Often I get to a point where I have a puzzle completely filled in except for a bunch of boxes with pairs of numbers that chain around the puzzle. I’ve photo copied them and tried picking one of the two numbers in a box and let the chain collapse according to that choice, and sure enough, one of the two tries completes the puzzle. The other winds up with a contradiction.
To me that’s a guess. I’ve been dying to find an analytical way of choosing the correct one.
Does that situation have a name?
Can anyone steer me to a site that might explain the way to analyze this situation?
Or can anyone explain it themselves?
There’s an interesting page here. Along the right side are a list of solving strategies, starting with “Hidden Singles” and continuing in increasing complexity. Clicking on the name will give a detailed description.
Be warned, it’s going to be a lot more complicated than you think.
Do you have an example Sudoku so we can use that solver to analyze it?
I’ve always considered this a feature of the more complex Sudoku puzzles, BTW. I thought they wanted you to have to guess a bit in order to make it harder.
The situation exactly as you describe, with every box containing a pair of possible numbers, is impossible in a well-formed Sudoku. If every box has a pair of possible numbers, it will always be the case that there are at least two solutions. I’m guessing that you overlooked one box that had three possibilities.
Might I recommend sudokuslam, it has a pretty good cheat function that will show you where and what logical step must be taken at any point. You can also enter in your own puzzle to solve alongside it. It helped my skills quite a bit to see some of the strategies in a graphical real-time example.