1. X is the n-dimensional projective space and G=PGL(n+1,k). This action is always doubly transitive and triply transitive for n=1.
2. X is the n-dimensional affine space and G=S L T, where S is a subgroup of the group of scalars, T is the group of translations, and L is either SL_n(k), Sp_n(k) (n even) or G_2 (n=6, char k=2) acting linearly. All these actions are doubly transitive.
This paper was my Diploma thesis in Erlangen.
Appeared in: Archiv der Mathematik 41 (1983) 438-446
Available files:
Scanned off-print
Original article (Springer-Verlag)