Definition:Set
Contents
Definition
A set is intuitively defined as any aggregation of objects, called elements, which can be precisely defined in some way or other.
We can think of each set as a single entity in itself, and we can denote it (and usually do) by means of a single symbol.
That is, anything you care to think of can be a set. This concept is known as the comprehension principle.
However, there are problems with the comprehension principle. If we allow it to be used without any restrictions at all, paradoxes arise, the most famous example probably being Russell's Paradox.
Hence some sources define a set as a 'well-defined' collection of objects, leaving the concept of what constitutes well-definition to later in the exposition.
Defining a Set
The elements in a set $S$ are the things that define what $S$ is.
If $S$ is a set, and $a$ is one of the objects in it, we say that $a$ is an element (or member) of $S$, or that $a$ belongs to $S$, or $a$ is in $S$, and we write $a \in S$.
If $a$ is not one of the elements of $S$, then we can write $a \notin S$ and say $a$ is not in $S$.
Thus a set $S$ can be considered as dividing the universe into two parts:
- all the things that belong to $S$
- all the things that do not belong to $S$.
Explicit Definition
A (finite) set can be defined by explicitly specifying all of its elements between the famous curly brackets, known as set braces: $\set {}$.
When a set is defined like this, note that all and only the elements in it are listed.
This is called explicit (set) definition.
It is possible for a set to contain other sets. For example:
- $S = \set {a, \set a }$
If there are many elements in a set, then it becomes tedious and impractical to list them all in one big long explicit definition. Fortunately, however, there are other techniques for listing sets.
Implicit Definition
If the elements in a set have an obvious pattern to them, we can define the set implicitly by using an ellipsis ($\ldots$).
For example, suppose $S = \set {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}$.
A more compact way of defining this set is:
- $S = \set {1, 2, \ldots, 10}$
With this notation we are asked to suppose that the numbers count up uniformly, and we can read this definition as:
- $S$ is the set containing $1$, $2$, and so on, up to $10$.
Explicit and implicit definition are collectively referred to as roster notation.
Definition by Predicate
An object can be specified by means of a predicate, that is, in terms of a property (or properties) that it possesses.
Whether an object $x$ possesses a particular property $P$ is either true or false (in Aristotelian logic) and so can be the subject of a propositional function $\map P x$.
Hence a set can be specified by means of such a propositional function:
- $S = \set {x: \map P x}$
which means:
or, more formally:
In this context, we see that the symbol $:$ is interpreted as such that.
Warning
It is important to distinguish between an element, for example $a$, and a singleton containing it, that is, $\set a$.
That is $a$ and $\set a$ are not the same thing.
While it is true that:
- $a \in \set a$
it is not true that:
- $a = \set a$
neither is it true that:
- $a \in a$
Uniqueness of Elements
A set is uniquely determined by its elements.
This means that the only thing that defines what a set is is what it contains.
So, how you choose to list or define the contents makes no difference to what the contents actually are.
Also known as
In the original translation by Jourdain of Georg Cantor's original work, this concept was called an aggregate. The term can be seen in subsequent works, but has now mostly been superseded by the term set.
Sometimes the terms class, family or collection are used. In some contexts, the term space is used. However, beware that these terms are usually used for more specific things than just as a synonym for set.
On this website, the terms class, family and space are not used as synonyms for set, being reserved specifically for the concepts to which they apply.
Point Set
A set whose elements are all (geometric) points is often called a point set.
In particular, the Cartesian coordinate plane and complex plane can each be seen referred to as a two-dimensional point set.
Examples
Set of Living People
Let $P$ denote the set of living people.
Then:
\(\displaystyle \text {The person reading this web page}\) | \(\in\) | \(\displaystyle P\) | |||||||||||
\(\displaystyle \text {Julius Caesar}\) | \(\notin\) | \(\displaystyle P\) | |||||||||||
\(\displaystyle -4\) | \(\notin\) | \(\displaystyle P\) |
Positive Integers Less than 10
The (strictly) positive integers less than $10$ form a set:
- $\set {1, 2, 3, 4, 5, 6, 7, 8, 9}$
Also see
- Results about set theory can be found here.
Historical Note
The concept of a set first appears in Bernhard Bolzano's posthumous ($1851$) work Paradoxien des Unendlichen (The Paradoxes of the Infinite).
The first investigation into the concept in any depth was made by Georg Cantor in his two papers called Beiträge zur Begründung der transfiniten Mengenlehre ($1895$ and $1897$).
It was Georg Cantor who, in $1874$, defined a set as being:
- a Many that allows itself to be thought of as a One.
