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Questions asking for the intuition behind some definition, conjecture, proof etc. In other words, questions designed to improve or to acquire understanding on a conceptual or intuitive level, as opposed to on a technical or formal level. When asking such a question it can be helpful to include a rough description of ones understanding of the subject at hand (on a technical level).

3 votes
Accepted

Explaining a comment: Difference between a transformation of points and a transformation of ...

Let $V$ be a finite-dimensional real vector space. Choosing a basis of $V$ amounts to giving an isomorphism $\phi : \mathbb{R}^n \to V$. Changing basis amounts to hitting $\mathbb{R}^n$ with an automo …
Qiaochu Yuan's user avatar
4 votes

Most helpful heuristic?

A sort-of heuristic in combinatorics is that if you can't figure out what to do with a set, take the free abelian group / vector space on that set and work with linear transformations instead of funct …
3 votes

Intuitive Example of a Jacobson Radical

The intuition I have about the nilradical (and by extension, the Jacobson radical) is that it measures how far R is from behaving like the ring of functions on a space. …
Qiaochu Yuan's user avatar
17 votes
3 answers
2k views

What is a reasonable finitary analogue of the statement that harmonic functions are smooth?

In my answer to this question on MU, I suggested that the OP think about the difference between real-differentiable and complex-differentiable functions by using a sort of finitary analogue. One way …
Qiaochu Yuan's user avatar
6 votes

Why is addition of observables in quantum mechanics commutative?

This story cannot possibly be correct as written, and I agree with Fabian Besnard about why: When you say "We can now define a sum and a product of observables. These are obtained by performing the t …
Qiaochu Yuan's user avatar
32 votes

What is a coalgebra intuitively?

Coalgebras appear naturally in combinatorics as describing ways one can decompose objects into other objects of the same type. For example, the coalgebra structure on $k[x]$ given by $$x^n \mapsto \s …
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

The physical intuition behind the orbit method comes from the notion of quantizing a classical system: the rough idea, as I understand it, is that orbits of the action of $G$ on $\mathfrak{g}^{\ast}$ should …
2 votes

Abstract nonsense versions of "combinatorial" group theory questions

Philosophically I think of the Sylow theorems as a statement about "localization of groups" at a prime. For abelian groups one can make this precise, since abelian groups, as Z-modules, can be realiz …
Qiaochu Yuan's user avatar
15 votes

Why the Dold-Thom theorem?

This won't involve any geometry, but here is a model-independent description of the situation as I understand it. I will not prove anything. The very short summary is that The infinite symmetric …
Qiaochu Yuan's user avatar
108 votes
10 answers
38k views

What is (co)homology, and how does a beginner gain intuition about it?

What's the intuition behind the definition of the boundary operator in simplicial homology? In what sense does homology count holes? …
10 votes

Proofs by induction

Let me see if I can address what I think is the underlying question. Probably most of the proofs by induction you've seen have been of the form "show that this identity $P(n)$ holds," where you are g …
Qiaochu Yuan's user avatar
4 votes

Heuristic explanation of why we lose projectives in sheaves.

We can turn the question around to ask: why do we have projectives in module categories? One answer is that we know we have a plentiful supply of projectives because free modules are projective. Abstr …
Qiaochu Yuan's user avatar
14 votes

Examples of using physical intuition to solve math problems

Mark Levi's book The Mathematical Mechanic is full of elementary and beautiful examples of this kind. Some examples are also given in this blog post by Yan Zhang. A classic example is a "proof" that …
50 votes
6 answers
6k views

Intuition for the last step in Serre's proof of the three-squares theorem

Bjorn Poonen, after presenting this proof in class, remarked that he had no intuition for why this should work. Does anyone have a reply? …
Qiaochu Yuan's user avatar

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