Let I and A be classes, F a functional relation between I and A. Then F is sometimes called a family of elements of A indexed by I (or with I as index class) and we write (F(i))i∈I instead of F. In particular, if E is a set then a family of elements of P(E) is called a family of subsets of E indexed by I. If F is such a family and we write X i = F(i) for each element i in I then we denote the family F by (X i )i∈I.
KeywordsMathematical Logic Identity Mapping Unique Mapping Pairwise Disjoint Projection Mapping
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