The maps are generated randomly but I know from Appel & Haken's work that four colours will always be sufficient. The number of countries can be varied. Your score depends on which level you have selected.

You can also use the 1, 2, 3 & 4 keys for colour selection.

Extension activities:

1. Explore colouring in with only three colours.

2. How can you tell in advance whether this is possible?

3. Martin suggests that "If a country has an odd number of neighbours, you will need four colours."

4. Check out Martin's idea - find a counter-example.

5. Refine Martin's idea so it is always true.

6. Let the rest of the class test your idea.

Appel Haken really needs the touchpad to function properly. However, these keys have been added:

* o = right click = reset
* square or spacebar = new round
* tick or enter = change level
* the number keys (1 to 4) will choose a colour

Appel Haken will now work properly without alteration in a non-Sugar environment.

I've now revisited all my activities. To make it easy to recognise these revisited versions, they all start at Version number 21. You can easily tell which version you have by pressing the v key - if this gives no response, then you have an older version.

• View Older Versions