The New Sudoku Players' Forum

[aran]

Another way of removing 4r5c4 :
459r128c4=6r1c4-(6=3)r1c6-(3=6)r4c6-(6=48)r6c4 (=>pair48 r5c6+r6c4) => <4>r5c4

Not a huge fans of chains like that, but I wonder why did you include the cells r14c6 when you can have it shorter in the opposite direction:

Code: Select all

+-------------------------+-------------------------+-------------------------+
| 34      24      249     |#4569    8       36      | 7       1       259     |
| 1       6       2489    |#459     49      7       | 2489    2459    3       |
| 7       348     5       | 1       349     2       | 489     469     689     |
+-------------------------+-------------------------+-------------------------+
| 345     1458    148     | 2       14      36      | 1489    4569    7       |
| 6       9       1478    |-478     5      *48      | 1248    3       28      |
| 2       134578  13478   |*468     1347    9       | 148     456     568     |
+-------------------------+-------------------------+-------------------------+
| 9       27      6       | 3       27      1       | 5       8       4       |
| 8       234     234     |#49      249     5       | 6       7       1       |
| 45      1457    147     | 78      6       48      | 3       29      29      |
+-------------------------+-------------------------+-------------------------+

4r5c4-(4=8)r5c6-(48=6)r6c4-(6=459)r128c4-4r5c4 => <4>r5c4

And I'm not a fan of any chain starting with xTrue=> : where you're condemned to find a loop to make any progress.
Furthermore since any elimination found by whatever means can be converted post hoc into a discontinuous nice loop of that sort, I find them inherently unsatisfying.

[hobiwan]

Suspect your question is directed to Gee, but I see an ALS nice loop and one net (due to a kraken cell) leading to 5 placements ...

01) r8c2 -4- r1c2 -2- r7c2 -7- als:r9c123 -4- r8c2 ==> r8c2<>4

02) kraken cell: either r8c2=5 or r8c2=3 or r8c2=2
r8c2 -5- r9c1 =5= r4c1
r8c2 -3- r8c3 =3= r6c3 -3- r4c1
r8c2 -2- r8c5 =2= r7c5 =7= r6c5 =3= r6c23 -3- r4c1 ==> r4c1<>3

I missed your post while playing with the puzzle. When using a second short ALS in your loop it get's even shorter:
r8c2 -4- ALS:r9c123 -7- ALS:r17c2 -4- r8c2 => r8c2<>4
or as ALS-XY:
A=r1c2 - {24}, B=r9c123 - {1457}, C=r7c2 - {27}, X,Y=2,7, Z=4 => r8c2<>4

[edit: changed X/Y/Z to match notation of XY-Wing]

[udosuk]

Another solution (from Gee's PMs) is a contradiction chain, which is I think is just a variation of udosuk's solution (resulting in assignment of a different cell):

r5c9=8 r5c7=2 r5c3=1 r5c4=7 r9c4=8 r9c6=4 r5c6=0 ==> r5c9=2

Your move is more direct, and quicker. Anyway I think you only need to consider 5 cells for your move:

r5c46=[74] => r9c46=[88] invalid
Therefore 8 @ r5,b5 is locked @ r5c46 => r5c9=2

I missed your post while playing with the puzzle. When using a second short ALS in your loop it get's even shorter:
r8c2 -4- ALS:r9c123 -7- ALS:r17c2 -4- r8c2 => r8c2<>4
or as ALS-XY:
A=r1c2 - {24}, B=r9c123 - {1457}, C=r7c2 - {27}, Y,Z=2,7, X=4 => r8c2<>4

or as ALS-xz:
A=r17c2={247}, B=r9c123={1457}, x=7, z=4 => r8c2<>4



aran, I really shouldn't talk about chains since I'm really not into them at all, and I really struggle to understand what everybody's different tastes about them.

[ttt]

Hi All,
I don’t solve this one, but I saw the solution for this here.
Too many things come to me this time, I can’t solve all of them on time so I forgot them by Vodka…

Very sorry for... nothing

ttt

[Bigtone53]

When sudoku puzzles were first published in the Daily Telegraph, the complier vigorously defended setting puzzles that required trial-and-error. I thought that this had stopped but perhaps one slipped by. I can see that it can be solved logically but the methods seem extreme for a puzzle in a daily paper.

[Bigtone53]

This is the link I was looking for, with Sue de Coq's comment particularly relevant. Sadly, Michael Mepham is no longer with us.

[DonM]

When sudoku puzzles were first published in the Daily Telegraph, the complier vigorously defended setting puzzles that required trial-and-error. I thought that this had stopped but perhaps one slipped by. I can see that it can be solved logically but the methods seem extreme for a puzzle in a daily paper.

This is the link I was looking for, with Sue de Coq's comment particularly relevant. Sadly, Michael Mepham is no longer with us.

That quote is from early 2005, well before the techniques that were later developed that avoid the need for T&E in many very difficult puzzles. The difficulty level of this puzzle is fairly typical of the Mepham Group Weekly Extremes that include this puzzle. Using today's available methods, they can be solved without guessing. IMO, if you follow ttt's link, you'll find what I think are the best approaches to manually solving Extreme #116 in a logical manner from beginning to end.