Erdős–Ko–Rado theorem

In mathematics, the Erdős–Ko–Rado theorem limits the number of sets in a family of sets for which every two sets have at least one element in common. Paul Erdős, Chao Ko, and Richard Rado proved the theorem in 1938, but did not publish it until 1961. It is part of the field of combinatorics, and one of the central results of
The theorem applies to families of sets that all have the same and are all subsets of some larger set of size One way to construct a family of sets with these parameters, each two sharing an element, is to choose a single element to belong to all the subsets, and then form all of the subsets of that contain the chosen element. The Erdős–Ko–Rado theorem states that when is large enough for the problem to be nontrivial this construction produces the largest possible intersecting families. When there are other equally-large families, but for larger values of only the families constructed in this way can be largest.
The Erdős–Ko–Rado theorem can also be described in terms of hypergraphs or independent sets in Kneser graphs. Several analogous theorems apply to other kinds of mathematical object than sets, including linear subspaces, permutations, and strings. They again describe the largest possible intersecting families as being formed by choosing an element and forming the family of all objects that contain the chosen element.

Statement

Suppose that is a family of distinct subsets of an set and that each two subsets share at least one element. Then the theorem states that the number of sets in is at most the binomial coefficient
The requirement that is necessary for the problem to be nontrivial: all sets intersect, and the largest intersecting family consists of all sets, with
The same result can be formulated as part of the theory of hypergraphs. A family of sets may also be called a hypergraph, and when all the sets are the same it is called an hypergraph. The theorem thus gives an upper bound for the number of pairwise overlapping hyperedges in an hypergraph with
The theorem may also be formulated in terms of graph theory: the independence number of the Kneser graph for is
This is a graph with a vertex for each subset of an set, and an edge between every pair of disjoint sets. An independent set is a collection of vertices that has no edges between its pairs, and the independence number is the size of the largest Because Kneser graphs have symmetries taking any vertex to any other vertex, their fractional chromatic number equals the ratio of their number of vertices to their independence number, so another way of expressing the Erdős–Ko–Rado theorem is that these graphs have fractional chromatic number

History

Paul Erdős, Chao Ko, and Richard Rado proved this theorem in 1938 after working together on it in England. Rado had moved from Berlin to the University of Cambridge and Erdős from Hungary to the University of Manchester, both escaping the influence of Nazi Germany; Ko was a student of Louis J. Mordell at However, they did not publish the result with the long delay occurring in part because of a lack of interest in combinatorial set theory in the 1930s, and increased interest in the topic in The 1961 paper stated the result in an apparently more general form, in which the subsets were only required to be size and to satisfy the additional requirement that no subset be contained in any A family of subsets meeting these conditions can be enlarged to subsets of size either by an application of or by choosing each enlarged subset from the same chain in a symmetric chain decomposition

Maximum and maximal families

Families of maximum size

A simple way to construct an intersecting family of sets whose size exactly matches the Erdős–Ko–Rado bound is to choose any fixed and let consist of all subsets that For instance, for 2-element subsets of the 4-element this produces the family
Any two sets in this family intersect, because they both The number of sets is, because after the fixed element is chosen there remain other elements to choose, and each set chooses of these remaining elements.
When this is the only intersecting family of this size. However, when, there is a more general construction. Each set can be matched up to its complement, the only set from which it is disjoint. Then, choose one set from each of these complementary pairs. For instance, for the same parameters above, this more general construction can be used to form the family
where every two sets intersect despite no element belonging to all three sets. In this example, all of the sets have been complemented from the ones in the first example, but it is also possible to complement only some of the sets.
families of the first type are the unique maximum families. In this case, a family of nearly-maximum size has an element which is common to almost all of its This property has been called although the same term has also been used for a different property, the fact that deleting randomly-chosen edges from the Kneser graph does not increase the size of its independent

Maximal intersecting families

An intersecting family of sets may be maximal, in that no further set can be added without destroying the intersection property, but not of maximum size. An example with and is the set of seven lines of the Fano plane, much less than the Erdős–Ko–Rado bound More generally, the lines of any finite projective plane of order form a maximal intersecting family that includes only sets, for the parameters The Fano plane is the case of this construction.
The smallest possible size of a maximal intersecting family of sets, in terms is unknown but at least Projective planes produce maximal intersecting families whose number of sets but for infinitely many choices of there exist smaller maximal intersecting families of
The largest intersecting families of sets that are maximal but not maximum have size
They are formed from an and an not by adding to the family of sets that include both and at least one element This result is called the Hilton–Milner theorem, after its proof by Anthony Hilton and Eric Charles Milner in

Proofs

The original proof of the Erdős–Ko–Rado theorem used induction The base case, follows easily from the facts that an intersecting family cannot include both a set and its complement, and that in this case the bound of the Erdős–Ko–Rado theorem is exactly half the number of all sets. The induction step for uses a method called shifting, of substituting elements in intersecting families to make the family smaller in lexicographic order and reduce it to a canonical form that is easier to
In 1972, Gyula O. H. Katona gave the following short proof using
It is also possible to derive the Erdős–Ko–Rado theorem as a special case of the Kruskal–Katona theorem, another important result in Many other proofs are

