4.3 Optimal Spanning Tree Our off-line multi-robot algorithms as well as the single-robot version in [9] use a spanning tree to create a circular path which completely cover the area. When building this spanning tree in a single robot system, the influence of the structure of the tree is almost irrelevant for the coverage time. This results from the fact that coverage time is linear in the size of the grid, since each cell except for the boundary cells is covered once, hence the total coverage time is n (the number of sub-cells). The structure of the tree may only affect efficiency due to the number of turns it requires, and other similar issues. On the other hand, in our multi-robot systems, the structure of the tree can have crucial consequences on the coverage time of the terrain. The choice of the spanning tree can change the robots’ initial positions from being concentrated, i.e., placed as a bundle, to being scattered along the spanning tree path - all without actually changing the physical initial position of the robots. That is mean that if the tree is appropriately built, the structure of the tree itself can substantially decrease the coverage time obtained by algorithms based upon it.
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