Stuck on LA Times Sudoku page (no registration required), a 5-star puzzle from Friday December 26th. Link. Given numbers are in green; my inserts are in black.
I always seem to get to this point and get stuck. There’s obviously a technique for getting past it, but the techniques (X-wings and swordfish) are hard to grasp. What am I looking for here?
R2C9 cannot be 6, because the only places that can be 6 in the second 3x3 square are in row2. That means that R2C9 can only be 1 or 8.
It is then possible to show by a chain of either/ors that if R6C7 is not 1, then R2C3 has to be 1, and vice-versa. This means that the two squares at R2C7 and R6C3 cannot be 1.
This opens up a chain of either/ors for the 1s that eliminate further candidates from being 1s. Some of the eliminated squares than have only two numbers left and can be scouted for either/or chains for other numbers, etc.
P.S., I couldn’t do this without a computer aid that shows me remaining candidates for each square.
I was using Simple Sudoku to display the candidates but it doesn’t solve it automatically. When I plug in Fish’s numbers I get the following results:
R7C2 is either 4 or 8. R3C2 is 1 or 4. R3C9 is 1 or 6. R5C9 is 6 or 8.
R5C2 can’t be 8 if R7C2 is 8. If it’s 4, then R3C2 has to be 1; R3C9 has to be 6; and R5C9 has to be 8. In which case R5C2 can’t be 8 anyway. I believe most Sudoku references call this type of proof “chaining”.