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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.

393 votes

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

"If a set X is smaller in cardinality than another set Y, then X has fewer subsets than Y." Althought the statement sounds obvious, it is actually independent of ZFC. The statement follows from the …
263 votes

What are the most misleading alternate definitions in taught mathematics?

Many topics in linear algebra suffer from the issue in the question. For example: In linear algebra, one often sees the determinant of a matrix defined by some ungodly formula, often even with specia …
174 votes
Accepted

Solutions to the Continuum Hypothesis

Since you have already linked to some of the contemporary primary sources, where of course the full accounts of those views can be found, let me interpret your question as a request for summary accoun …
Joel David Hamkins's user avatar
173 votes

Most 'unintuitive' application of the Axiom of Choice?

I have enjoyed the other answers very much. But perhaps it would be desirable to balance the discussion somewhat with a counterpoint, by mentioning a few of the counter-intuitive situations that can o …
148 votes
Accepted

Nontrivial theorems with trivial proofs

Bertrand Russell proved that the general set-formation principle known as the Comprehension Principle, which asserts that for any property $\varphi$ one may form the set $\lbrace\ x \mid \varphi(x)\ \ …
136 votes

Has philosophy ever clarified mathematics?

I find the case of Alan Turing's development of the concept of computatibility to be an example. Before Turing, the logicians had no clear concept of what it means to say that a function is computable …
135 votes
43 answers
38k views

What are the most attractive Turing undecidable problems in mathematics?

What are the most attractive Turing undecidable problems in mathematics? There are thousands of examples, so please post here only the most attractive, best examples. Some examples already appear on …
126 votes

The most outrageous (or ridiculous) conjectures in mathematics

W. Hugh Woodin, at a 1992 seminar in Berkeley at which I was present, proposed a new and ridiculously strong large cardinal concept, now called the Berkeley cardinals, and challenged the seminar audie …
115 votes

Examples of common false beliefs in mathematics

"Either you can prove the statement, or you can find a counterexample." This statement is usually applied to universal statements, those having the form $\forall x\ \varphi(x)$, where the concept o …
105 votes
Accepted

Have you solved problems in your sleep?

On several occasions it has happened that I have made a key insight while sleeping or drifting in and out of sleep. For example, one of the critical ideas in my paper Joel David Hamkins, Gap forcing, …
104 votes

Theorems with unexpected conclusions

My favorite example of this phenomenon is Goodstein's Theorem. Take any positive number $a_2$, such as the number $73$, and write it in complete base $2$, which means write it as a sum of powers of $2 …
95 votes

Examples of eventual counterexamples

The essence of the phenomenon of eventual counterexamples is that a certain pattern that holds among small numbers, turns out not to be universal. In the very best examples, such as the examples provi …
94 votes

Mistakes in mathematics, false illusions about conjectures

Computer designers and programmers dreamed, from the earliest days of the computer, of a computer that could play chess and win. Even Alan Turing had that dream, and designed turochamp, the first ches …
88 votes

Examples of common false beliefs in mathematics

"It is impossible in principle to well-order the reals in a definable manner." To be more precise, the belief I am talking about is the belief that well-orderings of the reals are provably chaotic in …
79 votes

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

"There is no definable well-ordering of the real numbers." Although many mathematicians simply believe this statement to be true, actually, it is independent of ZFC. In Goedel's constructible univer …

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