Feb 17 - Super Hard
Puzzle Copyright © Kevin Stone
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Reasoning
R9C9 can only be <2>
R1C9 can only be <9>
R7C7 can only be <7>
R2C9 can only be <3>
R8C7 can only be <5>
R8C8 can only be <9>
R3C6 is the only square in row 3 that can be <3>
R3C7 is the only square in row 3 that can be <6>
R4C5 is the only square in row 4 that can be <1>
R6C7 is the only square in row 6 that can be <3>
R6C5 is the only square in row 6 that can be <9>
R8C3 is the only square in row 8 that can be <3>
R9C5 is the only square in row 9 that can be <5>
R9C1 is the only square in column 1 that can be <4>
R9C2 is the only square in row 9 that can be <6>
R8C1 can only be <2>
R1C1 can only be <6>
R7C2 can only be <9>
R7C3 can only be <1>
Intersection of row 4 with block 4. The value <7> only appears in one or more of squares R4C1, R4C2 and R4C3 of row 4. 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 <24578> leaving <2458>
Intersection of column 7 with block 3. The value <8> only appears in one or more of squares R1C7, R2C7 and R3C7 of column 7. These squares are the ones that intersect with block 3. Thus, the other (non-intersecting) squares of block 3 cannot contain this value.
R1C8 - removing <8> from <1278> leaving <127>
R2C8 - removing <8> from <1248> leaving <124>
Intersection of block 5 with row 5. The values <247> only appears in one or more of squares R5C4, R5C5 and R5C6 of block 5. These squares are the ones that intersect with row 5. Thus, the other (non-intersecting) squares of row 5 cannot contain these values.
R5C2 - removing <24> from <2458> leaving <58>
R5C8 - removing <24> from <2458> leaving <58>
Squares R7C4, R7C6, R5C4 and R5C6 form a Type-2 Unique Rectangle on <24>.
R5C5 - removing <7> from <27> leaving <2>
Squares R1C7 (XY), R3C8 (XZ) and R1C5 (YZ) form an XY-Wing pattern on <7>. All squares that are buddies of both the XZ and YZ squares cannot be <7>.
R1C8 - removing <7> from <127> leaving <12>
R3C4 - removing <7> from <279> leaving <29>
R3C8 is the only square in row 3 that can be <7>
Squares R3C3, R3C4, R2C3 and R2C4 form a Type-4 Unique Rectangle on <29>.
R2C3 - removing <2> from <259> leaving <59>
R2C4 - removing <2> from <1289> leaving <189>
Squares R5C2 (XY), R2C2 (XZ) and R6C3 (YZ) form an XY-Wing pattern on <2>. All squares that are buddies of both the XZ and YZ squares cannot be <2>.
R6C2 - removing <2> from <248> leaving <48>
R3C3 - removing <2> from <29> leaving <9>
R4C2 - removing <2> from <2457> leaving <457>
R3C4 can only be <2>
R2C3 can only be <5>
R7C4 can only be <4>
R7C6 can only be <2>
R5C4 can only be <7>
R2C2 can only be <2>
R5C6 can only be <4>
R9C4 can only be <8>
R9C3 can only be <7>
R8C4 can only be <1>
R2C4 can only be <9>
R4C3 can only be <2>
R8C2 can only be <8>
R4C7 can only be <4>
R6C3 can only be <8>
R4C8 can only be <5>
R2C7 can only be <8>
R4C2 can only be <7>
R5C8 can only be <8>
R5C2 can only be <5>
R6C8 can only be <2>
R6C2 can only be <4>
R1C8 can only be <1>
R1C6 can only be <7>
R2C8 can only be <4>
R2C5 can only be <6>
R1C7 can only be <2>
R1C5 can only be <8>
R8C6 can only be <6>
R2C6 can only be <1>
R8C5 can only be <7>
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