LogFAQs > #962352105

joe40001 posted...

Because the digit in R5C5 can only appear in purple, that means it cannot appear in the blue ring above. And I think several sudoku people here like serious_cat could talk about the significance of that ring.

Phistomefel's ring didn't get me very far, but I came to the same conclusion with this equivalent set:

https://gamefaqs.gamespot.com/a/user_image/4/3/8/AAPWHiAAC4rW.jpg
The digit in the center square can only appear in columns 1, 2, 8 and 9 in rows 3, 4, 6 and 7 by either knight's move, diagonal, or plain sudoku rules. That requires the digit in the center square to be diagonally adjacent in those rows, thus eliminating the remaining possibilities left in the Phistomefel boxes. (Plus the additional row of red compared to blue means there's an extra complete set of digits 1-9 in the red squares in addition to the equivalent set.)