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Since there has been interesting strategy discussion on these puzzles and by popular request, I have opened an archive on the last thirty weeks. The discussion section is open for continued posting.
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It is quiet here.
Thus there is another EXTRA. It's a variation of an earlier EXTRA:
14 black cells:


10 clues:
A4=8/ B7=4/ C2=1, D1=6/ D8=3/
E2=7, E5=9/ G3=1/ H2=5, H7=8.


by: Christoph

"c" should be Christoph. Sorry.


by: hp

Thank you Chritoph nice and Relaxing puzzle
xwing on a Digit and two else on the two potences of the Digit and BCA of a Digit in the middle between 1 and 9 suffices for me


by: Stefan

Hope i'm right this time:


by: jgrab

Thank you; a rather easy one. I found several more or less hidden doubles rather than w-wings.


by: Christoph

thanks for transforming it into the correct pattern.


by: Ben

Can also be solved with Setti


by: mv

Nice puzzle. Thanks!


by: ThomasF

Add to this thread
Thanks Chrisohp, nice puzzle, solves with standard tool. Good entertainment.



by: bansalsaab

Thanks Stefan. I love UC and URs :-)


by: Candiru

Thanks for this one.
heavy fish-soup...
No Setti or UR needed


by: Berny

Thank you very much, Stefan!!!

Guess there are a few different ways to solve.
I used only one 3fish, no UR, but two settis and one large gap.


by: jgrab

Thank you, Stefan. For me one 3-fish, 2 x-wings and and Setti help.


by: Ben

There is of course an obvious single-cell UR that can be seen right from the start. We can immediately see that A1234 must contain an 8 to avoid multiple values in A9. But it still took until quite late in the puzzle when I could finally deduce A9.


by: bansalsaab

tried again and i was able to solve without UR. but UR was so simple and easy :-)


by: Augenweide

I wonder why some people find UR's doubtful. They often offer an elegant opportunity for a next step.


by: Leren

@ Augenweide

If you put SUDOKU UNIQUENESS CONTROVERSY into Google you will find an article about the so-called Uniqueness Controversy in Sudoku.

The issues raised apply equally well to all Latin square puzzles, including Str8ts.

I couldn't put the link in directly because unfortunately the forum software objected to it.


by: Christoph

Thanks, Stefan,
I used fishes and settis to solve this nice challenge. I did not use the UR-argumment in A9.


by: ThomasF

Thanks Stefan,
nice; Standardtools and UR


by: hp

I have Problems to solve.
After Applikation of bca4; swordfish on 7 and UR BC12 i could solve 10 Cells and stuck. Any hints??
Thanks in Advance


by: hp

Ok i saw now the 5fish on 8 that was decisive


by: Jan

Took me about 45 minutes by hand.

When checking with the solver, noted that only that 5-wing on 8 - which is there very early on after pruning the D7D6E6 chain and is really powerful - and in the end a 3-wing are really needed.


by: Frank

Add to this thread
Thanks to Stefan. This puzzle was very interesting for me. After using the solver and doing the usual eliminations you end up with 8 solved cells. Then you have 3 equivalent ways to the solution in the sense that the three ways lead to the same result: first the 5-fish on 8 mentioned by others, second the swordfish on 7 followed by the elimination of 9 in E6 and x-wing on 9 and the third way by setti on 8: there is no 8 in column 9 and only the rows C and D can have no 8. If there is no 8 in D then there is no 8 in column 6 and therefore no 8 in C.


Reply to this post
@Klaus and/or Stefan

Please see comments at the bottom of the last thread and show mercy


A UR in the beginning solved the puzzle for me.


by: hp

interesting Observation there could be no 78 in H89 because of AD89 and then the house of Cards collapse, still some Tools to the end but then solvable


by: Leren

Hi/Lo in Row H is obvious from the start. Clue 1 in E7 limits H7 to 2345, so H6789 is LO and H1234 is HI.


by: jgrab

@ Leren
but if H7=5, H6=6 and H89=78, H1-4 may well be low ...


by: Leren

Sorry my mistake - not so obvious.


by: kolumbus

@Laren, jgrab: I think it's the right track, however a little chain is needed (maybe this is hp's and mks's observation):
H89=high leads quickly (via HJ3=low, HJ4=low, D1-5 range) to UR on DH89 and contradiction on A89


by: Leren

I can see hp's argument. At 3 solved cells I have AD89 are all 678, so H89 can't be 78 to avoid a 6 cell 789 UR in ADH89. This really does determine Hi/Lo in row H and the puzzle solves relatively easily after that. Hopefully no rubbish this time :)


by: kolumbus

@Leren: Now I see it, so no chain is needed, thank you!


by: PAX

Following HP, it is not even an UR.


by: Leren


To be strictly correct, the potential 678 UR in ABH 89 only allows you to directly eliminate the 8's in H89 (but not the 7's). However this leaves no 8's in H6789, so it is LO and H1234 is HI.

In Sudoku parlance this UR has Type 4 Uniqueness. This means that if a UR candidate is False in a Roof cell it must be True in the other Roof cell. 8 in H8 forces 7 in H9 and 8 in H9 forces 7 in H8, but 7 in H8 does not force 8 in H9 and 7 in H9 does not force 8 in H8.


by: Klaus

Hi Leren,

i do not quite get this subtlety.

from the very start of this puzzle its clear due to the basic rules of Str8ts that 4 and 7 must not be present in H6789 at the same time, or, in other words, from the two digits 7 and 8 both or none have to be there.

so why is 7 forced by 8, but not the other way round?


by: Leren

Hi Klaus, I thought this question might come up.

