The full reasoning can be found below the Sudoku.
Jun 09 - Super Hard
Puzzle Copyright © Kevin Stone
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Reasoning
R4C4 can only be <4>
R5C1 can only be <4>
R6C4 can only be <8>
R4C6 can only be <3>
R6C6 can only be <2>
R5C3 can only be <5>
R4C2 can only be <6>
R5C5 can only be <7>
R4C8 can only be <5>
R6C2 can only be <3>
R6C8 can only be <6>
R5C2 can only be <1>
R2C9 is the only square in row 2 that can be <5>
R2C3 is the only square in row 2 that can be <6>
R2C2 is the only square in row 2 that can be <7>
R2C5 is the only square in row 2 that can be <2>
R3C8 is the only square in row 3 that can be <1>
R8C2 is the only square in row 8 that can be <5>
R9C8 is the only square in row 9 that can be <7>
R8C3 is the only square in row 8 that can be <7>
Intersection of row 8 with block 9. The value <2> only appears in one or more of squares R8C7, R8C8 and R8C9 of row 8. These squares are the ones that intersect with block 9. Thus, the other (non-intersecting) squares of block 9 cannot contain this value.
R7C7 - removing <2> from <23468> leaving <3468>
R7C8 - removing <2> from <2348> leaving <348>
Squares R8C4<19>, R8C6<14> and R9C5<49> in block 8 form a comprehensive naked triplet. These 3 squares can only contain the 3 possibilities <149>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the block.
R7C5 - removing <4> from <346> leaving <36>
R8C5 - removing <49> from <3469> leaving <36>
Squares R3C2 and R3C5 in row 3 and R9C2 and R9C5 in row 9 form a Simple X-Wing pattern on possibility <9>. All other instances of this possibility in columns 2 and 5 can be removed.
R1C2 - removing <9> from <489> leaving <48>
R1C5 - removing <9> from <489> leaving <48>
R1C8 is the only square in row 1 that can be <9>
Squares R1C2 and R1C5 in row 1 and R9C2 and R9C5 in row 9 form a Simple X-Wing pattern on possibility <4>. All other instances of this possibility in columns 2 and 5 can be removed.
R3C2 - removing <4> from <2489> leaving <289>
R3C5 - removing <4> from <489> leaving <89>
R7C2 - removing <4> from <248> leaving <28>
Squares R7C5, R8C5, R7C7 and R8C7 form a Type-4 Unique Rectangle on <36>.
R7C7 - removing <3> from <3468> leaving <468>
R8C7 - removing <3> from <23468> leaving <2468>
Squares R1C2 (XY), R9C2 (XZ) and R2C1 (YZ) form an XY-Wing pattern on <9>. All squares that are buddies of both the XZ and YZ squares cannot be <9>.
R8C1 - removing <9> from <89> leaving <8>
R3C2 - removing <9> from <289> leaving <28>
R8C9 can only be <3>
R2C1 can only be <9>
R7C2 can only be <2>
R8C5 can only be <6>
R5C9 can only be <8>
R2C4 can only be <1>
R2C6 can only be <4>
R8C4 can only be <9>
R8C6 can only be <1>
R1C5 can only be <8>
R7C3 can only be <4>
R3C2 can only be <8>
R7C8 can only be <8>
R3C3 can only be <2>
R9C2 can only be <9>
R7C7 can only be <6>
R2C8 can only be <3>
R9C5 can only be <4>
R7C5 can only be <3>
R1C2 can only be <4>
R3C5 can only be <9>
R2C7 can only be <8>
R5C8 can only be <2>
R3C7 can only be <4>
R8C7 can only be <2>
R5C7 can only be <3>
R8C8 can only be <4>
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