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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.

9 votes

Examples of ZFC theorems proved via forcing

In descriptive set theory, there is a significant number of results that have been established using forcing; typically, dichotomy theorems such as Silver's (a $\Pi^1_1$ equivalence relation has only …
Martin Sleziak's user avatar
17 votes

Well known theorems that have not been proved

I am not sure whether this qualifies as "well-known". Anyway, in set theory, in the study of the partition calculus (transfinite generalizations of Ramsey's theorem), effort centered for a while in st …
Andrés E. Caicedo's user avatar
21 votes

Which journals publish expository work?

There is a new journal of the European Mathematical Society that seems perfect for these articles: EMS Surveys in Mathematical Sciences. The description at the link reads: The EMS Surveys in Mathemat …
Martin Sleziak's user avatar
6 votes

Most memorable titles

Marginalia to a theorem of Silver (see also this link) by Keith I. Devlin and R. B. Jensen, 1975. A humble title and yet, undoubtedly, one of the most important papers of all time in set theory.
Martin Sleziak's user avatar
4 votes

Atlas-like websites on specific areas of mathematics

A database of number fields, by Jürgen Klüners and Gunter Malle. (Note this is not the same as the one mentioned in this answer.) The site also provides links to similar databases.
Andrés E. Caicedo's user avatar
24 votes

What definitions were crucial to further understanding?

In set theory, definitely the notion of a Woodin cardinal. First, it is not an entirely straightforward notion to guess. Significant large cardinals were up to that point defined as critical points …
Andrés E. Caicedo's user avatar
46 votes

Solutions to the Continuum Hypothesis

(1) Patrick Dehornoy gave a nice talk at the Séminaire Bourbaki explaining Hugh Woodin's approach. It omits many technical details, so you may want to look at it before looking again at the Notices pa …
Andrés E. Caicedo's user avatar
35 votes

Proofs of the uncountability of the reals

I thought about this question a while ago, while teaching a topics course. Since one can easily check that $${}|{\mathbb R}|=|{\mathcal P}({\mathbb N})|$$ by a direct construction that does not involv …
Community's user avatar
  • 1
91 votes

Proofs that require fundamentally new ways of thinking

The method of forcing certainly fits here. Before, set theorists expected that independence results would be obtained by building non-standard, ill-founded models, and model theoretic methods would be …
Stefan Geschke's user avatar
23 votes

Examples of common false beliefs in mathematics

In descriptive set theory, we study properties of Polish spaces, typically not considered as topological spaces but rather we equip them with their "Borel structure", i.e., the collection of their Bor …
Community's user avatar
  • 1
11 votes

Important open problems that have already been reduced to a finite but infeasible amount of ...

This is an elaboration of a comment on Suvrit's answer. Ramsey numbers can be defined for (infinite) ordinals, just as in the finite case: $r(\alpha,\beta)$ is the least $\gamma$ such that for any $ …
Andrés E. Caicedo's user avatar
31 votes

What notions are used but not clearly defined in modern mathematics?

There are several examples in set theory; the three I mention are related so I will include them in a single answer rather than three. 1) Large cardinal notion. I have seen in print many times t …
Andrés E. Caicedo's user avatar
8 votes

Books you would like to read (if somebody would just write them…)

AD${}^+$ by Hugh Woodin.
Andrés E. Caicedo's user avatar
7 votes

Constructions unique up to non-unique isomorphism

In recent work in set theory the concept of "canonical structure" has emerged, in connection with combinatorial work on pcf theory. The idea is that there are many constructions that depend on the axi …
Andrés E. Caicedo's user avatar
13 votes

Open problems in Euclidean geometry?

In recent years there have been a good amount of surveys and publications on "computational" or "combinatorial" geometry, and looking at them may give you a good idea of current questions. Specificall …
Andrés E. Caicedo's user avatar

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