  Let G be a semisimple group over an algebraically closed field of very good
characteristic for G. In the context of geometric invariant theory, G. Kempf
has associated optimal cocharacters of G to an unstable vector in a linear
G-representation. If the nilpotent element X in Lie(G) lies in the image of the
differential of a homomorphism SL(2) --> G, we say that homomorphism is optimal
for X, or simply optimal, provided that its restriction to a suitable torus of
SL(2) is optimal for X in Kempf's sense.
  We show here that any two SL(2)-homomorphisms which are optimal for X are
conjugate under the connected centralizer of X. This implies, for example, that
there is a unique conjugacy class of principal homomorphisms for G. We show
that the image of an optimal SL(2)-homomorphism is a completely reducible
subgroup of G; this is a notion defined recently by J-P. Serre. Finally, if G
is defined over the (arbitrary) subfield K of k, and if X in Lie(G)(K) is a
K-rational nilpotent element whose p-th power is 0, we show that there is an
optimal homomorphism for X which is defined over K.
