Impossible pattern chaining in an SE 9.0 challenge from Reddit

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Here's a puzzle generated by Sudoku.Coach and posted by u/ddalbabo on Reddit. SC's "Beyond Hell" grids are always a treat, often on the very edge of requiring Forcing Chains.

Discussion link


Puzzle string: 010000800025060000780001904000300000050100009800000010000020540090800106003600000

Sudoku.Coach

SE = 9.0 according to SukakuExplainer. YZF says 8.8. Not sure what's going on there

After basics

(7)r6c49 = (4)r6c4 - (4=9)r2c4 - r2c1 = r4c1 - r6c3 = (9)r6c5 => r6c5<>7

What immediately jumps out to me is this Bivalue Oddagon of {25}. We can build an AIC off the guardians and eliminate a 7. I couldn't find any other more useful chains using this so it's back to the drawing board for now.

The next move uses a DoF>1 AIC so read this article if you haven't already.


(4)r6c7 = [(4)r8c5 = r9c6 - r9c2 = r4c2 - (4=26)r46c7 - (2=7)r9c7 - (7=4)r9c2 - r9c6 = (4)r8c5] => r6c5<>4

You could call this a Kraken ALS? The 2 DoF component here is (4)r46c7. It can be in either of the ALS cells, or neither. The AIC ties up these three possibilities into a common outcome. It also proves a virtual strong link, (4)r6c7 = (4)r8c5, which may be useful later.

AIC: (4=9)r2c1 - r1c3 = r6c3 - (9=5)r6c5 - r8c5 = (5)r8c1 => r8c1<>4


Kraken Cell: (6)r5c1 = r4c1 - (6)r4c7 = [(2)r9c1 = r9c7 - (2=4)r4c7 - r4c2 = r9c2 - (4=52)r9c16] => r5c1<>2


(7)r6c49 = (4)r6c4 - r6c7 = (4-7)r9c2 = r4c2 - r4c9 = (7)r6c9 => r6c36<>7

The virtual strong link from earlier comes in handy and allows me to get something worthwhile out of the Bivalue Oddagon. This move breaks through the puzzle and brings it down to SE=8.3. Probably could have used it earlier too.

Let's speed through the rest.

(7)r5c3 = r8c3 - r8c8 = (7-3)r2c8 = (3-8)r5c8 = (8)r5c5 => r5c5<>7

(6=4)r5c1 - (4=7)r4c2 - r5c3 = (7)r5c6 => r5c6<>6

(5=9)r6c5 - r6c3 = r1c3 - r2c1 = r2c4 - (9=7)r7c4 - r6c4 = (7)r6c9 => r6c9<>5

Similarly, (2)r1c9 = (2-5)r3c8 = (5-8)r4c8 = r4c5 - (8=4)r5c5 - r8c5 = r8c3 - r1c3 = r2c1 - (4=9)r2c4 - (9=7)r7c4 - r6c4 = (7)r6c9 => r6c9<>2

UR Type 7

(2)r1c9 = (2-5)r4c9 = (5-8)r4c8 = r5c8 - (8=4)r5c5 - (4=52)r36c4 => r1c4<>2

(2)r3c4 = r6c4 - (2=49)r16c3 - r8c3 = r8c5 - (4=8)r5c5 - r5c8 = (8-5)r4c8 = (5-2)r4c9 = r1c9 => r1c6,r3c8<>2

XY-Wing

(9)r1c3 = r6c3 - (9=5)r6c5 - r6c4 = (5)r1c4 => r1c4<>9

(7=2)r8c8 - (2=8)r4c8 - (8=7)r4c5 - r4c2 = (7)r9c2 => r8c3,r9c7<>7

STTE

Over all it was a good puzzle. Took me about an hour and most of that was spent making images & writing for the blog lol. 




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