A Sudoku You'll Never Solve

• by
• Jeroen De Meyer

Thursday, November 12, 2015

A couple of months ago, a friend of mine showed me a Sudoku puzzle and said 'You can't solve that one'. I've always liked puzzles but never really paid much attention to Sudoku, so when I tried to solve it, I had to admit he was right... I don't know about you but I really don't like it when people tell me that I can't do something, especially when they're right. So I started digging in the logic behind Sudoku puzzles.

The first thing I learned is that there's a minimum number of clues a Sudoku must have, and it's been proven by Professor Gary McGuire, a Mathematician from University College Dublin in Ireland, that this number is 17. The proof wasn't easy. It took McGuire and his team 7 million core-CPU hours on a supercomputer (the equivalent of a single computer running for 7 million hours) and a year of actual elapsed time.

50,000 Sudoku with only 17 clues

As my friend's Sudoku also had 17 clues, I knew exactly what I wanted to do now. Let's make a website that generates 17 clues Sudoku puzzles and solves them for you, when you're not able to.

After a lot of reading and coding, I launched sudokus.co.nf, a website where you can try to solve nearly 50,000 Sudoku with only 17 clues. The list of nearly 50,000 seemed to contain every possible, valid 17 clues Sudoku grid. To give you an idea, if you'd solve one each day it would take you over 130 years to solve them all.

The second thing I noticed, is that although a Sudoku with lesser clues is not necessarily more difficult, the number of clues should to be taken into consideration when determining the difficulty of a grid. As a result the list contained a lot of (really) hard Sudoku, much harder than the one my friend gave me. If you don't believe me try solving the above puzzle (solution here) or visit sudokus.co.nf for more. I bet you can't solve them.

About the Author

Jeroen De Meyer is a Computer Science student from Belgium, with a great interest in logic. In his spare time he likes to program solutions to all kind of puzzles.

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