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Finite set
In mathematics, a finite set is a set that has a finite number of elements. Informally, a finite set is a set which one could in principle count and finish counting. For example,
- {2,4,6,8,10}{displaystyle {2,4,6,8,10}}
is a finite set with five elements. The number of elements of a finite set is a natural number (a non-negative integer) and is called the cardinality of the set. A set that is not finite is called infinite. For example, the set of all positive integers is infinite:
- {1,2,3,…}.{displaystyle {1,2,3,ldots }.}
Finite sets are particularly important in combinatorics, the mathematical study of counting. Many arguments involving finite sets rely on the pigeonhole principle, which states that there cannot exist an injective function from a larger finite set to a smaller finite set.
Contents
1 Definition and terminology
2 Basic properties
3 Necessary and sufficient conditions for finiteness
4 Foundational issues
5 Set-theoretic definitions of finiteness
5.1 Other concepts of finiteness
6 See also
7 Notes
8 References
9 External links
Definition and terminology
Formally, a set S is called finite if there exists a bijection
- f:S→{1,…,n}{displaystyle fcolon Srightarrow {1,ldots ,n}}
for some natural number n. The number n is the set's cardinality, denoted as |S|. The empty set {} or Ø is considered finite, with cardinality zero.[1][2][3][4]
If a set is finite, its elements may be written — in many ways — in a sequence:
- x1,x2,…,xn(xi∈S,1≤i≤n).{displaystyle x_{1},x_{2},ldots ,x_{n}quad (x_{i}in S,1leq ileq n).}
In combinatorics, a finite set with n elements is sometimes called an n-set and a subset with k elements is called a k-subset. For example, the set {5,6,7} is a 3-set – a finite set with three elements – and {6,7} is a 2-subset of it.
(Those familiar with the definition of the natural numbers themselves as conventional in set theory, the so-called von Neumann construction, may prefer to use the existence of the more elegant bijection f:S→n{displaystyle fcolon Srightarrow n}
Basic properties
Any proper subset of a finite set S is finite and has fewer elements than S itself. As a consequence, there cannot exist a bijection between a finite set S and a proper subset of S. Any set with this property is called Dedekind-finite. Using the standard ZFC axioms for set theory, every Dedekind-finite set is also finite, but this implication cannot be proved in ZF (Zermelo Fraenkel axioms with the axiom of choice removed) alone.
The axiom of countable choice, a weak version of the axiom of choice, is sufficient to prove this equivalence.
Any injective function between two finite sets of the same cardinality is also a surjective function (a surjection). Similarly, any surjection between two finite sets of the same cardinality is also an injection.
The union of two finite sets is finite, with
- |S∪T|≤|S|+|T|.{displaystyle |Scup T|leq |S|+|T|.}
In fact:
- |S∪T|=|S|+|T|−|S∩T|.{displaystyle |Scup T|=|S|+|T|-|Scap T|.}
More generally, the union of any finite number of finite sets is finite. The Cartesian product of finite sets is also finite, with:
- |S×T|=|S|×|T|.{displaystyle |Stimes T|=|S|times |T|.}
Similarly, the Cartesian product of finitely many finite sets is finite. A finite set with n elements has 2n distinct subsets. That is, the
power set of a finite set is finite, with cardinality 2n.
Any subset of a finite set is finite. The set of values of a function when applied to elements of a finite set is finite.
All finite sets are countable, but not all countable sets are finite. (Some authors, however, use "countable" to mean "countably infinite", so do not consider finite sets to be countable.)
The free semilattice over a finite set is the set of its non-empty subsets, with the join operation being given by set union.
Necessary and sufficient conditions for finiteness
In Zermelo–Fraenkel set theory without the axiom of choice (ZF), the following conditions are all equivalent:[citation needed]
S is a finite set. That is, S can be placed into a one-to-one correspondence with the set of those natural numbers less than some specific natural number.- (Kazimierz Kuratowski) S has all properties which can be proved by mathematical induction beginning with the empty set and adding one new element at a time. (See below for the set-theoretical formulation of Kuratowski finiteness.)
