Notes:Delta complex/Formal attempt

Formal attempt

We try and keep everything combinatorial, so keep an abstract simplicial complex in the back of your mind, and a simplex as being like [ilmath]\{a,b,c\} [/ilmath] for a triangle and such.

Notations:

• Let [ilmath]\#(n):\eq\{1,\ldots,n\}\subset\mathbb{N} [/ilmath] - I did want to use [ilmath]C(n)[/ilmath] for "count" or "consecutive" but given the context that'd be a poor choice!
  • Consider [ilmath]\#(n)[/ilmath] as a poset in its own right (in fact a total order is in play) with the "usual" ordering on [ilmath]\mathbb{N} [/ilmath] it inherits. This is a standard substructure construction.
• Let [ilmath]K[/ilmath] be our Delta complex, let us sidestep defining exactly what this is now, as a tuple of sets.
• Let [ilmath]S_n(K)[/ilmath] be the set of [ilmath]n[/ilmath]-simplices of [ilmath]K[/ilmath]
• Let [ilmath]I(m,n)[/ilmath] be defined to be equal the collection of all injective monotonic functions of the form [ilmath]f:\#(m+1)\rightarrow\#(n+1)[/ilmath]
  • The [ilmath]+1[/ilmath] comes from the definition: [ilmath]\text{Dim}(\sigma):\eq\vert\sigma\vert - 1\in\mathbb{N} [/ilmath] - we take care with the case [ilmath]\sigma\eq\emptyset[/ilmath] as I'm developing a framework including this and come up with 2 "null objects" that do not alter the theory, for now [ilmath]\text{Dim}(\emptyset)\eq -1[/ilmath] will do. It wont matter.
• [ilmath]\Delta^m[/ilmath] be the standard [ilmath]m[/ilmath]-simplex in [ilmath]\mathbb{R}^{m+1} [/ilmath]
• [ilmath]G(n,m)[/ilmath] - this is our goal, it's a collection of a bunch of maps of the form [ilmath]G:S_n(K)\rightarrow S_m(K)[/ilmath] {{Caveat|Notice the flip of [ilmath]n[/ilmath] and [ilmath]m[/ilmath]) with certain properties.
  • Our goal is to find a bijection, say [ilmath]F:I(m,n)\rightarrow G(n,m)[/ilmath]

First stab

• Let [ilmath]m,n\in\mathbb{N} [/ilmath] be given such that [ilmath]m\le n[/ilmath].
  • Let [ilmath]f\in I(m,n)[/ilmath] be given, so [ilmath]f:\#(m+1)\rightarrow\#(n+1)[/ilmath] is an injection and is monotonic - as per the definition of [ilmath]I(m,n)[/ilmath].
    • We associate [ilmath]f[/ilmath] with [ilmath]L_f:\mathbb{R}^{m+1}\rightarrow\mathbb{R}^{n+1} [/ilmath] which is a linear map defined by its action on a basis as [ilmath]L_f(e_i):\eq e_{f(i)} [/ilmath] where [ilmath]e_i\in\mathbb{R}^\text{whatever} [/ilmath] is a tuple that has [ilmath]0[/ilmath] in every entry except the [ilmath]i^\text{th} [/ilmath] which has [ilmath]1[/ilmath]; as usual.
      • It is fairly easy to see that [ilmath]\text{Ker}(M_f)\eq\{0\} [/ilmath], then by "a linear map is injective if and only if its kernel is trivial" and "the image of a linear map is a vector subspace of the codomain" wee see that:
        • [ilmath]L_f:\mathbb{R}^{m+1}\rightarrow L_f(\mathbb{R}^{m+1})[/ilmath] is a linear isomorphism
      • As [ilmath]\mathbb{R}^{m+1} [/ilmath] is finite dimensional we see that [ilmath]L_f[/ilmath] is a continuous map