Index set

In mathematics, an index set is a set whose members label (or index) members of another set.^[1][2] 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 $J\subset {\mathbb {N}}$, 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 ${\mathbf {1}}_{r}\colon {\mathbb {R}}\rightarrow \{0,1\}$ given by

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

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.^[3]

See also

• Friendly-index set
• Indexed family

References

1. Weisstein, Eric. "Index Set". Wolfram MathWorld. Wolfram Research. Retrieved 30 December 2013.
2. Munkres, James R. Topology. Vol. 2. Upper Saddle River: Prentice Hall, 2000.
3. Goldreich, Oded (2001). Foundations of Cryptography: Volume 1, Basic Tools. Cambridge University Press. ISBN 0-521-79172-3.