- -- Georg Cantor, A. Fraenkel and E. Zermelo, Gesammelte Abhandlungen (Berlin: Springer-Verlag, $1932$)
This definition was directly inspired by a problem raised by Bernhard Riemann in his paper Ueber die Darstellbarkeit einer Function durch eine trigonometrische Reihe of $1854$, on the subject of Fourier series.
Internationalization
Set is translated:
In French: | ensemble | |||
In German: | Menge | (literally: aggregate) |
Sources
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- 1951: J.C. Burkill: The Lebesgue Integral ... (next): Chapter $\text {I}$: Sets of Points
- 1951: Nathan Jacobson: Lectures in Abstract Algebra: Volume $\text { I }$: Basic Concepts ... (next): Introduction $\S 1$: Operations on Sets
- 1955: John L. Kelley: General Topology ... (previous) ... (next): Chapter $0$: Preliminaries: Sets
- 1959: E.M. Patterson: Topology (2nd ed.) ... (previous) ... (next): Chapter $\text {II}$: Topological Spaces: $\S 8$. Notations and definitions of set theory
- 1960: Paul R. Halmos: Naive Set Theory ... (previous) ... (next): $\S 1$: The Axiom of Extension
- 1963: Morris Tenenbaum and Harry Pollard: Ordinary Differential Equations ... (previous) ... (next): Chapter $1$: Basic Concepts: Lesson $2 \text A$: The Meaning of the Term Set
- 1964: W.E. Deskins: Abstract Algebra ... (next): Chapter $1$: A Common Language: $\S 1.1$ Sets
- 1964: Steven A. Gaal: Point Set Topology ... (previous) ... (next): Introduction to Set Theory: $1$. Elementary Operations on Sets
- 1964: Walter Rudin: Principles of Mathematical Analysis (2nd ed.) ... (previous) ... (next): Chapter $1$: The Real and Complex Number Systems: Introduction: $1.3$. Notation
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- 1966: Richard A. Dean: Elements of Abstract Algebra ... (previous) ... (next): $\S 0.2$
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- 1968: Ian D. Macdonald: The Theory of Groups ... (next): Appendix: Elementary set and number theory
- 1970: Avner Friedman: Foundations of Modern Analysis ... (next): $\S 1.1$: Rings and Algebras
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- 1975: T.S. Blyth: Set Theory and Abstract Algebra ... (previous) ... (next): $\S 1$. Sets; inclusion; intersection; union; complementation; number systems
- 1975: Bert Mendelson: Introduction to Topology (3rd ed.) ... (previous) ... (next): Chapter $1$: Theory of Sets: $\S 2$: Sets and Subsets
- 1975: W.A. Sutherland: Introduction to Metric and Topological Spaces ... (previous) ... (next): Notation and Terminology
- 1977: K.G. Binmore: Mathematical Analysis: A Straightforward Approach ... (next): $\S 1$: Real Numbers: $\S 1.1$: Set Notation
- 1977: Gary Chartrand: Introductory Graph Theory ... (next): Appendix $\text{A}.1$: Sets and Subsets
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- 1980: D.J. O'Connor and Betty Powell: Elementary Logic ... (previous) ... (next): $\S \text{III}$: The Logic of Predicates $(1): \ 3$: Quantifiers
- 1982: P.M. Cohn: Algebra Volume 1 (2nd ed.) ... (previous) ... (next): Chapter $1$: Sets and mappings: $\S 1.2$: Sets
- 1993: Keith Devlin: The Joy of Sets: Fundamentals of Contemporary Set Theory (2nd ed.) ... (previous) ... (next): $\S 1$: Naive Set Theory: $\S 1.1$: What is a Set?
- 1996: H. Jerome Keisler and Joel Robbin: Mathematical Logic and Computability ... (previous) ... (next): Appendix $\text{A}.1$: Sets
- 1999: András Hajnal and Peter Hamburger: Set Theory ... (previous) ... (next): $1$. Notation, Conventions: $1$
- 2000: James R. Munkres: Topology (2nd ed.) ... (previous) ... (next): $1$: Set Theory and Logic
- 2008: David Joyner: Adventures in Group Theory (2nd ed.) ... (previous) ... (next): Chapter $1$: Elementary, my dear Watson: $\S 1.2$: Elements, my dear Watson
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- 1983: George F. Simmons: Introduction to Topology and Modern Analysis ... (next): $\S 1$: Sets and Set Inclusion
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- 2012: M. Ben-Ari: Mathematical Logic for Computer Science (3rd ed.) ... (next): Appendix $\text{A}.1$: Definition $\text{A}.1$