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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.

-1 votes

Each mathematician has only a few tricks

A trick that is used daily is Zorn's Lemma. Sure, every mathematician knows it, but it certainly helped prove non-trivial propositions and it is in daily use. I would consider it in a list of the Top …
0 votes

Elementary + short + useful

I would show the students the Cantor-Schroeder-Bernstein Theorem and tell them they can themselves contribute to the list of elementary by providing a solution to the following open problems: Does th …
11 votes

Examples of eventual counterexamples

One could reasonably conjecture that there are no positive integers $a,b,c$ satisfying $$\frac{a}{b+c} + \frac{b}{a+c} + \frac{c}{a+b} = 4.$$ I say "reasonably", because the smallest integers satisfyi …
5 votes

Not especially famous, long-open problems which anyone can understand

If $2^x$ and $3^x$ are integers for some positive real number $x$, does this imply that $x\in\mathbb{N}$?
3 votes

Prominent non-mathematical work of mathematicians

It has been noted already that Noam Elkies is an accomplished composer. What I find at least as extraordinary about him is that he can hum-whistle some of Bach's two-part inventions. (Anecdotal "evide …
1 vote
1 answer
359 views

Examples of "irregularities" in mathematics, other than prime numbers [closed]

Prime numbers are the prime example (no pun intended) for something that arises apparently without describable patters; we know that infinitely many exist, that gaps between them can be arbitrarily la …
34 votes
9 answers
5k views

Decision problems for which it is unknown whether they are decidable

In computability theory, what are examples of decision problems of which it is not known whether they are decidable?
4 votes

Which mathematical definitions should be formalised in Lean?

Why formalize complex objects like topological groups when so much fun can be had with natural, simple objects like undirected graphs? There's also tons of open questions involving these simple beings …
2 votes

Tweetable Mathematics

Erdös-Faber-Lovasz conjecture: If $n$ copies of $K_n$ have pairwise intersection of $\leq 1$, you can color all points with $n$ colors.
6 votes

Concepts in topology successfully transferred to graph theory and combinatorics with non-tri...

Infinite graphs have been used as a "discrete version" of topological spaces, for instance infinite Cayley graphs as a discretisation of homogeneous spaces). Gromov constructed homogeneous spaces out …
4 votes
1 answer
321 views

Maximality without Zorn

When confronted with finding an object that is maximal with regard to some ordering relation, most of us have the reflex to use Zorn's Lemma. I am interested in instances of proving the existence of …
Dominic van der Zypen's user avatar
20 votes

Examples of common false beliefs in mathematics

False belief: ${\cal P}(\omega)$ has only countable chains with respect to $\subseteq$. It seems mind-boggling to me that you can start with $\emptyset$, and "add stuff" uncountably many …
11 votes

Proposals for polymath projects

A conjecture that can be stated in so simple terms that it is hard to classify, is Frankl's Union-Closed Sets Conjecture. It would be fantastic to see this solved.
1 vote

What are examples of good toy models in mathematics?

Boolean algebras are toy models for distributive lattices, which in turn are toy models for lattices in general (and partially ordered sets).
5 votes

Solving algebraic problems with topology

Using Priestley duality for distributive lattices and compact, totally disconnected ordered topological spaces, many purely algebraic questions have been solved using quite simple topological tools. F …

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