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 Dec16 awarded Caucus Dec2 comment Examples of mathematical discoveries which were kept as a secret Gauss also didn't publish the Fast Fourier Transform. Mar9 comment Can numbers exist outside of the number space? Ordinals and cardinals are extensions of natural numbers but not integers. Jan21 comment How do I convince my students that the choice of variable of integration is irrelevant? In logic it is an axiom, so IMHO it is fine to use intuition to justify it. Note that you have to be careful not to capture other variables which are free inside the function or integral. Aug6 awarded Nice Answer Oct23 comment Integral solutions of $x^2+y^2+1=z^2$ You are not trying to solve Project Euler problem 224, are you? projecteuler.net/problem=224 Oct9 comment Why is it called a 'ring', why is it called a 'field'? And it is still "Körper" in German. Oct3 comment How to derive a function to approximate $\sqrt{3}$? It is much older than Newton's method: en.wikipedia.org/wiki/… Sep12 comment Mathematical operation with unique result These are of course ordered pairs, edited to make that explicit. Sep12 revised Mathematical operation with unique result added 8 characters in body Sep9 answered Mathematical operation with unique result Sep9 comment In set theory, how are real numbers represented as sets? You have to exclude at least <0,0> in the construction of Q. It would be related to all other pairs so transitivity would fail. Aug31 comment What is the result of infinity minus infinity? This infinity of a denumerable set of things is normally written ω (omega), it is the least infinite ordinal (there are many many more infinite ordinals). Aug26 comment Who invented $\vee$ and $\wedge$, $\forall$ and $\exists$? They also had funny names as mnemoics for their inference rules, such as Barbara, Celarent, etc, where the vowels encoded the sentence forms. Jul28 awarded Yearling Jul24 comment Real world applications of Pythagoras' Theorem Please list what you already have. Jul20 comment divisibility of ABCD+DCBA by 11 For an odd number of digits the middle digit will lead to a summand 2 * d * 10^k, so it can only work for divisibility by multiples of 2 and/or 5. Jul14 answered Are all numbers real numbers? Jul14 comment Are all numbers real numbers? The rationals don't have gaps, they are dense. Jun19 comment Finding the 10001th Prime @Luboš Please delete your comment, it is a spoiler for those who still want to solve the problem.