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Questions that are about research in mathematics, or about the job of a research mathematician, without being mathematical problems or statements in the strictest sense. Do not use this tag for easy or supposedly easy mathematical questions.

208 votes
72 answers
51k views

What are your favorite instructional counterexamples?

Related: question #879, Most interesting mathematics mistake. But the intent of this question is more pedagogical. In many branches of mathematics, it seems to me that a good counterexample can be w …
195 votes

Most harmful heuristic?

This isn't really a heuristic, but I hate "functions are formulas". For most students it takes a really long time to think of a function as anything other than an algebraic expression, even though nat …
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 …
138 votes
Accepted

What is entropy, really?

Here is a simple story one can tell about the entropy $$H = -\sum_{i=1}^n p_i \log p_i$$ of a discrete probability distribution. Suppose you wanted to describe how surprised you are upon learning …
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 …
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 …
99 votes

Your favorite surprising connections in mathematics

From an essay of Arnol'd: Jacobi noted, as mathematics' most fascinating property, that in it one and the same function controls both the presentations of a whole number as a sum of four squares and t …
96 votes
16 answers
18k views

Why is it a good idea to study a ring by studying its modules?

This is related to another question of mine. Suppose you met someone who was well-acquainted with the basic properties of rings, but who had never heard of a module. You tell him that modules genera …
Qiaochu Yuan's user avatar
95 votes

Famous mathematical quotes

Combinatorics is an honest subject. No adèles, no sigma-algebras. You count balls in a box, and you either have the right number or you haven’t. You get the feeling that the result you have discove …
87 votes

What non-categorical applications are there of homotopical algebra?

As a student, I'm always looking for organizing principles in mathematics to help me keep track of all of the mathematics I learn. It's easy to get lost in a deluge of definitions unless I organize th …
76 votes

Why do so many textbooks have so much technical detail and so little enlightenment?

I absolutely agree that this is a question worth asking. I have only recently come to realize that all of the abstract stuff I've been learning for the past few years, while interesting in its own ri …
63 votes

Favorite popular math book

Title: Godel, Escher, Bach: an Eternal Golden Braid Author: Douglas Hofstadter Short Description: It's mildly debatable whether this is in fact a book about mathematics, but any mathematician who …
61 votes

What do named "tricks" share?

How about the following (which I think applies to some of these tricks but not others): a trick is something whose usefulness is not fully captured by any particular set of hypotheses, so it would lim …
59 votes
Accepted

Are there any "homotopical spaces"?

No.
Qiaochu Yuan's user avatar
59 votes

What are the most overloaded words in mathematics?

Normal Normal distribution Normal vector Normal space Normal extension Normal subgroup Normal operator Normal convergence

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