# Fenced Fences

Anthem
Posers and Puzzles 18 Aug '11 19:43
1. Anthem
The Ferocious Camel
18 Aug '11 19:431 edit
You have a bunch of sections of fence (however many you need), all the same length, all straight and impossible to bend. Your goal is to build two closed fences, one containing the other, such that the inner fence uses a greater number of sections than the outer one.

What is the minimum number of sections that you need to achieve your goal?

(note: a section of fence can have an arbitrarily small, but positive, thickness)
2. 18 Aug '11 22:38
Originally posted by Anthem
You have a bunch of sections of fence (however many you need), all the same length, all straight and impossible to bend. Your goal is to build two closed fences, one containing the other, such that the inner fence uses a greater number of sections than the outer one.

What is the minimum number of sections that you need to achieve your goal?

(note: a section of fence can have an arbitrarily small, but positive, thickness)
One thing you didn't specify is whether the fence sections have to be touching end-to-end. In other words can a fence section have a part that is dangling, not enclosing anything, while another section pokes it in its side. If so, I have constructed a toothpick example that I think is the minimum,
a rhombus of 4 enclosing a weird arrangement of 5
but it's hard to describe and impossible to really prove that I did it because of the olives or maybe it was the gin. ðŸ™‚
3. Anthem
The Ferocious Camel
19 Aug '11 00:39
If we did allow dangling sections, then yes,
a four sided figure enclosing a five sided figure
could work, but that was not what was intended.

No dangling parts, both fences must form a single closed loop which contains all sections of the fence.
4. AThousandYoung
All My Soldiers...
19 Aug '11 05:013 edits
6 piece Y shape inside 3 piece triangle total 9
5. wolfgang59
howling mad
19 Aug '11 05:29
Originally posted by AThousandYoung
[hidden]6 piece Y shape inside 3 piece triangle total 9[/hidden]
I dont think that will work because the OP said the fences do have thickness - in which case your inner shape just pops out. I think you need to add a section to your outer fence.
6. 19 Aug '11 06:37
How about, making the outer fence a regular pentagon ABCDE.

For inner fencing,
Place a section MN that touches AB at M and BC at N.
Place a section NO that touches BC at N and CD at O.
Place a section OP that touches CD at O and DE at P.
Place a section PQ that touches DE at P and EA at Q.
Now, place sections QR and RM (in V shape) to complete the inner fencing (MNOPQR).

Assuming thickness is very small (epsilon) the points for the inner fencing can be chosen arbitrarily inside the pentagon.
7. 19 Aug '11 11:09
It may not be the minimal solution but I reckon 14 pieces is do-able without the outer fence and the inner fence touching. By a 2x1 rectangle on the outside and sort of two deformed w-shapes in the middle to ensure both shapes are closed loops.

smartarin: I think your idea assumes is right however it rests on the assumption that you can fit a square of lengths n into a pentagon of lengths n, which I think you can. So that would make your answer better than mine.
8. 19 Aug '11 13:30
But then logically we can extend your idea to having a warped triangle inside a square.

So there's a square outside and a triangle with two extra sides so you get a zig-zag pattern at one of the edges, meaning you'd only need 9 lengths of fence.
9. Anthem
The Ferocious Camel
19 Aug '11 13:39
smartarin - Correct! Good job.