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I am trying to pick up advanced sudoku techniques by practicing with “difficult” puzzles as suggested by my sudoku app.

In this puzzle, I am stuck in the position as shown below. I think I can make use of forcing chains to break the puzzle, but I am unable to pinpoint the exact cell I should look at.

Is it possible to find a forcing chain in this puzzle which greatly simplifies the position? I neither wish to bifurcate, nor resort to a computer solver.

Sudoku Puzzle

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    $\begingroup$ As a comment, you're method of partially filling in unknown squares seems very confusing to me. I either leave them completely blank or fill in all possible options still available $\endgroup$
    – Kevin
    Sep 28, 2020 at 14:33
  • $\begingroup$ @Kevin, I use small digits like that, but only within a square of 9. For instance in the top right square the two "7"s represent the only place within that square that 7 can occur (and so can be used to eliminate all "7s" in the other squares in that row). ¶ But, the "4" in the highlighted cell is the only 4 in that square, which is definitely confusing. $\endgroup$
    – Ray Butterworth
    Jun 5, 2022 at 13:01

1 Answer 1

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You can deduce that

there must be a 1 in column 7 row 1

because

missing values in row 1 are 1,3,4,6. Of these 6 already occurs in column 7, 4 must be in column 7 row 8 or 9 and 3 must be in column 9 row 1 or 2.

and

there is a 6 in column 6 row 4

because

it must be one of 5 or 6 but 5 would force a 6 in row 5 column 6 and a 5 in row 5 column 7 leaving no value for row 5 column 2.

this in turn

leaves only 5 for column 7 row 4

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