The Algebraic Structure of n-Punctured Ball Tangles

Abstract

We consider a class of topological objects in the 3-sphere $S^3$ which will be called $n$-punctured ball tangles. Using the Kauffman bracket at $A=e^{i π/4}$, an invariant for a special type of $n$-punctured ball tangles is defined. The invariant $F^n$ takes values in $PM_{2\times2^n}(\mathbb Z)$, that is the set of $2\times 2^n$ matrices over $\mathbb Z$ modulo the scalar multiplication of $\pm1$. This invariant leads to a generalization of a theorem of D. Krebes which gives a necessary condition for a given collection of tangles to be embedded in a link in $S^3$ disjointly. Furthermore, we provide the general formula to compute the invariant of the $k_1 + ... + k_n$-punctured ball tangle determined by $n,k_1,...,k_n$-punctured ball tangles, respectively. Also, we consider various connect sums among punctured ball tangles and provide the formulas to compute their invariants. We also address the question of whether the invariant $F^n$ is surjective onto $PM_{2\times2^n}(\mathbb Z)$. We will show that the invariant $F^n$ is surjective when $n=0$. When $n=1$, $n$-punctured ball tangles will be also called spherical tangles. We show that ${\rm det} F^1(S)\equiv 0$ or 1 {\rm mod} 4 for every spherical tangle $S$. Thus, $F^n$ is not surjective when $n=1$. In addition, we introduce monoid structures on the class of 0-punctured ball tangles and the class of spherical tangles and show that the group generated by the elementary operations on $PM_{2\times2}(\mathbb Z)$ induced by those on the spherical tangles is isomorphic to a Coxeter group.