This is a Slitherlink puzzle, with a twist. The configuration of dots that the loop passes through must exhibit 180 degree rotational symmetry about the center of the puzzle. If a dot is used by the loop, its symmetrical opposite must be also, and vice versa. (Some dots are red; this is only as a reference and has no bearing on the rules.)

Puzzle 318

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EDIT (5/20/2011 9:26 AM): Only a cosmetic change; the puzzle is the same. Made some of the dots red so that it’s easier to identify the symmetric pairs.

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### 10 Responses to “Puzzle 318 (Slitherlink) [Point-symmetric]”

1. rob Says:

Wow. I’m through, but with quite a bit of trial and error and guessing. This was the hardest puzzle for me that you’ve posted in quite a while.

I have a feeling I was missing some logical deductions. My main approach was assuming that some dot is off the loop and finding a contradiction, thus proving some pair of dots is on the loop. But that information was often hard to make use of.

• MellowMelon Says:

The intended solution is quite involved and requires some serious scrounging around for any segments you can possibly draw at a given moment. I imagine it’s quite easy to miss one and lose the trail. It’s indeed a really hard puzzle, which surprised me given that it’s a 10 by 10 Slitherlink in a variation that seems harmless at first glance.

2. y Says:

Nice. I think the puzzle should have been presented without the center dot. I didn’t use that deduction beyond that dot.

• MellowMelon Says:

I don’t follow. Wouldn’t taking the center dot out make the break-in obvious?

• y Says:

The solution may have started from the center break-in, but I don’t think that was the main point of the puzzle. Removing the center dot would have just made the rest of the puzzle a bit easier by making it easier to keep track of the symmetry. When I said I didn’t use that deduction beyond the center dot, I meant counting the number of even and odd marked dots.

• MellowMelon Says:

It’s true it wasn’t the main point of the puzzle, but I think it would have detracted from the quality to take it out entirely. I see your point about making the symmetry easier to see though.

EDIT: I’ve updated the puzzle to try to address what you said.

• Rob Says:

Is it possible to deduce early on whether the center dot is on the loop? I seem to remember that staying open until quite late in my solve.

3. David Scherzinger Says: