Quasi-nearly subharmonic functions and quasiconformal mappings
  We prove that the composition of a quasi-nearly subharmonic function and a quasiregular mappings of bounded multiplicity is quasi-nearly subharmonic. Also, we prove that if $u\circ f$ is quasi-nearly subharmonic for all quasi-nearly subharmonic $u$ and $f$ satisfies some additional conditions, then $f$ is quasiconformal. Similar results are further established for the class of regularly oscillating functions. 
Pekka Koskela and Vesna Manojlovi\'c
math.FA