Now that it's more and more difficult to find Sudoku grids that the CyberJerry Sudoku Analyzer (the 'Analyzer') can't analyze step by step, the great Sudoku Challenge is also becoming more difficult. To help out a bit, below are four Sudoku grids that the Analyzer can't analyze step by step. You just have to figure out how to solve one of these analytically (no guesswork) to qualify as a successful Sudoku Challenger. Click on any of the grids to bring it up in the Analyzer. Both it and you should be able to solve several cells. But at some point, the Analyzer gets stuck and can't give a hint. Can your brain keep analyzing beyond that point, and solve the puzzle? If so, click on the corresponding 'Answer Challenge' link and tell me how you did it. As before, I promise to publish your success.

With my brain, I know how to analyze and solve one of the grids below; am not telling which one. But programming the Analyzer to do it would be a bear, and am very unlikely to tackle that job in the near future. Even less likely for the other three, which my brain can't (yet) figure out. So you have lots of time to meet this challenge. Good luck!

When the Unique Rectangle technique is unavoidable

Fri June 19 2020 9:24pmSudoku

A recent round of minor enhancements and corrections has made the CyberJerry Sudoku Analyzer capable of analyzing increasingly tough Sudokus. Two results: the "New Puzzle" control now offers a "Genius" level, and the "Hint" feature may offer extremely complex hints, containing many interdependent sub-steps of a variety of advanced strategies. The added focus on complex Sudokus has had another unforeseen result: the possibility of encountering what I call the "**Unavoidable Avoidable Rectangle**". (Not being able to find any mention of the phenomenon in any other website, I claim the right to assign this name to it.) Let me explain:

At any given point in working a valid Sudoku puzzle, there may exist only one way to solve one unsolved cell, and so proceed one step closer to a full solution. But many times there may be two, three, or more ways to solve the same cell or different cell(s). In such a situation, the Sudoku Analyzer will generally prefer a simple hint over a more complex one. But, whether simple or complex, the Analyzer always offers just one hint, randomly selecting from any number of possible hints at the given point. If you press the "Hint" button repeatedly at the same point, you may be randomly offered a wide variety of ways to proceed. To give you all possible hints at one time would be confusing, and would also be a departure from the design decision to proceed, as much as possible, as a human brain would proceed.

One of the advanced strategies is called the "Unique Rectangle" technique. A few Sudoku experts call it "Avoidable Rectangle", since it is based upon the "deadly pattern" that cannot exist in a valid single-solution Sudoku.

Ironically, in some complex Sudokus, this technique may be unavoidable. That is, at a given point in a given Sudoku, there may exist several ways to proceed. If one way involves the Unique Rectangle strategy, and you don't notice it, or choose to follow a different way, you may solve a different cell, inadvertantly cause the deadly pattern to disappear, and, with it, the Unique Rectangle step. In some cases, this means that subsequent hints may be much more difficult than if the Unique Rectangle technique had been used. In some rare cases, the Sudoku may become impossible to solve analytically without backtracking to the missed Unique Rectangle hint step.

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The Sudoku grid shown above illustrates this. Navigate to that Sudoku and click the "Hint" button. The Analyzer will offer you a hint consisting of 3 parts. The third part is a Unique Rectangle, case C, based upon avoiding the 'deadly pattern' in rectangle b2-b3-i2-i3. If you follow this hint, you will solve for cells h3 or i5, either one of which will make the rest of this Sudoku easy to solve.*

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But there is an alternative, at first sight easier. Instead of the Unique Rectangle, you could notice the Naked Pair in cells b2-b3 and use that to solve cell a2. Now you ask for another hint. This time there is only one hint available, a two-part hint involving a different Unique Rectangle (case A), at a1-a6-b1-b6, allowing you to solve cell a1. Next another hint with a case A at b2-b3-i2-i3, solving cells i2 or b3.*

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At this point, the original deadly pattern at b2-b3-i2-i3 has disappeared, and you will have to labor through some very complex steps to solve cells h3 or i5, which have turned out to be key cells in solving the whole Sudoku.
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In this respect, your human brain may have a distinct advantage over the Sudoku Analyzer, which doesn't remember past hint steps. By design, every point in the Sudoku is treated as a fresh start, calling for its own set of hints. Nor does the Analyzer bother much with hint steps that have not been randomly selected, as mentioned above. I may try to make the Sudoku Analyzer smarter in this regard. But in the meantime, be careful about overlooking a Unique Rectangle hint. It may end up being unavoidable and essential to reaching a full solution.

0 comments | rev. Thu Jun 25 7:40pm |

This blog article serves as both another bug report and as a candidate for the Sudoku Challenge .

This time, the Sudoku Analyzer cannot solve the Sudoku step-by-step. It should be able to. This is a known bug, recently discovered by yours truly, and which yours truly is working on, and hope to have fixed soon.

In the meantime, if you think you can solve this Sudoku analytically (no guesswork), (read article)

1 comment | rev. Fri Jun 5 9:45pm |

This Sudoku puzzle has me scratching my head.

The Sudoku Analyzer solves it step by step, giving clues along the way. Which is to say, it is not one of those occasional Sudokus which can only be solved in trial-end-error guesswork manner. But at a couple points, the Sudoku Analyzer gives very complex hints where simple hints are possible. This is not according to design. Then, in at least one very complex hint, several secondary hint steps are omitted, without which the complex hint is incomplete.

What's especially baffling is that these (read article)

0 comments | rev. Fri May 22 7:59pm |

The Great CyberJerry Sudoku Challenge has been answered

Wed August 1 2018 8:10pmSudoku

7-plus months ago this blog unveiled the great CyberJerry Sudoku Challenge. (See Jerry's Blog article of Dec 14 2017 for details.) In brief, the challenge is to provide a step-by-step analytical solution to a Sudoku that has CyberJerry's Sudoku Analyzer stumped. To meet the Challenge, the Sudoku must qualify thus:

(read article)

- 1. The Sudoku has exactly one solution.
- 2. CyberJerry's 'Analyze' button reports that the Sudoku cannot be rated, and at some point the 'Hint' button fails to provide a Hint.
- 3. You can describe a step-by-step analytical way to solve the Sudoku. You need only do so at the point(s) where the Sudoku Analyzer fails to provide a Hint. Note

3 comments | rev. Dec 4 2018 12:34pm |

After several years of development, testing, debugging, and improving the strategies, I am ready to proclaim that my Sudoku Analyzer is the best on the web, that it can analyze and give a step-by-step solution to even the toughest Sudoku.

**Please note that this does NOT entail simply giving a solution, but in providing a step-by-step analytical method to solve.**And so today I am unveiling the great CyberJerry Sudoku Challenge.

To be sure, I am still discovering some rare Sudokus for which the CyberJerry Sudoku Analyzer cannot give step-by-step analytical hints. But for those Sudokus, neither can anyone else. Or so I claim. (read article)

0 comments | rev. Dec 4 2018 7:58pm |