Feb 09 - Hard
Puzzle Copyright © Kevin Stone
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Reasoning
R8C9 can only be <9>
R1C4 is the only square in row 1 that can be <5>
R3C3 is the only square in row 3 that can be <3>
R7C7 is the only square in row 7 that can be <6>
R8C5 is the only square in row 8 that can be <5>
R3C7 is the only square in column 7 that can be <9>
Squares R9C7 and R9C8 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.
R9C4 - removing <4> from <23489> leaving <2389>
R9C6 - removing <4> from <23489> leaving <2389>
Intersection of row 3 with block 2. The values <124> only appears in one or more of squares R3C4, R3C5 and R3C6 of row 3. These squares are the ones that intersect with block 2. Thus, the other (non-intersecting) squares of block 2 cannot contain these values.
R1C6 - removing <4> from <4678> leaving <678>
R2C4 - removing <4> from <489> leaving <89>
R2C5 - removing <4> from <47> leaving <7>
R2C6 - removing <4> from <46789> leaving <6789>
R1C7 is the only square in row 1 that can be <7>
R7C6 is the only square in row 7 that can be <7>
Intersection of block 5 with row 5. The values <234> only appears in one or more of squares R5C4, R5C5 and R5C6 of block 5. These squares are the ones that intersect with row 5. Thus, the other (non-intersecting) squares of row 5 cannot contain these values.
R5C1 - removing <234> from <12348> leaving <18>
R5C3 - removing <2> from <12678> leaving <1678>
R5C7 - removing <24> from <124> leaving <1>
R5C9 - removing <4> from <4678> leaving <678>
R5C1 can only be <8>
R2C1 can only be <4>
R2C9 can only be <6>
R1C2 can only be <1>
R5C9 can only be <7>
R1C8 can only be <4>
R5C3 can only be <6>
R4C9 can only be <4>
R1C3 can only be <8>
R1C6 can only be <6>
R4C8 can only be <3>
R9C8 can only be <5>
R4C2 can only be <9>
R4C7 can only be <2>
R6C9 can only be <8>
R6C3 can only be <2>
R6C1 can only be <3>
R6C7 can only be <5>
R9C3 can only be <9>
R4C1 can only be <1>
R6C8 can only be <6>
R9C7 can only be <4>
R9C2 can only be <3>
R7C3 can only be <1>
R4C3 can only be <7>
R6C2 can only be <4>
R8C1 can only be <2>
R7C5 can only be <4>
R7C4 can only be <9>
R5C5 can only be <2>
R8C6 can only be <3>
R8C4 can only be <1>
R5C6 can only be <4>
R3C5 can only be <1>
R5C4 can only be <3>
R3C6 can only be <2>
R2C4 can only be <8>
R2C6 can only be <9>
R9C4 can only be <2>
R3C4 can only be <4>
R9C6 can only be <8>
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