Niltriangular subalgebras of the Chevalley algebras and their generalizations

We study some problems concerned with ideals and automorphisms of niltriangular subalgebras of classical Lie type Chevalley algebras over a field $K$ and of their non-finitary generalizations and also automorphisms of adjoint group. We characterize (for Lie type $A_{n-1})$ every Lie ideal of algebra $NT(n,K)$ of all niltriangular $n\times n$ matrices by a selection of constants from $K$. When $K=GF(q)$, this gives a combinatorial expression of number of Lie ideals and, for a simple $q$, also the number of normal subgroups in unitriangular group $UT(n,q)$.