Total curvature and isotopy of graphs in $R^3$

Abstract

Knot theory is the study of isotopy classes of embeddings of the circle $S^1$ into a 3-manifold, specifically $R^3$. The Fáry-Milnor Theorem says that any curve in $R^3$ of total curvature less than $4π$ is unknotted. More generally, a (finite) graph consists of a finite number of edges and vertices. Given a topological type of graphs $Γ$, what limitations on the isotopy class of $Γ$ are implied by a bound on total curvature? What does ``total curvature" mean for a graph? We define a natural notion of net total curvature of a graph $Γ$ in $R^3$, and prove that if $Γ$ is homeomorphic to the $θ$-graph, then the net total curvature of $Γ$ \geq 3π$; and if it is $< 4π$, then $Γ$ is isotopic in $R^3$ to a planar $θ$-graph. Further, the net total curvature $= 3π$ only when $Γ$ is a convex plane curve plus a chord. We begin our discussion with piecewise smooth graphs, and extend all these results to continuous graphs in the final section. In particular, we show that continuous graphs of finite total curvature are isotopic to polygonal graphs.