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Feb 28 - Super Hard

## Reasoning

R2C2 can only be <1>

R5C2 can only be <7>

R6C3 can only be <1>

R8C2 can only be <6>

R8C8 can only be <3>

R4C3 can only be <2>

R8C5 can only be <2>

R5C1 can only be <8>

R3C1 is the only square in row 3 that can be <7>

R5C6 is the only square in row 5 that can be <2>

R6C6 is the only square in row 6 that can be <8>

R9C1 is the only square in row 9 that can be <2>

R9C3 is the only square in row 9 that can be <3>

Squares R7C1 and R7C3 in row 7 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 row.

R7C7 - removing <4> from <4678> leaving <678>

R7C9 - removing <4> from <478> leaving <78>

Squares R6C5 and R9C5 in column 5 form a simple naked pair. These 2 squares both contain the 2 possibilities <67>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the column.

R4C5 - removing <7> from <1479> leaving <149>

Intersection of column 9 with block 9. The value <7> only appears in one or more of squares R7C9, R8C9 and R9C9 of column 9. These squares are the ones that intersect with block 9. Thus, the other (non-intersecting) squares of block 9 cannot contain this value.

R7C7 - removing <7> from <678> leaving <68>

R9C7 - removing <7> from <467> leaving <46>

Squares R2C5 and R2C8 in row 2 and R5C5 and R5C8 in row 5 form a Simple X-Wing pattern on possibility <9>. All other instances of this possibility in columns 5 and 8 can be removed.

R1C5 - removing <9> from <1459> leaving <145>

R4C5 - removing <9> from <149> leaving <14>

Squares R4C4 and R4C5 in row 4 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 row.

R4C6 - removing <1> from <179> leaving <79>

R4C7 - removing <4> from <479> leaving <79>

Squares R4C4 and R4C5 in block 5 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 block.

R5C4 - removing <4> from <345> leaving <35>

R5C5 - removing <4> from <459> leaving <59>

Squares R7C1, R7C3, R1C1 and R1C3 form a Type-1 Unique Rectangle on <45>.

R1C3 - removing <45> from <459> leaving <9>

Squares R1C7<48>, R7C7<68> and R9C7<46> in column 7 form a comprehensive naked triplet. These 3 squares can only contain the 3 possibilities <468>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the column.

R3C7 - removing <4> from <349> leaving <39>

Squares R2C5 (XY), R4C5 (XZ) and R3C6 (YZ) form an XY-Wing pattern on <1>. All squares that are buddies of both the XZ and YZ squares cannot be <1>.

R1C5 - removing <1> from <145> leaving <45>

R1C9 is the only square in row 1 that can be <1>

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

R7C7 can only be <6>

R7C4 can only be <1>

R9C7 can only be <4>

R9C9 can only be <7>

R9C5 can only be <6>

R7C9 can only be <8>

R7C6 can only be <7>

R4C4 can only be <4>

R4C6 can only be <9>

R6C5 can only be <7>

R4C5 can only be <1>

R3C4 can only be <5>

R4C7 can only be <7>

R3C6 can only be <1>

R5C5 can only be <5>

R6C7 can only be <3>

R5C4 can only be <3>

R1C5 can only be <4>

R6C4 can only be <6>

R3C7 can only be <9>

R5C9 can only be <4>

R1C1 can only be <5>

R2C5 can only be <9>

R2C8 can only be <4>

R5C8 can only be <9>

R3C9 can only be <3>

R3C3 can only be <4>

R7C1 can only be <4>

R7C3 can only be <5>

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