## yet another difficult sudoku

Advanced methods and approaches for solving Sudoku puzzles

### yet another difficult sudoku

I am stuck yet again with another sudoku which I am trying to solve and would really appreciate some help with :

This is where I am with it so far :

* * * 2 * 7 6 * 3
* * 3 6 1 * 4 * 7
* 6 * 4 3 8 5 * 1

3 1 6 * 8 2 * 4 *
2 * * * 6 1 3 * 8
* 8 * 3 4 * 2 1 6

6 7 9 8 * 4 1 3 *
4 * * * 7 3 8 6 *
* 3 * * * 6 * * 4

The solution to the puzzle is :

541 297 683
823 615 497
967 438 521

316 782 945
294 561 378
785 349 216

679 854 132
452 173 869
138 926 754

Hana.
hana somekh

Posts: 28
Joined: 30 July 2005

According to sudoku.sourceforge.net, It can't be solved without use of trial and error techniques.

Note: to save time, I have now pre-emptively appended the rest of this thread to the end of my post.

But that's just because the solver you use is incompetent. In fact this puzzle can be solved very easily with the XYZ-turbo-lesser-spotted-coloured-swordfish method in 6s and 12s at sector ZZ9 Plural Z Alpha.

But that method's basically just trial and error!

No it isn't! Don't be so stupid!

Is too.

Is not.

Is too.

etc ...
PaulIQ164

Posts: 533
Joined: 16 July 2005

you missed off: "I don't think that's what the solver reports"

Things have moved on since rubylips wrote that solver, we now have advanced, powerful, non-T&E methods available, such as colouring.

(Or do lifeforms from sector ZZ9 plural Z alpha not use anything beyond xwings?)

Colouring on some 9s should do the trick with this puzzle BTW.

Simes
simes

Posts: 324
Joined: 11 March 2005
Location: UK

Simes,

Which 9's please ?

Thanks, Hana
hana somekh

Posts: 28
Joined: 30 July 2005

Code: Select all
`Colouring 5: double exclusion found, eliminating 5 from r9c4, (r4c4 => r4c9 => r5c8 => r9c8)Colouring 9: double exclusion found, eliminating 9 from r2c2, (r5c2 => r5c4 => r6c6 => r2c6)column 2: naked pair 25 in cells r2c2, r8c2 - updating candidates for cell(s) r1c2, r5c2Colouring 9: double exclusion found, eliminating 9 from r1c1, (r6c1 => r6c6 => r2c6 => r1c5)Colouring 9: double exclusion found, eliminating 9 from r1c8, (r1c2 => r5c2 => r5c4 => r6c6 => r2c6 => r1c5)`

<disclaimer>It's early days for my colouring code, especially the chain reporting!</disclaimer>

Simes
simes

Posts: 324
Joined: 11 March 2005
Location: UK

Simes,

You wrote - Colouring 9: double exclusion found, eliminating 9 from r1c8, (r1c2 => r5c2 => r5c4 => r6c6 => r2c6 => r1c5)

This is where I am lost, could you possibly explain this ?

Highly appreciate,
Hana.
hana somekh

Posts: 28
Joined: 30 July 2005

PaulIQ164 wrote:According to sudoku.sourceforge.net, It can't be solved without use of trial and error techniques.

Her request was:

"I am stuck yet again with another sudoku which I am trying to solve and would really appreciate some help with"

Your reply doesn't address her post. In this case, she didn't even ask, as she did in her last post, for "help in finishing off the following sudoku logically".

Her replies show that she has been helped by and appreaciates the posts that DO answer her questions.

TSA: How do I get to the airport?
SaulIQ164: You can't get there without taking the freeway, according to my 5 year old map.
TSA: Uh, I want to get to the airport. Which freeway?
Sames: Drive north on Elm to 1st, turn left.
SaulIQ164: Elm street is just like a freeway.
TSA: Wha?
tso

Posts: 798
Joined: 22 June 2005

I'm not sure why you want the third colouring explaining and not the first -what is it you don't understand?

Assuming you've performed the first two colourings on 9, as the chain below doesn't exist until two other 9s are removed from candidates for other cells.

r1c2 => r5c2 => r5c4 => r6c6 => r2c6 => r1c5
is a chain of conjugate cells. So alternate links in this chain hold 9, and the others don't. But, we don't yet know exactly which links do hold 9. So, it could be r1c2 and r5c4 and r2c6 or, it could be r5c2 and r6c6 and r1c5. But, we know that either r1c2 or r1c5 will hold 9 as they're an odd number of links apart, so whichever particular cell it is, it excludes the 9 being in r1c8.

(Conjugate cells are cells that are linked because they are the only two cells with a particular candidate in the same unit. One of the cells must old the number, and the other must not.)

My explanation of colouring is here

Simes
simes

Posts: 324
Joined: 11 March 2005
Location: UK

You can look for two Turbot fish patterns in the 5's. This will allow you to eliminate 5 as candidate from two cells, and it's easy from there.
Nick70

Posts: 156
Joined: 16 June 2005

Simes,

I have eliminated the 9 from row 1 column 8 by colouring, thank you for pointing that and therefore row 1 column 8 as well as row 2 column 1 now have the digit 8 in them. However, that has still not solved the puzzle.

So, Nick,

I have found only ONE turbot fish, eliminating the 5 from row 1 column 3 and have still to find the other turbot fish !!! Still looking for it !
hana somekh

Posts: 28
Joined: 30 July 2005

hana somekh wrote:I have found only ONE turbot fish, eliminating the 5 from row 1 column 3 and have still to find the other turbot fish !!! Still looking for it !

Interesting, I don't see a Turbot that would allow to remove the 5 in r1c3. If there is one, then there are (at least) THREE Turbots
Nick70

Posts: 156
Joined: 16 June 2005

Nick,

The turbot fish I have is :

r2c6 to r6c6 strong line
r6c3 to r6c6 strong line
r1c5 to r2c6 strong line
r6c3 to r1c3 weak line
r1c3 to r1c5 weak line
hana somekh

Posts: 28
Joined: 30 July 2005

Nick,

These are the possibilities for the digit 5 on my puzzle :

5** *5* ***
*5* **5 ***
*** *** ***

*** 5** **5
**5 5** *5*
**5 **5 ***

*** *5* **5
*55 5** **5
5** *5* *5*
hana somekh

Posts: 28
Joined: 30 July 2005

hana somekh wrote:However, that has still not solved the puzzle.
It's all easy after that. What about r9c3, r9c5, r7c5...

S
simes

Posts: 324
Joined: 11 March 2005
Location: UK

Simes,

r9c3 = 8
r9c5 = eiether 2,5 or 9
r7c5 = 2 or 5

I'm not sure how to proceed ....
hana somekh

Posts: 28
Joined: 30 July 2005

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