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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.

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 …
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 …
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 …
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 $n$.
-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 …