A minimum spanning tree is a feasible but not usually optimal solution to the Steiner tree problem. The Steiner ratio is the largest possible ratio between the total length of a minimum spanning tree and the total length of a minimum Steiner tree.
In the metric Steiner tree problems, the Steiner ratio is 2. Therefore an algorithm that finds a minimum spanning tree is a polynomial-time factor-2 approximation algorithm for the metric Steiner tree problem.
In the Euclidean Steiner tree problem, the Steiner ratio is conjectured to be . Despite earlier claims of a proof, the conjecture is still open.
Read more about this topic: Steiner Tree Problem
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