Minimum Cost Homomorphism to Oriented Cycles with Some Loops

Karimi, M. and Gupta, A.

    For digraphs D and H, a homomorphism of D to H is a mapping f : V(D)->V(H) such that uv in A(D) implies f(u)f(v) in A(H). Suppose D and H are two digraphs, and c_i(u), u in V (D), i in V (H), are nonnegative real costs. The cost of the homomorphism f of D to H is \Sigma_u in V (D) cf(u)(u). The minimum cost homomorphism for a fixed digraph H, denoted by MinHOM(H), asks whether or not an input digraph D, with nonnegative real costs ci(u), u in V(D), i in V(H), admits a homomorphism f to H and if it admits one, find a homomorphism of minimum cost. The minimum cost homomorphism problem seems to offer a natural and practical way to model many optimization problems such as list homomorphism problems, retraction and precolouring extension problems, chromatic partition optimization, and applied problems in repair analysis. Our interest is in proving a dichotomy for minimum cost homomorphism problem: we would like to prove that for each digraph H, MinHOM(H) is polynomial-time solvable, or NP-hard. We say that H is a digraph with some loops, if H has at least one loop. For reflexive digraphs H (every vertex has a loop) the complexity of MinHOM(H) is well understood. In this paper, we obtain a full dichotomy for MinHOM(H) when H is an oriented cycle with some loops. Furthermore, we show that this dichotomy is a remarkable progress toward a dichotomy for oriented graphs with some loops.