This post checkpoints Ardv2 78, the second review puzzle in A.D. Ardson’s Sudoku Very Hard collection. The puzzle skates by basic and the bv scan, but is tamed on the X-panel.
Along with regular collection reviewing posts, I’m revising the first year’s posts 2011-2012 to bring graphics, tracing and solving examples up to date. That makes the opportunity to take a serious look at puzzles like Ardv2 78 especially valuable, as holistic tests of the fundamentals and order of battle of Sysudoku.
Here the policy of subset discovery in basic Sysudoku is rewarded with a naked triple in the closure of line marking.
The trace and the line marked grid illustrate the movement of the focus of attention. The last row to be line marked is r1. The marking fills the last cell of c2 and that is the time to look for subsets in c2. The NW box is locked at the same time. The time to look for subsets in the boxes of the West tower is in the follow up to line marking of c3. Subsets in the N, C and S banks are sought in the follow up of the c8 closure.
The next point of interest in Ardv2 78 is its remarkable resistance to the bv scan, with such a rich bv field. Here are the possible XYZ-hinges rejected.
A related symptom of the same disease is the lack of productive XY-wings along an extensive XY Railway. Here almost all of the duplicated candidates are in the same unit.
But before I go looking for a Berthier t-chain, I need to get out the X-panel and the crayons.
Help is on the way. First the 3-panel, grouped 3-chain gives up two candidates and a clue before its group breaks up. Actually no, that doesn’t work, because the c1 link is a wink. Reader Dov Mittleman caught that one. Removing that, a 4-panel slink chain allows itself to be interpreted as an AIC, for another removal.
Then comes a swordfish in parade formation on the 6-panel.
After some follow up, the 9-panel weighs in with a column-wise swordfish. The fish removal triggers two box/line interactions.
The crayons have the last word. I started with blue 1r5c2, and the cluster just spreads from there. Two green 3’s are forced into r9, and the blue army sweeps the field.
The extensive cluster tells you something about why 78 is “very hard”. A large region of the grid is dominated by blue/green bv. This region can’t distinguish between two solutions.
Did an Amazon drone fly in the window with your volume 2 of 400 Ardson very hard Sudoku puzzles yet? If not, here is a copy of Ardv2 118 for next week.