Aspects of area formulas by way of Luzin, Rad\'o, and Reichelderfer on   metric measure spaces
  We consider some measure-theoretic properties of functions belonging to a Sobolev-type class on metric measure spaces that admit a Poincar\'e inequality and are equipped with a doubling measure. The properties we have selected to study are those that are related to area formulas. 
Niko Marola and William P. Ziemer