the paper was rejected, and I was told that I should submit to another journal since my paper was not within the journal's scope. The two papers both target similar applications. Furthermore, the second paper uses some work in the first paper, making substantial improvements at solving a much more difficult problem. Based on the reviewer's comments (and my own personal assessment), I do not believe that the work presented in the second paper was "incremental" in nature. Far from it. Two of the three reviewers gave positive reviews. The third reviewer, however, mentioned the "outside of journal's scope" issue and the associate editor and editor-in-chief both sided with the 3rd reviewer, and the paper was stopped dead in its tracks.  I'm scratching my head trying to come to terms with how in the world my first paper could be accepted by this journal, but the second paper focused on the same application space and achieving substantial improvements on a much more difficult problem (backed up with measured results) could be rejected. Any ideas? I want to know, if this bug is fixed, because all previous versions of Ubuntu didn't work for me. So I just want to know, should i leave Windows 10 and go for Ubuntu 17.10. Thanks! Why doesn't logging out of Stack Overflow log me out of Gmail? Why does logging in to Stack Overflow keep me signed in to my Gmail email after logging out and regardless of the Stay signed in checkbox?  Steps to reproduce:   Browse to  and confirm you are logged out Browse to  and confirm you are logged out Click login Click Google Enter Email/Password and confirm Stay signed in is unchecked Click Sign in Click logout (up top) Click Logout (down below) Browse to  and confirm you are still logged in Click Sign out &lt;- Shouldn't clicking Logout on Stack Overflow be doing the same thing as this? Browse to  and confirm you are logged out     Answers are making sense but my frustration is that clicking Logout (twice) on StackOverflow essentially does nothing. If I proceed to click login and Gmail I am immediately authenticated because I still have an active Gmail cookie. In order to effectively logout I need to click Logout twice on Stack Overflow and then manually browse to  and Sign out. How can I hide username and computer in terminal command prompt?   In Terminal it says  Last login: Mon Jan 13 00:00:14 on ttys000 Whatever:~ UserName$    Is it possible to show just the current folder and $ sign? Is there a general method (which can be implemented by hand) to finding the $n$-th roots of $2 \times 2$ matrices? Is there a similar method for a general $m \times m$ matrix? (for $n &gt; 1$ and $n\in\mathbb{Z}$) How is Selenium RC different from Selenium Web driver and some good pointers for a naive beginner to learn to scripting please? Thanks! How to fix very slow Ubuntu booting? Latin Square and Graph Theory How many &quot;$1$&quot; can be selected at most, provided that only one from each row and column of the attached matrix $A$ is selected?(That is, when one &quot;$1$&quot; is selected, no other element can be selected than the row and column where that &quot;$1$&quot; is located.)  Answer: I think $6$ can be chosen.  How can I give this answer in connection with graph theory? I think it has to do with the Latin Square(or Rectangle). But I can't connect and explain. Please help me. I thought a lot, but I could not establish the relationship completely. Thanks... Cyclic Quadrilateral Problems What is the motivation behind axiomatically forcing the underpinning group of a ring to be abelian? Noncommutative rings are vastly more complex than commutative ones, so I am assuming that allowing the additive operation to be noncommuting would just make matters worse. Is there something deeper here, or is this a restriction for the sake of convenience and simplicity?