## Sudoku - difficulty classification

Everything about Sudoku that doesn't fit in one of the other sections

### Sudoku - difficulty classification

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?
swaatacba

Posts: 40
Joined: 09 February 2017

### Re: Sudoku - difficulty classification

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

m_b_metcalf
2017 Supporter

Posts: 11689
Joined: 15 May 2006
Location: Berlin

### Re: Sudoku - difficulty classification

Hi Mike

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

Stephen
swaatacba

Posts: 40
Joined: 09 February 2017

### Re: Sudoku - difficulty classification

Mike

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

Stephen
swaatacba

Posts: 40
Joined: 09 February 2017

### Re: Sudoku - difficulty classification

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 (17.79 KiB) Viewed 311 times

In cell R1C8 we have 2 possible solutions - either 5 or 8
Wrong_Analysis-Answer A.png (11.46 KiB) Viewed 311 times

Wrong_Analysis-Answer B.png (12.38 KiB) Viewed 311 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.
swaatacba

Posts: 40
Joined: 09 February 2017

### Re: Sudoku - difficulty classification

First solution has less 4s on the top row, leaving room for 1.
dobrichev
2016 Supporter

Posts: 1811
Joined: 24 May 2010

### Re: Sudoku - difficulty classification

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.
swaatacba

Posts: 40
Joined: 09 February 2017