Alex Nelson
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 Dec 27 comment The difference between $\Delta x$, $\delta x$ and $dx$ Well, $\delta x$ means different things depending on the context. For example, it has a particular meaning in variational calculus, and a completely different one in functional calculus... Dec 24 comment Learning math: still paper and pen @AndreySokolov A pop review and a more thorough answer (with many references) at the personal productivity stackexchange. I think from there, you can find additional references to studies done... Dec 19 comment Handbook of mathematical drawing? Honestly, learning how to draw in general helped me with technical diagrams and mathematical figures. Perhaps it's the only way... Nov 23 comment How to find this limit: $A=\lim_{n\to \infty}\sqrt{1+\sqrt{\frac{1}{2}+\sqrt{\frac{1}{3}+\cdots+\sqrt{\frac{1}{n}}}}}$ I'm interested: how did you derive your approximation? (I ask because I wouldn't have a clue where to begin with deriving it...) Nov 6 comment Derive the equation of motion from Lagrangian of a particle moving in an electromagnetic field Uh, perhaps plug the Lagrangian into the Euler-Lagrange equations...? Oct 21 comment General Linear Group of a vector space Well, to be technical, you only need to pick a basis to determine the components of the matrix representing the isomorphism. Oct 20 comment Maclaurin series for $e^z /\cos z$. Perhaps some words or explanations would help this jumble of equations make sense... Oct 20 comment Converging or diverging series? One quick trick for (a) is to note if $a_{n}=\sqrt{n}\cos^{2}(n)/(n^{2}-2)$, then $0\leq a_{n}\leq \sqrt{n}/(n^{2}-2)$, since $0\leq\cos^{2}(n)\leq1$. Sep 28 comment Question on category theory Well, you describe a "locally-small" category, where the $\hom(-,-)$ are sets. They can be "bigger" collections (classes), which would give us a "large" category (example of a large cat: Set). THEN the question is: are you talking about the category of small categories, or of large categories? (Or both?) Aug 14 comment Summing the series $\sum_{n=1}^{\infty} \int_0^{\infty} \frac{\mathrm dx}{n(1+x^3)^n}$ @Norbert, you are too clever for my feeble old mind! ;) Thanks for the explanation, I appreciate it greatly. Aug 13 comment Summing the series $\sum_{n=1}^{\infty} \int_0^{\infty} \frac{\mathrm dx}{n(1+x^3)^n}$ I'm old and forgetful, so I have a question: when you use your clever trick $\log(1+q)=\sum\dots$, setting $q=(1+x^{3})^{-1}$, don't you need to be careful with the limits of integration and take $\int^{\infty}_{\epsilon}\dots\,\mathrm{d}x$, then take the limit as $\epsilon\to0^{+}$? [I know this may be pedantic, but as I said -- I am old and forgetful, and I forgot whether you can skip it or if it's really necessary!] Aug 9 comment Understanding mathematical set theory syntax @user2666425: the one-to-one correspondence is between the powerset of $A$ and $2^{A}$. There is not necessarily a one-to-one correspondence between the cardinality of $A$ with $2$! There is equal cardinalities between $P(A)$ -- the powerset of $A$ -- and $2^{A}$, the set of characteristic functions for all subsets of $A$. Aug 9 revised Understanding mathematical set theory syntax added 461 characters in body Aug 9 answered Understanding mathematical set theory syntax Aug 9 revised Understanding mathematical set theory syntax Improved TeX Aug 9 suggested approved edit on Understanding mathematical set theory syntax Aug 6 revised Cohomology ring of $U(n)$ Improved TeX Aug 6 suggested approved edit on Cohomology ring of $U(n)$ Aug 6 comment Closed form for this continued fraction Rigorously speaking, $f(x)$ doesn't converge when $x\leq0$. If I recall correctly, this is by Worpitzky's theorem...I would be rather excited if I am wrong, though, and certainly do not rule it out! Aug 5 comment Closed form for this continued fraction I think he means "closed-form expression" the continued fraction...I'm guessing/hoping-since-I-already-answered ;p...