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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.

-2 votes

Examples of theorems with proofs that have dramatically improved over time

Liouville's Theorem that there exists a transcendental number had its proof greatly improved by Cantor who showed that a mere counting argument suffices. Liouville's argument needs facts about how ra …
22 votes

Results true in a dimension and false for higher dimensions

The sausage conjecture: Which way of arranging $M$ unit balls in $\mathbb{R}^n$ minimises the content of their convex hull? For $M$ small the answer is always to arrange them along a line, so that t …
30 votes

Decision problems for which it is unknown whether they are decidable

In Conway's Game of Life, the problem of deciding whether a given pattern with finitely many live cells is a Garden of Eden (i.e. whether it lacks a predecessor). The main obstacle is that there could …
5 votes

Atlas-like websites on specific areas of mathematics

The Cunningham Project seeks to factor the numbers $b^n \pm 1$ for $b = 2$, $3$, $5$, $6$, $7$, $10$, $11$, $12$, up to high powers $n$.
16 votes

Big list of comonads

Given a topology on a set $X$, let $2^X$ be the poset of subsets of $X$ ordered by inclusion. Then the interior operator for the topology is a comonad on $2^X$. In fact the topologies on $X$ correspon …