## Pitkow’s Grand Master Sudoku

This is a short review of the GM section of Sudoku vol. 3 , by Xaq Pitkow, published by Chronicle Books.  The puzzles vary between basic and moderately advanced levels

I pre-selected 10 GM and 25 Super GM puzzles for the review.  Here’s the review table for the GM puzzles :Before you ask about GM 55, I was disappointed to discover puzzles 51-59 had been replaced by extra copies of puzzles 91-97.  I substituted 100 for 55.  A pretty smudgy black mark for Chronicle Books.

As to results, no box marking collapses, but inconsistency persists.  Four of ten collapsed in line marking.  Four of the five were reasonably entertaining for sysudokies, loaded with bv and collapsing short of AIC chains.  The exception was GM 95 which emerged from  bv scan with one number dominating the candidate field. It proved to be classic case for LPO pattern enumeration, as we’ll see in an upcoming post.

Here I will report a couple of highlight advanced events that show what the advanced GM puzzles offer, and checkpoint the basic solving of the GM 5 puzzle suggested in the last post.  After the report on GM 95, there will be a review of five Super GM puzzles.  I pre- selected only 5 from the limited collection of 25 puzzles.

GM 35 was staggered by the blows captured in this snapshot. In orange, you see an ordinary 679-wing with a removal in r6c1. But we have gone further  many times, and are prepared to find how two more 6-candidates manage to see the 67 wing with grouped forcing chains. Now do you see the red diamond removals?  Think boxline.

And above the wing, note the APE r3c45 =31+81+83+86!

The resulting naked triple in W generates W7 and a naked pair 38 in r3, enabling a unique rectangle removal  to collapse GM 35.

More arresting events occurred in GM 85. Basic solving followed a recurring GM pattern, with many early clues.

Less typical was line marking on rows only, and an X-wing in the process:

Then, when there was little else on the grid, three distinct techniques insisted on the removal of a single, apparently indecisive candidate.

First in order was APE r6c78 = 12+36+96, then a regular 139-wing, and cranking up the bv map, a remote pair.

Coloring then revealed what was so decisive about this  little 1-candidate. Its removal allows the cluster to force two blue 2-candidates in c8, proving green and thereby immediately collapsing GM 85.

Let’s conclude with your checkpoint trace on the basic solving of GM 5. Going on from there, I found a couple of dead XYZ-wings, but had better luck along the XY-tracks, finding three ANL.

The gold key, however, is the Sue de Coq with an alternative possibly missing. Your mission for next post is to verify that it is not missing, and to show the verifying contradiction on the grid.

Sue then slays the GM, an act not allowed in real chess life, although we duffers feel like it sometimes.  To us, GM’s are monsters.

To keep things moving, I have another assignment for next post, the basic solving for GM 95. Chronicle Books shouldn’t mind if I further plug their little spiral bound by enabling you to fully participate right now in a very instructive journey, even if you don’t yet have your copy of Xaq Pitkow’s Sudoku, v.3.