A simple sequence of numbers, which I encountered during intermediate school, in my math book. Any person that finished intermediate school possesses the necessary toolbox required to solve this puzzle. Yet, so far, I have never found anyone that could solve it. I have had many collegues and friends try (and most of them are engineers), but it still remains unsolved.
I have to admit, I didn't solve it either, at the age of 14, but I certainly had the matemathical knowledge to be able to solve it, and the puzzle have been stuck in my mind ever since. It's really great (in my mind).
The start of the sequence is:
8 5 4 9 1 7 6 ...
Considering all the problem solvers here, I assume it will be solved pretty quick :) But it would be fun if it remained unsolved for just a little longer :)