## Unsolvable Sudoku PuzzlesAll too often I try to solve a Sudoku and end up believing it is impossible to solve and give up. I go through every strategy and trick I can think of and still can not find a square to solve. ## MistakesOne reason that my puzzle may be 'unsolvable' is that at some stage an incorrect allocation has been made. This may not be immediately obvious when you made the mistake. Here is an example of a puzzle which can not be solved, consider square In this case the mistake was assigning
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see it in Sudoku Dragon click here... SudokuDragon will spot this incorrect allocation using its hint facility and let you use undo to go all the way back and correct the square allocation. Here is another example unsolvable puzzle. In this case we have two squares Going back and 'Undoing' all the allocations, it turns out that allocating Sudoku Dragon has a hint that will show any incorrect allocations that you have made. ## Multiple SolutionsAnother example of a puzzle that can not be solved is when it has multiple legitimate solutions. Following all the rules you end up with two or more different solutions from the same starting squares. Here is the puzzle and two solutions, both of which fulfill all the rules. We have highlighted the squares that differ in the solution.
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see it in Sudoku Dragon click here... A puzzle with multiple solutions can often be created by accidentally missing off a square when copying a puzzle from a book or newspaper. Or it may be the case that it is a badly formed puzzle, and the person/program generating it has made a mistake. It is hard to devise puzzles that are challenging enough and yet have a unique solution. ## Hard PuzzlesSometimes we shout 'unsolvable puzzle' where we fail to spot the way forward. Consider this 'hard' puzzle, after solving all the 'easy' squares it reaches this point.
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see it in Sudoku Dragon click here... There are no more obvious squares to solve. However it is still solvable as there are more complex strategies available that yield more squares, in this case it is hidden twins and shared subgroups that permit further squares to be allocated. So often it is not spotting possible usage of a rule that can make you think you have an unsolvable puzzle. After you have solved a few more squares you may well get stuck again. This time the twin; shared subgroup rules although excluding some possibilities do not allow any more squares to be solved In fact the full force of alternative pair analysis needs to be used to be able to solve the next square. A look at the squares where These hard puzzles may seem 'unsolvable' to all but the committed Sudoku expert. We sometimes receive puzzles that our users consider 'unsolvable', but with a bit of study they can usually be solved. To get the full Sudoku solution, including hints, easy printing and unlimited puzzle generation download our Sudoku puzzle solver for a free 23 day trial. Any comments on this page ? Click here to contribute Copyright © 2005-2015 Sudoku Dragon |