A Bypass 3-Fill on the Bus

This post reports an extension of the Sysudoku bypass to include 3-cell fills, and describes the recently republished collection of 534 puzzles, Michael Rios’ Mensa Sudoku, © 2005.

It’s great to take flight, and take in what you can absorb of another culture.  Margie and I recently had that privilege, with Virginia, an accomplished mother hen and lecturer on Mayan history and culture, and Hector, a helpful and confidence inspiring bus driver, on Caravan’s tour of Guatemala. It was a bus sojourn. The country is not that large, but there is a lot of riding time along mountainous roads between stops. And of course, I came armed with the latest Sudoku book, one that popped up on the Barnes and Noble shelf just before the trip. It was Mensa Sudoku, by Michael Rios, and by the same publishers as Peter Gordon’s Puzzlewright Guide to Solving Sudoku, © 2006 Sterling Publishing, under the imprint of Puzzlewright Press. The “Guide” originally was the “Mensa Guide”, but that association was dropped before my confirming review  of Gordon’s solving advice and Frank Longo’s more deserving puzzle collection within it.

Michael describes the collection as starting out easy and getting harder as you go. Being wary of my pen to paper capability on a bus, I only wanted to find the point the puzzles began to require box marking, and let PowerPoint take it from there, back home on my laptop.  I started doing the most knee acessible series 1,2, 5, 6, . . . then dropped back every fourth 13, 17, 21,  .  .  ., then to  every 20th: 201, 221, 241, .  .  . , still on the bypass. This series continued to 381, then closed to every 10, 391 to 531. To my surprise, the bypass solved them all.

As to the possible significance of this, I realized that in the airports, plane and bus, I had slipped into the habit of going for the 3f: (three cell) fills in the bypass, rather than deferring them until line marking. And on this collection at least, this habit paid off handsomely.

Not only that, but I was spotting these line fills quickly with an obvious trick, well suited to the bypass, which I institutionalized immediately with this maroon and grey plaque:

That is quick and simple enough to add to the bypass. I’ll revise earlier posts to do that. This means the 3-fill line joins the wall, the square, and four cornered box as triggers for immediate box marking action. Try it out.

Let’s now have a look at the bypass 3-fill in Mensa 531. You might have started with:


That’s a good start, recognizing the C wall implies E2, and picking up the 3-fill line r6.

The forth line on the c8 fill depends on which qualifying feature you use: r6c8 sees 3 and 8, but also, r3c8 and r5c8 both see a 3.

 Sweet, but actually, we’ve already missed one. You could go back to r4 as a 3-fill generated by C1 and see how many 3-fills turn up in what could the toughest of the review puzzles.

 So, the bottom line is that Michael Rios’ Mensa collection is a basic collection, good for travel, but lacking in advanced challenges. But that’s OK. There’s a time and place for everything.

Next week, we begin the review of a collection which may be more like KrazyDad’s SuperTough and Insane collections in stretching Sysudoku principles. It’s one of a number of “very hard level” collections by A.D. Ardson published in 2016. The review posts start with Sudoku Very Hard Puzzles, Volume 2, number 38. Perhaps you’d like to have your implementation of Sysudoku for a checkpoint


About Sudent

My real name is John Welch. I'm a happily married, retired professor (computer engineering), timeshare traveling, marathon running father of 3 wonderful daughters and granddad to 7 fabulous grandchildren. The blog is about Sudoku solving. It covers how to start, basic solving to find candidates efficiently, and advanced solving methods in an efficient order of battle. It is about human solving methods, not computer solving.
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2 Responses to A Bypass 3-Fill on the Bus

  1. Gerald Asp says:

    Is there a clue missing from
    Sudoku Very Hard Puzzles, Volume 2, number 38?

    I’ve run the puzzle through Andrew Stuart’s Sudoku Solver (as a check that I’ve transcribed the puzzle correctly) and am getting:
    Number of solutions: 63
    (recursed 32552 times)

    Here’s a copy of the file that I am importing.
    400 000 200
    058 200 030
    000 500 008

    004 032 109
    009 815 700
    201 740 300

    800 000 000
    060 008 590
    005 000 003


    • Sudent says:

      Gerald, you’re amazing and absolutely right. A 7 belongs in r7c6! My post of May 30 will be devoted to a curious result of your discovery. That is, that without that given, my posted solution reachesbthe Ardson, apparently unique solution. The process included a Sysudoku innovation. Was it mistaken? Maybe you, or some other astute reader will have the answer for that post.

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