Sudoku Puzzle #92 & #93

By: Dan LeKander

Volume 16, Issue 10, October 2021

Once again, after August, the River has become really quiet.  Boat traffic is a fraction of summertime levels.  September and October as well as May and June are my favorite months for enjoying the grand St. Lawrence River.

Clueless?

As a bonus each month this year we will start with a Sudoku puzzle in progress, where it appears there are no more obvious or not-so-obvious clues.  Can you find the missing clue in Puzzle #92?

Puzzle #92

(The answer follows below after the conclusion of Puzzle #93, the feature puzzle for October)

Feature Puzzle # 93


This “impossible” puzzle was chosen to illustrate the importance of taking your time when you think you have all obvious answers and not-so-obvious answers filled in and it is time to fill in the options for the unsolved cells.   Finding that really not-so-obvious clue can possibly save you an immense amount of time in solving the puzzle.  We will see this in our feature puzzle #93 below  . . .

Puzzle # 93

DAN’S 8-STEP APPROACH TO SOLVING ALL SUDOKU PUZZLES

Once you have  completed the puzzle, to the extent that you have filled-in all obvious  answers and have written all potential options across the top of the unsolved cells (PUZZLE PREPARATION), Dan recommends the following Steps to complete the puzzle.

Step 1:  Sudoku Pairs, Triplets and Quads – See September 2015
Step 2:  Turbos & Interaction – See October 2015
Step 3:  Sudoku Gordonian Rectangles and Polygons – See November 2015
Step 4:  XY-Wings & XYZ Wings – See December 2015
Step 5:  X-Wings – See January 2016

Step 6:  DAN’S YES/NO CHALLENGE
Step 7:  DAN’S CLOSE RELATIONSHIP CHALLENGE
Step 8:  AN EXPANSION OF STEP 7Steps  1-5 are relatively common techniques and are explained in the TI LIFE  articles above. Steps 6-8 are covered in detail, in Dan’s book.


PUZZLE PREPARATION

Prior to utilizing Steps 1-8, complete the 5 Steps of Puzzle Preparation …

  1. FILL IN DATA FROM OBSERVATIONS
  2. FILL IN OBVIOUS ANSWERS
  3. FILL IN NOT-SO-OBVIOUS ANSWERS
  4. MARK UNSOLVED CELLS WITH OPTIONS THAT CANNOT EXIST IN THOSE CELLS

FILL IN THE OPTIONS FOR THE UNSOLVED CELLS

We will complete all of the first 4 steps in the order we observe them, until we conclude all Puzzle Preparation Step 1-4 clues.

The first thing we observe is that C6R1=9 and C1R8=4.   That is it for obvious answers.

In box 2 a 4 can only exist as an option in C4R2 or C6R2; therefore, a 4 cannot exist as an  option in C2R2, C7R2 or C8R2.  Indicate this by placing a small 4 in the bottom of those two cells

In box 4 a 1 can only exist as an option in C2R5 or C3R5; therefore, a 1 cannot exist as an option in C7R5 or C9R5.

In box 6 a 9 can only exist as an option in C7R4 or C7R5; therefore, a 9 cannot exist as an op-tion in C7R7 or C7R8.

In box 6 an 8 can only exist as an option in C9R4 or C9R5; therefore, an 8 cannot exist as an option in C9R7, C9R8 and C9R9.

In box 7 a 3 can only exist as an option in C1R9 or C2R9; therefore, a 3 cannot exist as an option in C6R9, C8R9 and C9R9.

In box 7 a 6 can only exist as an option in C3R7 or C3R8; therefore, a 6 cannot exist as an option in C3R1, C3R3, C3R4 and C3R5.

Now your grid should look like Example #93.1 below:

Example #93.1

Now the magic question arises!  What are we missing here?  See if you can figure this out on your own before reading further.

