dailysudoku.com

[Mogulmeister]

[TomC]

X wing on 3's solves this

[Mogulmeister]

[Mogulmeister]

Just for a diversion but to encourage those of you who should be exploring the dark side of sudoku:

If you say r6c1 = 3 you will put two 3's in box 2.

R6c1 < > 3

Which is the same elimination you might make with the X wing and the Swordfish but provides a different way of looking at the puzzle.

[TomC]

r4c7 <> 3

as then r4c9=6, r8c9=4, r3c7=4, r3c1=2, r6c1=3 leads to no 3's in box 5

[Mogulmeister]

I've shown the above contradictions in pictorial form - they either put too many 3's in one box or remove them altogether in another.

You can also form a loopback to your start point which also shows that the number you started with is wrong.

a)Start in the red square and make it 3 then follow the pink squares:

r6c1 IS 3, r2c1 < > 3, r2c4 = 3, r1c6 < >3, r6c6 = 3 and r6c1 (red) NOT 3.

Start with 3 follow the loop around and the 3 you began with is out.

b)Start in the green square and make it 3 then follow the blue squares:

r4c7 IS 3 (not 6), r8c7 = 6 (not 4), r3c7 = 4, r3c1 = 2, r6c1 = 3 (not 2)
r5c1 = not 3, r5c7 = 3 and r4c7 NOT 3.

Again, start with 3 follow the loop around and the 3 you began with is out.