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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.

137 votes

Suggestions for special lectures at next ICM

How about a lecture on proof assistants/formal proofs? Most mathematicians are still skeptical of the value of proof assistants, and it's certainly true that proof assistants are still very difficult …
131 votes

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

The lonely runner conjecture. As Wikipedia puts it: Consider $k + 1$ runners on a circular track of unit length. At $t = 0$, all runners are at the same position and start to run; the runners' sp …
121 votes

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

Gourevitch's conjecture1: $$\sum_{n=0}^\infty \frac{1+14n+76n^2+168n^3}{2^{20n}}\binom{2n}{n}^7 = \frac{32}{\pi^3}.$$ 1Jesús Guillera: About a New Kind of Ramanujan-Type Series; Experimental Mathemati …
121 votes

Nonequivalent definitions in Mathematics

Not a word but a piece of notation: Sometimes I have seen $\subset$ used to mean "is a proper subset of" while other times I have seen it used to mean "is a subset of".
96 votes

Examples of interesting false proofs

$$e^i = (e^i)^{(2\pi/2\pi)} = (e^{2\pi i})^{1/2\pi} = 1^{1/2\pi} = 1.$$ I first saw this one many years ago, written on the wall of a bathroom stall in the Princeton University math department.
86 votes
Accepted

Proofs of the uncountability of the reals

Mathematics isn't yet ready to prove results of the form, "Every proof of Theorem T must use Argument A." Think closely about how you might try to prove something like that. You would need to set up …
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75 votes

How would you have answered Richard Feynman's challenge?

There's a certain gaming/sporting aspect to Feynman's challenge that works in his favor. First of all, as phrased, the challenge gives him a 50/50 shot at being right even if he guesses randomly. Al …
69 votes

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

There are infinitely many primes $p$ such that the repeating part of the decimal expansion of $1/p$ has length $p-1$. First explicitly asked by Gauss, now generally thought of as a corollary of Artin …
68 votes

What are some reasonable-sounding statements that are independent of ZFC?

Harvey Friedman has devoted a large portion of his career to finding "natural" statements that are unprovable in ZFC. One example is given at the end of Martin Davis's article "The incompleteness the …
60 votes

Examples of common false beliefs in mathematics

False belief: "There are no known sub-exponential time algorithms for NP-complete problems." This one is tricky for a couple of reasons. The first is that the term "sub-exponential" is sometimes def …
58 votes

Mistakes in mathematics, false illusions about conjectures

Before Erdős and Selberg found an elementary proof of the prime number theorem, G. H. Hardy had predicted that the discovery of such a elementary proof would be cause "for the books to be cast aside a …
57 votes

Old books you would like to have reprinted with high-quality typesetting

I have some experience resurrecting old math books and I want to make a few comments about copyright. First, it is definitely true that except for very old books, someone owns the copyright. Typicall …
51 votes

Request for examples: verifying vs understanding proofs

Don Zagier has a well-known paper, A one-sentence proof that every prime $p\equiv 1\pmod 4$ is a sum of two squares. An undergraduate mathematics major should be able to verify that this proof is cor …
51 votes

Examples of common false beliefs in mathematics

False belief: Saying that ZFC is consistent is the same as saying that if ZFC proves "there are infinitely many twin primes" (for example) then there really are infinitely many twin primes. Everybod …
45 votes

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

The Kneser–Poulsen conjecture in dimension 3: An arrangement of (possibly overlapping) unit balls in space is tighter than a second arrangement of the same balls if, for all $i$ and $j$, the distance …

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