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Questions designed to generate a "big list" of certain results, examples, conjectures, etc. via many individual answers, each contributing one or a few instances. Such a question should typically be in Community Wiki mode (CW); after asking, please, flag for moderators attention requesting the question to be made CW.

12 votes

Vector spaces without natural bases

The solution space of a homogeneous (ordinary or partial) linear differerential equation has no natural basis.
8 votes

Do you read the masters?

I enjoyed reading Gauss's Allgemeine Auflösung der Aufgabe die Theile einer gegebenen Fläche auf einer andern gegebnen Fläche so abzubilden, dass die Abbildung dem Abgebildeten in den kleinsten Theile …
2 votes

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

First, note that a mapping between metric spaces is continuous if and only if the inverse image of an open set is always open. There are various concepts for metric spaces that you can likewise find e …
31 votes

What are the worst notations, in your opinion?

I rather dislike the notation $$\int_{\Omega}f(x)\,\mu(dx)$$ myself. I realize that just as the integral sign is a generalized summation sign, the $dx$ in $\mu(dx)$ would stand for some small measurab …
2 votes

Various concepts of "closure" or "completion" in mathematics

Just about every form of compactification. The compactification of a compact space is itself, and a compactification had better be compact or it shouldn't be called a compactification. The same thing …
115 votes

Probabilistic proofs of analytic facts

One nice example is Bernstein's proof of the Weierstrass theorem. This proof analyses a simple game: Let $f$ be a continuous function on $[0,1]$, and run $n$ independent yes/no experiments in which th …
Harald Hanche-Olsen's user avatar
61 votes

Favorite popular math book

Title: Logicomix: An epic search for truth Authors: Apostolos Doxiadis, Christos Papadimitriou Artists: Alecos Papadatos, Annie Di Donna Short description: A comic book biography of Bertrand Russe …
52 votes

Favorite popular math book

Title: Mathematics: A very short introduction Author: Timothy Gowers Short description: As the title says, very short. Gives the non-mathematical reader a good idea what mathematics is all about in …
36 votes

Famous mathematical quotes

“Having thus refreshed ourselves in the oasis of a proof, we now turn again into the desert of definitions.” – Th. Bröcker & K. Jänich, Introduction to differential topology (p.25)
56 votes

What is convolution intuitively?

What is the operator $C_f\colon g\mapsto f*g$? Consider the translation operator $T_y$ defined by $T_y(g)(x)=g(x-y)$, and look at $f*g(x)=\int_{\mathbb{R}}f(y)g(x-y) \, dy$. Rewriting this as an opera …
21 votes

Dimension leaps

The wave equation behaves differently in even and odd space dimensions. In odd-dimensional space, radial waves satisfy a modified version of the one-dimensional wave equation. In particular, Huygens' …