- (Paul Stäckel) S can be given a total ordering which is well-ordered both forwards and backwards. That is, every non-empty subset of S has both a least and a greatest element in the subset.
- Every one-to-one function from P(P(S)) into itself is onto. That is, the powerset of the powerset of S is Dedekind-finite (see below).[5]
- Every surjective function from P(P(S)) onto itself is one-to-one.
- (Alfred Tarski) Every non-empty family of subsets of S has a minimal element with respect to inclusion.[6] (Equivalently, every non-empty family of subsets of S has a maximal element with respect to inclusion.)
S can be well-ordered and any two well-orderings on it are order isomorphic. In other words, the well-orderings on S have exactly one order type.
If the axiom of choice is also assumed (the axiom of countable choice is sufficient[7][citation needed]), then the following conditions are all equivalent:
S is a finite set.- (Richard Dedekind) Every one-to-one function from S into itself is onto.
- Every surjective function from S onto itself is one-to-one.
S is empty or every partial ordering of S contains a maximal element.
Foundational issues
Georg Cantor initiated his theory of sets in order to provide a mathematical treatment of infinite sets. Thus the distinction between the finite and the infinite lies at the core of set theory. Certain foundationalists, the strict finitists, reject the existence of infinite sets and thus recommend a mathematics based solely on finite sets. Mainstream mathematicians consider strict finitism too confining, but acknowledge its relative consistency: the universe of hereditarily finite sets constitutes a model of Zermelo–Fraenkel set theory with the axiom of infinity replaced by its negation.
Even for those mathematicians who embrace infinite sets, in certain important contexts, the formal distinction between the finite and the infinite can remain a delicate matter. The difficulty stems from Gödel's incompleteness theorems. One can interpret the theory of hereditarily finite sets within Peano arithmetic (and certainly also vice versa), so the incompleteness of the theory of Peano arithmetic implies that of the theory of hereditarily finite sets. In particular, there exists a plethora of so-called non-standard models of both theories. A seeming paradox, non-standard models of the theory of hereditarily finite sets contain infinite sets --- but these infinite sets look finite from within the model. (This can happen when the model lacks the sets or functions necessary to witness the infinitude of these sets.) On account of the incompleteness theorems, no first-order predicate, nor even any recursive scheme of first-order predicates, can characterize the standard part of all such models. So, at least from the point of view of first-order logic, one can only hope to describe finiteness approximately.
More generally, informal notions like set, and particularly finite set, may receive interpretations across a range of formal systems varying in their axiomatics and logical apparatus. The best known axiomatic set theories include Zermelo-Fraenkel set theory (ZF), Zermelo-Fraenkel set theory with the Axiom of Choice (ZFC), Von Neumann–Bernays–Gödel set theory (NBG), Non-well-founded set theory, Bertrand Russell's Type theory and all the theories of their various models. One may also choose among classical first-order logic, various higher-order logics and intuitionistic logic.
A formalist might see the meaning[citation needed] of set varying from system to system. Some kinds of Platonists might view particular formal systems as approximating an underlying reality.
Set-theoretic definitions of finiteness
In contexts where the notion of natural number sits logically prior to any notion of set, one can define a set S as finite if S admits a bijection to some set of natural numbers of the form {x|x<n}{displaystyle {x|x<n}}
Various properties that single out the finite sets among all sets in the theory ZFC turn out logically inequivalent in weaker systems such as ZF or intuitionistic set theories. Two definitions feature prominently in the literature, one due to Richard Dedekind, the other to Kazimierz Kuratowski. (Kuratowski's is the definition used above.)
