h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

A couple of weeks ago I wrote a journal about the Listener crossword - an especially tough but rewarding puzzle which appears in the Saturday edition of the Times newspaper.

Four times a year, they publish a mathematical puzzle rather than a word one, and this week's puzzle was one of those, so I thought I'd give a flavour of it.

The setter's instructions were very long. The grid was cross-shaped like a solitaire puzzle, with one cell for each peg. The middle cell was blanked out. There was a series of clues towards the numbers which were needed to fill the grid. These needed to be entered in hexadecimal - i.e. not only using the digits 0 to 9, but the letters A,B,C,D,E,F too. The clues were generic - there were lists of which numbers were odd, which ones had all letters, which were divisible by 7, etc. It took a lot of trial and error to complete this, but that was when the fun really started.

You then needed to play a game of solitaire using the hexadecimal digits as pegs. With each move, a peg jumps over another one, which is removed. If you got it right, the final peg is in the middle hole of the grid. Not only that, but the removed pegs needed to spell out a particular sequence of numbers, followed by three thematic words, which had to be written below the grid.

The mathematical Listener puzzles are generally easier than the word puzzles, and it was the case again this time - I managed to complete it this morning.

if the puzzle's answers are numbers, then why call it a crossword?

For 48 weeks of the year it is word-based. I'm just covering one of their occasional mathematical ones.

[Amy P]

I would still prefer the word ones.

My wife and I do the Irish Times Sudoku (moderately fiendish). She often gets it quicker than I do, though she claims to be unmathematical and I claim to be pretty competent at maths. She says Sudoku isn't maths, and I say it is. True, any symbols could be used instead of numbers, but it is an exercise in logic.

How do we do it simultaneously? I long ago designed and saved a document consisting of six empty sudoku grids that I print off as needed.

Sudoku certainly is mathematics - it lies within the branch they call recreational mathematics.

Here's the Mathworld entry on Sudoku:

http://mathworld.wolfram.com/Sudoku.html

It's a nice question though; what defines it as mathematical? Not the fact that it is recreational, for a start.