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Search options answers only not deleted created 2012-09-28 - 2013-09-28
237 votes

Awfully sophisticated proof for simple facts

Seen on https://legauss.blogspot.com/2012/05/para-rir-ou-para-chorar-parte-13.html Theorem: $5!/2$ is even. Proof: $5!/2$ is the order of the group $A_5$. It is known that $A_5$ is a non-abelian simpl …
214 votes
Accepted

Non-amenable groups with arbitrarily large Tarski number?

It is indeed an open problem, as Misha said. But here is a solution. In E. Golod, Some problems of Burnside type. 1968 Proc. Internat. Congr. Math. (Moscow, 1966) pp. 284-289. Izdat. ”Mir”, Moscow, G …
user avatar
129 votes
Accepted

Why do primes dislike dividing the sum of all the preceding primes?

Here is a heuristic argument that there is nothing to explain: The probability that $p$ divides the sum of the preceding primes is $1/p$. So the expected number of primes less than $10^9$ with this p …
David E Speyer's user avatar
111 votes
Accepted

Does Physics need non-analytic smooth functions?

As a physicist "in nature" perhaps I can give a few examples that illustrate how non-analytic functions can appear in physics and counter the idea that physicists do not worry about the justification …
Jeff Harvey's user avatar
  • 5,546
103 votes
Accepted

Why might André Weil have named Carl Ludwig Siegel the greatest mathematician of the 20th ce...

No one with any familiarity with his work can doubt that Siegel was one of the greatest mathematicians of the 20th century. Weil was a decisive, opinionated man -- just the type of person who would h …
102 votes

Who wrote up Banach's thesis?

Here is a quote from the article by Krzysztof Ciesielski: On Stefan Banach and some of his results. Banach J. Math. Anal. 1 (2007), no. 1, 1–10. There is a curious story how Banach got his Ph …
98 votes
Accepted

Philosophy behind Yitang Zhang's work on the Twin Primes Conjecture

My understanding of this, which is essentially cobbled together from the various news accounts, is as follows: Let $\pi(x;q,a)$ denote the number of primes less than $x$ congruent to $a\bmod q$, and …
98 votes

What is convolution intuitively?

I prefer sound to Terry Tao's light. Listen to my voice through a wall. At each moment in time, you hear not just what I am saying now, but also some reverberation from what I said moments ago. So if …
96 votes

Can the Poisson summation formula break?

Exercise 15 of Chapter VI (Section 1) of Katznelson's book "An introduction to Harmonic analysis" gives an example of a continuous $L^1$ function $f$ where both sides of the Poisson summation formula …
Lucia's user avatar
  • 43.7k
95 votes

Examples of great mathematical writing

True story: When I was about to move to Stony Brook to start my PhD, one of my professors took me aside to tell me "You know, when I was a student Milnor was god, and Morse Theory was the bible." I fo …
93 votes
Accepted

Are there proofs that you feel you did not "understand" for a long time?

As an undergraduate, I learned the Sylow theorems in my algebra classes but could never retain either the statement or proof of these theorems in memory except for short periods of time (and in partic …
90 votes
Accepted

Is rigour just a ritual that most mathematicians wish to get rid of if they could?

I was not going to write anything, as I am a latecomer to this masterful troll question and not many are likely going to scroll all the way down, but Paul Taylor's call for Proof mining and Realizabil …
88 votes

Why is it hard to prove that the Euler Mascheroni constant is irrational?

Philosophically, there is essentially only one way to prove that a number is irrational/transcendental, which is to use the fact that there is no integer between 0 and 1. That is, one assumes that th …
Timothy Chow's user avatar
  • 82.6k
87 votes

$\prod_{n=1}^{\infty} n^{\mu(n)}=\frac{1}{4 \pi ^2}$

We have $$\frac{1}{\zeta(s)} = \sum_{n=1}^{\infty} \frac{\mu(n)}{n^s} \quad \mbox{for} \ Re(s)>1.$$ Taking the derivative with respect to $s$, we get the following $$- \frac{\zeta'(s)}{\zeta(s)^2} = - …
David E Speyer's user avatar
86 votes
Accepted

Famous vacuously true statements

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