# Definition:Indexing Set/Family of Subsets

## Contents

## Definition

Let $S$ be a set.

Let $I$ be an indexing set.

For each $i \in I$, let $S_i$ be a corresponding subset of $S$.

Let $\left \langle{S_i}\right \rangle_{i \mathop \in I}$ be a family of subsets of $S$ indexed by $I$.

Then $\left \langle{S_i}\right \rangle_{i \mathop \in I}$ is referred to as an **indexed family of subsets (of $S$ by $I$)**.

## Also defined as

As the set of subsets of $S$ is the power set $\mathcal P \left({S}\right)$ of $S$, $\left \langle{S_i}\right \rangle_{i \mathop \in I}$ can also be defined as a mapping from some indexing set $I$ into the power set $\mathcal P \left({S}\right)$ of $S$.

## Also known as

It is common to drop the word *indexed* and refer merely to a **family of subsets**.

### Notation

The family of elements $x$ of $S$ indexed by $I$ is often seen with one of the following notations:

- $\left \langle {x_i} \right \rangle_{i \mathop \in I}$

- $\left({x_i}\right)_{i \mathop \in I}$

- $\left\{{x_i}\right\}_{i \mathop \in I}$

There is little consistency in the literature, but $\left({x_i}\right)_{i \mathop \in I}$ is perhaps most common.

The subscripted $i \in I$ is often left out, if it is obvious in the particular context.

## Note on Terminology

It is a common approach to blur the distinction between an indexing function $x: I \to S$ and the indexed family $\left\langle{x_i}\right\rangle_{i \mathop \in I}$ itself, and refer to the mapping as the indexed family.

This approach is in accordance with the definition of a mapping as a relation defined as an as ordered triple $\left({I, S, x}\right)$, where the mapping is understood as being *defined* to include its domain and codomain.

However, on $\mathsf{Pr} \infty \mathsf{fWiki}$ the approach is taken to separate the concepts carefully such that an indexed family is defined as:

- the set of terms of the indexed set

together with:

- the indexing function itself

denoting the combination as $\left\langle{x_i}\right\rangle_{i \mathop \in I}$.

The various approaches in the literature can be exemplified as follows.

*There are occasions when the range of a function is deemed to be more important than the function itself. When that is the case, both the terminology and the notation undergo radical alterations. Suppose, for instance, that $x$ is a function from a set $I$ to a set $X$. ... An element of the domain $I$ is called an***index**, $I$ is called the**index set**, the range of the function is called an**indexed set**, the function itself is called a**family**, and the value of the function $x$ at an index $i$, called a**term**of the family, is denoted by $x_i$. (This terminology is not absolutely established, but it is one of the standard choices among related slight variants...) An unacceptable but generally accepted way of communicating the notation and indicating the emphasis is to speak of a family $\left\{{x_i}\right\}$ in $X$, or of a family $\left\{{x_i}\right\}$ of whatever the elements of $X$ may be; when necessary, the index set $I$ is indicated by some such parenthetical expression as $\left({i \in I}\right)$. Thus, for instance, the phrase "a family $\left\{{A_i}\right\}$ of subsets of $X$" is usually understood to refer to a function $A$, from some set $I$ of indices, into $\mathcal P \left({X}\right)$.- 1960: Paul R. Halmos:
*Naive Set Theory*: $\S 9$: Families

- 1960: Paul R. Halmos:

*Occasionally, the special notation for sequences is also employed for functions that are not sequences. If $f$ is a function from $A$ into $E$, some letter or symbol is chosen, say "$x$", and $f \left({\alpha}\right)$ is denoted by $x_\alpha$ and $f$ itself by $\left({x_\alpha}\right)_{\alpha \mathop \in A}$. When this notation is used, the domain $A$ of $f$ is called the set of***indices**of $\left({x_\alpha}\right)_{\alpha \mathop \in A}$, and $\left({x_\alpha}\right)_{\alpha \mathop \in A}$ is called a**family of elements of $E$ indexed by $A$**instead of a function from $A$ into $E$.- 1965: Seth Warner:
*Modern Algebra*: $\S 18$: The Natural Numbers

- 1965: Seth Warner:

*Let $I$ and $E$ be sets and let $f: I \to E$ be a mapping, described by $i \mapsto f \left({i}\right)$ for each $i \in I$. We often find it convenient to write $x_i$ instead of $f \left({i}\right)$ and write the mapping as $\left({x_i}\right)_{i \mathop \in I}$ which we shall call a***family of elements of $E$ indexed by $I$**. By abuse of language we refer to the $x_i$ as the**elements of the family**.*...**As we have already mentioned, many authors identify a mapping with its graph, thereby identifying the family $\left({x_i}\right)_{i \mathop \in I}$ with the set $\left\{{\left({i, x_i}\right); i \in I}\right\}$. In the case where the elements of the family are all distinct, some authors go even further and identify the mapping $\left({x_i}\right)_{i \mathop \in I}$ with its image $\left\{{x_i; i \in I}\right\}$.*- 1975: T.S. Blyth:
*Set Theory and Abstract Algebra*: $\S 6$. Indexed families; partitions; equivalence relations

- 1975: T.S. Blyth:

Some authors are specific about the types of objects to which this construction is applied:

*Let $\mathcal A$ be a nonempty collection of sets. An***indexing function**for $\mathcal A$ is a surjective function $f$ from some set $J$, called the**index set**, to $\mathcal A$. The collection $\mathcal A$, together with the indexing function $f$, is called an**indexed family of sets**. Given $\alpha \in J$, we shall denote the set $f \left({\alpha}\right)$ by the symbol $\mathcal A_\alpha$. And we shall denote the indexed family itself by the symbol $\left\{{\mathcal A_\alpha}\right\}_{\alpha \mathop \in J}$, which is read as "the family of all $\mathcal A_\alpha$, as $\alpha$ ranges over $J$."- 2000: James R. Munkres:
*Topology*(2nd ed.): $\S 5$: Cartesian Products

- 2000: James R. Munkres:

## Sources

- 1962: Bert Mendelson:
*Introduction to Topology*... (previous) ... (next): $\S 1.4$: Indexed Families of Sets - 1975: T.S. Blyth:
*Set Theory and Abstract Algebra*... (previous) ... (next): $\S 6$