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Sudoku Solution Path

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R1C7 can only be <5>
R4C6 is the only square in row 4 that can be <8>
R1C3 is the only square in column 3 that can be <8>
Squares R4C1 and R5C3 in block 4 form a simple locked 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 block.
   R5C2 - removing <4> from <469> leaving <69>
   R6C1 - removing <14> from <1469> leaving <69>
Squares R1C8<46>, R1C9<469> and R3C9<49> in block 3 form a comprehensive locked triplet. These 3 squares can only contain the 3 possibilities <469>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the block.
   R2C8 - removing <4> from <148> leaving <18>
   R2C9 - removing <49> from <1489> leaving <18>
Squares R4C1 and R4C9 in row 4 and R7C1 and R7C9 in row 7 form a Simple X-Wing pattern on possibility <1>. All other instances of this possibility in columns 1 and 9 can be removed.
   R2C9 - removing <1> from <18> leaving <8>
   R6C9 - removing <1> from <14> leaving <4>
   R9C1 - removing <1> from <124567> leaving <24567>
   R9C9 - removing <1> from <12356> leaving <2356>
R2C8 can only be <1>
R3C9 can only be <9>
R5C8 can only be <2>
R1C9 can only be <6>
R9C8 can only be <6>
R1C8 can only be <4>
R8C8 can only be <8>
R6C6 is the only square in row 6 that can be <1>
R5C6 is the only square in column 6 that can be <4>
R5C3 can only be <1>
R5C7 can only be <3>
R9C3 can only be <4>
R4C1 can only be <4>
R9C7 can only be <1>
R4C9 can only be <1>
R3C1 can only be <7>
R3C5 can only be <4>
R2C2 is the only square in row 2 that can be <4>
R7C1 is the only square in row 7 that can be <1>
R9C9 is the only square in row 9 that can be <3>
Squares R7C5, R7C9, R8C5 and R8C9 form a Type-1 Unique Rectangle on <25>.
   R8C5 - removing <25> from <257> leaving <7>
R9C6 can only be <9>
R1C6 can only be <7>
R6C4 is the only square in row 6 that can be <7>
R9C2 is the only square in row 9 that can be <7>
Squares R2C1 and R2C5 in row 2 and R6C1 and R6C5 in row 6 form a Simple X-Wing pattern on possibility <9>. All other instances of this possibility in columns 1 and 5 can be removed.
   R1C1 - removing <9> from <239> leaving <23>
   R5C5 - removing <9> from <569> leaving <56>
The puzzle can be reduced to a Bivalue Universal Grave (BUG) pattern, by making this reduction:
   R8C1=<56>
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
   R8C1 - removing <56> from <256> leaving <2>
R8C2 can only be <6>
R8C9 can only be <5>
R1C1 can only be <3>
R9C1 can only be <5>
R5C2 can only be <9>
R7C9 can only be <2>
R9C4 can only be <2>
R4C4 can only be <3>
R7C5 can only be <5>
R1C4 can only be <9>
R2C1 can only be <9>
R1C2 can only be <2>
R5C4 can only be <5>
R2C5 can only be <3>
R6C1 can only be <6>
R4C5 can only be <2>
R5C5 can only be <6>
R6C5 can only be <9>


 

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