Motivated by physical and topological applications, we study representations of the group $mathcal{LB}_3$ of motions of $3$ unlinked oriented circles in $mathbb{R}^3$. Our point of view is to regard the three strand braid group $mathcal{B}_3$ as a subgroup of $mathcal{LB}_3$ and study the problem of extending $mathcal{B}_3$ representations. We introduce the notion of a emph{standard extension} and characterize $mathcal{B}_3$ representations admiting such an extension. In particular we show, using a classification result of Tuba and Wenzl, that every irreducible $mathcal{B}_3$ representation of dimension at most $5$ has a (standard) extension. We show that this result is sharp by exhibiting an irreducible $6$-dimensional $mathcal{B}_3$ representation that has no extensions (standard or otherwise). We obtain complete classifications of (1) irreducible $2$-dimensional $mathcal{LB}_3$ representations (2) extensions of irreducible $3$-dimensional $mathcal{B}_3$ representations and (3) irreducible $mathcal{LB}_3$ representations whose restriction to $mathcal{B}_3$ has abelian image.
تحميل البحث