Speaker: Nivedita Bhaskhar
Date: December 20, 2018
Affiliation: UCLA, USA

Abstract: Any central simple algebra $A$ over a field $K$ is a form of a matrix algebra. Further $A/K$ comes equipped with a reduced norm map which is obtained by twisting the determinant function. Every element in the commutator subgroup $[A*, A*]$ has reduced norm 1 and hence lies in SL$_1(A)$, the group of reduced norm one elements of $A$. Whether the reverse inclusion holds was formulated as a question in 1943 by Tannaka and Artin in terms of the triviality of the reduced Whitehead group SK$_1(A) := \mathrm{SL}_1(A)/[A*,A*]$. <p> One can define Whitehead groups more generally for any isotropic group and it turns out that the Tannaka-Artin question is a special case of the well-known Kneser-Tits conjecture. The Whitehead group detects the non-rationality of the underlying variety of the algebraic group and therefore is an interesting albeit difficult invariant to study. In this talk, we discuss these connections to rationality questions and trace the progress towards answering the Tannaka-Artin question, which becomes especially interesting over low cohomological dimension fields.