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Questions designed to get an overview of a specific subject or body of results or to understand the relations among similar definitions, techniques or concepts appearing in different sub-fields of mathematics. While such questions by their very nature sometimes cannot be made very narrow and focused, it can be helpful to keep in mind that the design of MathOverflow does not make it a good fit for questions that are too broad.

27 votes

Why the triangle inequality?

The triangle inequality is natural. In any setting where the metric is related to some kind of optimization problem, for example if $d(a, b)$ measures the "length" of the "shortest path" between point …
9 votes

What's so special about $1$-categories?

You should increase your category level if you think it'll help you understand something you're thinking about. Otherwise, don't. To the extent that there's something special about $1$-categories, it …
8 votes

Surprising and Useful Physical Intuition for Mathematical Objects

There are several examples at the number theory and physics archive. To get you started let me mention the statistical-mechanical interpretation of the Riemann zeta function as the partition function …
15 votes

Surprising and Useful Physical Intuition for Mathematical Objects

Kirillov's orbit method in representation theory establishes a correspondence (which is not exact in general) between irreducible unitary representations of a Lie group $G$ and orbits of the action of …
8 votes

What is the high-concept explanation on why real numbers are useful in number theory?

Someone once suggested on MO that this is because on the one hand Matiyasevich's theorem shows that no algorithm can solve Diophantine equations over $\mathbb{Z}$ (and the corresponding result is not …
27 votes

What advanced area of mathematics can be delved into with only basic calculus and linear alg...

Stillwell's Naive Lie theory was essentially written as an answer to this question. I quote from the introduction: It seems to have been decided that undergraduate mathematics today rests on tw …
39 votes

Describe a topic in one sentence.

Complex Analysis: Taylor series behave the way you want them to in real analysis.
59 votes
Accepted

Are there any "homotopical spaces"?

No.
Qiaochu Yuan's user avatar
31 votes

Your favorite surprising connections in mathematics

It is possible to compute the Betti numbers of a smooth complex variety $X(\mathbb{C})$ by computing the cardinality of $X(\mathbb{F}_{p^n})$ for a prime $p$ with good reduction and a finite number of …
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 …
31 votes

Theorems that are 'obvious' but hard to prove

Subgroups of free groups are free. The plausible argument is that any relation satisfied in a subgroup must somehow translate to a relation satisfied in the larger group. Nowadays I guess most peopl …
13 votes

Justifying a theory by a seemingly unrelated example

If you're a combinatorialist and you want to know the asymptotics of a sequence $a_n$ with a nice generating function $A(z) = \sum_{n \ge 0} a_n z^n$, the very first thing you should do is find out if …
5 votes

Justifying a theory by a seemingly unrelated example

Suppose you are interested in random walks on an extremely structured graph such as a hypercube graph or a cycle graph. If your graph happens to be the Cayley graph of an abelian group $G$, as in bot …
9 votes

Generalizations of "standard" calculus

I can answer your last question, at least. The derivative acts as a shift operator on Taylor series, so the operator $\frac{d}{dx} - 1$ acts as the forward difference on Taylor series. So their eige …
Qiaochu Yuan's user avatar
10 votes

Why are modular forms interesting?

Lots of good answers so far, and I hope that someone talks about Moonshine at some point. Let me just briefly mention an application which hasn't been mentioned so far to class field theory. Special …

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