## The Peter Gordon Fishing Guide

We leave November and basic solving with a review of Peter Gordon’s advanced solving recommendations in his Mensa® Guide to Solving Sudoku, now re-issued as the PuzzleWright Guide etc. After a brief summary of the Guide’s general failings, this post examines Gordon’s Chapter 7, The X-wing Family. It’s about regular fish.

Basic solving is by nature a fixed sequence of techniques. The Sysudoku basic review found Gordon’s Guide inadequate for harder puzzles, because it depends on cell by cell number scanning, an inefficient, repetitive, and boring process for enumerating candidates.   But at least, Gordan puts his “scanning” process first to some degree. I was inspired by this and by Wayne Gould’s NPM, to adopt a sysudokie version, the dublex bypass, as a new preliminary phase of box marking.

The Gordon Guide review continues now into advanced solving. Unfortunately, it demonstrated that his treatment of advanced solving is far too incomplete to regarded as a true “Guide”. While claiming “ingenious tricks that many experts don’t know”, it actually omits too many tricks that experts know well, and offers very little that is new. Worse still, the Gordon Guide treatment of the techniques it does cover is woefully superficial. Peter simply demonstrates the technique at work, avoiding the embodied advanced concepts entirely.  In Gordon’s Guide, techniques are not built upon common logical concepts. You can only hope to recognize another instance of any technique from the Guide in the sea of candidates before you. Peter finds them, but he never tells you how. If you don’t see one, never mind. Just use a Gordonian trial and error method and you’ll soon have the answer.

Sorry to be yet again be delivering bad news on published Sudoku instruction, but the remainder of this review will still be of interest. I promise to stay off my soapbox and to demonstrate the particulars in a matter of fact manner. There’s a gem among the rubble. And you will find many helpful links to advanced Sysudoku posts relevant to Gordon Guide examples.

So let’s do it. Chapter 7 on regular fish is where advanced solving starts in in the Gordan Guide to Sudoku Solving.

The X-wing is Gordon’s first advanced method, to be undertaken with all candidates in place. To sysudokies the X-wing is the only basic fish, most often and almost automatically revealed in line marking. The Guide’s Example 13 is a good example.

In the last post, readers were invited to hop into Gordon’s boat and follow his Guide to an X-wing. To get to Peter’s stuck point, shown here, “candidate free” neighbor numbers were considered necessary. Now did you complete the candidates by number scanning, a.k.a. “one-choice”? Sysudokies have slink marking, but the Guide reader is stuck with the candidate lists shown here.

No problem, though. Gordon finds the X-wing, and everything else, for the reader. The “Guide” doesn’t deal with the finding process at all, for anything.

I’m sure you found the 7-wing regardless of your choice of marking, but with line slinks marked, it leaps out at you, once you have the candidates. But wait. In the SSOB, this puzzle never gets that far.

In line marking, on row 4 of 3f: r9, c5. 4f: r2, r4, the 7-wing shows up as we find cells for the number 7. Clearly, there are no more locations for 7 in r3 either. We mark the 7-wing to await the marking of line 3. In the placement of numbers 167 in r3, 6r3c3 is a naked single, and the collapse begins. We never went door to door, looking for the 7-wing. It came knocking.

When we move to X-panel territory, Guide omissions become more serious. Peter illustrates swordfish, jellyfish, and squirmbag, cheerfully finding each one among candidate soups of growing complexity. To see how misguided this is, just try your hand at finding a jellyfish on the 1-panel of Example 15-2. Guide readers are supposed to be doing what you’re doing, but with the 1’s embedded in those cell lists of varying lengths. The answer will be waiting in the next post.

As you search, note that the Guide never suggests anything like the X-panel. Peter just tells you where the jelly is, and reminds you that it’s four places for 1’s. You get a conversational acquaintance with Sudoku terminology, but that’s it.

If you have the Guide, don’t even try for Peter’s squirmbag. I thought I would show how the squirmbag with two 6-clues showing would be replaced by a 6-wing, but Gordon’s grid of completion candidates has a hidden single in row 2 that destroys all candidates that would be removed by the squirmbag, and starts the collapse. Isn’t somebody supposed to check these things?

The Guide tackle box has nothing else. No finned, kraken, or sashimi fish. Does any expert not know how to use these? I’m all right with the omission of mutant fish or sudopods. I’ve never needed them myself, but experts know about them.

Next post, we see what Peter Gordon does with a well known advanced technique he names for himself.