Hard sudoku 4

A hard nine by nine. Somewhere in this grid there is a step that scanning will not find: a pair locked into one line, or four cells sitting at the corners of a rectangle. Everything before and after it is ordinary work, and there is still exactly one answer.

35847823926859778916356469
Show the answer key
912735486384926715657418293549172368863549127271863954196354872735281649428697531

What is on this sheet

Given numbers
26
Difficulty
Hard
Hardest step needed
Y-wing
Grid
9 × 9

About this puzzle

Hard means 25 to 29 given numbers and at least one advanced step — an X-wing, a Y-wing, a box-line reduction. The level is measured by solving, not declared: the generator throws away any grid that its solver can only finish by guessing. The answer key is on the second page.

This particular grid starts with 26 of the 81 numbers already printed, and the hardest technique it asks for is: Y-wing.

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