# Projection (set theory)

In set theory, a projection is one of two closely related types of functions or operations, namely:

• A set-theoretic operation typified by the jth projection map, written ${\displaystyle \mathrm {proj} _{j}}$, that takes an element ${\displaystyle {\vec {x}}=(x_{1},\ \ldots ,\ x_{j},\ \ldots ,\ x_{k})}$ of the Cartesian product ${\displaystyle (X_{1}\times \cdots \times X_{j}\times \cdots \times X_{k})}$ to the value ${\displaystyle \mathrm {proj} _{j}({\vec {x}})=x_{j}}$.[1]
• A function that sends an element x to its equivalence class under a specified equivalence relation E,[2] or, equivalently, a surjection from a set to another set.[3] The function from elements to equivalence classes is a surjection, and every surjection corresponds to an equivalence relation under which two elements are equivalent when they have the same image. The result of the mapping is written as [x] when E is understood, or written as [x]E when it is necessary to make E explicit.