The hypercentral structure of the group of unitriangular automorphisms of a polynomial algebra

We describe the hypercentral structure of the group of unitriangular automorphisms of the polynomial algebra $K[x_1,\dots,x_n]$ for $n\ge 3$ and an arbitrary field $K$. This group is sometimes called the Borel group. The description implies the nonlinearity for this group over any field.