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DB Saturday Puzzle - April 15, 2006

 
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keith



Joined: 19 Sep 2005
Posts: 3182
Location: near Detroit, Michigan, USA

PostPosted: Sat Apr 15, 2006 12:53 pm    Post subject: DB Saturday Puzzle - April 15, 2006 Reply with quote

Here is today's David Bodycombe puzzle. A good one to do with pencil and paper.

Code:


Puzzle: DB041506  ******
+-------+-------+-------+
| . 2 . | 5 . . | . 7 4 |
| . 7 3 | . . . | . . . |
| 4 . . | . . . | 5 . 3 |
+-------+-------+-------+
| 9 . . | 2 . . | 3 . . |
| . 4 . | . 9 . | . 6 . |
| . . 8 | . . 4 | . . 1 |
+-------+-------+-------+
| 8 . 5 | . . . | . . 6 |
| . . . | . . . | 2 3 . |
| 7 1 . | . . 3 | . 9 . |
+-------+-------+-------+



See if you can find the Unique X-wing, and figure out what to do with it! (Also known as a Type-6 Unique Rectangle, it is an X-wing which is also a Unique Rectangle.)

Best wishes,
Keith
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Marty R.



Joined: 12 Feb 2006
Posts: 5176
Location: Rochester, NY, USA

PostPosted: Sat Apr 15, 2006 6:03 pm    Post subject: Reply with quote

Quote:
See if you can find the Unique X-wing, and figure out what to do with it! (Also known as a Type-6 Unique Rectangle, it is an X-wing which is also a Unique Rectangle.)

After the usual stuff, I used coloring to eliminate an "8." In thinking back, I don't think it contributed to the solution, but that's OK, I don't get to use coloring that often.

Then I found a rectangle. I never heard of a "Unique X-Wing" or a "Type-6 Unique Rectangle." I don't think I could have done anything with it as an X-Wing, but as a Unique Rectangle I was able to solve a cell outside of the rectangle, and that opened things up.

Fun stuff.
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keith



Joined: 19 Sep 2005
Posts: 3182
Location: near Detroit, Michigan, USA

PostPosted: Sat Apr 15, 2006 8:11 pm    Post subject: The details Reply with quote

Code:

Puzzle: DB041506  ******
+-------+-------+-------+
| . 2 . | 5 . . | . 7 4 |
| . 7 3 | . . . | . . . |
| 4 . . | . . . | 5 . 3 |
+-------+-------+-------+
| 9 . . | 2 . . | 3 . . |
| . 4 . | . 9 . | . 6 . |
| . . 8 | . . 4 | . . 1 |
+-------+-------+-------+
| 8 . 5 | . . . | . . 6 |
| . . . | . . . | 2 3 . |
| 7 1 . | . . 3 | . 9 . |
+-------+-------+-------+


After the usual standard moves (actually, only forced and pinned squares, or naked and hidden singles), you get to:

Code:

+----------------+----------------+----------------+
| 1    2    9    | 5    3     8   | 6    7    4    |
| 5    7    3    | 46   246   26  | 1    8    9    |
| 4    8    6    | 79e  1     79f | 5    2    3    |
+----------------+----------------+----------------+
| 9    5    17   | 2    68    16  | 3    4    78   |
| 3    4    17   | 18   9     5   | 78   6    2    |
| 2    6    8    | 3    7     4   | 9    5    1    |
+----------------+----------------+----------------+
| 8    3    5    | 479g 24i   279h| 47j  1    6    |
| 6    9    4    | 178  58a   17  | 2    3    578c |
| 7    1    2    | 468k 4568b 3   | 48   9    58d  |
+----------------+----------------+----------------+


Unique Rectangles:

You cannot have a solution where the corners of a rectangle contained within two blocks are

X Y

Y X

because this is not unique. You can interchange X and Y and still have a valid solution.

It follows that you cannot have the possibilities

XY XY

XY XY

as the corners of a rectangle contained within two blocks. Either of these (the solution or the possibilities) is called the "Deadly Pattern", to be avoided.

There are two unique rectangles in the position above, and both can be solved with reasoning involving "strong links" or "conjugate squares".

Rectangle 1:

Look at the cells labeled abcd. The Deadly Pattern is <58>. However, abcd is also an X-wing on <5>. Either the diagonal squares ad are both <5>, or bc are both <5>. If bc are <5>, ad must be <8>, and there is a Deadly Pattern. So, bc must both be <5>.

Rectangle 2:

Look at the cells labeled efgh. The Deadly Pattern is <79>. Looking only at the rectangle, at least one of gh cannot be <7> or <9>.

