Dan Neely
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 Jul 22 comment Help me solve my father's riddle and get my book back @becko Leaf is valid as well: "something suggestive of a leaf ... a part of a book or folded sheet containing a page on each side" merriam-webster.com/dictionary/leaf Jul 1 comment Is $n! + 1$ often a prime? @CaptainCodeman In general (ie excluding cases where an easy option is available), proving a number isn't prime is mostly done using algorithms that have a certain probability of finding a factor if one exists. If those algorithms fail to find a factor after enough iterations that the odds of one existing are very low, a very computationally intensive (on the order of a day of computer time to complete, but better than the naive brute force approach) algorithm is used to do the final proof. primes.utm.edu/prove/prove4.html Dec 12 comment How to calculate $(a+b)(c+d)$ @anorton I remember it helping some of my peers in pre-algebra (algebra 1?) in Junior High; but IIRC most of the big beneficiaries were struggling with algebra generally and in future years ended up dropping back to less challenging math classes. Nov 5 comment Are all mathematicians human calculators? Is g(x, y, z) in @Izkata 's meme a useful equation for some problem, or was it just something created for the lulz. Nov 5 comment Are all mathematicians human calculators? @LucM I prefer ODing on Asimov... "Nine times seven, thought Shuman with deep satisfaction, is sixty-three, and I don't need a computer to tell me so. The computer is in my own head. And it was amazing the feeling of power that gave him." Oct 21 comment Are there any open mathematical puzzles? @TylerHilton Probably not what you meant; but trying to find a counter example via brute force methods is one of the biggest credit fountains among BOINC distributed computing projects. boincstats.com/en/stats/projectStatsInfo Sep 26 comment Surprising identities / equations @JackM Disagree. This type of approach can be explained to anyone with basic algebra skills. Infinite sums require calculus to understand. Aug 12 comment Rigour in mathematics @Samprity was that who you intended to link to, or were you mistaken about what he was first at? He lived several thousand years after the Babylonian civilization ended and they did have a solution for quadratic equations. The article you linked to indicates that he could have priority for developing, but not realizing the value of, some introductory calculus concepts. Aug 12 comment Rigour in mathematics @AndresCaicedo Could you give an example of the type of curve you consider the Jordan curve theorem non-obvious with? Jun 13 comment Pair of compasses drawing a square (from children's fiction) @ulidtko nice. Unfortunately my mouse hand isn't steady enough to draw a good square. Jun 13 comment Pair of compasses drawing a square (from children's fiction) @MJD a square can be described by a set of 4 piecewise equations. ex X=0, X=1, y=0, y=1; where 0 <= x <= 1 and 0 <= y <= 1 Jun 3 comment Which one is bigger: $\;35{,}043 × 25{,}430\,$ or $\,35{,}430 × 25{,}043\;$? @Hurkyl Training your intuition will allow getting the answer much faster than doing a longhand calculation when you don't have access to a calculator/computer. Jun 3 comment Which one is bigger: $\;35{,}043 × 25{,}430\,$ or $\,35{,}430 × 25{,}043\;$? +1 for being generalizable to when the two pairs don't have the same sum. May 31 comment How can I find the surface area of a normal chicken egg? @jwg it depends on the egg. My parents have kept ~ two dozen chickens for the last 20 years; the range of "normal" eggs (excluding ex ones with 2 yolks) I've seen ranges from about 80% of the width of the curve shown to about 60% of the height of it. May 24 comment Conditional probability with ages Where is this city, and can I get a few free decades added to my life expectancy just by moving there? :) May 14 comment Why do the French count so strangely? @mau With the answers/discussion spanning multiple languages; unless there's a generic linguistics SE I'm not sure where it would go. May 6 comment Prove by mathematical induction that $1 + 1/4 +\ldots + 1/4^n \to 4/3$ @GitGud mathematicians, or people who saw the question in the hot question dropdown and voted for the answer with a pretty picture that they thought they understood? May 2 comment Repeating Decimals I normally demonstrate this technique by subtracting 100x from 10000x instead. Functionally they're equivalent; but making the decimal part go away completely in a single step seems to result in more people getting the concept immediately. Apr 26 comment Why is every answer of $5^k - 2^k$ divisible by 3? @SalvadorDali I think you're worrying too much; the same happens everywhere and the huge numbers of upvotes for questions asking "something every programmer should know" certainly hasn't kept stackoverflow itself from becoming one of the top programming help sites. If you're really concerned though, I suggest taking the issue from here to Meta.SE since its a network wide phenomena. Apr 26 comment Why is every answer of $5^k - 2^k$ divisible by 3? @SalvadorDali that happens all over on SE; some simpler questions get lots of upvotes from people who understand the subject matter well enough to say "hey that's neat"; but who ignore questions that they don't understand well enough to vote on. Once a question gets voted onto the hot question dropdown it's going to be seen by a huge number of people most of whom don't know math beyond basic calculus and whose voting will swamp that of the mathematicians who frequent this site.