Dec 09 - Hard
Puzzle Copyright © Kevin Stone
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Reasoning
R6C5 can only be <7>
R6C7 can only be <8>
R4C5 can only be <4>
R5C5 can only be <6>
R5C2 is the only square in row 5 that can be <8>
R1C3 is the only square in column 3 that can be <6>
R9C9 is the only square in column 9 that can be <8>
R2C2 is the only square in block 1 that can be <3>
Squares R3C2 and R3C8 in row 3 form a simple naked pair. These 2 squares both contain the 2 possibilities <27>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the row.
R3C4 - removing <27> from <2678> leaving <68>
R3C5 - removing <2> from <289> leaving <89>
R3C6 - removing <7> from <679> leaving <69>
Squares R4C2 and R4C7 in row 4 form a simple naked pair. These 2 squares both contain the 2 possibilities <57>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the row.
R4C3 - removing <5> from <359> leaving <39>
R4C8 - removing <57> from <3579> leaving <39>
Intersection of row 2 with block 2. The value <4> only appears in one or more of squares R2C4, R2C5 and R2C6 of row 2. These squares are the ones that intersect with block 2. Thus, the other (non-intersecting) squares of block 2 cannot contain this value.
R1C4 - removing <4> from <2457> leaving <257>
R1C6 - removing <4> from <1347> leaving <137>
Squares R4C7<57>, R5C7<457> and R5C9<47> in block 6 form a comprehensive naked triplet. These 3 squares can only contain the 3 possibilities <457>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the block.
R5C8 - removing <57> from <1357> leaving <13>
R2C8 is the only square in column 8 that can be <5>
R2C5 can only be <2>
R8C5 can only be <1>
R1C6 is the only square in row 1 that can be <1>
R1C5 is the only square in row 1 that can be <3>
R1C4 is the only square in row 1 that can be <5>
R9C1 is the only square in row 9 that can be <1>
R5C1 can only be <7>
R5C9 can only be <4>
R1C1 can only be <2>
R4C2 can only be <5>
R5C7 can only be <5>
R1C9 can only be <7>
R3C2 can only be <7>
R1C7 can only be <4>
R3C8 can only be <2>
R7C8 can only be <6>
R4C7 can only be <7>
R5C3 can only be <3>
R9C7 can only be <2>
R5C8 can only be <1>
R4C3 can only be <9>
R6C8 can only be <9>
R6C3 can only be <2>
R4C8 can only be <3>
R7C6 can only be <4>
R8C8 can only be <7>
R9C3 can only be <5>
R9C4 can only be <7>
R6C2 can only be <1>
R7C2 can only be <2>
R2C6 can only be <7>
R8C6 can only be <6>
R8C4 can only be <2>
R3C6 can only be <9>
R9C5 can only be <9>
R2C4 can only be <4>
R9C6 can only be <3>
R3C5 can only be <8>
R3C4 can only be <6>
R7C5 can only be <5>
R7C4 can only be <8>
R8C2 can only be <4>
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