Now, look at the other squares in R7 and B7. One of gh must be <9>, there are no other possible squares to hold <9>. So, neither of gh can be <7>. Then, there are two paths:

1. j must be <7> (in R8), etc.

2. gih are now a triple in <249> (in B8), and we can eliminate <4> from b and k.


I had fun with this one!

Best wishes,

Keith
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keith



Joined: 19 Sep 2005
Posts: 3182
Location: near Detroit, Michigan, USA

PostPosted: Sat Apr 15, 2006 8:22 pm    Post subject: Coloring on 8 Reply with quote

Marty,

Your coloring removes the <8> from square "b" in my previous post, thus destroying the first (Type-6) Unique Rectangle.

The Rectangle logic is cool, but placing the last two <5>'s in this puzzle is not immensely helpful.

Keith
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Marty R.



Joined: 12 Feb 2006
Posts: 5176
Location: Rochester, NY, USA

PostPosted: Sat Apr 15, 2006 9:34 pm    Post subject: Reply with quote

Keith,

Correct, the coloring removed the "8" from cell b, thus destroying the rectangle. It's questionable whether I would have otherwise noticed it, and it's almost certain I wouldn't have found that the deadly pattern remained if one of the choices was used. My mind is not yet trained in reasoning out that configuration.

As to the 79 rectangle, it was immediately apparent that one of the corners had to be a "2" or a "4", but I thought it wasn't enough to do anything. But then when I noticed the "9s" were locked, I realized the "7s" had to go, after which I noticed the single "7" before I saw the newly formed 249 triple.

As an aside, did I read recently that the principles of Unique Rectangles don't apply if the corners occupy four boxes, i.e., the corners must be in only two boxes?

P.S. After posting the above, I noticed in your explanation you made reference to two boxes, so I guess that means the answer to my question must be "yes."
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alanr555



Joined: 01 Aug 2005
Posts: 193
Location: Bideford Devon EX39

PostPosted: Sun Apr 16, 2006 9:39 am    Post subject: Reply with quote

Code:

> As an aside, did I read recently that the principles of Unique
> Rectangles don't apply if the corners occupy four boxes, i.e.,
> the corners must be in only two boxes?

> P.S. After posting the above, I noticed in your explanation
> you made reference to two boxes, so I guess that means
> the answer to my question must be "yes."

CARE is needed here.
If the rectangle is a TRUE rectangle
(ie involving only two columns and two rows)
then it does not matter in how many boxes it is contained.

However, if there are other relationships involved (as may
be the case with only two boxes involved) then greater
care is needed and, indeed, the principles of the unique
rectangle may apply even to trapezoids or other
quadrilaterals - but it is likely that "Strong Links" will
be evident in such cases so that rectangle principles
are highly unlikely to be first or second port of call.

Essentially, if there is a constraint of a unique solution,
an "interloper" must be part of the solution if exclusion
of such interloper would leave a "deadly" rectangle
with the same pair of possibilities at each vertex.

If there is more than one interloper, then at least one
of them must be retained but further inspection will
be needed to determine which one.

Alan Rayner  BS23 2QT
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David Bryant



Joined: 29 Jul 2005
Posts: 559
Location: Denver, Colorado

PostPosted: Sun Apr 16, 2006 12:02 pm    Post subject: The vertices must lie in just two boxes Reply with quote

Alan R wrote:
If the rectangle is a TRUE rectangle (ie involving only two columns and two rows) then it does not matter in how many boxes it is contained.

I don't think this is right, Alan.

Suppose the "vertices" lie in four separate boxes. Just to make it concrete let's suppose that the "pair" appearing at each vertex is {1, 2}.

What happens if we reverse the values around the "rectangle"? Well, in box 1 we remove a "1" and insert a "2". In another box we remove a "2" and insert a "1".

After this transformation box 1 contains two "2"s, and the second box contains two "1"s. That's not a valid solution, so the vertices of the "non-uniqe rectangle" cannot lie in four different boxes. dcb
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Marty R.



Joined: 12 Feb 2006
Posts: 5176
Location: Rochester, NY, USA

PostPosted: Tue Apr 18, 2006 3:55 pm    Post subject: Reply with quote

Coincidentally, just yesterday I finished a puzzle that had a four-box rectangle. Note the "deadly pattern" at the four corners. This puzzle brought into focus for me the whole rectangle issue and illustrates what David is saying. Sorry about the non-grid.

Code:
291645873
587329146
463817295
615983724
832754619
749162358
928576431
154238967
376491582
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