When there is no visible solution left (and usually there is, but it is hidden), one can resort to trial and error, which consists of choosing a number in a certain cell (preferably one that contains only a pair of candidates) and testing it. The choice of one of two possible candidates in a cell does not work properly when there is not a reasonable number of cells with only two candidates.
Did you finish the course? The question that remains is: do these techniques solve Sudoku puzzles considered extremely difficult?
Then it’s time to demonstrate this on the GALILEU Sudoku, considered one of the hardest in the world.
