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Search options answers only not deleted not community wiki created 2013-09-28 - 2014-09-28
229 votes
Accepted

Perfectly centered break of a perfectly aligned pool ball rack

This question was cross-posted on Math Stack Exchange. Here is a copy of my answer for it there. This is it.  The perfectly centered billiards break.  Behold. Setup This break was computed i …
Jim Belk's user avatar
  • 8,483
162 votes
Accepted

Is it possible to express $\int\sqrt{x+\sqrt{x+\sqrt{x+1}}}dx$ in elementary functions?

The answer is 'no'. Making the substitution $$ x = \frac{(t-1)(t-5)(t^2+2t+5)}{16t^2}, $$ one finds $$ {\textstyle\sqrt{x+\sqrt{x+\sqrt{x+1}}}\,\mathrm{d}x} = \frac{(t^2-2t+5)(t^2-5)\sqrt{t^4{-}2t^2 …
Robert Bryant's user avatar
111 votes
Accepted

History of "without loss of generality"

I think one reason JSTOR doesn't have “loss of generality” before 1831 is that fewer scientists wrote in English. But one finds (with minor variants merged, translations *starred, and year first publi …
Francois Ziegler's user avatar
108 votes

Is it possible to express $\int\sqrt{x+\sqrt{x+\sqrt{x+1}}}dx$ in elementary functions?

I'm adding a separate answer for the general question that the OP asked, which settles the question in the negative for all $n>2$ (and gives an alternate proof for $n=3$ to the one I gave above). Rec …
Robert Bryant's user avatar
105 votes

The Arnold – Serre debate

I was there. Arnol'd is one of my big mathematical heros, but I found the whole thing really sad. It was in French, but my French is decent. Arnol'd began his part with a phrase I've heard him say be …
Richard Montgomery's user avatar
94 votes
Accepted

A hard integral identity on MathSE

I have proved this equality by means of Cauchy’s Theorem applied to an adequate function. Since my solution is too long to post it here, I posted it in arXiv: Juan Arias de Reyna, Computation of a De …
juan's user avatar
  • 7,024
84 votes
Accepted

How did "normal" come to mean "perpendicular"?

normalis already meant right-angled in classical Latin; for example, angulus normalis appears in the first century text De institutione oratoria (volume XI, paragraph 3.141) by Marcus Fabius Quintilia …
Carlo Beenakker's user avatar
68 votes
Accepted

Are the primes normally distributed? Or is this the Riemann hypothesis?

These questions on the spacings between primes are expected to be true, but are far from being proved. They are not directly related to RH, but seem to encode other relations among zeros. The conjec …
Lucia's user avatar
  • 43.7k
66 votes
Accepted

There's something strange about $\sqrt{\big(j(\tau)-1728\big)d}$

"Numerology" such as you've observed is explained in the paper Gross, B.H., and Zagier, D.: On singular moduli, J. reine angew. Math. 355 (1985), 191$-$220. MR772491 (86j:11041) which gives mor …
Noam D. Elkies's user avatar
65 votes

Absolute value inequality for complex numbers

In general, once you've proven an inequality like this in ${\bf R}$ it holds automatically in any Euclidean space (including ${\bf C}$) by averaging over projections. ("Inequality like this" = inequa …
Noam D. Elkies's user avatar
65 votes

A topological concept dual to compactness

When looking for a dual concept we should be careful not to be tricked by a shallow symmetry. I will not comment on your definition of anti-compactness. Instead I would like to explain what the "true" …
Andrej Bauer's user avatar
  • 48.8k
61 votes
Accepted

What fraction of the integer lattice can be seen from the origin?

The limiting ratio is $\frac{1}{\zeta(2)} = \frac{6}{\pi^2}= 0.6079271018540266286632767792\ldots$ The question is easily seen to be equivalent to "What is the probability that two integers are relati …
Pete L. Clark's user avatar
61 votes
Accepted

How feasible is it to prove Kazhdan's property (T) by a computer?

Using the $\Delta^2- \epsilon \Delta$ approach, Tim Netzer and I have verified Kazhdan's property (T) for ${\rm SL}(3,\mathbb Z)$. For the standard generators $e_{ij}$ ($i\neq j$) we can show a spectr …
Andreas Thom's user avatar
  • 25.5k
60 votes
Accepted

Computing $\prod_p(\frac{p^2-1}{p^2+1})$ without the zeta function?

I found a proof of $$5 \sum_{m=1}^{\infty} \frac{1}{m^4} = 2 \left( \sum_{n=1}^{\infty} \frac{1}{n^2} \right)^2$$ by rearranging sums and wrote it up. The argument is just 1.5 pages, the other 4.5 ar …
David E Speyer's user avatar
59 votes
Accepted

Function extensionality: does it make a difference? why would one keep it out of the axioms?

I am going to draw heavily from Github discussion on HoTT book pull request 617. There are different kinds of equality. Let us say that equality is "intensional" if it distinguishes objects based on …
Andrej Bauer's user avatar
  • 48.8k

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