The Universe of Discourse


Thu, 29 Nov 2018

How many kinds of polygonal loops?

Take !!N!! equally-spaced points on a circle. Now connect them in a loop: each point should be connected to exactly two others, and each point should be reachable from the others. How many geometrically distinct shapes can be formed?

For example, when !!N=5!!, these four shapes can be formed:


Four displays, each with five dark gray dots arranged at the vertices
of a regular pentagon.  In each display the dots are connected with
purple lines, but each in a different order.  If the dots were
numbered 0-1-2-3-4 in clockwise order, the four figures have purple
lines connecting them, respectively, in the orders
0-1-2-3-4-0, 0-1-3-2-4-0, 0-2-1-4-3-0, and 0-2-4-1-3-0.  The first of
these is a plain pentagon, and the last is a five-pointed star.  The
middle two are less symmetric.

(I phrased this like a geometry problenm, but it should be clear it's actually a combinatorics problem. But it's much easier to express as a geometry problem; to talk about the combinatorics I have to ask you to consider a permutation !!P!! where !!P(i±1)≠P(i)±1!! blah blah blah…)

For !!N<5!! it's easy. When !!N=3!! it is always a triangle. When !!N=4!! there are only two shapes: a square and a bow tie.

But for !!N=6!!, I found it hard to enumerate. I think there are nine shapes but I might have missed one, because I know I kept making mistakes in the enumeration, and I am not sure they are all corrected:

This time
there are nine displays, each with six dots.  The connections are,
respectively, 
012345 (a hexagon), 015432, 032145 (two diamonds), 
015234 (highly irregular), 014523 (three triangles that share a vertex
in the center), 013254, 023154, 031254, and 035142.

It seems like it ought not to be hard to enumerate and count these, but so far I haven't gotten a good handle on it. I produced the !!N=6!! display above by considering the compositions of the number !!6!!:

Composition How many
loops?
6 1
1+5
2+4 1
3+3 1
1+1+4
1+2+3 1
2+2+2 2
1+1+1+3
1+1+2+2 1
1+2+1+1 1
1+1+1+1+2
1+1+1+1+1+1 1
Total9 (?)

(Actually it's the compositions, modulo bracelet symmetries — that is, modulo the action of the dihedral group.)

But this is fraught with opportunities for mistakes in both directions. I would certainly not trust myself to hand-enumerate the !!N=7!! shapes.

[ Addendum: For !!N=6!! there are 12 figures, not 9. For !!N=7!!, there are 39. Further details. ]


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