This post reports an alternative solution, without my pattern analysis, to Antoine Alary’s special More Extreme 200, based on a Sysudoku reader’s innovative use of a little known technique, the double common ALS pair.
Two ALS sharing two restricted common numbers are a closed set, i.e. candidates of each number they share being a toxic set. My friend Gordon Fick commented on my 3/14 post, Finishing More Extreme 200, pointing out a double common pair of ALS, and then demonstrating how to gain additional removals by comparing the two ways that the true candidates of the restricted common numbers can be shared. I’m adding this technique to my extreme repertoire. I’m calling it Fick’s trick, but that may last only until Gordon comes up with another gem.
Gordon shares my interest in human engineered Sudoku solving, and brings well grounded experience to the sysudokie reader challenges. He shares my upbringing at the hands of Andrew Stuart, but is also a student of Hoduko’s Bernhard Hobiger. He comes at extreme puzzles with an open mind and a complementary point of view.
This helped early on More Extreme 200. While I was dazzled by the double numbered finned 78-wing, and missed a ER assisted ANL (Almost Nice Loop). Gordon saw the two ER corners for the square AIC. I’m showing it as a grouped forcing chain, but ER makes it easier to spot.
After the double wing, while I went to pattern analysis, Gordon searched the ALS field. His attention was first drawn to this ALS toxic set with common 6 and a victim 3-candidate. It’s generally known as an ALS-XZ. An excellent example.
Regardless of which gets which, the RC partners are both toxic sets, and each ALS is locked in its unit, with the unrestricted numbers in each ALS also forming toxic sets.
Here, both commons are grouped.
The new bv r1c878 brings a series of naked pairs, with two remote pairs, and an immediate collapse of More Extreme 200.
With this mode of solution, I believe More Extreme 200 still ranks in the Sysudoku extreme class. It is very difficult to find ALS toxic set removals among the large numbers of ALS in the typical puzzle. Thanks, Gordon, for this effective reminder to look for them.
Next week the review continues with Weekly Extreme 431.