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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.

9 votes

Are infinite dimensional constructions needed to prove finite dimensional results?

Serre and Cartan wrote a Comptes Rendus note Un Théorème de finitude concernant les variétés analytiques compactes where they proved that all cohomology complex vector spaces $H^q(X,\mathcal F)$ of an …
Georges Elencwajg's user avatar
35 votes

Universal definition of tangent spaces (for schemes and manifolds)

Consider the real line $\mathbb R$ and $C^1_0$ , the ring of germs of continuously differentiable functions at zero. Now take the ideal $M$ of germs vanishing at zero. The Zariski cotangent space $M/M …
Georges Elencwajg's user avatar
44 votes

Theorems that are 'obvious' but hard to prove

That $\mathbb R^n$ has topological dimension $n$. In a similar vein that affine space $\mathbb A^n_k$ over a field $k$ has Zariski dimension $n$.
38 votes
Accepted

Justifying a theory by a seemingly unrelated example

[In front of a blackboard, in an office at Real College] Skeptic: And why should I care about holomorphic functions? Holomorphic enthusiast:$\;$ Can you compute $\quad$ $\sum_{n={-\infty}}^{\infty …
16 votes

Justifying a theory by a seemingly unrelated example

Let us call "division algebra over $\mathbb R$" a finite-dimensional vector space $A$ equipped with a bilinear map $A \times A \to A: (a,b) \mapsto a \bullet b$ , such that $a\bullet b=0$ implies $a= …
14 votes

Fundamental Examples

In the theory of holomorphic functions of several variables, Hartogs's theorem that any holomorphic function on a punctured open set of $\mathbb C^n$ ($n\geqslant 2$) can holomorphically be continued …