
Randomized and Approximate Algorithms 547
Approximation of TSP without triangle inequality
Lemma 4 Unless NP = P, there is no polynomial-time algorithm for general TSP with ratio ρ ≥ 1.
Proof The proof is by contradiction. Suppose that A is a polynomial-time algorithm that approximates
TSP for some ρ ≥ 1. Then A can be used to solve the HAM-CYCLE problem in polynomial-time, as
follows. Let G = (E, V) be an instance of Hamiltonian cycle. Let G′ = (V, E′) be a complete graph with costs
c(u, v) = 1 if (u, v) ∈ E and ρ|V | + 1 otherwise. Thus, if G has a Hamiltonian-cycle, then G′ has a tour of cost
|V |. Otherwise, any tour of G′ has cost