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For questions in Mathematics Education as a scientific discipline. For more hands-on questions on teaching Mathematics, please use the tag teaching. There is also a Stack Exchange community http://matheducators.stackexchange.com/

278 votes

Examples of common false beliefs in mathematics

I don't know if this is common or not, but I spent a very long time believing that a group $G$ with a normal subgroup $N$ is always a semidirect product of $N$ and $G/N$. I don't think I was ever sho …
115 votes

What are your favorite instructional counterexamples?

A polynomial $p(x) \in \mathbb{Z}[x]$ is irreducible if it is irreducible $\bmod l$ for some prime $l$. This is an important and useful enough sufficient criterion for irreducibility that one might w …
86 votes

Sophisticated treatments of topics in school mathematics

The angle addition formula $\tan(\alpha + \beta) = \frac{\tan(\alpha) + \tan(\beta)}{1 - \tan(\alpha) \tan(\beta)}$ for tangent gives one of the simplest nontrivial examples of a formal group law, nam …
73 votes

Sophisticated treatments of topics in school mathematics

It's common in calculus classes and textbooks to state that the antiderivative of $\frac{1}{x}$ is $\log |x| + C$, where $C$ is a constant. This is incorrect: $C$ need only be a locally constant funct …
55 votes

Cool problems to impress students with group theory

An obvious choice is the enumeration of orbits of finite group actions, which show up everywhere in middle- and high-school competitions in disguise. The "cute" example here is coloring a cube or a r …
38 votes

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

The first proof of Tychonoff's theorem I learned, from the Alexander subbase theorem, was completely mysterious to me. I didn't understand it at all. In particular I didn't really understand what the …
35 votes

What should be offered in undergraduate mathematics that's currently not (or isn't usually)?

I think undergraduates should take problem-solving classes. I don't think such classes are widely available, but bright students who didn't do a lot of problem-solving in high school would definitely …
34 votes

Examples of common false beliefs in mathematics

The quotient $G/Z(G)$ of a group by its center is centerless. I definitely thought this until it was pointed out to me in a Lie theory textbook that this wasn't true in general, but is true for (edit …
32 votes

How to present mathematics to non-mathematicians?

There is this nice quote whose wording I can't quite recall. It is something like "physics is the study of the laws of God. Mathematics is the study of the laws even God must follow." I think there …
32 votes

Real-world applications of mathematics, by arxiv subject area?

math.CO Combinatorics Combinatorics finds applications in computer science, especially in the run-time analysis of algorithms. It has also in recent years found applications in physics, at least in …
27 votes

How should one present curl and divergence in an undergraduate multivariable calculus class?

As far as explaining the formulas for div and curl, you should be able to do this starting with the definitions given in the Wikpedia articles by taking the corresponding integrals on rectangles and b …
Qiaochu Yuan's user avatar
25 votes

Taylor's theorem and the symmetric group

One way is to use a combinatorial definition of the derivative. Let $A(z) = \sum a_n z^n$ be a power series. In combinatorics, where $A$ is likely to be an ordinary generating function, $a_n$ is likel …
Qiaochu Yuan's user avatar
25 votes

Why is a topology made up of 'open' sets?

This is an attempt to synthesize ideas that have appeared in other answers, for example sigfpe's and Tim Perutz's. Feel free to edit if you think the ideas can be better expressed. The idea I want t …
24 votes

Interesting results in algebraic geometry accessible to 3rd year undergraduates

This isn't a result so much as a perspective, but it is one of the main reasons I first got interested in algebraic geometry. In basic algebraic number theory you learn that some extensions of the in …
Qiaochu Yuan's user avatar
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 …

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