Dewdney's column in this month's Scientific American presents a puzzle
which he claims to be comparable in difficulty to a Rubik's cube. (Like he
claims there are people who can do the cube but haven't done this thing.)
Interested Cube-Hackers might find it diverting to give it a try. It
didn't take me long to devise a sufficient set of tools for solving it
using only pencil-and-paper. Personally I think it was significantly
easier than a cube, but perhaps it is harder than the majority of other
permutation puzzles I have gotten my hands on in the last few years.
(But perhaps not. I did this without having a physical model of it in my
hands, so perhaps that has caused me to overlook something.)
You know, I don't recall ever hearing anybody speculate about just what
makes a permutation puzzle interesting and/or difficult. I guess the group
has to be large and have a large diameter, and there should be a scarcity
of short identities...