quality:correction
Operations on indexed families

Index sets are often used in sums and other similar operations. For example, if (a i) i ∈ I {\displaystyle \left(a_{i}\right)_{i\in I}} is an indexed family of numbers, the sum of all those numbers is denoted by

When (A i) i ∈ I {\displaystyle \left(A_{i}\right)_{i\in I}} is a family of sets, the union of all those sets is denoted by

Likewise for intersections and Cartesian products.