This post steps through the most fiendish of the 10 puzzles preselected for the review of Brian Challenger’s Super Fiendish Sudoku, SF 67. , and then adds one of the bypass victims, SF 77.
Looking at the trace, you might think basic is easy,
but the long fill strings reveal a tough line marking.
Advanced solving starts with hidden unique rectangles of type 2b. The floor to ceiling slinks project a victim clue into a reversible rectangle of four clues.
Then, after an X-chain ANL,
a regular 127-wing is extended into an inference chaining i127-wing.
The midfield action continues with a 4-BARN (Bent Almost Restricted 4-Set),
and an ungainly looking APE in which 5r4c9 cannot be combined with 1, 2 or 7 in r4c8 within sight of ALS 157 and ALS 2457.
The APE removal is telling,
leading to an XY railway of interconnected nice loops.
No removals result, but a nice loop generated cluster is expanded by a trap and wraps blue in r2 and c4, for the collapse of Super Fiendish 67.
In his comment to this post, forum member strmckr provided this quick resolution to SF 67 immediately after line marking. He terms it an ALS W-wing.
The restricted common is a forcing chain. Since one ALS will lose it’s 9’s. Any candidate seeing all of any other value in both ALS must see a true value. 1r1c9 does just that, and the removal collapses SF 67. With all the ALS in every grid, can you hope to spot such an event? Not really, but you can see the appeal of writing code that can find every instance of it.
Super Fiendish 77 might have presented similar advanced challenges, but was too vulnerable to Sysudoku Basic. It takes a while but the collapse is relentless.
Next week, the Super Fiendish review ends with 87 and 97. For the next review, get a copy of J.B. Nakamoto’s Sudoku Puzzle Book, Extreme Level.
The 67 and 77 solutions: