Searching for a line on the plane with n unit speed robots is a classic
online problem that dates back to the 50's, and for which competitive ratio
upper bounds are known for every n≥1. In this work we improve the best
lower bound known for n=2 robots from 1.5993 to 3. Moreover we prove that the
competitive ratio is at least 3 for n=3 robots, and at least
1/cos(π/n) for n≥4 robots. Our lower bounds match the best upper
bounds known for n≥4, hence resolving these cases. To the best of our
knowledge, these are the first lower bounds proven for the cases n≥3 of
this several decades old problem.Comment: This is an updated version of the paper with the same title which
will appear in the proceedings of the 23rd International Conference on
Principles of Distributed Systems (OPODIS 2019) Neuchatel, Switzerland, July
17-19, 201