Bracket width of current Lie algebras

Abstract

The length of an element $z$ of a Lie algebra $L$ is defined as the smallest number $s$ needed to represent $z$ as a sum of $s$ brackets. The bracket width of $L$ is defined as supremum of the lengths of its elements. Given a finite-dimensional simple Lie algebra $\mathfrak g$ over an algebraically closed field $k$ of characteristic zero, we study the bracket width of current Lie algebras $L=\mathfrak g\otimes A$. We show that for an arbitrary $A$ the width is at most 2. For $A=k[[t]]$ and $A=k[t]$ we compute the width for algebras of types A and C.