The Ideal Stratum and Deformation Persistence of Knot Types

Abstract: We introduce a persistent geometric framework for knot types based on normalized spaces of representatives. For a knot type $K$ and a scale parameter $Λ>0$, we consider the space [ Y_Λ(K)= R_{1,Λ}(K) ] of representatives of $K$ with thickness at least $1$ and length at most $Λ$, modulo orientation-preserving reparametrization and rigid motions. This space may be viewed as a normalized moduli-type space of unparametrized representatives of the knot type $K$, equipped with a ropelength sublevel filtration. We equip $Y_Λ(K)$ with an extended pseudometric defined by the infimum of swept areas of admissible deformations. This leads to a deformation-theoretic notion of admissible components and hence to a natural $0$-dimensional persistence module as $Λ$ increases. We show that the first birth time of this persistence is exactly the ropelength of the knot type. Accordingly, the layer [ I(K)=Y_{Rop(K)}(K) ] appearing at the first birth is distinguished; we call it the ideal stratum of $K$. From the moduli-theoretic viewpoint, the ideal stratum is the minimizer locus of the ropelength functional on the normalized representative space. In this way, ideal knots are interpreted not merely as minimizers of ropelength, but as the initial stratum of a persistent shape profile associated with the knot type. We also introduce ideal admissible components and ideal merge scales, which suggest further geometric invariants beyond ropelength.