Thesiss

For digraphs D and H, a homomorphism of D to H is a mapping f : V (D)→V( H) such that uv ∈ A( D) implies f(u)f( v) ∈ A(H). Suppose D and H are two digraphs, and ci( u), u ∈ V (D), i ∈ V(H), are nonnegative integer costs. The cost of the homomorphism f of D to H is ∑u∈V (D) cf(u)…