Abstract: In this paper, we determine the lower central and derived series for the braid groups of the sphere. We are motivated in part by the study of Fadell-Neuwirth short exact sequences, but the problem is important in its own right.
The braid groups of the -sphere were studied by Fadell, Van Buskirk and Gillette during the 1960s, and are of particular interest due to the fact that they have torsion elements (which were characterised by Murasugi). We first prove that for all , the lower central series of the -string braid group is constant from the commutator subgroup onwards. We obtain a presentation of , from which we observe that is a semi-direct product of the quaternion group of order by a free group of rank . As for the derived series of , we show that for all , it is constant from the derived subgroup onwards. The group being finite and soluble for , the critical case is for which the derived subgroup is the above semi-direct product . By proving a general result concerning the structure of the derived subgroup of a semi-direct product, we are able to determine completely the derived series of which from onwards coincides with that of the free group of rank , as well as its successive derived series quotients.
1. E. Artin, Theorie der Zöpfe, Abh. Math. Sem. Univ. Hamburg 4 (1925), 47-72.
26. -, The classification and the conjugacy classes of the finite subgroups of the sphere braid groups, Algebraic and Geometric Topology 8 (2008), 757-785.
27. -, The lower central and derived series of the braid groups of the finitely-punctured sphere, to appear in J. Knot Theory and its Ramifications.
28. -, Classification of the virtually cyclic subgroups of the pure braid groups of the projective plane, preprint October 2007, arXiv:math.GT/0710.5940.
29. -, Classification of the virtually cyclic subgroups of the sphere braid groups, work in progress.
30. -, Classification of the finite and virtually cyclic subgroups of the braid groups of the projective plane, work in progress.
32. E. A. Gorin and V Ja. Lin, Algebraic equations with continuous coefficients and some problems of the algebraic theory of braids, Math. USSR Sbornik 7 (1969), 569-596.
E. Artin, Theorie der Zöpfe, Abh. Math. Sem. Univ. Hamburg 4 (1925), 47-72.
-, The classification and the conjugacy classes of the finite subgroups of the sphere braid groups, Algebraic and Geometric Topology 8 (2008), 757-785.
-, The lower central and derived series of the braid groups of the finitely-punctured sphere, to appear in J. Knot Theory and its Ramifications.
-, Classification of the virtually cyclic subgroups of the pure braid groups of the projective plane, preprint October 2007, arXiv:math.GT/0710.5940.
-, Classification of the virtually cyclic subgroups of the sphere braid groups, work in progress.
-, Classification of the finite and virtually cyclic subgroups of the braid groups of the projective plane, work in progress.
E. A. Gorin and V Ja. Lin, Algebraic equations with continuous coefficients and some problems of the algebraic theory of braids, Math. USSR Sbornik 7 (1969), 569-596.