I am reading the following paper:[on quantum information theory][1]
Does anybody understand the estimate at the bottom of page 7
∥ρ−ρn∥≤2∥(idA−Pn)⊗idRρ(Pn⊗idR)∥1+2rn?
I have troubles understanding what happened in this step. Can anybody explain this one to me?
[1]: https://arxiv.org/pdf/1706.00361.pdf