Sudoku Puzzle © Kevin Stone

R3C4 can only be <1>

R3C6 can only be <3>

R4C4 can only be <9>

R6C4 can only be <8>

R4C6 can only be <2>

R7C4 can only be <7>

R6C6 can only be <5>

R6C3 can only be <6>

R7C6 can only be <9>

R5C5 can only be <7>

R6C7 can only be <2>

R4C3 can only be <1>

R4C7 can only be <6>

R8C7 is the only square in row 8 that can be <7>

R2C3 is the only square in row 2 that can be <7>

R3C9 is the only square in row 3 that can be <7>

R8C3 is the only square in row 8 that can be <9>

R7C2 is the only square in column 2 that can be <1>

R3C2 is the only square in column 2 that can be <8>

R1C7 is the only square in row 1 that can be <8>

R3C3 is the only square in column 3 that can be <5>

R3C8 is the only square in column 8 that can be <2>

Squares R9C1 and R9C3 in row 9 form a simple locked pair. These 2 squares both contain the 2 possibilities <23>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the row.

R9C5 - removing <2> from <25> leaving <5>

R9C7 - removing <3> from <1359> leaving <159>

R7C5 can only be <2>

R7C7 is the only square in column 7 that can be <3>

Intersection of column 7 with block 3. The value <4> 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.

R2C8 - removing <4> from <456> leaving <56>

Squares R2C2 and R2C8 in row 2 and R8C2 and R8C8 in row 8 form a Simple X-Wing pattern on possibility <6>. All other instances of this possibility in columns 2 and 8 can be removed.

R7C8 - removing <6> from <456> leaving <45>

The puzzle can be reduced to a Bivalue Universal Grave (BUG) pattern, by making this reduction:

R1C1=<29>

These are called the BUG possibilities. In a BUG pattern, in each row, column and block, each unsolved possibility appears exactly twice. Such a pattern either has 0 or 2 solutions, so it cannot be part of a valid Sudoku

When a puzzle contains a BUG, and only one square in the puzzle has more than 2 possibilities, the only way to kill the BUG is to remove both of the BUG possibilities from the square, thus solving it

R1C1 - removing <29> from <269> leaving <6>

R1C5 can only be <4>

R1C9 can only be <9>

R3C1 can only be <9>

R7C1 can only be <8>

R2C2 can only be <4>

R1C3 can only be <2>

R3C5 can only be <6>

R9C9 can only be <1>

R3C7 can only be <4>

R2C7 can only be <5>

R8C2 can only be <6>

R2C8 can only be <6>

R5C7 can only be <1>

R8C8 can only be <4>

R5C9 can only be <5>

R9C7 can only be <9>

R7C9 can only be <6>

R7C3 can only be <4>

R5C1 can only be <3>

R7C8 can only be <5>

R9C3 can only be <3>

R5C3 can only be <8>

R9C1 can only be <2>