### Rubik's Cube

In the case of puzzles, consider the well-known

*Rubik’s Cube*puzzle. Hungarian Enro Rubik invented it in 1974, when he discovered it was difficult to realign the colours to match on all six sides of a coloured spinning cube. He was not even sure if he would ever be able to return his invention to its original position! Since then the standard 3x3x3 version and more recently 4x4x4 version has been popularised worldwide.

It can be shown that there are approximately different arrangements of the standard Rubik’s cube and, of course only one of those is the desired solution. So it is not surprising that many enthusiasts have struggled for hours to solve a scrambled cube without ever finding a solution. However, experienced players are well aware that a speedier solution can be found which involves only about 30 turns. Part of the strategy recognises that the smaller faces can be grouped respectively as corner-cube, side-cube or centre-cube faces. These groups can then be considered to follow different rules.

In general, an NxNxN spinning cube will require different solution strategies as the value of N increases. Rubik’s Revenge corresponds to the case when N = 4. Developed by Péter Sebestény, was briefly called the *Sebestény* cube until a last-minute decision changed the puzzle’s name to attract fans of the original Rubik’s cube. The *Professors Cube* (also known as *Rubik’s Professor*) is a 5x5x5 version developed by Udo Krell.

In the 3x3x3 version of the Rubik’s cube, there are eight corner-cubes each with three faces (total **24**), twelve side-cubes each with two faces (total **24**) and six centre-cubes with one face (total **6**), making a total of **54 visible faces**. Have a go at the **Rubik’s cube** puzzle below and see how far you can get to:

**Applet courtesy of www.flashgamehq.com**