dailysudoku.com

[David Bryant]

This is a continuation from the topic "A Very Tough Minimal Sudoku."

[alanr555]

> [Anyway, after 35 moves I arrived at the following position:

> *8/9* 1 4 *7/9* 5 3/8 . . .
> 5 7 3 1/9 2 6 4 1/9 8
> *8/9* 6 2 4 *7/8* . 3/7 . 5
> where the "XY-Wing" formation is marked with asterisks.

This XY-Wing can be viewed in TWO ways.
The skill is in spotting where one occurs.

One way is to find a quadrilateral of which three corners are composed
each of any two digits from three. Then the fourth corner CANNOT be
the digit that is NOT in its opposite corner but IS in the other two.

Here the three corners could be r1c1,r1c4,r3c5 This leads to digit 8
not be possible for r3c1 (ie must be 9)

Alternatively the three corners could be r1c4,r3c5,r3c1. This leads to
digit 9 being impossible for r1c1 (ie must be 8)

Note: This example can give information only about r1c1 and r3c1 as
these two cells have identical candidates and so CANNOT form the
"second" corner of a quadrilateral which meets the rule above.
(eg 8/9,8/9,7/8 does NOT have any digit which in the first and third
positions but is not in the second. Similarly 8/9,8/9,7/9 is not one).

> PS One can also look at the "XY-Wing" in reverse, which is actually
> easier for me to see. If we assume r1c1 = 9 we obtain a contradiction:
> r1c1 = 9 ==> r3c1 = 8 ==> r3c5 = 7 ==> r1c4 = 9 tilt! -- two "9"s in > row 1.

This is true - and useful in formulating a logical rule - but it invokes the
big confront of "IF-THEN". My impression is that postulation 0f a value
for a cell is, BY AGREEMENT, not regarded as within the ranges of
solution methods being promoted on THIS site (or needed for any of
the Daily sudoku puzzles) - however much it may be tolerated or even
promoted on OTHER sites.

ie - the XY-wing method is a means of resolving a potential "IF-THEN"
without actually invoking the possibility of backtrack if an incorrect
postulation is made!!

Alan Rayner BS23 2QT