Fleshing Out Stuart’s ALS Node Figure 30.1


Andrew Stuart’s The Logic of Sudoku, in a chapter on Almost Locked Sets, presents an example of an ALS node in a confirming Almost Nice Loop, as Figure 30.1. This post fills in the solution, before and after the series of four grids in the AIC Hinges page of the Sysudoku Guide, that illustrate the flexibility of the ALS node in AIC.

As is the case throughout The Logic of Sudoku,  solving to Figure 30.1 starts with candidates  scanned directly from the given clues. Sysudoku Basic usually gets to advanced in a more entertaining way, and with more information.

When basic uncovers no clues, as in this case, there’s as many candidates.

Needing to account for a few candidates missing, and I went through the bv scan with the same level of success: none.

But next, on the 1-panel, a finned swordfish succeeded. The three kraken victims escaped.

Then on the 4-panel, a skyscraper triggered a Nc6 boxline, accounting for the remaining missing candidates in Figure 30.1.

 

 

 

 

Getting to the 8-panel, there is a kraken 8-wing and a Cc4 boxline. These removals come before the AIC scan in Sysudoku, but  do not affect Andrew’s example.

X-chains and fish are best at cutting through sucn a sea of candidates, and the same disconnected bv field that frustrated the bv scan makes coloring ineffective.

So it’s time to add the hinges and search out some AIC. For the results, go to

Guide/Sysudoku Advanced/AIC Hinges, where, under the heading The ALS node, the page continues with Figure 30.1 and two more grids, in which the innocent looking r12c3 ALS 245 is wired to produce a  confirming ANL, a grouped elimination ANL, and a nice loop.

Coming back from  that display, one AIC hinge gets the last laugh, as one of the winks makes the decisive elimination.  This type of ANL, with the victim seeing a toxic set member with a hinge wink,  deserves a distinct name,

and I propose the “hinged ANL”.

The hinged ANL is generated naturally in the effort to keep a hinge initiated chain going until something happens.

After working through the puzzle of Andrew’s ALS node example, I was hoping for another one in his immediately following challenge puzzle 17.  Next post is a report on that one, in which I got more than enough challenge. If you want to get your discoveries before reading about them, here’s Logic 17.

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About Sudent

My real name is John Welch. I'm a happily married, retired professor (computer engineering), timeshare traveling, marathon running father of 3 wonderful daughters and granddad to 7 fabulous grandchildren. The blog is about Sudoku solving. It covers how to start, basic solving to find candidates efficiently, and advanced solving methods in an efficient order of battle. It is about human solving methods, not computer solving.
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