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The maximum four point condition matrix of a tree

Combinatorics 2023-08-17 v1 Discrete Mathematics

Abstract

\newcommand{\Max}{\mathrm{Max4PC}} The Four point condition (4PC henceforth) is a well known condition characterising distances in trees TT. Let w,x,y,zw,x,y,z be four vertices in TT and let dx,yd_{x,y} denote the distance between vertices x,yx,y in TT. The 4PC condition says that among the three terms dw,x+dy,zd_{w,x} + d_{y,z}, dw,y+dx,zd_{w,y} + d_{x,z} and dw,z+dx,yd_{w,z} + d_{x,y} the maximum value equals the second maximum value. We define an (n2)×(n2)\binom{n}{2} \times \binom{n}{2} sized matrix \MaxT\Max_T from a tree TT where the rows and columns are indexed by size-2 subsets. The entry of \MaxT\Max_T corresponding to the row indexed by {w,x}\{w,x\} and column {y,z}\{y,z\} is the maximum value among the three terms dw,x+dy,zd_{w,x} + d_{y,z}, dw,y+dx,zd_{w,y} + d_{x,z} and dw,z+dx,yd_{w,z} + d_{x,y}. In this work, we determine basic properties of this matrix like rank, give an algorithm that outputs a family of bases, and find the determinant of \MaxT\Max_T when restricted to our basis. We further determine the inertia and the Smith Normal Form (SNF) of \MaxT\Max_T.

Keywords

Cite

@article{arxiv.2308.08237,
  title  = {The maximum four point condition matrix of a tree},
  author = {Ali Azimi and Rakesh Jana and Mukesh Kumar Nagar and Sivaramakrishnan Sivasubramanian},
  journal= {arXiv preprint arXiv:2308.08237},
  year   = {2023}
}
R2 v1 2026-06-28T11:56:50.564Z