Automorphisms and normal structure of unipotent subgroups of finitary Chevalley groups

The description of the automorphisms of an unipotent subgroup $U$ of a Chevalley group over a field $K$ known earlier for $\operatorname{char}K\ne2,3$ (Gibbs, 1970) was completed in 1990 together with a solution of problem (1.5) from A. S. Kondrat’ev's survey (Usp. Mat. Nauk, 1986). In the present paper, $\operatorname{Aut}U$ is described for the case of finitary Chevalley groups. For a Chevalley group of classical type, it is proved that any large Abelian subgroup from $U$ is conjugate to a normal subgroup in $U$. It is shown that this is not so in the general case; therefore, problem (1.6) from Kondrat'ev's survey about large Abelian subgroups in $U$ is reduced to listing the exceptions. Large Abelian normal subgroups were listed by the authors earlier.