The full reasoning can be found below the Sudoku.
Mar 25 - Super Hard
Puzzle Copyright © Kevin Stone
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Reasoning
R9C1 can only be <6>
R1C1 can only be <3>
R8C1 can only be <9>
R5C1 can only be <8>
R2C1 can only be <7>
R1C2 is the only square in row 1 that can be <6>
R3C7 is the only square in row 3 that can be <7>
R3C8 is the only square in row 3 that can be <9>
R7C7 is the only square in row 7 that can be <9>
R4C3 is the only square in row 4 that can be <9>
R5C3 can only be <6>
R5C7 can only be <2>
R5C9 can only be <9>
R4C4 is the only square in row 4 that can be <2>
R9C8 is the only square in row 9 that can be <8>
Intersection of row 1 with block 3. The value <5> only appears in one or more of squares R1C7, R1C8 and R1C9 of row 1. These squares are the ones that intersect with block 3. Thus, the other (non-intersecting) squares of block 3 cannot contain this value.
R2C7 - removing <5> from <145> leaving <14>
R2C9 - removing <5> from <1245> leaving <124>
Intersection of row 3 with block 2. The values <34> only appears in one or more of squares R3C4, R3C5 and R3C6 of row 3. These squares are the ones that intersect with block 2. Thus, the other (non-intersecting) squares of block 2 cannot contain these values.
R2C4 - removing <4> from <458> leaving <58>
Squares R7C5<34>, R8C4<345>, R8C6<135> and R9C5<14> in block 8 form a comprehensive naked quad. These 4 squares can only contain the 4 possibilities <1345>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the block.
R7C4 - removing <345> from <34568> leaving <68>
R7C6 - removing <35> from <3568> leaving <68>
Intersection of block 8 with row 8. The value <5> only appears in one or more of squares R8C4, R8C5 and R8C6 of block 8. These squares are the ones that intersect with row 8. Thus, the other (non-intersecting) squares of row 8 cannot contain this value.
R8C3 - removing <5> from <235> leaving <23>
R8C7 - removing <5> from <145> leaving <14>
R8C9 - removing <5> from <1245> leaving <124>
Squares R2C7 and R8C7 in column 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 column.
R4C7 - removing <1> from <156> leaving <56>
R6C7 - removing <14> from <1456> leaving <56>
R6C8 is the only square in row 6 that can be <4>
R4C8 is the only square in block 6 that can be <1>
Squares R1C5 and R1C9 in row 1 and R9C5 and R9C9 in row 9 form a Simple X-Wing pattern on possibility <1>. All other instances of this possibility in columns 5 and 9 can be removed.
R2C9 - removing <1> from <124> leaving <24>
R3C5 - removing <1> from <1234> leaving <234>
R8C9 - removing <1> from <124> leaving <24>
Squares R2C9 and R8C9 in column 9 form a simple naked pair. These 2 squares both contain the 2 possibilities <24>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the column.
R1C9 - removing <2> from <125> leaving <15>
R9C9 - removing <4> from <145> leaving <15>
Intersection of block 9 with row 8. The value <4> only appears in one or more of squares R8C7, R8C8 and R8C9 of block 9. These squares are the ones that intersect with row 8. Thus, the other (non-intersecting) squares of row 8 cannot contain this value.
R8C4 - removing <4> from <345> leaving <35>
R3C4 is the only square in column 4 that can be <4>
Squares R2C3 and R2C9 in row 2 and R8C3 and R8C9 in row 8 form a Simple X-Wing pattern on possibility <2>. All other instances of this possibility in columns 3 and 9 can be removed.
R3C3 - removing <2> from <128> leaving <18>
R7C3 - removing <2> from <235> leaving <35>
Squares R7C3 (XY), R7C5 (XZ) and R9C2 (YZ) form an XY-Wing pattern on <4>. All squares that are buddies of both the XZ and YZ squares cannot be <4>.
R9C5 - removing <4> from <14> leaving <1>
R7C2 - removing <4> from <245> leaving <25>
R9C9 can only be <5>
R1C5 can only be <2>
R9C2 can only be <4>
R1C9 can only be <1>
R7C8 can only be <2>
R1C8 can only be <5>
R3C5 can only be <3>
R2C7 can only be <4>
R2C9 can only be <2>
R8C7 can only be <1>
R8C9 can only be <4>
R7C5 can only be <4>
R7C2 can only be <5>
R7C3 can only be <3>
R4C2 can only be <7>
R8C3 can only be <2>
R4C6 can only be <6>
R6C2 can only be <1>
R4C7 can only be <5>
R7C6 can only be <8>
R6C4 can only be <3>
R6C7 can only be <6>
R6C3 can only be <5>
R3C2 can only be <2>
R6C6 can only be <7>
R8C4 can only be <5>
R7C4 can only be <6>
R3C6 can only be <1>
R8C6 can only be <3>
R2C4 can only be <8>
R2C3 can only be <1>
R3C3 can only be <8>
R2C6 can only be <5>
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