When considering whether a UR digit in a Roof cell forces another UR digit into the other Roof cell, I've limited the forcing considerations only to the compartment containing the Roof cells (For the purposes of coding this move into my solver). What you are considering is a special case where the UR roof cells are part of a two 4 cell compartment arrangement.

As it turns out, after removing the 8's I reconsider the two 4 cell compartment arrangement in Row H and remove the 6's and 7's from H6789 and the 1234's from H1234.

So the subtlety, if there is any, is whether you consider the removal of the 7's to be a part of a complex UR setup or an immediate follow-on move. I think that's just a matter of style over substance.


by: jgrab

I must say that I tried to understand this case - in the absence of pre-existing rules - by everyday logic.
If H6789 is high, then H7=5, H6=6, H89=78.
D89=678; A789=678.
Now, if A7 <> 7, then x-wing 7 AH89, and no 7 left for D89.
So the interesting case is A7=7 and A89=68. x-wing on 8 makes D89=67. And now ADH89 is a situation that can not be uniquely resolved.
So H6789=low.


by: Leren

As an addendum to my previous post, and noting that Jan said last week that he didn't like URs, I moved them back to their default position just before chains. The puzzle takes longer to solve, but by the time the 789 UR hits, the contents of H6789 have been reduced somewhat by intervening moves, and the 7 in H9 is removed as well as the two 8's, because the contents of H6789 alone require it.


by: Klaus

Hi Leren,

this i dont understand. 7 and 8 are removed from H6789 either by UR or by a little chain, as jgrab did. but what means by its content alone?


by: Leren

@ Klaus

With URs "demoted" in my solver's solving order to their "normal" place just before chains the contents of H6789 are 26, 35, 178 and 478. If one of H89 is 8 the other is 7 as before, exposing the 678 UR. If H9 is 7 then there is no 4 in H6789, so it's range must be 5678 and there is only one place the 8 can go, in H8 again exposing the 678 UR, so out goes the 7.

With UR's "promoted" and H9 = 7 it is possible to have H9 = 7, H6 = 6, H8 = 5 and H7 = 4 (again I stress that this ignores the fact that the second compartment in the row also has 4 cells).

So this UR move as I've coded it does not depend on there being two 4 cell compartments in the same row, and when promoted doesn't slow up the solution as I've explained above.

I wonder what Jan has to say about this puzzle. Early placement of the UR move allows for a quick and elegant solution.


by: Klaus

Ok Leren,

Think i understand. a bit confusing to me whether we are talking about the puzzle or how your solver works.
Not quite the same.


by: PAX

Sorry for the confusion, it is an UR, no doubt.
But my error created an interesting discussion :)
No new puzzle for cold and windy Cape Town?


by: jgrab

I also think that we are through with it. A new one would be great, wouldn't it?


by: KDN

Full Support! I'm lost at work, nothing to do, everybody is on holiday


by: Berny

Add to this thread
Full support too!

Article created on 27-June-2010.
This page was last modified on 17-April-2011.
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  • 36 min with the H hint
       - Boerny - 7 hours ago
  • 15 min. Not very complicated
       - D-Day - 1 day ago
  • 30' but only with the hint, without I couldn't solve. Thank you! But now very nice
       - barbara - 2 days ago
  • 35' thank you for hint about row H
       - hildi - 4 days ago
  • lucky guess on high low in row h
       - silke - 4 days ago
  • 38' after correcting a stupid mistake
       - selecti - 5 days ago
  • 28:36, 3 lucky guesses
       - dj - 5 days ago
  • 1very nice hour
       - chris - 5 days ago
  • 27' - without the hint on row 8 i would have been lost ...
       - Axel - 5 days ago
  • 12' - that was pretty easy, just a few settis and you're done
       - earnie - 5 days ago
  • 11'23 second try, after high/low in row H
       - easydoesit - 5 days ago
  • 45' - 2xUR
       - SlowThinker - 5 days ago
  • not the ordinary type of UR.
       - Traherne - 6 days ago
  • 39 min. Took awhile to see the UR.
       - mv - 6 days ago
  • 14:45 by hand - nice but a bit simple...many singles and an x-wing or two.
       - Jan - 6 days ago
  • 20 min - nice and straightforward! :-)
       - Stef - 6 days ago
  • 24' nice and easy
       - KDN - 6 days ago
  • The question high/low in row H is decided by UR!!
       - Maily - 6 days ago
  • 17 min.Because of the comments(hard to ignore) I made a hi/lo decision on row H right at the start and this greatly shortened the time.
       - mparmen - 6 days ago
  • 18:00 - high/low on row H
       - malue74 - 6 days ago
  • 20 min - UR argument, no chain needed
       - ghw - 6 days ago
  • 19 '; hi/lo-decision or UR lead to the same result. After that it's easy
       - Candiru - 6 days ago
  • 29:50 the right high/low decision in row H and a few settis - now ready for a sunny sunday
       - joe reitzl - 6 days ago
  • 48 min. high/low on roh H was the key. very nice one.
       - turbo - 6 days ago
  • 35 min with a chain
       - jgrab - 6 days ago
  • 19 min, with a guess in row H
       - Alex - 6 days ago
  • 25' but needed a chain
       - Loser - 6 days ago
  • 20' very nice, B3 was the key
       - hrw - 6 days ago
  • 23' quite easy, no chain needed
       - ThomasF - 6 days ago
  • 45 min, lucky guess on row H
       - un - 6 days ago