Feb 19 - Very Hard
Puzzle Copyright © Kevin Stone
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Reasoning
R5C3 can only be <1>
R5C7 can only be <2>
R6C5 can only be <3>
R8C2 can only be <3>
R8C8 can only be <5>
R6C9 can only be <7>
R6C6 can only be <2>
R4C9 can only be <5>
R8C1 can only be <8>
R4C5 can only be <1>
R6C1 can only be <6>
R4C6 can only be <7>
R8C9 can only be <1>
R4C4 can only be <6>
R6C4 can only be <4>
R4C1 can only be <2>
R1C1 is the only square in row 1 that can be <1>
R7C3 is the only square in row 7 that can be <7>
R7C7 is the only square in row 7 that can be <6>
R9C8 can only be <2>
R1C9 is the only square in row 1 that can be <2>
R1C7 is the only square in row 1 that can be <3>
R3C1 is the only square in row 3 that can be <3>
R9C4 is the only square in row 9 that can be <1>
R9C9 is the only square in row 9 that can be <3>
Squares R2C1 and R2C9 in row 2 form a simple naked pair. These 2 squares both contain the 2 possibilities <49>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the row.
R2C2 - removing <9> from <679> leaving <67>
Intersection of row 1 with block 2. The value <8> only appears in one or more of squares R1C4, R1C5 and R1C6 of row 1. These squares are the ones that intersect with block 2. Thus, the other (non-intersecting) squares of block 2 cannot contain this value.
R3C5 - removing <8> from <589> leaving <59>
Intersection of column 1 with block 7. The value <5> only appears in one or more of squares R7C1, R8C1 and R9C1 of column 1. These squares are the ones that intersect with block 7. Thus, the other (non-intersecting) squares of block 7 cannot contain this value.
R9C3 - removing <5> from <456> leaving <46>
Squares R2C1 and R2C9 in row 2 and R7C1 and R7C9 in row 7 form a Simple X-Wing pattern on possibility <4>. All other instances of this possibility in columns 1 and 9 can be removed.
R3C9 - removing <4> from <489> leaving <89>
R9C1 - removing <4> from <459> leaving <59>
Squares R2C2, R2C8, R1C2 and R1C8 form a Type-1 Unique Rectangle on <67>.
R1C2 - removing <67> from <679> leaving <9>
R9C2 can only be <6>
R2C1 can only be <4>
R2C9 can only be <9>
R7C1 can only be <5>
R3C3 can only be <5>
R3C9 can only be <8>
R3C5 can only be <9>
R1C3 can only be <6>
R3C7 can only be <4>
R7C9 can only be <4>
R7C5 can only be <8>
R9C1 can only be <9>
R5C5 can only be <5>
R9C6 can only be <5>
R9C7 can only be <8>
R9C3 can only be <4>
R2C2 can only be <7>
R1C6 can only be <8>
R1C8 can only be <7>
R1C4 can only be <5>
R5C6 can only be <9>
R2C8 can only be <6>
R5C4 can only be <8>
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