In [Matsusaka, T. Polarized varieties, fields of moduli and generalized Kummer varieties of polarized abelian varieties. Amer. J. Math. 80 1958 45--82.] it is proved that a non-singular projective variety has a maximal algebraic group of automorphisms (that is, every group which acts on the variety by automorphisms contains a maximal algebraic subgroup)

In [Matsusaka, T. Polarized varieties, fields of moduli and generalized Kummer varieties of polarized abelian varieties. Amer. J. Math. 80 1958 45--82.] it is proved that a non-singular projective variety has a maximal algebraic group of automorphisms (that is, every group which acts on the variety by automorphisms contains a maximal algebraic subgroup)

[Matsumura, Hideyuki; Oort, Frans. Representability of group functors, and automorphisms of algebraic schemes. Invent. Math. 4 1967 1--25.] shows the automorphism group (of an algebraic proper scheme over a field) is representable by a group scheme.