The math of the Rubik's cube

Erik Demaine's collection of Rubik's-cube-type puzzles includes cubes with five, six, and seven squares to a row, as well as one of the original cubes, signed by its inventor (close-up below).Photo: Dominick Reuter Last August, 30 years after the Rubik’s cube first appeared, an international team of researchers proved that no matter how scrambled a cube got, it could be solved in no more than 20 m