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The base_link of the robot should be defined in /odom frame. And /odom should be defined in either world/map. So if using a standard mobile platform like turtlebot or Pioneer the frame /odom is published itself. Like you said for using multiple robots you have to tf_prefix. And all the robots should be defined w.r.t frame which is common between them like map or world.

Try using tf_echo between world and base_link to confirm whether tf is getting published or not.

Hope this helps..