If you figured this out, great.  If not, please look at row 2.   What do you see?  Yes, a hidden pair.  The only unsolved cells in row 2 that could be a 2 and 5 are C1R2 & C7R2.  We will mark our grid as such, giving us Example #93.2 below:

Example #93.2

This completes puzzle preparation, so we will fill in the options for the unsolved cells, giving us Example #93.3 below:

Example 93.2


Steps 1-5

I always check to see if there are any possibilities of a potentially lucrative Step 6 exercise, to give a fast conclusion to the puzzle.   If you recall from previous articles, look for a number 1-9 where three cells have that number as an answer, and the three cells are not in the same row, column or box!  Looking at the puzzle above, we see that numbers 2 and 5 fit that description.  So, we will now proceed to Step 6:  Dan’s Yes-No Challenge with Example #93.4 below:

Example #93.4

As you can see above, column 3 has just two unsolved cells with a 2 as an option, meeting our criteria for a Step 6 exercise.  We call them our starter cells, and one of the these two cells must be a 2.  We will first assume C3R1 is a 2 and see which other unsolved cells can or cannot be a 2, and mark them with a “Y” for “yes” or an “N” for “no”.  If C3R1=2 (Y), then C1R1, C1R2 & C9R1 =N.  Then, C7R2=Y and C7R7=N.

Next, we will assume C3R4 is a 2 and mark it with a “y”.   Then, we will mark which other cells can and cannot be a 2 with a “y” or “n”.  Follow the markings above.

Cells with a N,n designation indicate that those cells cannot be a 2 regardless of which of the two starter cells is a 2, and the 2 can be dropped as an option for those cells.

The cells with a Y,y designation indicates that that cell is a 2 regardless of which starter cell is a 2; therefore, C7R2=2.   It follows that C1R2=5.  Then C3R5=5, C6R6=5.

The 5’s are quickly retired, then the 1’s and 2’s retire and the puzzle is a downhill bike ride from there, giving us the completed puzzle in Example #93.5 below:

Example #93.5

And Puzzle #92?

Clue for Puzzle #92 …  check out box 8.   What do you observe?
In box 8 a 2 can exist as an option only in C4R7 or C5R7; therefore, a 2 cannot be an option in C3R7.  Now focus on column 3.  A 2 can only exist as an option in C3R9.   C3R9=2, and then C3R6=6.]

May the gentle winds of Sudoku be at your back.

By Dan LeKander

Editor's Note:  October 2021!

Dan, when not out fishing, and his proofreader (and, as they say, better half) Peggy, give us a new challenge each month. This month we have 92  & 93 = getting very close to 100.  So stay tuned as we will certainly have a celebration.

It was back in January 2016, when we published a final article in Dan's Series of Steps to learn the logic of Sudoku –  he asked if we would like a puzzle to solve every month … this editor said an enthusiastic… Yes, please!

If you have not already done so, I suggest you purchase Dan’s book: “3 Advanced Sudoku Techniques, That Will Change Your Game Forever!” Purchase of a book includes a 50-page blank grid pad, 33 black and two green tokens, to assist with Step 6.…

I also encourage you to write to Dan and tell him how his system is helpful!

The book is available online at ebay.com.

Be sure to read the TI Life's review of Dan's book by Jesse Kahn published in Jun 2015.

Here are links to all past Sudoku Puzzle Challenge beginning: February 2016, March 2016, April 2016, May 2016, June 2016, July 2016, August 2016, September 2016, October 2016, November 2016, December 2016, January 2017, February 2017, March 2017, April 2017, May 2017, June 2017, July 2017, August 2017, September 2017, October 2017, November 2017, December 2017, January 2018, February 2018, March 2018, April 2018, May 2018, June 2018, July 2018, August 2018, September 2018, October 2018, November 2018, December 2018, January 2019, February 2019, March 2019, April 2019, May 2019, June 2019, July 2019, August 2019, September 2019, October 2019, November 2019, December 2019, January 2020, February 2020, April 2020, May 2020,  June 2020 and July 2020, August 2020,  September 2020, October 2020, November 2020 and December 2020, January 2021, February 2021, March 2021, April 2021, May 2021, June 2021, July 2021 , August 2021 and September 2021.  (And like last month just for fun, go back to, say, May 2017 and see if you can solve the puzzle easily - and let us know.)

Posted in: Volume 16, Issue 10, October 2021, Sports, Current


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