1    2    3     4    5    6     7    8    9
   +---------------+---------------+---------------+
A  |  6   137   .  |  .   159   8  |  .    .    .  |
B  | 179   5   379 |  4    19   2  |  6    37   8  |
C  |  .    78   4  |  3    59   6  |  .   257   1  |
   |---------------+---------------+---------------|
D  |  .    .    1  | 257   6    .  |  .    9    .  |
E  | 247   9   237 |  27   8   347 |  1    6    5  |
F  |  .   347   6  |  .    .    .  |  8    .    .  |
   |---------------+---------------+---------------|
G  |  3    2    8  |  6    7    45 |  45   1    9  |
H  |  .    6   579 |  .    .    1  |  .    .    .  |
I  |  .   147  579 |  .    .    .  |  .    .    6  |
   +---------------+---------------+---------------+
Since A4 is the only cell in its box that can be 7, it must.
By process of elimination, E4 must be 2.
By process of elimination, D4 must be 5.
Since F4 is the only cell in its column that can be 1, it must.
Since F1 is the only cell in its row that can be 5, it must.
Since A3 is the only cell in its column that can be 2, it must.
By process of elimination, C1 must be 8.
By process of elimination, C2 must be 7.
Since D1 is the only cell in its box that can be 2, it must.
Since D2 is the only cell in its box that can be 8, it must.
Both H4 and I4 (same box) can only be 8 or 9.
   So H5 can't be 9.
Both H4 and I4 (same box) can only be 8 or 9.
   So I5 can't be 9.
Both H4 and I4 (same box) can only be 8 or 9.
   So I6 can't be 9.
Since B8 is the only cell in its row that can be 7, it must.
Both B1 and B5 (same row) can only be 1 or 9.
   So B3 can't be 9.
By process of elimination, B3 must be 3.
By process of elimination, E3 must be 7.