The full reasoning can be found below the Sudoku.
Mar 17 - Hard
Puzzle Copyright © Kevin Stone
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Reasoning
R1C3 is the only square in row 1 that can be <9>
R1C7 is the only square in row 1 that can be <6>
R2C6 is the only square in row 2 that can be <9>
R3C1 is the only square in row 3 that can be <6>
R6C8 is the only square in row 6 that can be <6>
R4C3 is the only square in row 4 that can be <6>
R8C6 is the only square in row 8 that can be <5>
R8C4 is the only square in row 8 that can be <6>
R8C2 is the only square in column 2 that can be <3>
R7C8 is the only square in block 9 that can be <7>
Intersection of row 5 with block 4. The value <8> only appears in one or more of squares R5C1, R5C2 and R5C3 of row 5. These squares are the ones that intersect with block 4. Thus, the other (non-intersecting) squares of block 4 cannot contain this value.
R6C1 - removing <8> from <124578> leaving <12457>
R6C2 - removing <8> from <1248> leaving <124>
R1C2 is the only square in column 2 that can be <8>
R2C9 is the only square in row 2 that can be <8>
R2C8 is the only square in row 2 that can be <5>
Squares R4C6 and R4C8 in row 4 form a simple naked pair. These 2 squares both contain the 2 possibilities <23>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the row.
R4C1 - removing <2> from <12457> leaving <1457>
R4C2 - removing <2> from <124> leaving <14>
R4C4 - removing <23> from <2347> leaving <47>
R4C5 - removing <3> from <349> leaving <49>
R4C9 - removing <23> from <12359> leaving <159>
R4C6 is the only square in block 5 that can be <3>
R4C8 can only be <2>
R9C8 can only be <3>
R5C9 can only be <3>
R5C7 can only be <7>
R6C7 can only be <1>
R8C3 is the only square in column 3 that can be <7>
Squares R3C2 and R3C6 in row 3 form a simple naked pair. These 2 squares both contain the 2 possibilities <12>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the row.
R3C7 - removing <2> from <234> leaving <34>
Squares R5C1 and R5C3 in block 4 form a simple naked pair. These 2 squares both contain the 2 possibilities <28>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the block.
R6C1 - removing <2> from <2457> leaving <457>
R6C2 - removing <2> from <24> leaving <4>
R4C2 can only be <1>
R3C2 can only be <2>
R3C6 can only be <1>
R2C1 can only be <1>
R9C6 can only be <8>
R6C6 can only be <2>
R7C5 can only be <3>
R8C1 can only be <4>
R6C4 can only be <7>
R7C4 can only be <1>
R3C5 can only be <4>
R8C9 can only be <1>
R9C1 can only be <2>
R7C9 can only be <2>
R9C3 can only be <1>
R9C7 can only be <4>
R5C1 can only be <8>
R7C3 can only be <8>
R3C7 can only be <3>
R4C5 can only be <9>
R1C4 can only be <2>
R2C7 can only be <2>
R4C9 can only be <5>
R6C5 can only be <8>
R4C1 can only be <7>
R6C9 can only be <9>
R5C3 can only be <2>
R6C1 can only be <5>
R4C4 can only be <4>
R1C9 can only be <4>
R2C4 can only be <3>
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