The full reasoning can be found below the Sudoku.
Nov 24 - Very Hard
Puzzle Copyright © Kevin Stone
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Reasoning
R3C4 can only be <9>
R5C5 can only be <8>
R7C4 can only be <2>
R9C4 can only be <7>
R9C6 can only be <4>
R1C4 can only be <8>
R9C8 can only be <1>
R9C9 can only be <9>
R3C6 can only be <6>
R7C5 can only be <6>
R9C1 can only be <8>
R9C2 can only be <2>
R3C5 can only be <4>
R1C6 can only be <7>
R7C6 can only be <3>
R7C8 can only be <4>
R8C5 can only be <9>
R7C2 can only be <1>
R2C5 can only be <2>
R1C1 is the only square in row 1 that can be <1>
R4C3 is the only square in row 4 that can be <1>
R4C7 is the only square in row 4 that can be <8>
R2C8 is the only square in row 2 that can be <8>
R6C3 is the only square in row 6 that can be <9>
R2C2 is the only square in row 2 that can be <9>
R8C7 is the only square in row 8 that can be <2>
R2C1 is the only square in column 1 that can be <6>
R5C2 is the only square in column 2 that can be <7>
R5C7 can only be <6>
R8C8 is the only square in column 8 that can be <7>
R8C9 can only be <6>
R1C8 is the only square in row 1 that can be <6>
Squares R1C9 and R4C9 in column 9 form a simple naked pair. These 2 squares both contain the 2 possibilities <45>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the column.
R2C9 - removing <4> from <347> leaving <37>
R6C9 - removing <4> from <347> leaving <37>
Intersection of block 1 with column 2. The value <5> only appears in one or more of squares R1C2, R2C2 and R3C2 of block 1. These squares are the ones that intersect with column 2. Thus, the other (non-intersecting) squares of column 2 cannot contain this value.
R8C2 - removing <5> from <345> leaving <34>
Intersection of block 4 with column 1. The value <4> only appears in one or more of squares R4C1, R5C1 and R6C1 of block 4. These squares are the ones that intersect with column 1. Thus, the other (non-intersecting) squares of column 1 cannot contain this value.
R8C1 - removing <4> from <345> leaving <35>
Squares R3C2 and R5C3 form a remote naked pair. <35> can be removed from any square that is common to their groups.
R2C3 - removing <3> from <34> leaving <4>
R2C7 can only be <7>
R1C2 can only be <5>
R2C9 can only be <3>
R6C7 can only be <4>
R6C9 can only be <7>
R3C8 can only be <5>
R3C2 can only be <3>
R5C8 can only be <3>
R1C9 can only be <4>
R5C3 can only be <5>
R6C1 can only be <3>
R4C9 can only be <5>
R8C2 can only be <4>
R4C1 can only be <4>
R8C3 can only be <3>
R8C1 can only be <5>
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