





Bowman's Bingo This is the last of the "last resort" strategies, often derided as Trial and Error. Bowman’s Bingo suffers from the problem of choosing a starting cell as we’ll see, but its one of the few bifurcating strategies that works well for a human, if you’ve got the equipment. MadOverLord came up with the idea and named it after Doug Bowman for his work on Colouring. The Bingo comes from the translucent Bingo chips that work best as place markers. You can do this with bits of paper on a large grid as well as chips  but all must be marked with the numbers 1 to 9 and the sides must be distinguished between up and down. Your grid should have all the candidates marked  the point where you’ve become stuck. We’re going to test for a contradiction that allows us to eliminate a candidate. Choose a cell you want to test and a number in that cell that’s a possible candidate. Place a chip with that number on the cell face down. To "flip" a chip is to turn it over. Now the following steps should be repeated:
If you end up with all your chips face up and no contradiction then you’re not a winner. Clear the chips and try another cell and candidate. If you’ve managed to cover ALL the unsolved squares with faceup chips then you’ve lucked into the solution. Say "Bingo" out load! Bowman’s Bingo doesn’t solve all ‘bifurcating’ Sudokus but if applied thoroughly it will crack more than 80% of them. It’s not a panacea like Tabling or Nishio but it is easier to do and will work better if you are down to your last twenty or so unsolved squares. As a logical process this follows on from many of the chaining techniques discussed earlier. However, when chains branch they are called ‘nets’ and Forcing Nets are closest to this strategy. All candidates on the board have one of four states. They are either Untested, Killed, Up or Down. Chips on the board make a candidate either Up or Down. When you flip a chip to turn it up you are saying “that’s the solution" for that cell. This "Kills" all the other candidates in that cell and you should mentally clear any other down chips on that cell. This is important since we don’t want two upchips (two solutions) on the same cell. Here’s an Evil Sudoku which can now be solved with other strategies but I retain it as the example because redoing images and example is too much work :)
Personally I think the method as originally described is a little over methodological, since we have just drawn a forcing net. But for me it crosses the line into 'trial and error' because it is so arbitrary  both the initially selected cell and the branching off in every direction. It is a true bifurcation method. But it is presented here as one method of searching for eliminations when all else fails. 
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