A set S is called Dedekind infinite if there exists an injective, non-surjective function f:S→S{displaystyle f:Srightarrow S}
Kuratowski finiteness is defined as follows. Given any set S, the binary operation of union endows the powerset P(S) with the structure of a semi-lattice. Writing K(S) for the sub-semi-lattice generated by the empty set and the singletons, call set S Kuratowski finite if S itself belongs to K(S).[8] Intuitively, K(S) consists of the finite subsets of S. Crucially, one does not need induction, recursion or a definition of natural numbers to define generated by since one may obtain K(S) simply by taking the intersection of all sub-semi-lattices containing the empty set and the singletons.
Readers unfamiliar with semi-lattices and other notions of abstract algebra may prefer an entirely elementary formulation. Kuratowski finite means S lies in the set K(S), constructed as follows. Write M for the set of all subsets X of P(S) such that:
X contains the empty set;- For every set T in P(S), if X contains T then X also contains the union of T with any singleton.
Then K(S) may be defined as the intersection of M.
In ZF, Kuratowski finite implies Dedekind finite, but not vice versa. In the parlance of a popular pedagogical formulation, when the axiom of choice fails badly, one may have an infinite family of socks with no way to choose one sock from more than finitely many of the pairs. That would make the set of such socks Dedekind finite: there can be no infinite sequence of socks, because such a sequence would allow a choice of one sock for infinitely many pairs by choosing the first sock in the sequence. However, Kuratowski finiteness would fail for the same set of socks.
Other concepts of finiteness
In ZF set theory without the axiom of choice, the following concepts of finiteness for a set S are distinct. They are arranged in strictly decreasing order of strength. In other words, if a set S meets one of the criteria in this list, it meets all of the criteria which follow that one. In the absence of the axiom of choice, the reverse implications are all unprovable. If the axiom of choice is assumed, then all of these concepts are equivalent.[9] (Note that none of these definitions need the set of finite ordinal numbers to be defined first. They are all pure "set-theoretic" definitions in terms of the equality and element-of relations, not involving ω.)
I-finite. Every non-empty set of subsets of S has a ⊆-maximal element. (This is equivalent to requiring the existence of a ⊆-minimal element. It is also equivalent to the standard numerical concept of finiteness.)
Ia-finite. For every partition of S into two sets, at least one of the two sets is I-finite.
II-finite. Every non-empty ⊆-monotone set of subsets of S has a ⊆-maximal element.
III-finite. The power set P(S) is Dedekind finite.
IV-finite. S is Dedekind finite.
V-finite. ∣S∣ = 0 or 2⋅∣S∣ > ∣S|.
VI-finite. ∣S∣ = 0 or ∣S∣ = 1 or ∣S∣2 > ∣S∣.
VII-finite. S is I-finite or not well-orderable.
The forward implications (from strong to weak) are theorems within ZF. Counter-examples to the reverse implications (from weak to strong) are found using model theory.[10]
Most of these finiteness definitions and their names are attributed to Tarski 1954 by Howard & Rubin 1998, p. 278. However, definitions I, II, III, IV and V were presented in Tarski 1924, pp. 49, 93, together with proofs (or references to proofs) for the forward implications. At that time, model theory was not sufficiently advanced to find the counter-examples.
See also
- FinSet
- Ordinal number
- Peano arithmetic
Notes
^ Apostol (1974, p. 38)
^ Cohn (1981, p. 7)
^ Labarre (1968, p. 41)
^ Rudin (1976, p. 25)
^ The equivalence of the standard numerical definition of finite sets to the Dedekind-finiteness of the power set of the power set was shown in 1912 by Whitehead & Russell 2009, p. 288. This Whitehead/Russell theorem is described in more modern language by Tarski 1924, pp. 73–74.
^ Tarski 1924, pp. 48–58, demonstrated that his definition (which is also known as I-finite) is equivalent to Kuratowski's set-theoretical definition, which he then noted is equivalent to the standard numerical definition via the proof by Kuratowski 1920, pp. 130–131.
