The full reasoning can be found below the Sudoku.
Apr 13 - Hard
Puzzle Copyright © Kevin Stone
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Reasoning
R4C6 can only be <8>
R4C1 can only be <3>
R1C6 can only be <4>
R1C4 can only be <2>
R9C6 can only be <6>
R6C6 can only be <1>
R1C5 can only be <8>
R2C5 can only be <6>
R5C6 can only be <9>
R1C3 is the only square in row 1 that can be <3>
R3C2 is the only square in row 3 that can be <7>
R4C5 is the only square in row 4 that can be <2>
R6C5 is the only square in row 6 that can be <3>
R5C8 is the only square in row 5 that can be <3>
R8C7 is the only square in row 8 that can be <3>
R5C2 is the only square in column 2 that can be <1>
R9C5 is the only square in column 5 that can be <7>
R7C7 is the only square in column 7 that can be <7>
Squares R9C4 and R9C7 in row 9 form a simple naked pair. These 2 squares both contain the 2 possibilities <45>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the row.
R9C1 - removing <4> from <1249> leaving <129>
R9C3 - removing <5> from <159> leaving <19>
R9C9 - removing <45> from <2459> leaving <29>
Squares R5C1 and R6C1 in column 1 form a simple naked pair. These 2 squares both contain the 2 possibilities <68>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the column.
R3C1 - removing <8> from <1248> leaving <124>
R7C1 - removing <6> from <2469> leaving <249>
Intersection of column 7 with block 3. The values <18> only appears in one or more of squares R1C7, R2C7 and R3C7 of column 7. These squares are the ones that intersect with block 3. Thus, the other (non-intersecting) squares of block 3 cannot contain these values.
R3C9 - removing <8> from <2468> leaving <246>
Intersection of column 8 with block 9. The value <5> only appears in one or more of squares R7C8, R8C8 and R9C8 of column 8. These squares are the ones that intersect with block 9. Thus, the other (non-intersecting) squares of block 9 cannot contain this value.
R7C9 - removing <5> from <24569> leaving <2469>
R9C7 - removing <5> from <45> leaving <4>
R9C4 can only be <5>
R2C7 can only be <8>
R2C3 can only be <9>
R3C7 can only be <1>
R3C3 can only be <8>
R1C7 can only be <5>
R4C4 can only be <7>
R8C5 can only be <4>
R1C9 can only be <9>
R1C1 can only be <1>
R9C9 can only be <2>
R2C8 can only be <2>
R9C3 can only be <1>
R2C2 can only be <4>
R3C8 can only be <6>
R3C9 can only be <4>
R8C8 can only be <5>
R3C1 can only be <2>
R4C9 can only be <5>
R6C4 can only be <6>
R5C9 can only be <8>
R5C1 can only be <6>
R6C9 can only be <7>
R6C1 can only be <8>
R5C4 can only be <4>
R8C2 can only be <8>
R5C5 can only be <5>
R8C3 can only be <6>
R7C8 can only be <9>
R9C1 can only be <9>
R7C9 can only be <6>
R7C2 can only be <2>
R7C1 can only be <4>
R7C3 can only be <5>
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