dailysudoku.com

[Glassman]

Having trouble with this. I solved it and proved uniqueness by starting at the derived end of the only non-trivial pair on the grid. Whichever way around I started this pair, the cell in between came out with the same value.

I could not find a way of solving this without resorting to trial and error, and now cannot find a way retrospectively.

I would welcome enlightenment.

Glassman

[Earl]

I solved it by the interconnection of the conjugate pairs of <1> in r3c5- r3c7 and r7c5-r7c8, and r3c7-r8c7. If either r3c5 or r3c7 is 1, r7c8 must be 1. The rest is simple elimination.

Is this called a skyscraper?

Earl

[PouLeeps]

Hi all,
I'm very new to Sudoku. I'm stuck at this point. Thanks.
[code]
+--------------+------------+--------------+
| 16 2 59 | 17 1579 3 | 4 167 8 |
| 168 368 389 | 2 179 4 | 5 1367 67 |
| 4 35 7 | 6 15 8 | 13 2 9 |
+--------------+------------+--------------+
| 2 9 6 | 8 4 1 | 7 5 3 |
| 7 58 58 | 9 3 2 | 6 4 1 |
| 3 1 4 | 5 6 7 | 9 8 2 |
+--------------+------------+--------------+
| 68 7 38 | 4 18 9 | 2 136 5 |
| 9 3468 1 | 37 2 5 | 38 367 467 |
| 5 348 2 | 137 178 6 | 138 9 47 |
+--------------+------------+--------------+
How do I align the columns?

[Mesmin]

[Steve R]

This is the sticking point:

[David]

I think the fastest route to solving this one is the option chosen by Earl. Yes, it is called a 'skyscraper', although I normally refer to it as 'strong links'

[Marty R.]

[sdq_pete]

[cgordon]

I eventually solved it with skyscrapers on 1s in col 4 and 7. Then on 3s in col 4 and 7. Then with an xy wing using nos 1-8-6 in R2 & R7, Col 1 and R7C8 which eliminated the 1 from R2C8. I probably got lucky - but I think it was brilliant.

[Asellus]

Starting from Peter's position, I applied the X-Wing on <7> in R1 and R8. This leaves 13 and 67 pairs in Box 3.

Next, color wrapping <1> starting at R2C8 produces opposite polarity for the <1>s in R3C7 and R7C5, eliminating the <1> at R3C5.

Essentially the same solution as others have posted, I believe. The term "Skyscraper" used on this board seems to apply both to what I learned as the simplest forms of Color Wraps and Color Wings (without extended wrapping).

[cgordon]

Geez Mate! - Color Wraps and Color Wings (without extended wrapping).

No offense, but I actually think there is far too much "terminolgy" on this site. When I first joined there was Swordfish and colouring and all kinds of other stuff. Never did get to find out what it all meant. But since, I have learned that the only things I need to solve the "very hards" are Skyscrapers (probably called x wings). - xy wings and xyz wings.

I believe there is a need to simplify and standardise terminology.

[Marty R.]

[Mogulmeister]

Asellus is right - colour wraps/traps/wings are just a different name for a series of techniques which are perhaps slightly better known (eg skyscraper).

In any case, I have to agree with David: these naming conventions are all just variants on strong links (ie conjugate pairs). We know that one must be true and the other false. Colouring is used to highlight different polarities. The colour wrap uses the colours to show up polarities where there is a contradiction or impossibility.

Whatever formation/technique/naming convention you use, the underlying concepts are "strong links".

This was a classic skyscraper (horizontal!) which actually made 3 eliminations - and this, as has been explained, is distinct from an x-wing.

[Asellus]

I didn't want to set off a terminology crisis. Being new to this board, I just didn't want to claim "Skyscraper" when I was still not completely certain I had the concept accurately. The responses have shown my understanding to be correct: two parallel strong links ("polarity pairs") sharing a common end, essentially.

Since I was residing in Color Wrap/Wing Land, I saw two different entities in this, depending upon whether (Wrap) or not (Wing) there was also a strong link between the common ends. These have different logics, but produce the same effect when limited to four cells... hence, Skyscraper. And, in C W/W Land, the column/row restriction doesn't matter (any "House" will do, including Boxes), nor does the 4-cell restriction (the strong links can be extended in polarity chains).

But, these restrictions take care of the great majority of useful cases, so... Skyscraper it is!

(Unimportant Notes: A Skyscraper looks like a sort of lopsided X-Wing, which may account for that confusion. Since X-Wing is a species of fish and is unrelated to the XY-and XYZ-Wings, I think it should have a fishy name myself! Perhaps guppy or sardine.)

[cgordon]

I'm confused here. Are x-wings skyscrapers of equal height? ie squares.

Anyway - I found the Marty R example very helpful. I've been solving the very hards (incl this one) by xyz wings which I find difficult to locate.

[Mogulmeister]