Projection (set theory)
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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
, that takes an element 
of the cartesian product 
to the value 
.
- A function that sends an element x to its equivalence class under a specified equivalence relation E. 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.
