2005/09/24 tough puzzle from sudoku.com.au

1) Easy eliminations : a3=1%col, i7=1%col, g5=1%col,
lead to (for better reading, only remaining possibilities in still empty cells are written) :

2) Look at R4 : 578 are in abc4, so h4=9%cell, and it follows h5=7%block. Look at Ca : 3 is in a89, so no other 3 in Bb8, and it follows f7=3%row. After easy eliminations : e2=3%col, c3=3%row, h5=7%block, b5=3%row, we're at :

3 ) Look at 7 in Bh8 : it's in g78, so no 7 in g3%col, and then d3=7%row. Follow easy eliminations (e8=7%col, e4=1%col, f4=2%row, g3=2%row, g6=8%cell, h6=3%cell, i6=2%cell, i9=3%col, a8=3%row, h3=6%block, f3=8%cell, g9=6%row, h2=8%col, g2=5%block, g8=9%cell, g7=7%cell) up to :

4) Look at 45 in R9 : they are in ah9 so that f9=9%cell. Also note that in Bb8, 26 are in bc8. Follow then easy eliminations (f9=9%cell, d5=9%col, d9=8%col, e9=2%cell, e5=8%col) up to :

5) Look at two only possibles in e1 : whether e1=4 (in which f2=1%cell), or e1=5 (in which f8=5%col) : f8<>1 (heptagon). Now easy eliminations : f2=1%col, d8=1%row and here we are :

6) Look at 5 in Be5 : whether e6=5, in which e1=4%cell, or d6=5, in which d7=4%cell : d2=2%cell (heptagon). But now 4 in R2 is also in Bb2, so a1=7%cell and easy eliminations lead now to unique solution

(Details if needed : i1=9%cell, a4=5%cell, c4=8%cell, b4=7%cell, a9=4%cell, b7=9%cell, c7=5%cell, d7=4%cell, d2=2%cell, b2=4%cell, c1=2%cell, c2=9%cell, f8=5%cell, h8=4%cell, h9=5%cell, b8=2%col, c5=4%col, f5=6%cell, f1=4%cell, e1=5%cell, d1=6%cell, e6=4%cell, d6=5%cell, i2=7%row, c8=6%block, b6=6%block, b1=8%cell).

See equivalent puzzles : 09/24, 09/25.