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For questions related to teaching mathematics. For questions in Mathematics Education as a scientific discipline there is also the tag mathematics-education. Note you may also ask your question on http://matheducators.stackexchange.com/.

174 votes

What are the most misleading alternate definitions in taught mathematics?

Here's another algebra peeve of mine. The definition of a normal subgroup in terms of conjugation is pretty strange until it's explained that normal subgroups are the ones you can quotient by. Again …
121 votes

What are the most misleading alternate definitions in taught mathematics?

In my experience, introductory algebra courses never bother to clarify the difference between the direct sum and the direct product. They're the same for a finite collection of abelian groups, which …
33 votes

Demystifying complex numbers

If the students have had a first course in differential equations, tell them to solve the system $$x'(t) = -y(t)$$ $$y'(t) = x(t).$$ This is the equation of motion for a particle whose velocity vect …
33 votes

Why should one still teach Riemann integration?

I haven't really thought this through, but how does one actually compute integrals? For the Riemann integral one can either prove the fundamental theorem and have a large table of derivatives handy, …
24 votes

Is Euclid dead?

As long as this question is open I might as well throw in my two cents. I think it is not useful to teach Euclidean geometry to high school students. Here are some reasons I can think of for people to …
22 votes

Integrating powers without much calculus

This may not be in the spirit of what you want, but... by scaling arguments it suffices to establish that $\int_0^1 x^p dx = \frac{1}{p + 1}$. Consider the following probabilistic argument (not entire …
Qiaochu Yuan's user avatar
22 votes

Why should one still teach Riemann integration?

Paul Siegel mentioned that to teach undergrads the Lebesgue integral you have to spend half the semester on measure theory. This is actually not true. There is a self-contained definition of the Leb …
20 votes

How to mentor an exceptional high school student?

Tell him about his other opportunities (although perhaps being on AoPS he is already aware of them). Summer programs like Ross PROMYS the Canada/USA Mathcamp HCSSiM and others come highly recomme …
18 votes

Collecting proofs that finite multiplicative subgroups of fields are cyclic

Let $G$ be a finite subgroup of $F^{\ast}$ of order $n$. Then all the elements of $G$ satisfy $x^n = 1$ in $F$. Since polynomials of degree $n$ over a field have at most $n$ roots, it follows that the …
16 votes
Accepted

Can this informal argument (for the fact that almost all reals in the unit interval are irra...

You can make sense of the uniform probability distribution on lots of infinite sets, notably any compact topological group $G$, where "uniform probability distribution" should mean "normalized Haar me …
Qiaochu Yuan's user avatar
14 votes

Applications of knot theory

Colin Adams' The knot book discusses the following applications: Knotting in DNA, Molecular knots, Statistical mechanics (e.g. the Potts model). Constructing invariants of knots is also related t …
13 votes

Math books for advanced high school students

The list I give undergraduates and strong high schoolers is here.
11 votes

Math History Question about the exponential function

Short answer: Most likely undefined. Long answer: The "naive" definition of $f(x) = a^x$ where $a, x \in \mathbb{R}$ and $a > 0$ is as follows. You know how to define $f(n)$ where $n$ is an intege …
Qiaochu Yuan's user avatar
10 votes

Category theory sans (much) motivation?

The Unapologetic Mathematician covers category theory before it covers group theory, so after a little more experience with abstraction you might want to expose your friend to the beginning posts. I …
9 votes

Elementary applications of linear algebra over finite fields

Suppose you want to compute the period of the Fibonacci sequence $\bmod p$. This reduces to examining the powers of the matrix $\left[ \begin{array}{cc} 1 & 1 \\\ 1 & 0 \end{array} \right]$ over $\mat …

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