Finite depth and Jacobson-Bourbaki correspondence
Abstract
Description
We introduce a notion of depth three tower of three rings C < B < A with depth two ring extension A | B recovered when B = C. If A = \End B_C and B | C is a Frobenius extension, this captures the notion of depth three for a Frobenius extension in arXiv:math/0107064 and arXiv:math/0108067, such that if B | C is depth three, then A | C is depth two (a phenomenon of finite depth subfactors, see arXiv:math/0006057). We provide a similar definition of finite depth Frobenius extension with embedding theorem utilizing a depth three subtower of the Jones tower. If A, B and C correspond to a tower of subgroups G > H > K via the group algebra over a fixed base ring, the depth three condition is the condition that subgroup K has normal closure K^G contained in H. For a depth three tower of rings, there is a pre-Galois theory for the ring \End {}_BA_C and coring (A ø_B A)^C involving Morita context bimodules and left coideal subrings. This is applied in two sections to a specialization of a Jacobson-Bourbaki correspondence theorem for augmented rings to depth two extensions with depth three intermediate division rings.
26 pp., depth three towers with new section on finite depth, and corrections
26 pp., depth three towers with new section on finite depth, and corrections