999_Springs wrote:Intrigued by the failure of Bob Hanson's solver to remove a single candidate and the fact that SE needs 12 sub-chains in some enormous first step, I ask these questions:

1. How far does Bob Hanson's ++depth actually go?

2. Do all puzzles of this sort of difficulty stump Bob Hanson's solver, or just this one?

3. Why choose r1c7#3 in Mauricio's puzzle? Is this the simplest elimination? (I can hardly believe this is.)

1) I don't know the answer here. You could test puzzles of increasing ER, determine the last to succeed and examine Sudoku Assistant's solving path to see if that gives a clue.

2) Other puzzles of this complexity will fox Bob Hanson's solver eg Easter Monster.

3) Of the 17 eliminations of ER 11.2 r1c7<>3 is the simplest.

In this cycle of the program SE finds 76 conclusions up to ER 11.6. The last Cell Forcing Chain having a mind numbing 116 dynamic nested chains supporting the 4 primary chains.

Applying all these conclusions simultaneously maintains symmetry, leaving a few singles to mop up. There are fluctuations in the logical sequences leading to symmetrical eliminations and hence different ERs and orders of priority.