A road is to be constructed between four towns situated in such a way they form square. So, what is the minimum length of path joining these towns? Given that length between two adjacent towns is 1 km , i.e. side of square is 1 km.
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This is the Steiner tree problem on the vertices of the square. It's well known that the answer is 1 + 3 times the side length of the square (the network can be seen on the Wikipedia page; the internal vertices have angles 1 2 0 ∘ everywhere).