I have a question
Covij=BiBjσm2 is the equition of market model in notes. I tried to find where it derive from.
since Bi= ρim*σi/σm, the equition can be Covij=ρim*σi*ρjm*σj. But this conflict with Covij=ρij*σi*σj.
Want to know why and where am I wrong. Thank you!