Aug 23 - Hard
Puzzle Copyright © Kevin Stone
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Reasoning
R2C6 can only be <2>
R6C4 can only be <6>
R8C4 can only be <8>
R4C6 can only be <8>
R6C6 can only be <9>
R1C5 can only be <6>
R4C4 can only be <5>
R2C4 can only be <7>
R8C6 can only be <1>
R3C5 can only be <3>
R5C5 can only be <2>
R2C9 is the only square in row 2 that can be <3>
R3C3 is the only square in row 3 that can be <9>
R7C1 is the only square in row 7 that can be <1>
R8C9 is the only square in row 8 that can be <2>
R1C3 is the only square in column 3 that can be <8>
R1C7 can only be <1>
R1C8 can only be <5>
R1C2 can only be <2>
Intersection of column 1 with block 1. The value <4> only appears in one or more of squares R1C1, R2C1 and R3C1 of column 1. These squares are the ones that intersect with block 1. Thus, the other (non-intersecting) squares of block 1 cannot contain this value.
R2C2 - removing <4> from <456> leaving <56>
Squares R3C7<468>, R5C7<78>, R7C7<47> and R9C7<468> in column 7 form a comprehensive naked quad. These 4 squares can only contain the 4 possibilities <4678>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the column.
R4C7 - removing <47> from <2347> leaving <23>
R6C7 - removing <4> from <234> leaving <23>
Intersection of block 6 with column 8. The values <14> only appears in one or more of squares R4C8, R5C8 and R6C8 of block 6. These squares are the ones that intersect with column 8. Thus, the other (non-intersecting) squares of column 8 cannot contain these values.
R2C8 - removing <4> from <46> leaving <6>
R9C8 - removing <4> from <4689> leaving <689>
R2C2 can only be <5>
R2C1 can only be <4>
R3C1 can only be <6>
R8C1 can only be <7>
R8C8 can only be <9>
R5C1 can only be <5>
R8C2 can only be <6>
R9C8 can only be <8>
R7C9 can only be <4>
R7C3 can only be <5>
R7C7 can only be <7>
R3C9 can only be <8>
R9C7 can only be <6>
R5C2 can only be <1>
R3C7 can only be <4>
R5C9 can only be <9>
R5C8 can only be <7>
R5C3 can only be <6>
R5C7 can only be <8>
R4C8 can only be <4>
R7C5 can only be <9>
R9C3 can only be <4>
R9C5 can only be <5>
R9C2 can only be <9>
R6C3 can only be <2>
R4C2 can only be <3>
R6C8 can only be <1>
R6C7 can only be <3>
R4C3 can only be <7>
R6C2 can only be <4>
R4C7 can only be <2>
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