Absolutely Nasty Sudoku Level 4
Designed for experienced puzzle solvers, this book features challenging, high-level Nasty Sudoku puzzles by Frank Longo to push your logical skills to the limit.
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Orbace Journal
A record of how Sudoku is solved, taught, and shaped. The Journal turns interesting games into substantial original lessons — giving players something useful to read before the next puzzle.
Su-Pu Studies
The studies, in order.
Lesson 08 · September 11, 2026 · 8 min
Solution Space, Not Candidate Space
Why ibtree's pencil digits trace a trial solution forward, not every possible candidate in every cell.
Lesson 07 · August 22, 2026 · 8 min
When the Branch Holds: Finishing Without Contradiction
What happens when your chosen candidate turns out to be correct? A look at the ibtree method's success path—and a practical verification technique for any completed grid.
Lesson 06 · August 2, 2026 · 8 min
Three-Way Fork in Box 9: A Double Contradiction
When three candidates compete for the same digit, a two-layer ibtree resolves the uncertainty—eliminating two false paths to prove the third.
Lesson 05 · July 30, 2026 · 9 min
A Puzzle Worth Remembering: Beauty Meets Ibtree
When a Hell-tier puzzle’s elegant initial state meets the ibtree method’s sharp forks, even a Hodoku 7820 becomes a study in creative leverage.
Lesson 04 · July 28, 2026 · 8 min
Taming the 'Hell' Puzzle: One Fork, Seven Minutes
When candidate reduction demands a dozen advanced techniques, ibtree finds an elegant path through a single well-chosen assumption.
Lesson 03 · July 30, 2026 · 8 min
The Three-Way Fork: Restructuring Multiple Branches
When three candidates vie for one position, treat it as two binary decisions—and watch a Hell-tier puzzle collapse.
Lesson 02 · July 31, 2026 · 8 min
The Knight's Move: A Second Proof by Contradiction
When two candidates in a box form a knight's-move pattern, one often acts as a chain trigger—tracing it forward quickly reveals which assumption leads to contradiction.
Lesson 01 · July 31, 2026 · 8 min
Two Homes for a Nine
When basic methods stall, look for a binary fork—two candidates in one box—and trace one forward to find your proof by contradiction.
On paper
Put your extreme Sudoku skills into practice
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Beyond Black Belt Sudoku
Step up your game with expert-level puzzles crafted by Frank Longo, built specifically for enthusiasts who have mastered standard difficulty levels.
View on AmazonWay Beyond Black Belt Sudoku
A masterclass in number placement, offering some of the most intricate and brain-bending Sudoku challenges available.
View on AmazonAbsolutely Nasty Sudoku: Even Nastier – Level 4
Take a thrilling leap past regular difficulty with a fresh collection of diabolical puzzles designed for dedicated solvers.
View on AmazonPuzzle Challenge
Found a puzzle that beat you? Send it in.
Submit a Sudoku you couldn't crack — your own or one you found elsewhere. If it checks out, we'll solve it, reconstruct the study, and publish it here with your credit (or anonymously, your call).
Submit a puzzle →Explore the journal
Learn without guessing
Rules, scanning, candidates, and a dependable first solving method.
Understand why it works
Formal patterns explained through real positions and useful eliminations.
What makes a grid satisfying?
Logical flow, validation, turning points, and thoughtful difficulty.
How the Journal is made
From a real solve to a useful lesson.
Select
Choose and verify a noteworthy solve.
Reconstruct
Rebuild the Su-Pu and draft the lesson.
Review
Check the logic against the playable position.
Publish
Release, link practice, and correct openly.