Braid Index Bounds Ropelength From Below

Abstract

For an un-oriented link $\mathcal{K}$, let $L(\mathcal{K})$ be the ropelength of $\mathcal{K}$. It is known that when $\mathcal{K}$ has more than one component, different orientations of the components of $\mathcal{K}$ may result in different braid index. We define the largest braid index among all braid indices corresponding to all possible orientation assignments of $\mathcal{K}$ the {\em absolute braid index} of $\mathcal{K}$ and denote it by $\textbf{B}(\mathcal{K})$. In this paper, we show that there exists a constant $a>0$ such that $L(\mathcal{K})\ge a \textbf{B}(\mathcal{K}) $ for any $\mathcal{K}$, {\em i.e.}, the ropelength of any link is bounded below by its absolute braid index (up to a constant factor).