Sudoku - difficulty classification

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Sudoku - difficulty classification

Postby swaatacba » Sat Feb 08, 2020 10:22 am

This morning's Sudoku from the UK Guardian newspaper https://www.theguardian.com/lifeandstyle/2020/feb/08/sudoku-4704-expert was classified "expert". The puzzle has 28 given values.

My Excel based software is designed to post those elements of the solution that can be solved simply by applying the Sudoku's definition rules, leaving me to scratch my head over the more sophisticated stuff. To my surprise, the repeated application of the definition rules solved the whole puzzle. https://drive.google.com/open?id=10FfLrMrFcyzGkZkmKM2HiRsgMwmciljz

Was the difficulty classification wrong?
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Re: Sudoku - difficulty classification

Postby m_b_metcalf » Sat Feb 08, 2020 11:45 am

swaatacba wrote:Was the difficulty classification wrong?

Yes, wildly. It's very, very, easy (SE rating 1.2).

Many clues are redundant. Removing four of them gives this minimal, and harder, puzzle:
Code: Select all
 . . . . . . 6 . .
 2 . . . . . . . .
 . . . 8 . . . . 4
 5 . . 4 2 . . . .
 . 1 . . . 9 . . .
 . . 3 . . 1 7 . .
 8 5 . 1 . . 3 . .
 . 7 2 . 6 . 5 . .
 . . 6 . 7 4 . 1 .


Regards,

Mike Metcalf
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Re: Sudoku - difficulty classification

Postby swaatacba » Sat Feb 08, 2020 2:07 pm

Hi Mike

Thank you. I'll try that. At least, I know the solution.

Stephen
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Re: Sudoku - difficulty classification

Postby swaatacba » Sat Feb 08, 2020 5:29 pm

Mike

Your revised puzzle was definitely a challenge. I hate hidden multiples. They are so difficult to spot.

Stephen
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Re: Sudoku - difficulty classification

Postby swaatacba » Tue Feb 11, 2020 8:03 pm

m_b_metcal
wrote
Many clues are redundant. Removing four of them gives this minimal, and harder, puzzle:


I thought that I had solved this harder puzzle, but there are 2 possible solutions.

The logic as far as here is consistent and unique.

Wrong_Analysis.png
Wrong_Analysis.png (17.79 KiB) Viewed 217 times


In cell R1C8 we have 2 possible solutions - either 5 or 8
Wrong_Analysis-Answer A.png
Wrong_Analysis-Answer A.png (11.46 KiB) Viewed 217 times

Wrong_Analysis-Answer B.png
Wrong_Analysis-Answer B.png (12.38 KiB) Viewed 217 times


Either choice delivers a solution that appears superficially valid. Notwithstanding my earlier belief that I had solved the puzzle, I can't be sure which is right.
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Re: Sudoku - difficulty classification

Postby dobrichev » Tue Feb 11, 2020 9:11 pm

First solution has less 4s on the top row, leaving room for 1.
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Re: Sudoku - difficulty classification

Postby swaatacba » Tue Feb 11, 2020 10:11 pm

First solution has less 4s on the top row, leaving room for 1.


Well spotted. (I should have put that solution through my checking program.)

Even though solution A looks right, I am not convinced by the logical processes.
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