Beautiful resolutions were given to this puzzle without revealing the myth announced by Denis.
For my part, I ended up finding a particularity, namely that it is possible to simplify it so that each cell has a backdoor (stte-backdoor). Here is how in detail.
After simplifying the puzzle by the basic techniques, I use successively 3 anti-tracks to proceed to eliminations and validations:
(-4r7c3)->9r7c3->2r7c7->5r8c7->48r38c3 => -4r5c3 => r5c1=4.
puzzle 1: Show
(-8r3c3)->4r3c3->9r7c3->2r7c7->5r8c7->8r8c3 = > -8r2569c3
puzzle 2: Show
The puzzle is then significantly simplified with the basic techniques (see puzzle3).
puzzle 3: Show
(-2r4c2)->2r4c1->1r4c5->1r9c4->6r9c6->2r8c6 => -2r8c2 => r9c3=7 by basic techniques.
puzzle 4: Show
The puzzle then presents one stte-backdoor per cell. These are the candidates marked in blue on the puzzle5.
puzzle 5: Show
The final resolution is obtained, for example, like this:
(-1r2c3)->[1r2c1->1r4c5->5r4c6->8r1c6]->8r9c1->5r8c3 => -5r2c3, stte.
puzzle 6: Show
Note that in this resolution, the (anti-track) chains used are short and do not exceed 6 sequences.
Cordialy
Robert
