Feb 14 - Super Hard
Puzzle Copyright © Kevin Stone
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Reasoning
R2C5 can only be <6>
R2C1 can only be <5>
R1C5 can only be <2>
R7C5 can only be <3>
R3C5 can only be <7>
R2C9 can only be <8>
R1C1 is the only square in row 1 that can be <6>
R6C7 is the only square in row 6 that can be <8>
R8C9 is the only square in row 8 that can be <7>
R5C1 is the only square in column 1 that can be <7>
R7C9 is the only square in column 9 that can be <9>
R7C2 is the only square in row 7 that can be <2>
R3C3 is the only square in row 3 that can be <2>
R3C2 is the only square in row 3 that can be <9>
R1C2 can only be <3>
R8C2 can only be <5>
R9C6 is the only square in row 9 that can be <5>
R9C4 is the only square in row 9 that can be <7>
R4C6 is the only square in row 4 that can be <7>
Squares R7C4 and R7C6 in row 7 form a simple naked pair. These 2 squares both contain the 2 possibilities <68>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the row.
R7C8 - removing <68> from <1468> leaving <14>
Squares R7C1 and R9C2 in block 7 form a simple naked pair. These 2 squares both contain the 2 possibilities <14>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the block.
R9C1 - removing <14> from <134> leaving <3>
R8C1 can only be <9>
Squares R4C4<69>, R5C6<46> and R6C6<49> in block 5 form a comprehensive naked triplet. These 3 squares can only contain the 3 possibilities <469>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the block.
R5C4 - removing <6> from <236> leaving <23>
R6C4 - removing <9> from <239> leaving <23>
Squares R4C2 and R7C8 form a remote naked pair. <14> can be removed from any square that is common to their groups.
R4C8 - removing <14> from <1456> leaving <56>
Squares R4C3<59>, R4C4<69> and R4C8<56> in row 4 form a comprehensive naked triplet. These 3 squares can only contain the 3 possibilities <569>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the row.
R4C7 - removing <6> from <146> leaving <14>
Squares R8C3, R9C3, R8C8 and R9C8 form a Type-3 Unique Rectangle on <68>. Upon close inspection, it is clear that:
(R8C8 or R9C8)<134>, R7C8<14> and R3C8<34> form a naked triplet on <134> in column 8. No other squares in the column can contain these possibilities
R1C8 - removing <1> from <15> leaving <5>
(R8C8 or R9C8)<134>, R7C8<14>, R3C8<34> and R1C8<15> form a naked quad on <1345> in column 8. No other squares in the column can contain these possibilities
R4C8 - removing <5> from <56> leaving <6>
R1C9 can only be <1>
R4C4 can only be <9>
R4C3 can only be <5>
R1C4 can only be <8>
R6C6 can only be <4>
R6C1 can only be <1>
R5C6 can only be <6>
R1C6 can only be <9>
R7C4 can only be <6>
R5C3 can only be <3>
R5C4 can only be <2>
R6C3 can only be <9>
R6C4 can only be <3>
R7C6 can only be <8>
R6C5 can only be <5>
R7C1 can only be <4>
R4C2 can only be <4>
R6C9 can only be <2>
R5C5 can only be <1>
R9C9 can only be <4>
R7C8 can only be <1>
R9C2 can only be <1>
R9C8 can only be <8>
R5C9 can only be <5>
R4C7 can only be <1>
R5C7 can only be <4>
R3C7 can only be <3>
R9C3 can only be <6>
R8C8 can only be <3>
R3C8 can only be <4>
R8C7 can only be <6>
R8C3 can only be <8>
R9C7 can only be <2>
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