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Common Answers

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Sudoku Solution Path

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R3C4 can only be <9>
R1C5 can only be <2>
R1C8 is the only square in row 1 that can be <5>
R4C5 is the only square in row 4 that can be <5>
R4C4 is the only square in row 4 that can be <3>
R4C3 is the only square in row 4 that can be <1>
R5C4 is the only square in row 5 that can be <7>
R8C2 is the only square in row 8 that can be <5>
R9C8 is the only square in row 9 that can be <8>
R3C8 can only be <3>
R3C2 can only be <6>
R3C6 can only be <8>
R2C5 can only be <6>
R2C7 is the only square in row 2 that can be <8>
R5C5 is the only square in row 5 that can be <8>
R5C8 is the only square in row 5 that can be <1>
R6C6 is the only square in row 6 that can be <6>
R5C9 is the only square in row 5 that can be <6>
R6C7 is the only square in row 6 that can be <4>
R4C7 can only be <9>
R4C6 can only be <4>
R8C7 can only be <3>
R7C8 is the only square in row 7 that can be <6>
R8C3 is the only square in row 8 that can be <6>
R9C5 is the only square in row 9 that can be <3>
R9C2 is the only square in column 2 that can be <4>
Intersection of block 8 with row 7. The value <2> only appears in one or more of squares R7C4, R7C5 and R7C6 of block 8. These squares are the ones that intersect with row 7. Thus, the other (non-intersecting) squares of row 7 cannot contain this value.
   R7C2 - removing <2> from <129> leaving <19>
Squares R1C2 and R7C2 in column 2 form a simple locked pair. These 2 squares both contain the 2 possibilities <19>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the column.
   R2C2 - removing <9> from <239> leaving <23>
   R5C2 - removing <9> from <239> leaving <23>
Intersection of block 7 with column 1. The value <2> only appears in one or more of squares R7C1, R8C1 and R9C1 of block 7. These squares are the ones that intersect with column 1. Thus, the other (non-intersecting) squares of column 1 cannot contain this value.
   R2C1 - removing <2> from <2379> leaving <379>
   R5C1 - removing <2> from <239> leaving <39>
Squares R2C3 and R6C3 in column 3, R6C5 and R8C5 in column 5 and R2C8 and R8C8 in column 8 form a Swordfish pattern on possibility <9>. All other instances of this possibility in rows 2, 6 and 8 can be removed.
   R2C1 - removing <9> from <379> leaving <37>
   R8C1 - removing <9> from <129> leaving <12>
   R2C9 - removing <9> from <479> leaving <47>
   R8C9 - removing <9> from <249> leaving <24>
The puzzle can be reduced to a Bivalue Universal Grave (BUG) pattern, by making this reduction:
   R1C1=<17>
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 <17> from <179> leaving <9>
R1C2 can only be <1>
R1C9 can only be <7>
R5C1 can only be <3>
R9C1 can only be <2>
R2C3 can only be <2>
R7C2 can only be <9>
R2C9 can only be <4>
R2C2 can only be <3>
R6C3 can only be <9>
R2C8 can only be <9>
R8C9 can only be <2>
R5C2 can only be <2>
R2C1 can only be <7>
R5C6 can only be <9>
R7C6 can only be <2>
R6C5 can only be <1>
R6C4 can only be <2>
R8C5 can only be <9>
R7C4 can only be <1>
R8C8 can only be <4>
R8C1 can only be <1>
R9C9 can only be <9>


 

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