What to fix the tranformation using AMCL?

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Hi guys!

I was following this tutorial ( https://www.fer.unizg.hr/_download/re... ) and something happened that I can't explain. When we use this code

rosrun amcl amcl _use_map_topic:=true scan:=/robot0/laser_0 _odom_frame_id:=/world _base_frame_id:=/robot0 _global_frame_id:=/world

shows one error but works.

I wanna know if someone knows how fix this error about do the transformation from laser→base_link and from base_link→odometry. And i wanna know if someone knows why we use _odom_frame_id:=/world.

The error is:

[ERROR] [1509645225.800510786, 658.227000000]: Ignoring transform from authority "unknown_publisher" with frame_id and child_frame_id  "world" because they are the same

Thanks all!