Index set

In mathematics, an index set I is another name for a function domain. A collection indexed by I, often written A_i for i in I (can be said 'for i running over I) is in effect a function A(i) into some codomain.

Index sets are often used in sums (sigma notation) and other such operations; and are common when the A_i are themselves sets rather than numbers, in indexed intersections and unions.

Nore generally, a functor can be considered as giving rise to an indexed family of objects in a category D, indexed by another category C, and related by morphisms depending on two indices.