Suppose I have n points in Rn−1. For example, 3 points in R2.
If I think of these as objects in a 2-category, and I think
of the x-direction of R2 as representing 'the flow of time as far
as the 1-cells are concerned', and the y-direction as 'the flow
of time as far as the 2-cells are concerned', then there's two possible sorts
of 2-cell that these points (together with the lines and triangle that lie between them) can represent. One where two morphisms 'compose'
across the 2-cell to yield a third, and one where one 'decomposes' into two morphisms.
More precisely, I want to observe the following things:
There is a categorical sense in which every cell lies
either in the domain or codomain of the higher-dimensional cells
it's part of. For example, a is in the domain of h, b is in the codomain of g, h is in the codomain of α,
f and g are both in the domain of α, k is in the codomain of β, etc.
But there is also a geometric sense in which we can take any cell
and ask whether it looks like it's in the domain or codomain of a higher dimensional cell.
For example, to ask whether h is in the codomain of α, we ask
whether
det⎝⎛axcxbxaycyby111⎠⎞<0
at least assuming the points are laid out as depicted, with cx−ax.
In general, if we have points q,p1,…,pn in Rn then
det⎝⎜⎜⎛p11p12⋮p1np21p22⋮p2n⋯⋯⋱⋯pn−11pn−12⋮pn−1n11⋮1⎠⎟⎟⎞det⎝⎜⎜⎜⎜⎛p11p12⋮p1nq1p21p22⋮p2nq2⋯⋯⋱⋯⋯pn1pn2⋮pnnqn11⋮11⎠⎟⎟⎟⎟⎞
is how far q is — in the n-coordinate — from the hyperplane determined by p1,…,pn.
Thus, this will be positive if it's on the `positive n-coordinate side' of the hyperplane,
and negative for the other.
So if we have a large sequence of points S=(S1,…,Sm)∈(R∞)m, then
we can extract some coarse information about how they are shaped, as some data
of type
Πn<m.ΠK:Pn(S).Πq∈(S∖K).{dom,cod}
That is, for any n<m and any n-sized subset p1,…pn⊆S and some
q∈S∖p1,…,pn not already in that subset,
we can project each of the p1,…,pn,q from R∞ down to just Rn by
taking their first n coordinates, and ask
whether the sign of the expression is above is positive or negative. We interpret this
as telling us whether the (n−1)-cell determined by p1,…pn is in the domain
or codomain of the n-cell determined by q,p1,…pn.
Question
For each m, how many distinct such functions can actually be attained by a suitable
choice of m points S? Call this ϕ(m).
I know ϕ(1)=1 and ϕ(2)=2 and ϕ(3)=12, (because of the two triangle diagrams
above, plus a factor of 3! for permutations of merely in which order the three vertices appear in S)
and I think ϕ(4)=576=4!⋅2⋅2⋅6, because of a choice of permutation
of the 4 vertices, a choice of how to flip all the 2-cells, a choice
of how to flip the 3-cell, and the fact that 6 of the following 8 diagrams
seem 'consistent' with a
downward-pointing 2-cell going 'out of the page' from the top 1-cell of each little
diagram to the bottom 1-cell. Where 'consistency' means that there is a 'least' and 'greatest'
1-path.
But I'm less confident about that.