A Corollary of Hamada and Ohmori's on Group Law over BIBD
  In this note, we present an interesting corollary of a theorem of Hamada and Ohmori. We prove that the complementary of PG(n,2) is the only design, up to an isomorphism, whose blocks form a group for the symmetric difference. 
Arnaud Bannier, Johann Barbier, Eric Filiol and Pierre Castel
math.CO