Nikoli’s X-treme Sudoku

Here we begin a series of reviews that are a clinic on Sysdudoku basic solving skills. Each collection of the survey will be introduced, with several puzzles checkpointed by 2-D traces. This post introduces the Nikoli Publishing top toughy, the “Very Difficult” section of X-treme Sudoku, published in the United States by Workman Publishing.

On the X-treme Sudoku cover, Nikoli bills the collection as “the toughest, roughest puzzles from the Japanese creators of the game”. If you read that as the toughest from Japanese creators, the claim is probably true, simply because there are no other Japanese creators of Sudoku puzzles. Nikoli fiercely defends its Japanese copyright on the game itself. The “Very Difficult” half of the X-treme collection does appear to be the toughest collection Nikoli has published.

But don’t read it to mean Nikoli created the game. Read “creators of the game” as “makers of puzzles”. Peter Gordon in his 2006 Mensa Guide to Solving Sudoku, details how Nikoli named the game, and where it was born in the United States in 1979. I’ll be reviewing Gordon’s guiding instruction in this book, as supported by Frank Longo puzzles, starting in December. Of course, the clinic is based on the beginner’s page and early, basic solving posts of this blog

xtreme 180 1Your homework puzzle, the X-treme 180, starts box marking very aggressively, showing how the immediate follow up of completed chutes and squares pays off.

The “1:” trace is below.

In this series, I am leaving each effect clue in candidate size font until it has a turn as a cause. The final number of unused effects is a measure of the difficulty of the puzzle. The more, the easier.

xtreme 180 1 trIf you missed some of the low hanging fruit, you can follow the trace to improve your marking technique. New readers can find directions for reading a 2-D trace on the trace page(link above). Also, this is a good time for a reminder that my email for requesting ©PowerPoint box marking grid template is on the tools page(another link above), along with detailed help. It’s a good way to save your solving states for later review or sharing. And it’s so easy.

If you had to upgrade your fruit picker after that checkpoint, try out your new version on box marking 2: through 7: before looking at the checkpoint below.

X-treme 180 is one of 10 X-tremes preselected for the review. I started with 165 and selected every 15th puzzle, reaching 300. The survey results are tabulated a bit differently than previously reviewed collections(Find It above), because in this basic series, advanced methods are not needed.

xtreme review tableX-treme 180 is one of four that collapsed (—-) in box marking. The slash notation for box marking survivors denotes marking clues/marks or locked sets. Parentheses enclose (clues,marks) over a range of scan numbers or for locked sets, such as naked(n) or hidden(h) singles(s), pairs(p) or triples(t).

xtreme 210One of the reviewed X-tremes, 210, almost made it to advanced Sysudoku solving, collapsing in the close after two naked triples.  I’ll checkpoint the box marking next time

If you’ve put off learning the 2-D trace conventions, now is the time. Many readers are checkpointing their Sysudoku blog homework. For maximum basic skills, you might want to buy a copy of X-treme Sudoku, and ask for my 2-D traces of all ten puzzles as a Word file from If you don’t have it already, you can ask for the ©PowerPoint grid template at the same time.

xtreme 180 7OK, here is the grid after box marking 2 -7: of X-treme 180.

Miss anything? Find its immediate cause in the trace below.

Next time, we’ll wrap up X-treme 180.





xtreme 180 7tr



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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