The full reasoning can be found below the Sudoku.
May 11 - Hard
Puzzle Copyright © Kevin Stone
Reasoning
R1C2 is the only square in row 1 that can be <9>
R2C5 is the only square in row 2 that can be <2>
R7C3 is the only square in row 7 that can be <6>
R8C1 is the only square in row 8 that can be <2>
R8C8 is the only square in row 8 that can be <6>
R9C9 is the only square in row 9 that can be <9>
R5C7 is the only square in row 5 that can be <9>
R2C8 is the only square in column 8 that can be <1>
R1C1 is the only square in block 1 that can be <6>
Squares R5C1 and R5C3 in row 5 form a simple naked pair. These 2 squares both contain the 2 possibilities <47>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the row.
R5C2 - removing <47> from <13467> leaving <136>
R5C5 - removing <7> from <13567> leaving <1356>
R5C8 - removing <4> from <345> leaving <35>
R5C9 - removing <4> from <345> leaving <35>
Squares R5C8 and R5C9 in row 5 form a simple naked pair. These 2 squares both contain the 2 possibilities <35>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the row.
R5C2 - removing <3> from <136> leaving <16>
R5C5 - removing <35> from <1356> leaving <16>
Squares R2C2, R8C2 and R9C2 in column 2 form a simple naked triplet. These 3 squares all contain the 3 possibilities <478>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the column.
R4C2 - removing <478> from <13478> leaving <13>
R6C2 - removing <78> from <3678> leaving <36>
Squares R4C2 and R4C4 in row 4 form a simple naked pair. These 2 squares both contain the 2 possibilities <13>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the row.
R4C6 - removing <13> from <137> leaving <7>
R4C8 - removing <3> from <2348> leaving <248>
R6C7 is the only square in column 7 that can be <7>
R5C3 is the only square in column 3 that can be <7>
R5C1 can only be <4>
R3C3 is the only square in column 3 that can be <4>
R2C9 is the only square in row 2 that can be <4>
R3C7 is the only square in column 7 that can be <5>
R1C5 is the only square in row 1 that can be <5>
R9C8 is the only square in row 9 that can be <5>
R5C8 can only be <3>
R8C9 can only be <8>
R5C9 can only be <5>
R1C8 can only be <8>
R1C9 can only be <3>
R7C7 can only be <4>
R6C8 can only be <2>
R6C3 can only be <8>
R4C8 can only be <4>
R4C7 can only be <8>
R4C3 can only be <2>
R9C2 is the only square in row 9 that can be <4>
R8C2 can only be <7>
R8C5 can only be <1>
R2C2 can only be <8>
R9C1 can only be <8>
R8C6 can only be <5>
R5C5 can only be <6>
R8C4 can only be <4>
R6C6 can only be <3>
R9C5 can only be <7>
R2C1 can only be <7>
R5C2 can only be <1>
R3C5 can only be <8>
R6C2 can only be <6>
R6C4 can only be <5>
R2C6 can only be <9>
R4C4 can only be <1>
R2C4 can only be <3>
R7C6 can only be <8>
R3C6 can only be <1>
R7C5 can only be <3>
R3C4 can only be <6>
R4C2 can only be <3>
R7C4 can only be <9>
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