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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 …
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 …
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 …
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 …
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 …
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 …
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 …
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 …
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 …
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 …
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" …
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 …
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 …
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 …
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 …