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I've moved onto "expert" sudoku's in my app (Sudoku.com on IOS) and whilst I am getting by solving some, others I'm having a hard time with.

With this one, I used a hint so the AI could note all the fields up for me but it hasn't put a note for 1 in this box. Why? I can't see anything which would rule it out for "certain". I know there are 2 possible 1's on the same line in block 9 but it also has the possibility of being on the line above so isn't certain in my mind.

Can someone explain why the AI can be sure no 1 should be in this box?

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  • $\begingroup$ I guess you can more or less just solve the sudoku, and in the end you'll get a contradiction for the other ordering of the 1+6 pair in R8C45. (To solve the sudoku, notice that column 5 has a 2+4 pair at the top, so box 2 will have a very useful 156 triple in column 6.) $\endgroup$
    – Bass
    Feb 6 at 15:48
  • $\begingroup$ @Bass I see a 4+5 and 2+4 in the top at column 5. Am I missing a 2+4 pair somewhere? $\endgroup$
    – webnoob
    Feb 6 at 18:40
  • $\begingroup$ @webnoob Ignoring the hints, just notice that column 6 already contains both 2 and 4, so the remaining two entries in box 2 must be 2 and 4. $\endgroup$
    – RobPratt
    Feb 6 at 18:48
  • $\begingroup$ Ah ofc, can't see the wood for the tree's now. Thanks @RobPratt $\endgroup$
    – webnoob
    Feb 6 at 18:59

1 Answer 1

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If there were a one in that cell, there would be no place left for the six to go in its column.

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    $\begingroup$ While this answers the question asked, it isn't very helpful. Why is that the only location for 6 in that column? How has 6 been ruled out from R1C5 and R2C5? $\endgroup$
    – fljx
    Feb 6 at 13:17
  • $\begingroup$ Yes, I agree. According to the notes, 6 can be in 2 places in that block so how have you deduced this? $\endgroup$
    – webnoob
    Feb 6 at 13:43
  • $\begingroup$ Ok, I see now that the 6 only has one place in that column but I guess I'm still struggling to see why a 6 couldn't go in some of the other cells i.e R4C5 and R1C5, R2C5 $\endgroup$
    – webnoob
    Feb 6 at 14:30
  • $\begingroup$ sudokuspoiler.com confirms that the puzzle (just the large black numbers) does have a unique solution, but doesn't explain its reasoning either. Wonder if anyone's built a site that explains its deductions step by step? $\endgroup$
    – Ed Murphy
    Feb 6 at 16:22
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    $\begingroup$ @JaapScherphuis you must have made a typo. Apart from a single inference chain (solved by "simple colouring" by the bot) which restricts the 2s in column 8, the solver doesn't need anything except the basic deductions to solve the puzzle. $\endgroup$
    – Bass
    Feb 7 at 4:03

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