Expert Killer Sudoku — Pure Cage Logic

Expert Killer Sudoku is rated 5.0 to 6.9, the diabolical tier, and the grid starts with no given digits. The cage sums alone fix the solution. If you finish hard killers comfortably and want a blank start, this is the level.

What this level asks

You begin from nothing but the cage layout. Open with cage combinations that allow only one digit set, and with the 45 rule on boxes and bands. Those first steps are the easy part. The rating comes from the hardest step a puzzle needs, so an expert killer can look gentle for a long time and then demand a chain of linked candidates.

With no givens, the sums are the only way in. A two-cell cage of 16 can only be 7 and 9, so both cells start with two candidates instead of nine.

16

Possible combinations: 1

• 79

Techniques you will meet

• The hard-level cage set, used repeatedly: cage splitting, cage compare and multi-cell innies and outies. Splitting cuts a cage that hangs over a box edge into two smaller sums. Compare sets two cages against each other so the cells inside cancel out and a simple relation between the outside cells remains.
• Classic diabolical tools once arithmetic gives nothing more: XY-Chain and unique rectangles.
• Naked and hidden subsets and wings in between.

Tips for this level

• Spend the first minutes on sums only. Fill each cell's notes from its cage before looking at lines and boxes.
• Record every 45-rule result in your notes straight away. You will need it again later.
• A wrong candidate can hide the next step for many moves, so recheck notes when a deduction looks odd.
• Rating follows the single hardest step, so one tricky move can set the level even when the rest was easy.

Next step

Done with expert? Master Killer raises the band to 7.0 and above. The levels above it build on the same habits: complete notes, and every sum used before any chain.