Two subspaces UUU and WWW of VVV are complementary if:
U∩W={0}U \cap W = \{0\}U∩W={0} and U+W=VU+W = VU+W=V
U∩W=VU \cap W = VU∩W=V
dim(U)+dim(W)=0\dim(U) + \dim(W) = 0dim(U)+dim(W)=0
U∪W=VU \cup W = VU∪W=V