The full reasoning can be found below the Sudoku.
Apr 19 - Hard
Puzzle Copyright © Kevin Stone
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Reasoning
R4C6 can only be <3>
R6C4 can only be <5>
R4C4 can only be <2>
R6C6 can only be <8>
R1C6 can only be <5>
R3C6 can only be <4>
R8C6 can only be <7>
R2C4 can only be <6>
R5C5 can only be <1>
R4C9 is the only square in row 4 that can be <1>
R6C1 is the only square in row 6 that can be <4>
R7C7 is the only square in row 7 that can be <2>
Intersection of column 9 with block 9. The value <6> only appears in one or more of squares R7C9, R8C9 and R9C9 of column 9. These squares are the ones that intersect with block 9. Thus, the other (non-intersecting) squares of block 9 cannot contain this value.
R8C8 - removing <6> from <3569> leaving <359>
R9C8 - removing <6> from <369> leaving <39>
Squares R9C4 and R9C8 in row 9 form a simple naked pair. These 2 squares both contain the 2 possibilities <39>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the row.
R9C1 - removing <39> from <1379> leaving <17>
R9C2 - removing <39> from <3679> leaving <67>
R9C3 - removing <9> from <1679> leaving <167>
R5C1 is the only square in column 1 that can be <3>
R3C1 is the only square in column 1 that can be <9>
Squares R4C2 and R9C2 in column 2 form a simple naked pair. These 2 squares both contain the 2 possibilities <67>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the column.
R1C2 - removing <7> from <278> leaving <28>
R2C2 - removing <7> from <278> leaving <28>
R5C2 - removing <7> from <279> leaving <29>
R7C2 - removing <6> from <3469> leaving <349>
R8C2 - removing <6> from <3469> leaving <349>
R2C1 is the only square in block 1 that can be <7>
R9C1 can only be <1>
R2C8 is the only square in row 2 that can be <1>
R2C7 is the only square in row 2 that can be <5>
R3C3 is the only square in row 3 that can be <1>
R5C3 is the only square in column 3 that can be <2>
R5C2 can only be <9>
R6C3 can only be <6>
R6C8 can only be <9>
R9C3 can only be <7>
R4C2 can only be <7>
R9C8 can only be <3>
R9C2 can only be <6>
R9C4 can only be <9>
R8C8 can only be <5>
R4C7 can only be <6>
R3C7 can only be <8>
R8C3 can only be <9>
R5C8 can only be <8>
R7C4 can only be <3>
R3C5 can only be <2>
R5C7 can only be <7>
R1C8 can only be <2>
R5C9 can only be <5>
R1C7 can only be <9>
R7C2 can only be <4>
R8C9 can only be <6>
R7C3 can only be <5>
R8C5 can only be <4>
R7C9 can only be <9>
R1C9 can only be <7>
R1C2 can only be <8>
R3C8 can only be <6>
R2C5 can only be <8>
R7C5 can only be <6>
R8C2 can only be <3>
R2C2 can only be <2>
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