dailysudoku.com

PeteGrant · Posted: Sun May 07, 2006 12:27 pm Post subject: WebSudoku 'Evil' Puzzle

Ran into a dead-end with an "Evil" puzzle from WebSudoku.com. Here's what I have so far:

Code:

+-----------------------------------+
| . . 5 | 6 7 . | . 9 4 |
| 9 . 4 | 3 5 . | . . . |
| . . 7 | 9 4 . | 5 8 . |
|-----------+-----------+-----------|
| . 5 2 | 7 3 . | 4 1 . |
| . . . | 4 . . | . 5 . |
| . 4 . | 8 . 5 | 7 . . |
|-----------+-----------+-----------|
| . 2 . | 5 8 4 | 1 . . |
| 4 . . | 1 . 3 | . . 5 |
| 5 1 . | 2 . 7 | . 4 . |
+-----------------------------------+

The Draw facility show that I should place a 9 in r4c6 but I fail to see the pattern that dictates this. Here's what I have as possibilities:

Code:

+------------------------------------------------------------------+
| 1238 38 5 | 6 7 128 | 23 9 4 |
| 9 678 4 | 3 5 128 | 26 267 12678 |
| 1236 36 7 | 9 4 12 | 5 8 1236 |
+------------------------------------------------------------------+
| 68 5 2 | 7 3 69 | 4 1 689 |
| 13678 36789 13689 | 4 12 69 | 23689 5 23689 |
| 136 4 1369 | 8 12 5 | 7 236 2369 |
+------------------------------------------------------------------+
| 367 2 369 | 5 8 4 | 1 367 3679 |
| 4 6789 689 | 1 69 3 | 2689 267 5 |
| 5 1 3689 | 2 69 7 | 3689 4 3689 |
+------------------------------------------------------------------+

What am I missing?

TIA,
Pete
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Janesville Assembly

ravel · Joined: 21 Apr 2006 Posts: 536

There is a box/line or box/box interaction (number 1), which leads to a hidden pair.

PeteGrant · Posted: Sun May 07, 2006 1:34 pm Post subject:

Thank you, ravel, for responding, but I don't see the hidden pair -- yet. Could you point out the cells that constitute the interaction?

Thanks
Pete
________
vapormatic vaporizer

Angel · Joined: 26 Mar 2006 Posts: 31

From the postion you posted, I would note that since 7 is already placed in box 1 you can eliminate it from the possibilities for r2c2. This then leads to a triple {3,6,8} in r1c2, r2c2, r3c2. Therefore there is a pair {1,2} in r1c1, r3c1. Also there is then a pair {1,2} in r3c1, r3c6, which reduces the possibilities for r3c9. Then looking at box 3 you will have another triple (and check the 8 which is already placed but still appears as a candidate).

Over to you after that.

ravel · Joined: 21 Apr 2006 Posts: 536

TKiel · Joined: 22 Feb 2006 Posts: 292 Location: Kalamazoo, MI

There is a swordfish on 9's that excludes 9 from r5c6, leaving r4c6 as the only place for a 9 in box 5 but it also leaves r5c6 as a naked single, which seems a more appropriate hint.

ravel · Joined: 21 Apr 2006 Posts: 536

Well spotted Smile

Looking for the swordfish i first saw the x-wing in rows 6 and 7, that kills the 9 in r4c9 => r4c6=9.

PeteGrant · Posted: Mon May 08, 2006 10:40 am Post subject:

Thank you all for your input. This was one of those cases where the more I looked, the more I missed Smile

BTW, WebSudoku's "evil" puzzles seem to be of about the same difficulty as DailySudoku's "hard". I haven't dared to try the "very hard" yet....
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