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VH alternative

 
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Steve R



Joined: 24 Oct 2005
Posts: 289
Location: Birmingham, England
PostPosted: Mon Mar 31, 2008 3:38 pm    Post subject: VH alternative Reply with quote
For those missing their VH fix, the Times puzzle 1823 is of a similar standard.

Steve
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storm_norm



Joined: 18 Oct 2007
Posts: 1741
PostPosted: Mon Mar 31, 2008 11:07 pm    Post subject: Reply with quote
its no picnic through the basic box/line eliminations, neither the triples that show up.

x-wing on 6 first.

then this grid...
color the 2

Code:
29     26     5      | 3      8      7      | 4      69     1
3      1      69     |R25     4      25     | 679    79     8
47     8      47     | 9      6      1      | 23     35     25
------------------------------------------------------------------
6      4     R27     | 8     -279    25     | 579    1      3
257    257    1      | 6      279    3      | 789    4789   47
8      9      3      | 57     1      4      | 57     2      6
------------------------------------------------------------------
2479   237    69     | 1      5      8      | 2367   347    247
1      257   G247    |G27     3      6      | 278    4578   9
257    36     8      | 4     R27     9      | 1      36     257


the coloring eliminates the 2 in r2c4

leaves this coloring grid on candidate 2, also a fishy pattern...

Code:
29     26     5      | 3      8      7      | 4      69     1
3      1      69     |G25     4     R25     | 679    79     8
47     8      47     | 9      6      1      | 23     35     25
------------------------------------------------------------------
6      4     R27     | 8      79    G25     | 579    1      3
257    257    1      | 6     R279    3      | 789    4789   47
8      9      3      | 57     1      4      | 57     2      6
------------------------------------------------------------------
2479   237    69     | 1      5      8      | 2367   347    247
1     -257   G247    |R27     3      6      |-278    4578   9
257    36     8      | 4     G27     9      | 1      36     257


eliminates the 2 in r8c2 and r8c7...

this is simply a swordfish in col 3,4,6

that leaves a hidden {2,3} pair in col 7

edit: I realize the swordfish is there from the start, just wanted to show the steps in the coloring pattern along the way.
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ravel



Joined: 21 Apr 2006
Posts: 536
PostPosted: Tue Apr 01, 2008 9:01 am    Post subject: Reply with quote
It seems that the Times still gets the puzzles from Wayne Gould, who has made sudoku popular all over the world some years ago. At that time the only known advanced techniques have been x-wing and swordfish - and i suppose that Wayne never updated his program for other methods.
So, if you are stuck there, look for swordfish. But i dont like to do that, especially not on paper, because the search is cumbersome and boring.
I did not try this puzzle on paper, but probably i would have solved it something like that (after x-wing, xy-wing and kite):
Code:
 *------------------------------------------------*
 |#29    26   5    | 3   8   7   | 4   #69   1    |
 | 3     1    69   | 25  4   25  | 69   7    8    |
 | 47    8    47   | 9   6   1   | 23   35   25   |
 |-----------------+-------------+----------------|
 | 6     4    27   | 8   9   25  | 57   1    3    |
 | 257   257  1    | 6   27  3   | 89  #89   4    |
 | 8     9    3    | 57  1   4   | 57   2    6    |
 |-----------------+-------------+----------------|
 |#2479  237  69   | 1   5   8   | 236  34   27   |
 | 1     257 *247  | 27  3   6   | 28  *458  9    |
 | 257   36   8    | 4   27  9   | 1    36   257  |
 *------------------------------------------------*
Useless skyscraper for 9, one of r7c1 and r5c8 must be 9.
r7c1=9 => r8c3=4
r5c8=9 => r8c8=8
In both cases r8c8<>4
[edit: corrected typo]

Last edited by ravel on Fri Apr 04, 2008 7:33 am; edited 1 time in total
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Victor



Joined: 29 Sep 2005
Posts: 207
Location: NI
PostPosted: Tue Apr 01, 2008 12:58 pm    Post subject: Reply with quote
I used to like Brainbashers SH, apart from the occasional times when swordfish appeared - and I didn't then know any alternatives. (Still don't always see the alternatives, tho' I did this time.) Finding a non-SF solution made this a nice puzzle.
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Steve R



Joined: 24 Oct 2005
Posts: 289
Location: Birmingham, England
PostPosted: Tue Apr 01, 2008 1:24 pm    Post subject: Reply with quote
Ravel may well be right about Wayne Gould’s puzzles. Certainly his book of extreme puzzles soon went in the bin because of an excess of swordfish. However, the Times’ puzzles have been supplied by sudokusolver.com for a year or so now. This one be solved with straightforward techniques, which is why I chose it.

Starting from Norm’s first grid

Code:
29     26     5      | 3      8      7      | 4      69     1
3      1      69     | 25     4      25     | 679    79     8
47     8      47     | 9      6      1      | 23     35     25
------------------------------------------------------------------
6      4      27     | 8      279    25     | 579    1      3
257    257    1      | 6      279    3      | 789    4789   47
8      9      3      | 57     1      4      | 57     2      6
------------------------------------------------------------------
2479   237    69     | 1      5      8      | 2367   347    247
1      257    247    | 27     3      6      | 278    4578   9
257    36     8      | 4      27     9      | 1      36     257

7 can be eliminated from r4c5 using the xy-wing pivoted on r4c6 (either r4c3 or r6c4 contains 7). That reveals a finned x-wing for 7 in columns 2 and 5, so it can be eliminated from r9c1 as well.

An xyz-wing then completes the puzzle.

Steve
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storm_norm



Joined: 18 Oct 2007
Posts: 1741
PostPosted: Fri Apr 04, 2008 5:28 am    Post subject: Reply with quote
ravel wrote:
It seems that the Times still gets the puzzles from Wayne Gould, who has made sudoku popular all over the world some years ago. At that time the only known advanced techniques have been x-wing and swordfish - and i suppose that Wayne never updated his program for other methods.
So, if you are stuck there, look for swordfish. But i dont like to do that, especially not on paper, because the search is cumbersome and boring.
I did not try this puzzle on paper, but probably i would have solved it something like that (after x-wing, xy-wing and kite):
Code:
 *------------------------------------------------*
 |#29    26   5    | 3   8   7   | 4   #69   1    |
 | 3     1    69   | 25  4   25  | 69   7    8    |
 | 47    8    47   | 9   6   1   | 23   35   25   |
 |-----------------+-------------+----------------|
 | 6     4    27   | 8   9   25  | 57   1    3    |
 | 257   257  1    | 6   27  3   | 89  #89   4    |
 | 8     9    3    | 57  1   4   | 57   2    6    |
 |-----------------+-------------+----------------|
 |#2479  237  69   | 1   5   8   | 236  34   27   |
 | 1     257 *247  | 27  3   6   | 28  *458  9    |
 | 257   36   8    | 4   27  9   | 1    36   257  |
 *------------------------------------------------*
Useless skyscraper for 9, one of r8c1 and r5c8 must be 9.
r8c1=9 => r8c3=4
r5c8=9 => r8c8=8
In both cases r8c8<>4


Ravel, shouldn't your cell be r7c1 ?? for the useless skyscraper??
so it should read "r7c1=9 => r8c3=4"
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ravel



Joined: 21 Apr 2006
Posts: 536
PostPosted: Fri Apr 04, 2008 7:31 am    Post subject: Reply with quote
Yes, thanks. But Steve R's solution is much better anyway Wink
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