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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.

0 votes

Unconventional types of induction

The following induction method is almost never mentioned: Step 1: Prove something is true for $n = i$, for $i$ with $1 \le i \le k-1$, where $k \ge 2$ may be arbitrarily chosen. Step 2: Assuming it' …
15 votes

What are some examples of ingenious, unexpected constructions?

I think Elkies' construction of numbers $a,b,c,d$, such that $a^4 + b^4 + c^4 = d^4$, disproving Euler's sum of powers conjecture, is quite impressive. Another nice construction from number theory is …
6 votes

Contest problems with connections to deeper mathematics

A problem on Tournament of the towns 2002 Fall/A-level: A sequence with first two terms equal to $1$ and $24$ respectively, is defined by the following rule: each subsequent term is equal to the smal …
1 vote

Theorems that are 'obvious' but hard to prove

Very good candidates for this question are theorems that amount to saying that some sequence behaves randomly in a way. Both the fact that the statements are obvious and the fact that they are usually …
15 votes

Awfully sophisticated proof for simple facts

An olympiad-type question I once tried to solve was: prove that all integers $>1$ can be written as a sum of two squarefree integers$^{[1]}$. The proof I came up with (which uses at least $3$ non-triv …