I have solved up to this point. How do I proceed without guessing?
closed as unclear what you're asking by Marius, JonMark Perry, Lolgast, Glorfindel, Mithrandir Feb 15 '18 at 9:42
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I don't 100% trust this logical device myself, but if the top-left cell is not a 2 then we have a paradox with the 4 and 8's against the Sudoku being valid (i.e. a single solution). So the top left cell is a 2.