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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.

33 votes
18 answers
6k views

Non-rigorous reasoning in rigorous mathematics

I was wondering what role non-rigorous, heuristic type arguments play in rigorous math. Are there examples of rigorous, formal proofs in which a non-rigorous reasoning still plays a central part? Her …
4 votes

Blackbox Theorems

The Lovasz Local Lemma gives a very simple criterion for when certain random events have positive probability. Almost all applications of the Lemma automatically have an algorithm to find such events. …
David Harris's user avatar
  • 3,475
2 votes

Random versions of deterministic problems

While it is not known that $P \neq NP$, it is known that for most oracles $A \subseteq \omega$ we have $P^A \neq NP^A$. This might be interpreted as "evidence" for the conjecture when $A = \emptyset$. …
David Harris's user avatar
  • 3,475
-1 votes

What are some correct results discovered with incorrect (or no) proofs?

Results in complexity theory such as $P \neq NP$. Philosophically, it makes sense that there is a difference between verification and search, and no-one has discovered a counterexample. (Note, tha …
David Harris's user avatar
  • 3,475
9 votes

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

Infinitesimals are almost in this category. Technically, calculus generally uses limits instead of infinitesimals. And there are logical systems (e.g. nonstandard analysis) in which genuine infinites …
David Harris's user avatar
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15 votes

Why are so few operations with arity bigger than 2?

Any $k$-ary relation can be expressed in terms of binary relations by means of projection maps, i.e. introduce new objects which correspond to $n$-tuples of the original objects ($n \leq k$), and intr …
David Harris's user avatar
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