Braids and Permutations

Abstract

E. Artin described all irreducible representations of the braid group B_k to the symmetric group S(k). We strengthen some of his results and, moreover, exhibit a complete picture of homomorphisms of B_k to S(n) for n<2k+1. We show that the image of such ahomomorphism f is cyclic whenever either (*) n<k\ne 4 or (**) f is irreducible and 6<k<n<2k. For k>6 there exist, up to conjugation, exactly 3 irreducible representations of B_k into S(2k) with non-cyclic images but they all are imprimitive. We use these results to prove that for n<k\ne 4 the image of any homomorphism from B_k to B_n is cyclic, whereas any endomorphism of B_k with non-cyclic image preserves the pure braid group PB_k. We prove also that for k>4 the intersection PB_k\cap B'_k of PB_k with the commutator subgroup B'_k=[B_k,B_k] is a completely characteristic subgroup of B'_k.