Question

The sign is used to express projection operation?​

Answer 1

In linear algebra and functional analysis, a projection is a linear transformation {\displaystyle P} from a vector space to itself such that {\displaystyle P^{2}=P}. That is, whenever {\displaystyle P} is applied twice to any value, it gives the same result as if it were applied once (idempotent). It leaves its image unchanged.[1] Though abstract, this definition of "projection" formalizes and generalizes the idea of graphical projection. One can also consider the effect of a projection on a geometrical object by examining the effect of the projection on points in the object.