The ultrahardcore ALS-wing survey continues with examples from UHC 47. The survey illustrates how ALS-wings are found as ALS are added to ALS maps.
Continuing the survey into a second review puzzle UHC 47, the reward is this well laid out, classic ALS-wing. It’s another rows vs columns building assignment and an ANL with value group terminals.
The orange ALS is added to the row map first, then red on the column map. Orange goes on red’s match list.
We did acknowledge that a column vs row scan is necessary. We get 4 as X, but no Z.
Now when the green ALS arrives, and a singles match occurs with red, we have 4: r9 orange on red’s match list. Having that, add a copy of the three ALS on your grid and mark the victim.
My two human oriented solvers have shown that many Death Blossoms (Sudokuwiki) to be ALS_XZ (Beeby). UHC 47 has an interesting case.
In this Death Blossom, all three of the stem cell candidates 679 are seen by value groups in two ALS. The victim 5r8c9 sees value groups in both ALS, so if the victim were true, it would lock all other value groups in the ALS, removing all three candidates from the stem cell.
Now to get back to the subject at hand, the Death Blossom has an ALS-wing cover. When the c9 green ALS is added, your scan matches the c5 red ALS on single 9. Your match list then connects the c5 red ALS to the c6 blue ALS and get to the exit on 9, where a fourth ALS has a value group matching a last added value group, and 5r8c9 sees the matching groups.
We just learned that an ALS-wing be part of a longer ALS chain, and that the match list is useful even if all ALS are on the same map.
But Stefan Heine gets the last word on this one. Besides the Death Blossom and the ALS-wing cover, there’s a third way to grab 5r8c9. It comes come with the addition of ALS c9 16/569, when it is found to be an ALS on a grouped AIC and its 5 value group gets to be a terminal of an ANL.
This suggests, that we follow up each new ALS addition to the map with a little mental AIC building. Take the ALS exit wink from every value group, and build towards the background AIC net, using the value group as a starter slink. And even when that succeeds, leave the ALS on the map, and go for the ALS_XZ and the ALS-wing.
Next week, we skip to ultrahardcore 179 in the ALS-wing survey. The survey continues to refine a systematic approach to ALS overload in DIY sudoku solving.