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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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An X-str8ts for people:



by: jgrab

Add to this thread
For whom else? For THE people?
Anyway: Thanks, mostly wings and one Setti.


Many here comment on how they need a small chain in the end to solve Andrew's puzzle. Which sounds OK to me.

In this particular case, though, the puzzle ends with something that looks like a BUG (Bi-value Universal Grave). In a real BUG all remaining cells (no matter how many) have 2 candidates and each occurs exactly 2 times per row/column. Whatever you choose would lead to a correct solution. A puzzle ending in an actual BUG, is therefore an invalid puzzle.

So to solve the puzzle, prevent a BUG-position.

In this particular puzzle you end up with all remaining cells except one having 2 candidates, each occurring twice in the row/column. From that single cell having more than 2 candidates, select the candidate that occurs more than 2x in the row/column. That is how you prevent the BUG-position. From then it is all singles.

This BUG-prevention approach is not a chain. Any short chain will lead toward the same outcome, though.


by: kolumbus

Hi Marc, sorry, I can't follow your approach. If we put J7==79, we get a BUG-position as you describe it, however NOT with multiple solutions, rather without any solution at all! So the uniqueness argument doesn't work here.
Obviously your BUG condition – "all remaining cells (no matter how many) have 2 candidates and each occurs exactly 2 times per row/column" – is not sufficient to define a multiple solution position.


by: Klaus

Hi kolumbus,

i think what Marc should have said (and probably he meant it) is:

the described BUG-situation has either no or two solutions. i think thats easy to prove.

so his arguemnt is not a pure UR-argument but still absolutely valide.


by: kolumbus

Hi Klaus,
maybe that's what Marc thought :) – what he wrote is: "Whatever you choose would lead to a correct solution ...", and with this argument he prevents the BUG position and chooses 6.
The correct digit, but not a logic conclusion ...


by: Klaus

Add to this thread
ok kolumbus,

so he forgot the trivial part in the definiton of a BUG. you are very strict today...

the argument is general and for me its nice. i didnt know (or think of) it, so i had to fumble around a bit.





by: kolumbus

Very nice one, thank you Leren.
First it seems very extreme, however there are just 5 beyond-solver-steps needed, one (not easy to spot) head chain included.


by: pax

With an UR consideration no head chain needed. Nice!


by: Cole

I agree with @pax. The UR is all that is required.


by: kolumbus

@ pax and Cole
thank you, I didn't spot the 1-cell-UR before – so just this single beyond-solver-step is needed, amazing ...


by: Cole


I am always amazed at you thoughtful comments so pleased to offer a different perspective - if only for this one time.


by: Leren

Where is the 1 cell UR? I can't see it.


by: Cole


Column 2. Determines the high low.


by: Leren

Add to this thread
Sorry for my stupidity but I still can't see it. Perhaps you can describe it in a bit more detail.





by: Toni

hmm... Seems like the puzzle has 3 solutions. At least I can't find errors in any of the three...


by: Toni

add-on: there should be a unique solution for any of the three scenarios F7 = 5 or 6 or 7


by: Stefan

Toni, you are right, it's not unique.


by: Leren


Sorry, a little slip up there. Try this instead.



by: Cole

@ Leren

This puzzle was great. Amazing how two settis open it right up after some CE. Loved it! Thank you!


by: jgrab

Add to this thread
Nice little exercise, thanks. After row E is quickly fixed, the rest flows smoothly.


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Article created on 27-June-2010.
This page was last modified on 17-April-2011.
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  • 15 min, logic and settis, fun!
       - un - 2 hours ago
  • settis and a short chain.
       - Benny_42 - 5 hours ago
  • 33' 2nd try
       - Uli - 6 hours ago
  • 24 min. - very nice one
       - Nessie - 7 hours ago
  • 14 min, straight forward
       - EC - 7 hours ago
  • 14:30 nice one.
       - JochenK - 7 hours ago
  • 21 minutes, nice
       - Aart - 7 hours ago
  • Nice settis
       - 7iron - 8 hours ago
  • 7 min - no Need for a chain, nice and easy
       - Susan - 8 hours ago
  • a short half hour on a lazy sunday afternoon
       - emil - 8 hours ago
  • a short half hour on a lazy sunday afternoon
       - emil - 8 hours ago
  • Interesting, the application of the Sudoku "BUG" rule to a Str8ts!
       - Jan - 8 hours ago
  • 20 min; I'm not sure if I really understand the "force unique solution" - but it works;-)
       - Drefplin - 8 hours ago
  • 12+' also chain at end. Didn't understand Marc's "force unique solution clue"
       - ttbass - 10 hours ago
  • 16 min. A few settis at the end but otherwise very easy
       - D-Day - 10 hours ago
  • 18 min very simple, but in the end I needed Marc
       - Christian - 10 hours ago
  • 17 min, not too hard
       - phil - 10 hours ago
  • Also needed a short chain at the end
       - mparmen - 11 hours ago
  • 16 min., same as Nessi,not more to say
       - susi - 11 hours ago
  • 32 min, the position of 4 was the key for me.
       - poblid - 11 hours ago
  • In the end force unique solution: All cells have 2 candidates left, only one has 3. Choose the one that occurs >2x in the row/column.
       - Marc - 11 hours ago
  • 13 min. - lot of singles and a little chain at the end - a real 5star next week would be nice
       - Nessi - 11 hours ago
  • 16 min setti final chain easy
       - juehe - 12 hours ago
  • 11 min, 2nd attempt, entertaining
       - fred - 12 hours ago
  • magic
       - val - 12 hours ago
  • done
       - Loser - 12 hours ago
  • 12 min with final head chain
       - jgrab - 12 hours ago
  • 15:27 Interesting fast starter with substantial gaining complexity towards the end. Settis, logic, careful elimination did it.
       - Korbinian Bär - 13 hours ago
  • 15min - short chain as the others...
       - COM - 14 hours ago
  • 20:40 nice.
       - abracadabra - 14 hours ago