^ Canada, A.; Drabek, P.; Fonda, A. (2005-09-02). Handbook of Differential Equations: Ordinary Differential Equations. Elsevier. ISBN 9780080461083..mw-parser-output cite.citation{font-style:inherit}.mw-parser-output q{quotes:"""""""'""'"}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:inherit;padding:inherit}.mw-parser-output .cs1-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/6/65/Lock-green.svg/9px-Lock-green.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-limited a,.mw-parser-output .cs1-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/d/d6/Lock-gray-alt-2.svg/9px-Lock-gray-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/a/aa/Lock-red-alt-2.svg/9px-Lock-red-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{font-size:100%}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}
^ The original paper by Kuratowski 1920 defined a set S to be finite when
P(S)∖{∅} = ⋂{X ∈ P(P(S)∖{∅}); (∀x∈S, {x}∈X) and (∀A,B∈X, A∪B∈X)}.
In other words, S is finite when the set of all non-empty subsets of S is equal to the intersection of all classes X which satisfy:
- all elements of X are non-empty subsets of S,
- the set {x} is an element of X for all x in S,
X is closed under pairwise unions.
Kuratowski showed that this is equivalent to the numerical definition of a finite set.
^ This list of 8 finiteness concepts is presented with this numbering scheme by both Howard & Rubin 1998, pp. 278–280, and Lévy 1958, pp. 2–3, although the details of the presentation of the definitions differ in some respects which do not affect the meanings of the concepts.
^ Lévy 1958 found counter-examples to each of the reverse implications in Mostowski models. Lévy attributes most of the results to earlier papers by Mostowski and Lindenbaum.
References
Apostol, Tom M. (1974), Mathematical Analysis (2nd ed.), Menlo Park: Addison-Wesley, LCCN 72011473
Cohn, Paul Moritz, F.R.S. (1981), Universal Algebra, Dordrecht: D. Reidel, ISBN 90-277-1254-9, LCCN 80-29568
Dedekind, Richard (2012), Was sind und was sollen die Zahlen?, Cambridge Library Collection (Paperback ed.), Cambridge, UK: Cambridge University Press, ISBN 978-1-108-05038-8
Dedekind, Richard (1963), Essays on the Theory of Numbers, Dover Books on Mathematics, Beman, Wooster Woodruff (Paperback ed.), Dover Publications Inc., ISBN 0-486-21010-3
Herrlich, Horst (2006), Axiom of Choice, Lecture Notes in Math. 1876, Berlin: Springer-Verlag, ISBN 3-540-30989-6
Howard, Paul; Rubin, Jean E. (1998). Consequences of the axiom of choice. Providence, Rhode Island: American Mathematical Society. ISBN 9780821809778.
Kuratowski, Kazimierz (1920), "Sur la notion d'ensemble fini" (PDF), Fundamenta Mathematicae, 1: 129–131
Labarre, Jr., Anthony E. (1968), Intermediate Mathematical Analysis, New York: Holt, Rinehart and Winston, LCCN 68019130
Lévy, Azriel (1958). "The independence of various definitions of finiteness" (PDF). Fundamenta Mathematicae. 46: 1–13.
Rudin, Walter (1976), Principles Of Mathematical Analysis (3rd ed.), New York: McGraw-Hill, ISBN 0-07-054235-X
Suppes, Patrick (1972) [1960], Axiomatic Set Theory, Dover Books on Mathematics (Paperback ed.), Dover Publications Inc., ISBN 0-486-61630-4
Tarski, Alfred (1924). "Sur les ensembles finis" (PDF). Fundamenta Mathematicae. 6: 45–95.
Tarski, Alfred (1954). "Theorems on the existence of successors of cardinals, and the axiom of choice". Nederl. Akad. Wetensch. Proc. Ser. A., Indagationes Math. 16: 26–32. MR 0060555.
Whitehead, Alfred North; Russell, Bertrand (February 2009) [1912]. Principia Mathematica. Volume Two. Merchant Books. ISBN 978-1-60386-183-0.
External links
- Barile, Margherita. "Finite Set". MathWorld.
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