RW wrote:Make the very obvious BUG-lite reductions...
Maybe I am missing something, but why can we exclude "2" from the indicated cells?
Regards, Carcul
vidarino wrote:Hmm, another side-effect of this phenomenon is that you could theorhetically find a hard puzzle, remove a clue, and make it easier.
However, you'd have to somehow verify that it still has a unique solution after the removal. Otherwise the use of uniqueness techniques would be invalid.
Carcul wrote:.. why can we exclude "2" from the indicated cells?
vidarino wrote:Hmm, another side-effect of this phenomenon is that you could theoretically find a hard puzzle, remove a clue, and make it easier. ....
Smythe Dakota wrote:To those who defend uniqueness methods as a valid solving technique, don't you now have another, similar, technique in your arsenal? For example, you could say something like "r4c3 cannot be a 7, because then" (through some complicated logic or other) "the 2 given in r8c5 would be redundant".
Ruud wrote:Smythe Dakota wrote:A math purist would probably say that uniqueness techniques should not be considered valid, and that part of the task in solving any puzzle is to establish uniqueness.
Many websites and newspapers that publish Sudokus explicitly state that "every puzzle has a unique solution that can be found by logic".
A math purist would IMO stick to the problem definition and not try to prove something given as an axiom.
Pat wrote:i and Smythe Dakota seem to be in the minority,
refusing to assume Uniqueness-Of-Answer
even when the puzzle is published with such a promise.
just imagine they had a misprint
and the puzzle as published does have 2 answers.
Ruud wrote:The real question is: Do you think a contestant at the Sudoku World Championships should be disqualified for using a technique that assumes a unique solution?
carcul wrote:RW wrote:Make the very obvious BUG-lite reductions...
Maybe I am missing something, but why can we exclude "2" from the indicated cells?
. abc . | . abc .
. abc . | . abc .
. abc . | . abc .
. ab . | . ab .
. bc . | . bc .
. ac . | . ac .
. ab . | . ab .
. ac . | . ac .
. bc . | . bc .
. bc . | . bc .
. ab . | . ab .
. ac . | . ac .
. ac . | . ac .
. ab . | . ab .
. bc . | . bc .
. bc . | . bc .
. ac . | . ac .
. ab . | . ab .
. ac . | . ac .
. bc . | . bc .
. ab . | . ab .
. 34 . | . 34 .
. 48 . | . 48 .
. 38 . | . 38 .
. 34 . | . 34 .
. 38 . | . 38 .
. 48 . | . 48 .
Pat wrote:just imagine they had a misprint
and the puzzle as published does have 2 answers.
Pat wrote:just imagine they had a misprint
and the puzzle as published does have 2 answers.
vidarino wrote:And RW; I'm fairly sure the 3x2 BUG-Lite was discussed in the BUG-Lite thread, if I recall correctly. I am 100% sure about having read about it *somewhere* before, at least, because I have added them to my solver.
Pat wrote:.... if you assume Uniqueness-Of-Answer, you may reach what you think is the only answer, where in fact the puzzle was invalid (had 2 answers). ....
Smythe Dakota wrote:Certainly, though, a solver PROGRAM should never assume uniqueness, since arbitrary grids can be plugged into such programs. The program should not only come up with a solution, but make sure that solution is unique (and say so if it isn't). Otherwise, the program just isn't doing the whole job.
Bill Smythe
6 . . | 3 1 9 | . . x
. 1 . | 2 . 4 | . 5 .
. . . | . . . | . . .
-------+-------+-------
1 8 . | . . . | . 6 9
7 . . | . 2 . | . . 1
2 4 . | . . . | . 7 5
-------+-------+-------
. . . | . . . | . . .
. 6 . | 1 . 8 | . 3 .
9 . . | 7 5 3 | . . 6