Dec 11 - Very Hard
Puzzle Copyright © Kevin Stone
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Reasoning
R4C7 is the only square in row 4 that can be <5>
R1C9 is the only square in row 1 that can be <5>
R4C3 is the only square in row 4 that can be <6>
R7C4 is the only square in column 4 that can be <1>
R3C6 is the only square in column 6 that can be <2>
R8C1 is the only square in column 1 that can be <2>
R3C9 is the only square in column 9 that can be <1>
R1C3 is the only square in row 1 that can be <1>
R1C7 is the only square in row 1 that can be <2>
R2C3 is the only square in row 2 that can be <2>
R3C7 is the only square in row 3 that can be <6>
R3C5 is the only square in row 3 that can be <9>
R2C7 is the only square in row 2 that can be <9>
R5C1 is the only square in row 5 that can be <1>
R6C9 is the only square in row 6 that can be <6>
Squares R3C1 and R3C3 in row 3 form a simple naked pair. These 2 squares both contain the 2 possibilities <78>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the row.
R3C4 - removing <7> from <347> leaving <34>
R3C8 - removing <78> from <3478> leaving <34>
Intersection of column 7 with block 6. The value <4> only appears in one or more of squares R4C7, R5C7 and R6C7 of column 7. These squares are the ones that intersect with block 6. Thus, the other (non-intersecting) squares of block 6 cannot contain this value.
R4C8 - removing <4> from <347> leaving <37>
Intersection of column 8 with block 3. The values <48> only appears in one or more of squares R1C8, R2C8 and R3C8 of column 8. These squares are the ones that intersect with block 3. Thus, the other (non-intersecting) squares of block 3 cannot contain these values.
R2C9 - removing <8> from <78> leaving <7>
R4C8 is the only square in column 8 that can be <7>
R1C4 is the only square in column 4 that can be <7>
R5C5 is the only square in column 5 that can be <7>
Squares R3C3<78>, R5C3<348>, R6C3<3478> and R8C3<34> in column 3 form a comprehensive naked quad. These 4 squares can only contain the 4 possibilities <3478>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the column.
R7C3 - removing <3478> from <345789> leaving <59>
R9C3 - removing <378> from <35789> leaving <59>
Squares R9C3<59>, R9C5<35> and R9C6<39> in row 9 form a comprehensive naked triplet. These 3 squares can only contain the 3 possibilities <359>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the row.
R9C1 - removing <3> from <378> leaving <78>
R9C7 - removing <3> from <1378> leaving <178>
Squares R3C1 and R9C1 in column 1 form a simple naked pair. These 2 squares both contain the 2 possibilities <78>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the column.
R7C1 - removing <78> from <3478> leaving <34>
Intersection of row 9 with block 8. The value <3> only appears in one or more of squares R9C4, R9C5 and R9C6 of row 9. These squares are the ones that intersect with block 8. Thus, the other (non-intersecting) squares of block 8 cannot contain this value.
R7C5 - removing <3> from <3456> leaving <456>
R7C6 - removing <3> from <349> leaving <49>
R8C5 - removing <3> from <346> leaving <46>
Squares R6C6 and R7C1 form a remote naked pair. <34> can be removed from any square that is common to their groups.
R7C6 - removing <4> from <49> leaving <9>
R7C3 can only be <5>
R9C6 can only be <3>
R9C5 can only be <5>
R6C6 can only be <4>
R4C4 can only be <3>
R9C3 can only be <9>
R4C1 can only be <4>
R3C4 can only be <4>
R3C8 can only be <3>
R2C5 can only be <8>
R1C8 can only be <8>
R7C1 can only be <3>
R7C9 can only be <8>
R8C3 can only be <4>
R7C7 can only be <7>
R5C9 can only be <3>
R8C5 can only be <6>
R8C2 can only be <1>
R7C5 can only be <4>
R1C5 can only be <3>
R2C8 can only be <4>
R5C3 can only be <8>
R6C7 can only be <8>
R6C2 can only be <7>
R5C7 can only be <4>
R7C2 can only be <6>
R9C7 can only be <1>
R8C7 can only be <3>
R3C3 can only be <7>
R6C3 can only be <3>
R9C2 can only be <8>
R9C1 can only be <7>
R3C1 can only be <8>
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