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nataraj · Posted: Fri Nov 02, 2007 9:18 am Post subject: Nov 2 vh - to each his own

Depending on tastes and personal preferences, today's puzzle could be solved

exclusively with xy-wings
78-57-58, pivot r3c2, and later
36-35-56, pivot r5c1

but also

exclusively with color(w)ings (kite/skyscraper) in "3" and "5"
3: -r1c1=r5c1-r5c7=r3c7-
3: -r5c1=r1c1-r1c9=r3c7- (plus -r5c7=r4c9-)
5: -r4c2=r3c2-r3c8=r6c8-

and, of course, any combination thereof ...

andras · Joined: 31 Oct 2007 Posts: 56 Location: Mid Wales

Yes, I saw the two xy-wings quickly and the rest just fell into place.

Got a busy day today, so it's nice not to feel too distracted!

John

Johan · Joined: 25 Jun 2007 Posts: 206 Location: Bornem Belgium

I found an ER for digit <3> in Box 4, eliminating <3> in R5C7, collapsing the whole puzzle.

cgordon · Joined: 04 May 2007 Posts: 769 Location: ontario, canada

Earl · Joined: 30 May 2007 Posts: 677 Location: Victoria, KS

cgordon,

Assuming the 3's in your gird are the only significant ones,
does the ER eliminate not only the 3 in R1C9, but also the 3 in R4C3?
Is that true of other ER's?
I have little experience with ER's

Earl

cgordon · Joined: 04 May 2007 Posts: 769 Location: ontario, canada

Earl: I just showed the 3's relevant to the ER. These are ALL the 3's I had at this point. Actually, now I'm looking at the grid - I'm having difficulty spotting Johan's ER elimination of R5C7 using Box 4. Though I do see a skyscraper on C17 that takes out<3> in R3C3 and R1C9 - so forget the ERs cos skyscrapers are surely the simpler route.

Marty R. · Posted: Fri Nov 02, 2007 3:05 pm Post subject:

I used the two XY-Wings.

Earl, I'm new to ERs too, but once I found an explanation of them that I understood, I've used a number of them. As far as I know, at least with the basic ER, only one cell can have an elimination since by definition the elimination occurs in a cell that sees both the ERI and the second cell of the strong link.

cgordon · Joined: 04 May 2007 Posts: 769 Location: ontario, canada

re'born · Joined: 28 Oct 2007 Posts: 80

Earl,

In this situation,

nataraj · Posted: Fri Nov 02, 2007 4:05 pm Post subject:

There are actually more ERs in the grid posted by cgordon (the one with all the 3s)

One in box 1 (the EMPTY rectangle being r23c12), and another one in box 4 (r46c12). In both cases the strong link is the same: r3c3=r3c7

I will try to explain the way I remember ERs using the lower one (box 4) as example:

The ER works like a grenade launcher.

the "sight" is the strong link. Through it one looks at the target in c7.
We drop the granade into the launcher (from our side of the sight r3c3 down c3) and it shoots out (row 5) to hit the target.

In less militaristic terms:
one cell of the strong link defines the column (c3) of the ER box (box 4)
within the box, all OTHER occurrences of "3" are confined to one row (row 5)

The other cell of our strong link and this row together form a pincer which eliminates 3 from r5c7.

BTW, in your grid I see anoter ER, this time it is rotated 90 degrees, the strong link is r3c7=r5c7, and the victim is r3c3:

if r5c7<>3 then r3c7=3 (strong link), then r3c3<>3 because it sees r3c7
if r5c7 IS 3 then none of r5c13=3, but then r4c3=3 and again r3c3<>3.

Asellus · Posted: Fri Nov 02, 2007 9:03 pm Post subject:

In nataraj's two XY Wing route, after that first XY Wing there is a BUG+2 that forces R7C3 to be <6>. It has the same effect as the second XY Wing.