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Theorem 35.2

For TSP with triangle inequality, Approx-TSP-Tour is an approximation algorithm with a ratio bound of 2.

If H is approximate tour, c(H) $\leq$ 2c(H*)


Removing an edge from a tour yields a spanning tree. Thus, if H* is the optimal tour and T is a MST(G), then c(T) $\leq$ c(H*).

Consider a full walk W of a MST with cost c(W).



W = a b d b e b a c a
c(W) = 2c(T) $\longrightarrow$ c(W) $\leq$ 2c(H*)

W is not a tour, but by triangle inequality, we can change w $\rightarrow$ x $\rightarrow$ w $\rightarrow$ y to w $\rightarrow$ x $\rightarrow$ y, without increasing cost to yield approximate tour H. Or, use the pre-order traversal of the MST.

c(H) $\leq$ c(W) $\leq$ 2c(H*)

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