Help please! Is this extreme puzzle unsolvable?

Post the puzzle or solving technique that's causing you trouble and someone will help

Help please! Is this extreme puzzle unsolvable?

Hi, I would be really grateful for some help with this puzzle which has me stumped. It is from the magazine PuzzleLife Sudoku Extreme.

I know from scanning the answers that the resolved cells are correct.

Using single digit colouring on the 1s I have tried eliminating 1 as a candidate from Row 2, Column 3; Row 5, Column 1; and Row 5, Column 4, as it appears to be a Colour Trap where these three cells can see opposite cells for a 1 in those rows or columns.

However, this appears to leave me with a puzzle which is unsolvable without resorting to trial by error.

Is there another solution?

Thanks

IMG_1197.JPG (158.22 KiB) Viewed 322 times
MarkRobert

Posts: 6
Joined: 07 March 2017

Re: Help please! Is this extreme puzzle unsolvable?

An X-Wing and a BUG+1 will reduce the puzzle to being solvable with just singles.

JasonLion
2017 Supporter

Posts: 640
Joined: 25 October 2007
Location: Silver Spring, MD, USA

Re: Help please! Is this extreme puzzle unsolvable?

745289613...5374282384619753.264..97.74.....295..728.45.78..3.14....3....637.42.9

An alternative to Jason's solution will avoid the BUG +1 situation, which some may find a bit obscure.

Code: Select all
`*-----------------------------------------------------*| 7    4    5     | 2    8    9     | 6    1    3     || 16   19   169   | 5    3    7     | 4    2    8     || 2    3    8     | 4    6    1     | 9    7    5     ||-----------------+-----------------+-----------------|| 3   a18   2     | 6    4    58    | 15   9    7     || 168  7    4     | 139  19   58    | 15   36   2     || 9    5   *6-1   | 13   7    2     | 8    36   4     ||-----------------+-----------------+-----------------|| 5   c29   7     | 8    29   6     | 3    4    1     || 4   b28  d19    | 19   25   3     | 7    58   6     || 18   6    3     | 7    15   4     | 2    58   9     |*-----------------------------------------------------*`

There is an XY chain in the cells marked a-b-c-d which proves that at least one of cells a or d must be 1, so Row 6 Column 3 must not be 1, so it solves to 6 and the rest of the puzzle solves in singles.

If you are unfamiliar with XY chains let me know and I'll explain further.

Leren
Leren

Posts: 3315
Joined: 03 June 2012

Re: Help please! Is this extreme puzzle unsolvable?

Thanks Leren, I haven't quite got to grips with XY chains yet. An explanation on how to identify and apply them would be great.
MarkRobert

Posts: 6
Joined: 07 March 2017

Re: Help please! Is this extreme puzzle unsolvable?

OK, I'll explain how this particular XY chain works in words and refer you to some teaching sites that have good general explanations and nicely worked examples.

An XY chain consists of a string of bi-value cells, where one candidate is common to adjacent cells in the chain and the adjacent cells can see each other.

The idea is to start and end on the same candidate, and you can then eliminate that candidate from cells that can see the first and last cells in the chain.

So, with this one, start by assuming that cell a is not 1, then it must be 8. So cell b can't be 8, so it must be 2. So cell c can't be 2 so it must be 9. So cell d can't be 9, so it must be 1.

So, summarizing this, if cell a is not 1, cell d is 1. You then reverse the argument and start with cell d and assume that it is not 1 and following the cells in order d-c-b-a you will conclude that cell a is 1.

What all this proves is that at least one of cell a and cell d must be 1. Since the target elimination cell, which I've marked with a * in the diagram can see both cells a and d it can't be 1.

This particular XY chain is of length 4 but they can be of just about any length. The shortest useful XY chain is of length 3 and is called an XY Wing, which you may have heard about.

The teaching site XY chain examples can be found here and here and just for good measure you can also read about XY Wings on the same sites here and here.

Leren

PS An alternative solution is to first apply the X Wing, as Jason suggested, as shown in the following diagram : The X Wing is in 1's in rows 68 / c34 in the cells marked * and eliminates the 1's in r2c3 and r5c4 as shown.

Code: Select all
`*-----------------------------------------------------*| 7    4    5     | 2    8    9     | 6    1    3     || 16   19   69-1  | 5    3    7     | 4    2    8     || 2    3    8     | 4    6    1     | 9    7    5     ||-----------------+-----------------+-----------------|| 3    18   2     | 6    4    58    | 15   9    7     || 168  7    4     | 39-1 19   58    | 15   36   2     || 9    5   *16    |*13   7    2     | 8    36   4     ||-----------------+-----------------+-----------------|| 5    29   7     | 8    29   6     | 3    4    1     || 4   28   *19    |*19   25   3     | 7    58   6     || 18   6    3     | 7    15   4     | 2    58   9     |*-----------------------------------------------------*`

This brings you to here, where a length 3 XY chain ie an XY Wing in cells a, b and c, solves r9c1 and the whole puzzle. I suspect that this was what the magazine had in mind as the preferred solution.

Code: Select all
`*-----------------------------------------------------*| 7    4    5     | 2    8    9     | 6    1    3     ||a16   19  b69    | 5    3    7     | 4    2    8     || 2    3    8     | 4    6    1     | 9    7    5     ||-----------------+-----------------+-----------------|| 3    18   2     | 6    4    58    | 15   9    7     || 168  7    4     | 39   19   58    | 15   36   2     || 9    5    16    | 13   7    2     | 8    36   4     ||-----------------+-----------------+-----------------|| 5    29   7     | 8    29   6     | 3    4    1     || 4    28  c19    | 19   25   3     | 7    58   6     ||*8-1  6    3     | 7    15   4     | 2    58   9     |*-----------------------------------------------------*`

An alternative move at this point, suggested by Jason, is to apply a BUG+1 move, which forces r5c1 to be 1. If you really wan't know why this is True, read about the BUG principle here .

Leren
Leren

Posts: 3315
Joined: 03 June 2012

Re: Help please! Is this extreme puzzle unsolvable?

D'oh. I can't believe I did not spot the X-Wing. Thanks Jason and Leren for the XY explanation. Definitely something I need to learn.
MarkRobert

Posts: 6
Joined: 07 March 2017