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Nov 27, 2021 at 15:27 answer added Manfred Weis timeline score: 0
Nov 27, 2021 at 14:44 comment added Manfred Weis Of course the inapproximability for $\lbrace 1, 2\rbrace$ edge weights is plausible, but there are also situations where the edgeweights allow for good approximation ratios, most prominently the planar Euclidean TSP and I am looking for heursitics for the problem stated as the above question that work well in specific situations.
Nov 22, 2021 at 20:30 comment added Manuel Lafond I'd conjecture that no good approx guarantee can be achieved. In an unweighted graph, deciding if there is a $\{1, 2\}$-spanning tree is NP-hard (that's the Hamiltonian path problem). This is used to show that finding a min-weight $\{1, 2\}$-spanning tree in a complete graph, i.e. TSP, can't be approximated by essentially any ratio (by replacing non-edges with edges of huge weight). The same ideas should apply to $\{1, 3\}$-spanning trees. It's probably NP-hard to find one in unweighted graphs, making the weighted complete version inapproximable. Requires proof though.
Nov 22, 2021 at 16:09 history asked Manfred Weis CC BY-SA 4.0