2005/09/28 tough puzzle from sudoku.com.au

Initial position :

1) Easy eliminations : h2=8%block, f8=6%block, h9=6%row, d9=4%row, e9=3%row, lead to (for better reading, only remaining possibilities in still empty cells are written) :

2) a) 1 in Bh2 is in R1, and so forbids {c1=1, e1=1, b1=1},
b) samely 2 in Bh2 is in Ci, and so forbids {i8=2, i7=2, i4=2, i5=2},
c) samely 3 in Be5 is in R5, and so forbids {a5=3, h5=3, i5=3, b5=3}, hence 36 in Ca are in a26 so a3=4%col .
d) 17 are in d78 so no other 17 in Be8 or in Cd, hence d3=5%cell and e2=1%block or col (remember that e1<>1).

Now come some more simple eliminations e8=2%col, e4=8%col, f7=8%col, f1=4%row, b1=3%row, i1=6%row, a2=6%cell, b6=6%row, a6=3%cell, b2=5%block, so here we are with 39 filled cells :

3) 79 are in c12 and nowhere else in Cc and Bb2. 27 are in i12 and nowhere else in Ci and Bh2, from which follow new simple eliminations : i5=9%cell, e6=9%row, b4=9%row, d2=9%col, c1=9%col, f2=3%row, d5=3%row, i2=2%row, i3=7%col, f3=2%row, c2=7%row, e1=7%row, f5=7%cell, i7=3%cell, h4=3%row, and here we are with 54 filled cells :

4) Look at 2 in column Cb : whether b9=2 (in which g9=7%row) or b5=2, h7=2 (%cell or %col). (Heptagon).

From here simple eliminations end to unique solution. (Details if needed : b5=2%row, c9=2%row, c3=8%col, b3=1%block, g4=2%row, a5=8%row, c6=1%block, h5=1%block, g1=1%block, h6=4%row, h8=7%col, g6=7%col, i8=4%row, c4=4%row, b9=7%row, b8=8%col, g9=8%col, d7=7%row, d8=1%block, a7=1%block, g7=9%row, a8=9%row, a9=5%row, g8=5%row, i4=5%row, h1=5%row).

Equivalent puzzles : 09/17, 09/21. That's all for today, folks...(over time to bed)