I see this page Ordinary least square and random projection, and I am thinking that how L^2$L^2$ integrable random variables be regarded as projections over a defined filtration \mathcal{F_n}sequence $\mathcal{F_n}$ ?
It seems that a vector space of square integrable random variables defined on the same probability space is not enough? Is Is there existing literature addressing a formalism of this projection view? Thank you!