Feb 17 - Hard
Puzzle Copyright © Kevin Stone
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Reasoning
R3C6 can only be <8>
R7C8 is the only square in row 7 that can be <6>
R3C4 is the only square in row 3 that can be <6>
R9C3 is the only square in row 9 that can be <6>
Intersection of row 7 with block 8. The value <4> only appears in one or more of squares R7C4, R7C5 and R7C6 of row 7. These squares are the ones that intersect with block 8. Thus, the other (non-intersecting) squares of block 8 cannot contain this value.
R8C5 - removing <4> from <14789> leaving <1789>
R9C4 - removing <4> from <457> leaving <57>
R9C5 - removing <4> from <124579> leaving <12579>
Intersection of row 9 with block 9. The value <4> only appears in one or more of squares R9C7, R9C8 and R9C9 of row 9. These squares are the ones that intersect with block 9. Thus, the other (non-intersecting) squares of block 9 cannot contain this value.
R8C7 - removing <4> from <1479> leaving <179>
R8C9 - removing <4> from <47> leaving <7>
R4C9 can only be <3>
Intersection of column 1 with block 4. The value <7> only appears in one or more of squares R4C1, R5C1 and R6C1 of column 1. These squares are the ones that intersect with block 4. Thus, the other (non-intersecting) squares of block 4 cannot contain this value.
R5C2 - removing <7> from <23789> leaving <2389>
Intersection of block 7 with column 2. The value <2> only appears in one or more of squares R7C2, R8C2 and R9C2 of block 7. These squares are the ones that intersect with column 2. Thus, the other (non-intersecting) squares of column 2 cannot contain this value.
R1C2 - removing <2> from <2378> leaving <378>
R5C2 - removing <2> from <2389> leaving <389>
Squares R2C3<248>, R6C3<28> and R8C3<48> in column 3 form a comprehensive naked triplet. These 3 squares can only contain the 3 possibilities <248>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the column.
R1C3 - removing <248> from <2348> leaving <3>
R4C3 - removing <8> from <158> leaving <15>
R5C3 - removing <28> from <12358> leaving <135>
R3C2 can only be <7>
R3C8 can only be <4>
R1C2 can only be <8>
R3C5 can only be <3>
R9C8 can only be <1>
R2C9 can only be <6>
R8C7 can only be <9>
R1C8 can only be <7>
R1C7 can only be <5>
R5C8 can only be <8>
R6C9 can only be <2>
R6C3 can only be <8>
R5C9 can only be <4>
R9C7 can only be <4>
R1C4 can only be <4>
R2C7 can only be <8>
R8C3 can only be <4>
R4C1 can only be <7>
R8C1 can only be <8>
R2C3 can only be <2>
R7C4 can only be <8>
R2C1 can only be <4>
R2C5 can only be <5>
R4C5 can only be <8>
R4C7 can only be <1>
R6C1 can only be <9>
R8C5 can only be <1>
R4C3 can only be <5>
R5C7 can only be <7>
R5C4 can only be <5>
R6C7 can only be <6>
R6C5 can only be <7>
R5C1 can only be <2>
R5C2 can only be <3>
R1C5 can only be <2>
R1C6 can only be <1>
R9C5 can only be <9>
R5C3 can only be <1>
R5C6 can only be <9>
R9C4 can only be <7>
R7C6 can only be <2>
R7C2 can only be <9>
R9C6 can only be <5>
R9C2 can only be <2>
R7C5 can only be <4>
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