Index set

Not to be confused with Indexed set.

In mathematics, an index set is a set whose members label (or index) members of another set. For instance, if the elements of a set A may be indexed or labeled by means of a set J, then J is an index set. The indexing consists of a surjective function from J onto A and the indexed collection is typically called an (indexed) family, often written as (A_j)_j∈J.

Examples

• An enumeration of a set S gives an index set $J \sub \mathbb{N}$, where f : J → S is the particular enumeration of S.
• Any countably infinite set can be indexed by $\mathbb{N}$.
• For $r \in \mathbb{R}$, the indicator function on r is the function $\mathbf{1}_r\colon \mathbb{R} \rarr \{0,1\}$ given by

$\mathbf{1}_r (x) := \begin{cases} 0, & \mbox{if } x \ne r \\ 1, & \mbox{if } x = r. \end{cases}$

The set of all the $\mathbf{1}_r$ functions is an uncountable set indexed by $\mathbb{R}$.

Other uses

In computational complexity theory and cryptography, an index set is a set for which there exists an algorithm I that can sample the set efficiently; e.g., on input 1ⁿ, I can efficiently select a poly(n)-bit long element from the set.

See also

• Friendly-index set
• Indexed family