OK, here's a thing: imagine you have in your hands a pocket calculator. (That probably gives you some idea of how many years ago it was that I discovered this.) The digits 0-9 are arranged on the keypad like this:
Code: Select all
7 8 9
4 5 6
1 2 3
0
Now, tap in any 4-digit number that conforms to the following rule: Pick a starting digit, and then imagine that digit as being at one corner of a regular 4-sided shape on the keypad. By "regular 4-sided shape", I mean a square, rectangle, parallelogram or rhombus. Tap in the 4-digit number formed by tracking the corners of that shape around the keypad, either clockwise or anticlockwise. For example:
- Starting with the 7 and tracing a clockwise square round the outer corners of the main block of 9 digits gives you 7931.
- Starting with the 8, you could trace an anticlockwise parallelogram to give 8129. Also, you can bring the 0 into play by entering 8206.
- Starting with the 5 and working clockwise, you could trace a square to give 5632, or a rhombus to give 5302.
- You can also start with a notional leading 0 to produce a 3-digit number such as (0)341.
Why?