4
$\begingroup$

There are $n$ men, standing one at each vertex of a convex $n$-gon. If they are allowed to move together along sides or diagonals of the polygon to reach another vertex, how many different ways are there to do so without meeting another one?

See OEIS A350599 for the first few numerical values.

$\endgroup$
7
  • 1
    $\begingroup$ By "not meeting each other," do you mean that their paths may not cross? Is a man allowed to stand still, or does every man move to a new vertex? $\endgroup$ Commented Mar 26, 2022 at 23:37
  • 1
    $\begingroup$ Probably a question better suited for math.stackexchange than mathoverflow, but in its current form it would be closed there in the blink of an eye. You'll want to read up on how to write a good question on that site before posting it there. Meanwhile, can you solve it for, say, $n=3,4,5$ and look for a pattern? $\endgroup$ Commented Mar 26, 2022 at 23:43
  • 1
    $\begingroup$ This problem looks okay to me. It is stated a little unclearly, but one can guess what is intended. $\endgroup$ Commented Mar 27, 2022 at 0:07
  • 3
    $\begingroup$ Actually I'm unable to guess the meaning of the question (what is meant by a "way to move"?...). $\endgroup$
    – YCor
    Commented Mar 30, 2022 at 18:21
  • $\begingroup$ @YCor intention is to find how many different ways are there for all to move simultaneously to another vertex without meeting another one. Here you need to form nonintersecting closed loops ( polygons). There are real life applications other than what I have mentioned here. $\endgroup$ Commented Mar 31, 2022 at 1:52

1 Answer 1

6
$\begingroup$

Assume that the paths may not cross and each man must move. Label the vertices $1,2,\dots,n$ in clockwise order. Let the man at vertex $i$ move to vertex $\pi(i)$, so $\pi$ is a permutation of $1,2,\dots,n$. If we draw an arrow from vertex $i$ to $\pi(i)$, then we get a disjoint union of noncrossing cycles of length $\geq 3$. We can obtain such cycles by choosing a noncrossing partition of the vertices with no blocks of size 1 and 2, and then orienting the boundary of the convex hull of each block in two ways. Thus in Exercise 5.35(b) of Enumerative Combinatorics, vol. 2, we should set $f(i)=2$ for $i\geq 3$ and $f(1)=f(2)=0$. If the desired answer is $h(n)$, then by this exercise we have $$ x+\sum_{n\geq 1}h(n)x^{n+1} = \left( \frac{x}{1+2\sum_{n\geq 3} x^n}\right)^{\langle -1\rangle} $$ $$ = \left( \frac{x(1-x)}{(1+x)(1-2x+2x^2)}\right)^{\langle -1\rangle}, $$ where $\langle -1\rangle$ denotes compositional inverse.

If a man is allowed to stand still, replace $1+2\sum_{n\geq 3}x^n$ by $1+x+2\sum_{n\geq 3}x^n$.

Possibly you can get some kind of explicit formula for $f(n)$ out of this, but it will be messy.

$\endgroup$
9
  • $\begingroup$ Yes as you suggested this is regarding how to form nonintersecting closed loops that means how many different ways are there to partition the set of vertices of a convex n - gon into nonintersecting directed polygons. I tried this as a research and the result published in OEIS A 350599 $\endgroup$ Commented Mar 27, 2022 at 2:58
  • $\begingroup$ R. P. Stanley, Enumerative Combinatorics, Wadsworth, Vol. 1, 1986, page 93, problem 7 is cited at oeis.org/A000266 which counts "the number of permutations in the symmetric group S_n whose cycle decomposition contains no transposition." This should give the number of ways if men are allowed to stand still. $\endgroup$ Commented Mar 27, 2022 at 22:20
  • $\begingroup$ @GerryMyerson: where does the noncrossing condition come in? $\endgroup$ Commented Mar 27, 2022 at 23:29
  • $\begingroup$ The way I read the question, it doesn't say the paths can't cross, it just says no two can be at the same place at the same time. That forces it to be a permutation, and rules out transpositions, but as long as they're careful with their timing it doesn't rule out crossing paths. $\endgroup$ Commented Mar 28, 2022 at 0:59
  • 1
    $\begingroup$ The intended interpretation is that the paths don't cross, as confirmed by OEIS A350599. $\endgroup$ Commented Mar 28, 2022 at 1:15

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .