The New Sudoku Players' Forum

[Wolfgang]

I like that one because it can be solved using colors (filtering on 9s) without having to resort to Nishio.

As we saw, coloring is an easy to use and mighty technique for hard puzzles. angusj, do you agree that xwing and swordfish are only special cases of coloring ?

[angusj]

angusj, do you agree that xwing and swordfish are only special cases of coloring ?

Yes, I probably do, however I'm not dogmatic about terminology.

Anyhow, on the topic of xwing and swordfish, these 3 puzzles may entertain .

Code: Select all

..6|9..|3..
5..|...|..4
...|73.|2.1
-----------
..1|4..|..2
8..|6.2|..5
4..|..7|1..
-----------
3.8|.49|...
1..|...|..3
..7|..3|9..


...|...|.87
.3.|1..|5.6
.8.|59.|3.1
-----------
..4|8..|.35
...|.4.|...
37.|..5|8..
-----------
8.7|.12|.5.
1.3|..4|.7.
94.|...|...


5.2|..6|...
...|8..|4..
9..|.4.|.61
-----------
...|..3|..7
8.1|.9.|6.5
2..|1..|...
-----------
68.|.2.|..9
..5|..4|...
...|6..|7.4

[Wolfgang]

Anyhow, on the topic of xwing and swordfish, these 3 puzzles may entertain .

Thanx for the puzzles, i solved the first with a (hopefully correct) long chain, the second with a short nishio to fix the 3 (and 6) in box 2, but i got stuck with the third one:

5 4 2 * 1 6 * * *
* * * 8 * * 4 * 2
9 * 8 * 4 * * 6 1

4 * * * * 3 * * 7
8 * 1 4 9 * 6 * 5
2 * * 1 * * * 4 *

6 8 4 * 2 * * * 9
7 * 5 * * 4 * * 6
* * * 6 * * 7 * 4

5____4_____2______379____1_____6______389____3789____38___
13___16____367____8______357___579____4______3579____2____
9____37____8______2357___4_____257____35_____6_______1____
4____569___69_____25_____568___3______1289___1289____7____
8____37____1______4______9_____27_____6______23______5____
2____569___3679___1______5678__578____389____4_______38___
6____8_____4______357____2_____157____135____135_____9____
7____129___5______39_____38____4______1238___1238____6____
13___129___39_____6______58____1589___7______1258____4____


Is it really solvable without nishio?

[Wolfgang]

..i solved the second with a short nishio to fix the 3 (and 6) in box 2...

this was a "lucky mistake" before, as i checked it now. It would need a rather long nishio like this:

3 in (1,6):
3 in (5,4), 2 in (5,1) and (6,4), 9 in (1,7) and (7,4) -> 6 in (8,4) -> 2 in (8,7)
8 in (2,5) -> 8 in (9,6), 5 in (8,5) -> 2 in (8,2)

[angusj]

Is it really solvable without nishio?

There's a hidden pair in box 4, then an xwing on 7s, then a little later a swordfish on 3s .

[Wolfgang]

There's a hidden pair in box 4 ...

Thanks, i have been already blind not to see it

[Wolfgang]

There's a hidden pair in box 4, then an xwing on 7s, then a little later a swordfish on 3s .

i wanted to go on with it now, but the hidden pair did not help me, in the situation above i cannot see an x-wing in 7 or anything else that could help me. Am i blind again?

[scrose]

5____4_____2______379____1_____6______389____3789____38___
13___16____367____8______357___579____4______3579____2____
9____37____8______2357___4_____257____35_____6_______1____
4____569___69_____25_____568___3______1289___1289____7____
8____37____1______4______9_____27_____6______23______5____
2____569___3679___1______5678__578____389____4_______38___
6____8_____4______357____2_____157____135____135_____9____
7____129___5______39_____38____4______1238___1238____6____
13___129___39_____6______58____1589___7______1258____4____

Look at where the 7's are in columns 3 and 5 of your candidates.

[Wolfgang]

Look at where the 7's are in columns 3 and 5 of your candidates.

i think, i cannot use that because if 7 is not in (2,3), it can go to (2,8), not only to (2,5)

[scrose]

i think, i cannot use that because if 7 is not in (2,3), it can go to (2,8), not only to (2,5)

In columns 3 and 5, the 7's will lie at either r2c3 and r6c5, or r2c5 and r6c3. This lets you eliminate all other candidate 7's from rows 2 and 6.

[Animator]

Actually that isn't true.

If r2c3 is not 7 then r6c3 has to be 7, and then there is only one place for the number 7 in column 5. Which is r2c5.

If r2c5 is not 7 then it has to go in r2c3. Because of the X-wing. It can never go in r2c8.