Density and Compression on Lattice-Knot Merge Trees

Abstract

Ropelength is usually studied as a minimization problem for one knot at a time. We instead use ropelength to filter the space of all realizations of a knot type. For a knot type $K$ and a budget $\Lambda$, let $Y_\Lambda(K)$ be the space of unit-thickness $C^{1,1}$ configurations of length at most $\Lambda$, modulo rigid motions and constant-speed reparametrization. We prove compact capture for smooth knot families, yielding homotopical and homological exhaustion of the ordinary knot space by finite ropelength levels. In the normalized Euclidean model we prove compactness of every $Y_\Lambda(K)$, right continuity of connected components, and attainment of connected merge levels; on components of the ideal stratum these levels define an ultrametric. We also prove that $Y_L(K)$ strongly deformation retracts onto its exact-length shell. This separates an attained connected merge scale from the path merge scale, the genuine min-max quantity for constrained deformation paths. For higher homotopy we introduce ropelength widths and relate them to known models of spaces of knots. We record mirror symmetry and the complete minimal-layer BFACF merge trees of $4_1$ and $6_3$ as certified discrete benchmarks, without identifying discrete and continuum barriers. Finally, we relate the path filtration to the companion swept-area construction: at fixed ropelength its infimal trace area defines a genuine extended metric on the corresponding moduli space, with flat-current and projected-area lower bounds. The paper also formulates open problems concerning Gordian phenomena, regularization, and quantitative topology of thick-knot spaces.