Bruno Joyal
54,711
reputation
Some fun answers of mine:
A real number $x$ such that $x^n$ and $(x+1)^n$ are rational is itself rational
A series of rationals whose every subseries is irrational
The prime spectrum of a Dedekind domain
The same integral, solved in another way
All elements in $\mathbb{Z}/n\mathbb{Z}$ are representable as sum of a square and a cube?
Algebraic varieties in $\mathbf C^n$ have no interior points