  A linear algebraic group G defined over a field k is called special if every
G-torsor over every field extension of k is trivial. In 1958 Grothendieck
classified special groups in the case where the base field is algebraically
closed. In this paper we describe the derived subgroup and the coradical of a
special reductive group over an arbitrary field k. We also classify special
semisimple groups, special reductive groups of inner type and special
quasisplit reductive groups over an arbitrary field k.