Related results

Generalizations

A generalization of the theorem applies to subsets that are required to have large intersections. This version of the theorem has three parameters:, the number of elements the subsets are drawn from,, the size of the subsets, as before, and, the minimum size of the intersection of any two subsets. For the original form of the Erdős–Ko–Rado theorem, In general, for large enough with respect to the other two parameters, the generalized theorem states that the size of a family of subsets is at
More precisely, this bound holds and does not hold for smaller values When the only families of this size are obtained by designating as the common intersection of all the subsets, and constructing the family of all subsets that include these designated The maximal size of a -intersecting family when was determined by Ahlswede and Khachatrian, in their Ahlswede–Khachatrian theorem.
The corresponding graph-theoretic formulation of this generalization involves Johnson graphs in place of Kneser For large enough values of and in particular both the Erdős–Ko–Rado theorem and its generalization can be strengthened from the independence number to the Shannon capacity of a graph: the Johnson graph corresponding to the subsets has Shannon
The theorem can also be generalized to families in which every subsets have a common intersection. Because this strengthens the condition that every pair intersects (for which these families have the same bound on their maximum size, when is sufficiently large. However, in this case the meaning of "sufficiently large" can be relaxed from

Analogs

Many results analogous to the Erdős–Ko–Rado theorem, but for other classes of objects than finite sets, are known. These generally involve a statement that the largest families of intersecting objects, for some definition of intersection, are obtained by choosing an element and constructing the family of all objects that include that chosen element. Examples include the following:
There is a -analog of the Erdős–Ko–Rado theorem for intersecting families of linear subspaces over finite fields. If is an intersecting family of subspaces of an vector space over a finite field of then
where the subscript marks the notation for the Gaussian binomial coefficient, the number of subspaces of a given dimension within a vector space of a larger dimension over a finite field of In this case, a largest intersecting family of subspaces may be obtained by choosing any nonzero vector and constructing the family of subspaces of the given dimension that all contain the chosen
Two permutations on the same set of elements are defined to be intersecting if there is some element that has the same image under both permutations. On an set, there is an obvious family of intersecting permutations, the permutations that fix one of the elements. The analogous theorem is that no intersecting family of permutations can be larger, and that the only intersecting families of size are the cosets of one-element stabilizers. These can be described more directly as the families of permutations that map some fixed element to another fixed element. More generally, for any and sufficiently large, a family of permutations each pair of which has elements in common has maximum size, and the only families of this size are cosets of pointwise Alternatively, in graph theoretic terms, the permutations correspond to the perfect matchings of a complete bipartite graph and the theorem states that, among families of perfect matchings each pair of which share edges, the largest families are formed by the matchings that all contain chosen Another analog of the theorem, for partitions of a set, includes as a special case the perfect matchings of a complete graph . There are matchings, where denotes the double factorial. The largest family of matchings that pairwise intersect has size and is obtained by fixing one edge and choosing all ways of matching the remaining vertices.
A partial geometry is a system of finitely many abstract points and lines, satisfying certain axioms including the requirement that all lines contain the same number of points and all points belong to the same number of lines. In a partial geometry, a largest system of pairwise-intersecting lines can be obtained from the set of lines through any single
A signed set consists of a set together with sign function that maps each element Two signed sets may be said to intersect when they have a common element that has the same sign in each of them. Then an intersecting family of signed sets, drawn from an universe, consists of at most
signed sets. This number of signed sets may be obtained by fixing one element and its sign and letting the remaining elements and signs
For strings of over an alphabet of two strings can be defined to intersect if they have a position where both share the same symbol. The largest intersecting families are obtained by choosing one position and a fixed symbol for that position, and letting the rest of the positions vary arbitrarily. These families consist of strings, and are the only pairwise intersecting families of this size. More generally, the largest families of strings in which every two have positions with equal symbols are obtained by choosing positions and symbols for those positions, for a number that depends on,, and, and constructing the family of strings that each have at least of the chosen symbols. These results can be interpreted graph-theoretically in terms of the
An unproven conjecture, posed by Gil Kalai and Karen Meagher, concerns another analog for the family of triangulations of a convex polygon with vertices. The number of all triangulations is a and the conjecture states that a family of triangulations every pair of which shares an edge has maximum An intersecting family of size exactly may be obtained by cutting off a single vertex of the polygon by a triangle, and choosing all ways of triangulating the remaining polygon.

Applications

The Erdős–Ko–Rado theorem can be used to prove the following result in probability theory. Let be independent 0–1 random variables with probability of being one, and let be any fixed convex combination of these variables. Then
The proof involves observing that subsets of variables whose indicator vectors have large convex combinations must be non-disjoint and using the Erdős–Ko–Rado theorem to bound the number of these
The stability properties of the Erdős–Ko–Rado theorem play a key role in an efficient algorithm for finding monochromatic edges in improper colorings of Kneser graphs. The Erdős–Ko–Rado theorem has also been used to characterize the symmetries of the space of phylogenetic trees.