Local mean dimension theory for sofic group actions

Using a local perspective, we introduce mean dimension pairs and give sufficient conditions of when every non-trivial factor of a continuous group action of a sofic group G has positive mean dimension. In addition, we prove that the mean dimension map is Borel, and that the set of subshifts with completely positive mean dimension of [0,1]^G, the full G-shift on the interval, is a complete coanalytic set in the set of all subshifts (hence not Borel). Our results are new even when the acting group is ℤ.