The rules are: Fill in each column, row and each region with the numbers 1-9. Some of the regions run off (for example) the top edge of the grid. They continue up from the bottom edge of the grid. Similarly with the left/right edges. The idea is that if you take a nice square, glue left side to right side, and then the top edge – now a circle – to the bottom edge you get a torus – and hence the adjective toroidal. Although if you too are a dabbler in the dark arts of pedantry, you will have noticed that actually a classic sudoku is also “toroidal”. But arguably not as hard. :)
All that is left to be said is this puzzle deserves a slot by itself. Back to two (or more) next week!
#007 toroidal sudoku – rated hardAll puzzles © Tom Collyer 2009
P.S. The next 2-3 Friday’s might suffer some delays too. I’d say there is approximately a 1/7 chance of the puzzles being published on a Friday, but I’ll nevertheless try to keep to the deadline!