Feb 22 - Very Hard
Puzzle Copyright © Kevin Stone
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Reasoning
R5C5 is the only square in row 5 that can be <9>
R5C6 is the only square in row 5 that can be <5>
Squares R6C2 and R6C6 in row 6 form a simple naked pair. These 2 squares both contain the 2 possibilities <47>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the row.
R6C4 - removing <4> from <148> leaving <18>
R6C5 - removing <7> from <378> leaving <38>
R6C8 - removing <7> from <1378> leaving <138>
Intersection of row 1 with block 1. The values <48> only appears in one or more of squares R1C1, R1C2 and R1C3 of row 1. These squares are the ones that intersect with block 1. Thus, the other (non-intersecting) squares of block 1 cannot contain these values.
R2C1 - removing <4> from <2457> leaving <257>
R2C2 - removing <4> from <24579> leaving <2579>
Intersection of column 9 with block 9. The value <8> 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.
R8C8 - removing <8> from <1589> leaving <159>
R9C8 - removing <8> from <589> leaving <59>
Intersection of block 4 with row 5. The value <3> only appears in one or more of squares R5C1, R5C2 and R5C3 of block 4. These squares are the ones that intersect with row 5. Thus, the other (non-intersecting) squares of row 5 cannot contain this value.
R5C9 - removing <3> from <137> leaving <17>
R2C9 is the only square in column 9 that can be <3>
Squares R5C7 and R5C9 in row 5 form a simple naked pair. These 2 squares both contain the 2 possibilities <17>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the row.
R5C1 - removing <7> from <2347> leaving <234>
R5C4 - removing <1> from <124> leaving <24>
R6C4 is the only square in column 4 that can be <1>
Squares R2C4 and R5C4 in column 4 form a simple naked pair. These 2 squares both contain the 2 possibilities <24>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the column.
R4C4 - removing <2> from <268> leaving <68>
R8C4 - removing <2> from <268> leaving <68>
Squares R5C7 and R5C9 in block 6 form a simple naked pair. These 2 squares both contain the 2 possibilities <17>. Since each of the squares must contain one of the possibilities, they can be eliminated from the other squares in the block.
R4C8 - removing <7> from <378> leaving <38>
Intersection of column 1 with block 1. The value <7> only appears in one or more of squares R1C1, R2C1 and R3C1 of column 1. These squares are the ones that intersect with block 1. Thus, the other (non-intersecting) squares of block 1 cannot contain this value.
R1C2 - removing <7> from <12457> leaving <1245>
R2C2 - removing <7> from <2579> leaving <259>
Intersection of column 8 with block 3. The value <7> only appears in one or more of squares R1C8, R2C8 and R3C8 of column 8. These squares are the ones that intersect with block 3. Thus, the other (non-intersecting) squares of block 3 cannot contain this value.
R1C7 - removing <7> from <167> leaving <16>
R1C9 - removing <7> from <12567> leaving <1256>
R3C7 - removing <7> from <1679> leaving <169>
R3C9 - removing <7> from <1267> leaving <126>
Intersection of block 2 with column 5. The value <6> only appears in one or more of squares R1C5, R2C5 and R3C5 of block 2. These squares are the ones that intersect with column 5. Thus, the other (non-intersecting) squares of column 5 cannot contain this value.
R4C5 - removing <6> from <23678> leaving <2378>
R9C5 - removing <6> from <2568> leaving <258>
Intersection of row 9 with block 9. The value <6> only appears in one or more of squares R9C7, R9C8 and R9C9 of row 9. These squares are the ones that intersect with block 9. Thus, the other (non-intersecting) squares of block 9 cannot contain this value.
R8C9 - removing <6> from <1568> leaving <158>
Squares R1C2 and R8C2 in column 2 and R1C8 and R8C8 in column 8 form a Simple X-Wing pattern on possibility <1>. All other instances of this possibility in rows 1 and 8 can be removed.
R1C1 - removing <1> from <124578> leaving <24578>
R8C1 - removing <1> from <1258> leaving <258>
R1C7 - removing <1> from <16> leaving <6>
R1C9 - removing <1> from <1256> leaving <256>
R8C9 - removing <1> from <158> leaving <58>
R3C5 is the only square in row 3 that can be <6>
R3C1 is the only square in row 3 that can be <7>
R9C9 is the only square in row 9 that can be <6>
R7C1 is the only square in column 1 that can be <1>
R8C8 is the only square in row 8 that can be <1>
R1C2 is the only square in row 1 that can be <1>
R8C2 is the only square in row 8 that can be <9>
R2C8 is the only square in row 2 that can be <9>
R9C8 can only be <5>
R3C7 can only be <1>
R3C9 can only be <2>
R5C7 can only be <7>
R3C3 can only be <9>
R1C9 can only be <5>
R5C9 can only be <1>
R7C7 can only be <4>
R7C3 can only be <8>
R9C7 can only be <9>
R1C8 can only be <7>
R8C9 can only be <8>
R1C5 can only be <2>
R7C5 can only be <5>
R7C9 can only be <7>
R8C4 can only be <6>
R1C3 can only be <4>
R9C5 can only be <8>
R2C4 can only be <4>
R2C6 can only be <7>
R5C4 can only be <2>
R6C6 can only be <4>
R4C6 can only be <6>
R6C2 can only be <7>
R8C6 can only be <2>
R4C4 can only be <8>
R8C1 can only be <5>
R6C5 can only be <3>
R1C1 can only be <8>
R5C3 can only be <3>
R4C8 can only be <3>
R4C5 can only be <7>
R6C8 can only be <8>
R5C1 can only be <4>
R9C3 can only be <2>
R4C2 can only be <2>
R2C1 can only be <2>
R9C2 can only be <4>
R2C2 can only be <5>
R9C1 can only be <3>
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